diff --git a/nimbus/db/aristo.nim b/nimbus/db/aristo.nim
index 95a092836..dd6b47756 100644
--- a/nimbus/db/aristo.nim
+++ b/nimbus/db/aristo.nim
@@ -25,8 +25,7 @@ import
 export
   append, read
 
-import aristo/aristo_desc/[
-  aristo_types_identifiers, aristo_types_structural]
+import aristo/aristo_desc/[desc_identifiers, desc_structural]
 export
   AristoAccount,
   PayloadRef,
diff --git a/nimbus/db/aristo/README.md b/nimbus/db/aristo/README.md
index 402a04431..6a1c6a6b7 100644
--- a/nimbus/db/aristo/README.md
+++ b/nimbus/db/aristo/README.md
@@ -402,7 +402,8 @@ assumed, i.e. the list with the single vertex ID *1*.
 
 This list is used to control the filters on the database. By holding some IDs
 in a dedicated list (e.g. the latest filters) one can quickly access particular
-entries without searching through the set of filters.
+entries without searching through the set of filters. In the current
+implementation this list comes in ID pairs i.e. the number of entries is even.
 
 ### 4.10 Serialisation record identifier identification
 
diff --git a/nimbus/db/aristo/aristo_constants.nim b/nimbus/db/aristo/aristo_constants.nim
index c2064c5f0..01706e506 100644
--- a/nimbus/db/aristo/aristo_constants.nim
+++ b/nimbus/db/aristo/aristo_constants.nim
@@ -12,7 +12,7 @@
 
 import
   eth/[common, trie/nibbles],
-  ./aristo_desc/aristo_types_identifiers
+  ./aristo_desc/desc_identifiers
 
 const
   EmptyBlob* = seq[byte].default
@@ -24,7 +24,7 @@ const
   EmptyVidSeq* = seq[VertexID].default
     ## Useful shortcut
 
-  EmptyFidSeq* = seq[FilterID].default
+  EmptyQidPairSeq* = seq[(QueueID,QueueID)].default
     ## Useful shortcut
 
   VOID_CODE_HASH* = EMPTY_CODE_HASH
@@ -36,4 +36,10 @@ const
   VOID_HASH_LABEL* = HashLabel(root: VertexID(0), key: VOID_HASH_KEY)
     ## Void equivalent for Merkle hash value
 
+  DEFAULT_QID_QUEUES* = [
+    (128,   0), ## Consecutive list of 128 filter slots
+    ( 64,  63), ## Overflow list, 64 filters, skipping 63 filters in-between
+    ( 64, 127), ## ..
+    ( 64, 255)]
+
 # End
diff --git a/nimbus/db/aristo/aristo_debug.nim b/nimbus/db/aristo/aristo_debug.nim
index fd3c0cf93..dd097a315 100644
--- a/nimbus/db/aristo/aristo_debug.nim
+++ b/nimbus/db/aristo/aristo_debug.nim
@@ -16,7 +16,8 @@ import
   results,
   stew/byteutils,
   "."/[aristo_constants, aristo_desc, aristo_hike, aristo_init],
-  ./aristo_init/[aristo_memory, aristo_rocksdb]
+  ./aristo_init/[memory_db, rocks_db],
+  ./aristo_filter/filter_desc
 
 export
   TypedBackendRef, aristo_init.to
@@ -85,11 +86,28 @@ proc ppVid(vid: VertexID; pfx = true): string =
   else:
     result &= "ø"
 
-proc ppFid(fid: FilterID): string =
-  if fid.isValid:
-    "%" & fid.uint64.toHex.stripZeros.toLowerAscii
-  else:
-    "ø"
+proc ppQid(qid: QueueID): string =
+  if not qid.isValid:
+    return "ø"
+  let
+    chn = qid.uint64 shr 62
+    qid = qid.uint64 and 0x3fff_ffff_ffff_ffffu64
+  result = "%"
+  if 0 < chn:
+    result &= $chn & ":"
+
+  if 0x0fff_ffff_ffff_ffffu64 <= qid.uint64:
+    block here:
+      if qid.uint64 == 0x0fff_ffff_ffff_ffffu64:
+        result &= "(2^60-1)"
+      elif qid.uint64 == 0x1fff_ffff_ffff_ffffu64:
+        result &= "(2^61-1)"
+      elif qid.uint64 == 0x3fff_ffff_ffff_ffffu64:
+        result &= "(2^62-1)"
+      else:
+        break here
+      return
+  result &= qid.uint64.toHex.stripZeros.toLowerAscii
 
 proc ppVidList(vGen: openArray[VertexID]): string =
   "[" & vGen.mapIt(it.ppVid).join(",") & "]"
@@ -450,8 +468,17 @@ proc pp*(lty: LeafTie, db = AristoDbRef()): string =
 proc pp*(vid: VertexID): string =
   vid.ppVid
 
-proc pp*(fid: FilterID): string =
-  fid.ppFid
+proc pp*(qid: QueueID): string =
+  qid.ppQid
+
+proc pp*(a: openArray[(QueueID,QueueID)]): string =
+  "[" & a.toSeq.mapIt("(" & it[0].pp & "," & it[1].pp & ")").join(",") & "]"
+
+proc pp*(a: QidAction): string =
+  ($a.op).replace("Qid", "") & "(" & a.qid.pp & "," & a.xid.pp & ")"
+
+proc pp*(a: openArray[QidAction]): string =
+  "[" & a.toSeq.mapIt(it.pp).join(",") & "]"
 
 proc pp*(vGen: openArray[VertexID]): string =
   vGen.ppVidList
diff --git a/nimbus/db/aristo/aristo_desc.nim b/nimbus/db/aristo/aristo_desc.nim
index 41bd42735..a4805c905 100644
--- a/nimbus/db/aristo/aristo_desc.nim
+++ b/nimbus/db/aristo/aristo_desc.nim
@@ -25,16 +25,14 @@ import
   std/[hashes, sets, tables],
   eth/common,
   ./aristo_constants,
-  ./aristo_desc/[
-    aristo_error, aristo_types_identifiers, aristo_types_structural]
+  ./aristo_desc/[desc_error, desc_identifiers, desc_structural]
 
-from ./aristo_desc/aristo_types_backend
+from ./aristo_desc/desc_backend
   import BackendRef
 
 # Not auto-exporting backend
 export
-  aristo_constants, aristo_error, aristo_types_identifiers,
-  aristo_types_structural
+  aristo_constants, desc_error, desc_identifiers, desc_structural
 
 type
   AristoTxRef* = ref object
@@ -108,8 +106,8 @@ func isValid*(lbl: HashLabel): bool =
 func isValid*(vid: VertexID): bool =
   vid != VertexID(0)
 
-func isValid*(fid: FilterID): bool =
-  fid != FilterID(0)
+func isValid*(qid: QueueID): bool =
+  qid != QueueID(0)
 
 # ------------------------------------------------------------------------------
 # Public functions, miscellaneous
@@ -120,8 +118,8 @@ func hash*(db: AristoDbRef): Hash =
   ## Table/KeyedQueue/HashSet mixin
   cast[pointer](db).hash
 
-# Note that the below `init()` function cannot go into
-# `aristo_types_identifiers` as this would result in a circular import.
+# Note that the below `init()` function cannot go into `desc_identifiers` as
+# this would result in a circular import.
 func init*(key: var HashKey; data: openArray[byte]): bool =
   ## Import argument `data` into `key` which must have length either `32`, or
   ## `0`. The latter case is equivalent to an all zero byte array of size `32`.
diff --git a/nimbus/db/aristo/aristo_desc/aristo_types_backend.nim b/nimbus/db/aristo/aristo_desc/desc_backend.nim
similarity index 86%
rename from nimbus/db/aristo/aristo_desc/aristo_types_backend.nim
rename to nimbus/db/aristo/aristo_desc/desc_backend.nim
index 578ce95fb..5fd25df9f 100644
--- a/nimbus/db/aristo/aristo_desc/aristo_types_backend.nim
+++ b/nimbus/db/aristo/aristo_desc/desc_backend.nim
@@ -16,7 +16,7 @@
 
 import
   results,
-  "."/[aristo_error, aristo_types_identifiers, aristo_types_structural]
+  "."/[desc_error, desc_identifiers, desc_structural]
 
 type
   GetVtxFn* =
@@ -30,7 +30,7 @@ type
       ## `Aristo DB` hash lookup value.
 
   GetFilFn* =
-    proc(fid: FilterID): Result[FilterRef,AristoError]
+    proc(qid: QueueID): Result[FilterRef,AristoError]
       {.gcsafe, raises: [].}
         ## Generic backend database retrieval function for a filter record.
 
@@ -39,10 +39,10 @@ type
       ## Generic backend database retrieval function for a the ID generator
       ## `Aristo DB` state record.
 
-  GetFasFn* =
-    proc(): Result[seq[FilterID],AristoError] {.gcsafe, raises: [].}
-      ## Generic backend database retrieval function for some administartion
-      ## of the filters (e.g. the top ID.)
+  GetFqsFn* =
+    proc(): Result[seq[(QueueID,QueueID)],AristoError] {.gcsafe, raises: [].}
+      ## Generic backend database retrieval function for some filter queue
+      ## administration data (e.g. the bottom/top ID.)
 
   # -------------
 
@@ -69,7 +69,7 @@ type
         ## values indicate that records should be deleted.
 
   PutFilFn* =
-    proc(hdl: PutHdlRef; vf: openArray[(FilterID,FilterRef)])
+    proc(hdl: PutHdlRef; qf: openArray[(QueueID,FilterRef)])
       {.gcsafe, raises: [].}
         ## Generic backend database storage function for filter records.
 
@@ -79,11 +79,11 @@ type
         ## Generic backend database ID generator state storage function. This
         ## function replaces the current generator state.
 
-  PutFasFn* =
-    proc(hdl: PutHdlRef; vs: openArray[FilterID])
+  PutFqsFn* =
+    proc(hdl: PutHdlRef; vs: openArray[(QueueID,QueueID)])
       {.gcsafe, raises: [].}
         ## Generic backend database filter ID state storage function. This
-        ## function replaces the current filter ID state.
+        ## function replaces the current filter ID list.
 
   PutEndFn* =
     proc(hdl: PutHdlRef): AristoError {.gcsafe, raises: [].}
@@ -106,14 +106,14 @@ type
     getKeyFn*: GetKeyFn              ## Read Merkle hash/key
     getFilFn*: GetFilFn              ## Read back log filter
     getIdgFn*: GetIdgFn              ## Read vertex ID generator state
-    getFasFn*: GetFasFn              ## Read filter ID state
+    getFqsFn*: GetFqsFn              ## Read filter ID state
 
     putBegFn*: PutBegFn              ## Start bulk store session
     putVtxFn*: PutVtxFn              ## Bulk store vertex records
     putKeyFn*: PutKeyFn              ## Bulk store vertex hashes
     putFilFn*: PutFilFn              ## Store back log filter
     putIdgFn*: PutIdgFn              ## Store ID generator state
-    putFasFn*: PutFasFn              ## Store filter ID state
+    putFqsFn*: PutFqsFn              ## Store filter ID state
     putEndFn*: PutEndFn              ## Commit bulk store session
 
     closeFn*: CloseFn                ## Generic destructor
diff --git a/nimbus/db/aristo/aristo_desc/aristo_error.nim b/nimbus/db/aristo/aristo_desc/desc_error.nim
similarity index 99%
rename from nimbus/db/aristo/aristo_desc/aristo_error.nim
rename to nimbus/db/aristo/aristo_desc/desc_error.nim
index e97288cf3..871b829d9 100644
--- a/nimbus/db/aristo/aristo_desc/aristo_error.nim
+++ b/nimbus/db/aristo/aristo_desc/desc_error.nim
@@ -188,7 +188,7 @@ type
     GetKeyNotFound
     GetFilNotFound
     GetIdgNotFound
-    GetFasNotFound
+    GetFqsNotFound
 
     # RocksDB backend
     RdbBeCantCreateDataDir
diff --git a/nimbus/db/aristo/aristo_desc/aristo_types_identifiers.nim b/nimbus/db/aristo/aristo_desc/desc_identifiers.nim
similarity index 86%
rename from nimbus/db/aristo/aristo_desc/aristo_types_identifiers.nim
rename to nimbus/db/aristo/aristo_desc/desc_identifiers.nim
index 75bfeaf00..9cf3d32b0 100644
--- a/nimbus/db/aristo/aristo_desc/aristo_types_identifiers.nim
+++ b/nimbus/db/aristo/aristo_desc/desc_identifiers.nim
@@ -23,9 +23,13 @@ type
   ByteArray32* = array[32,byte]
     ## Used for 32 byte hash components repurposed as Merkle hash labels.
 
-  FilterID* = distinct uint64
+  QueueID* = distinct uint64
     ## Identifier used to tag filter logs stored on the backend.
 
+  FilterID* = distinct uint64
+    ## Identifier used to identify a particular filter. It is generatied with the
+    ## filter.
+
   VertexID* = distinct uint64
     ## Unique identifier for a vertex of the `Aristo Trie`. The vertex is the
     ## prefix tree (aka `Patricia Trie`) component. When augmented by hash
@@ -84,21 +88,44 @@ func `==`*(a, b: VertexID): bool {.borrow.}
 func cmp*(a, b: VertexID): int {.borrow.}
 func `$`*(a: VertexID): string {.borrow.}
 
-func `==`*(a: VertexID; b: static[uint]): bool =
-  a == VertexID(b)
+func `==`*(a: VertexID; b: static[uint]): bool = (a == VertexID(b))
 
-# Scalar model extension for `IntervalSetRef[VertexID,uint64]`
-proc `+`*(a: VertexID; b: uint64): VertexID = (a.uint64+b).VertexID
-proc `-`*(a: VertexID; b: uint64): VertexID = (a.uint64-b).VertexID
-proc `-`*(a, b: VertexID): uint64 = (a.uint64 - b.uint64)
+# Scalar model extension as in `IntervalSetRef[VertexID,uint64]`
+func `+`*(a: VertexID; b: uint64): VertexID = (a.uint64+b).VertexID
+func `-`*(a: VertexID; b: uint64): VertexID = (a.uint64-b).VertexID
+func `-`*(a, b: VertexID): uint64 = (a.uint64 - b.uint64)
+
+# ------------------------------------------------------------------------------
+# Public helpers: `QueueID` scalar data model
+# ------------------------------------------------------------------------------
+
+func `<`*(a, b: QueueID): bool {.borrow.}
+func `<=`*(a, b: QueueID): bool {.borrow.}
+func `==`*(a, b: QueueID): bool {.borrow.}
+func cmp*(a, b: QueueID): int {.borrow.}
+func `$`*(a: QueueID): string {.borrow.}
+
+func `==`*(a: QueueID; b: static[uint]): bool = (a == QueueID(b))
+
+func `+`*(a: QueueID; b: uint64): QueueID = (a.uint64+b).QueueID
+func `-`*(a: QueueID; b: uint64): QueueID = (a.uint64-b).QueueID
+func `-`*(a, b: QueueID): uint64 = (a.uint64 - b.uint64)
 
 # ------------------------------------------------------------------------------
 # Public helpers: `FilterID` scalar data model
 # ------------------------------------------------------------------------------
 
+func `<`*(a, b: FilterID): bool {.borrow.}
+func `<=`*(a, b: FilterID): bool {.borrow.}
 func `==`*(a, b: FilterID): bool {.borrow.}
 func `$`*(a: FilterID): string {.borrow.}
 
+func `==`*(a: FilterID; b: static[uint]): bool = (a == FilterID(b))
+
+func `+`*(a: FilterID; b: uint64): FilterID = (a.uint64+b).FilterID
+func `-`*(a: FilterID; b: uint64): FilterID = (a.uint64-b).FilterID
+func `-`*(a, b: FilterID): uint64 = (a.uint64 - b.uint64)
+
 # ------------------------------------------------------------------------------
 # Public helpers: `HashID` scalar data model
 # ------------------------------------------------------------------------------
diff --git a/nimbus/db/aristo/aristo_desc/aristo_types_structural.nim b/nimbus/db/aristo/aristo_desc/desc_structural.nim
similarity index 99%
rename from nimbus/db/aristo/aristo_desc/aristo_types_structural.nim
rename to nimbus/db/aristo/aristo_desc/desc_structural.nim
index 54c9004d2..49b138d0e 100644
--- a/nimbus/db/aristo/aristo_desc/aristo_types_structural.nim
+++ b/nimbus/db/aristo/aristo_desc/desc_structural.nim
@@ -17,7 +17,7 @@
 import
   std/[sets, tables],
   eth/[common, trie/nibbles],
-  "."/[aristo_error, aristo_types_identifiers]
+  "."/[desc_error, desc_identifiers]
 
 type
   VertexType* = enum
diff --git a/nimbus/db/aristo/aristo_filter.nim b/nimbus/db/aristo/aristo_filter.nim
index e47fc59c4..017a27597 100644
--- a/nimbus/db/aristo/aristo_filter.nim
+++ b/nimbus/db/aristo/aristo_filter.nim
@@ -15,131 +15,9 @@
 import
   std/[sequtils, sets, tables],
   results,
-  ./aristo_desc/aristo_types_backend,
-  "."/[aristo_desc, aristo_get, aristo_vid]
-
-type
-  StateRootPair = object
-    be: HashKey
-    fg: HashKey
-
-# ------------------------------------------------------------------------------
-# Private helpers
-# ------------------------------------------------------------------------------
-
-proc getLayerStateRoots(
-    db: AristoDbRef;
-    layer: LayerRef;
-    chunkedMpt: bool;
-      ): Result[StateRootPair,AristoError] =
-  ## Get the Merkle hash key for target state root to arrive at after this
-  ## reverse filter was applied.
-  var spr: StateRootPair
-
-  spr.be = block:
-    let rc = db.getKeyBE VertexID(1)
-    if rc.isOk:
-      rc.value
-    elif rc.error == GetKeyNotFound:
-      VOID_HASH_KEY
-    else:
-      return err(rc.error)
-
-  block:
-    spr.fg = layer.kMap.getOrVoid(VertexID 1).key
-    if spr.fg.isValid:
-      return ok(spr)
-
-  if chunkedMpt:
-    let vid = layer.pAmk.getOrVoid HashLabel(root: VertexID(1), key: spr.be)
-    if vid == VertexID(1):
-      spr.fg = spr.be
-      return ok(spr)
-
-  if layer.sTab.len == 0 and
-     layer.kMap.len == 0 and
-     layer.pAmk.len == 0:
-    return err(FilPrettyPointlessLayer)
-
-  err(FilStateRootMismatch)
-
-# ------------------------------------------------------------------------------
-# Private functions
-# ------------------------------------------------------------------------------
-
-proc merge(
-    db: AristoDbRef;
-    upper: FilterRef;                          # Src filter, `nil` is ok
-    lower: FilterRef;                          # Trg filter, `nil` is ok
-    beStateRoot: HashKey;                      # Merkle hash key
-      ): Result[FilterRef,(VertexID,AristoError)] =
-  ## Merge argument `upper` into the `lower` filter instance.
-  ##
-  ## Comparing before and after merge
-  ## ::
-  ##   current                     | merged
-  ##   ----------------------------+--------------------------------
-  ##   trg2 --upper-- (src2==trg1) |
-  ##                               | trg2 --newFilter-- (src1==trg0)
-  ##   trg1 --lower-- (src1==trg0) |
-  ##                               |
-  ##   trg0 --beStateRoot          | trg0 --beStateRoot
-  ##                               |
-  ##
-  # Degenerate case: `upper` is void
-  if lower.isNil:
-    if upper.isNil:
-      # Even more degenerate case when both filters are void
-      return ok FilterRef(nil)
-    if upper.src != beStateRoot:
-      return err((VertexID(1),FilStateRootMismatch))
-    return ok(upper)
-
-  # Degenerate case: `upper` is non-trivial and `lower` is void
-  if upper.isNil:
-    if lower.src != beStateRoot:
-      return err((VertexID(0), FilStateRootMismatch))
-    return ok(lower)
-
-  # Verify stackability
-  if upper.src != lower.trg or
-     lower.src != beStateRoot:
-    return err((VertexID(0), FilStateRootMismatch))
-
-  # There is no need to deep copy table vertices as they will not be modified.
-  let newFilter = FilterRef(
-    src:  lower.src,
-    sTab: lower.sTab,
-    kMap: lower.kMap,
-    vGen: upper.vGen,
-    trg:  upper.trg)
-
-  for (vid,vtx) in upper.sTab.pairs:
-    if vtx.isValid or not newFilter.sTab.hasKey vid:
-      newFilter.sTab[vid] = vtx
-    elif newFilter.sTab.getOrVoid(vid).isValid:
-      let rc = db.getVtxUBE vid
-      if rc.isOk:
-        newFilter.sTab[vid] = vtx # VertexRef(nil)
-      elif rc.error == GetVtxNotFound:
-        newFilter.sTab.del vid
-      else:
-        return err((vid,rc.error))
-
-  for (vid,key) in upper.kMap.pairs:
-    if key.isValid or not newFilter.kMap.hasKey vid:
-      newFilter.kMap[vid] = key
-    elif newFilter.kMap.getOrVoid(vid).isValid:
-      let rc = db.getKeyUBE vid
-      if rc.isOk:
-        newFilter.kMap[vid] = key # VOID_HASH_KEY
-      elif rc.error == GetKeyNotFound:
-        newFilter.kMap.del vid
-      else:
-        return err((vid,rc.error))
-
-  ok newFilter
-
+  "."/[aristo_desc, aristo_get, aristo_vid],
+  ./aristo_desc/desc_backend,
+  ./aristo_filter/[filter_desc, filter_helpers]
 
 # ------------------------------------------------------------------------------
 # Public helpers
diff --git a/nimbus/db/aristo/aristo_filter/filter_desc.nim b/nimbus/db/aristo/aristo_filter/filter_desc.nim
new file mode 100644
index 000000000..525f520c6
--- /dev/null
+++ b/nimbus/db/aristo/aristo_filter/filter_desc.nim
@@ -0,0 +1,117 @@
+# nimbus-eth1
+# Copyright (c) 2021 Status Research & Development GmbH
+# Licensed under either of
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
+#    http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or
+#    http://opensource.org/licenses/MIT)
+# at your option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+import
+  ../aristo_desc
+
+type
+  StateRootPair* = object
+    ## Helper structure for analysing state roots.
+    be*: HashKey                   ## Backend state root
+    fg*: HashKey                   ## Layer or filter implied state root
+
+  # ----------------
+
+  QidAction* = object
+    ## Instruction for administering filter queue ID slots. The op-code is
+    ## followed by one or two queue ID arguments. In case of a two arguments,
+    ## the value of the second queue ID is never smaller than the first one.
+    op*: QidOp                     ## Action, followed by at most two queue IDs
+    qid*: QueueID                  ## Action argument
+    xid*: QueueID                  ## Second action argument for range argument
+
+  QidOp* = enum
+    Oops = 0
+    SaveQid                        ## Store new item
+    HoldQid                        ## Move/append range items to local queue
+    DequQid                        ## Store merged local queue items
+
+  QidLayoutRef* = ref object
+    ## Layout of cascaded list of filter ID slot queues where a slot queue
+    ## with index `N+1` serves as an overflow queue of slot queue `N`.
+    q*: array[4,QidSpec]
+
+  QidSpec* = tuple
+    ## Layout of a filter ID slot queue
+    size: uint                     ## Capacity of queue, length within `1..wrap`
+    width: uint                    ## Instance gaps (relative to prev. item)
+    wrap: QueueID                  ## Range `1..wrap` for round-robin queue
+
+  QidSchedRef* = ref object of RootRef
+    ## Current state of the filter queues
+    ctx*: QidLayoutRef             ## Organisation of the FIFO
+    state*: seq[(QueueID,QueueID)] ## Current fill state
+
+const
+  DefaultQidWrap = QueueID(0x3fff_ffff_ffff_ffffu64)
+
+  QidSpecSizeMax* = high(uint32).uint
+    ## Maximum value allowed for a `size` value of a `QidSpec` object
+
+  QidSpecWidthMax* = high(uint32).uint
+    ## Maximum value allowed for a `width` value of a `QidSpec` object
+
+# ------------------------------------------------------------------------------
+# Private helpers
+# ------------------------------------------------------------------------------
+
+func max(a, b, c: int): int =
+  max(max(a,b),c)
+
+# ------------------------------------------------------------------------------
+# Public helpers
+# ------------------------------------------------------------------------------
+
+func to*(a: array[4,tuple[size, width: int]]; T: type QidLayoutRef): T =
+  ## Convert a size-width array to a `QidLayoutRef` layout. Over large
+  ## array field values are adjusted to its maximal size.
+  var q: array[4,QidSpec]
+  for n in 0..3:
+    q[n] = (min(a[n].size.uint, QidSpecSizeMax),
+            min(a[n].width.uint, QidSpecWidthMax),
+            DefaultQidWrap)
+  q[0].width = 0
+  T(q: q)
+
+func to*(a: array[4,tuple[size, width, wrap: int]]; T: type QidLayoutRef): T =
+  ## Convert a size-width-wrap array to a `QidLayoutRef` layout. Over large
+  ## array field values are adjusted to its maximal size. Too small `wrap`
+  ## values are adjusted to its minimal size.
+  var q: array[4,QidSpec]
+  for n in 0..2:
+    q[n] = (min(a[n].size.uint, QidSpecSizeMax),
+            min(a[n].width.uint, QidSpecWidthMax),
+            QueueID(max(a[n].size + a[n+1].width, a[n].width+1, a[n].wrap)))
+  q[0].width = 0
+  q[3] = (min(a[3].size.uint, QidSpecSizeMax),
+          min(a[3].width.uint, QidSpecWidthMax),
+          QueueID(max(a[3].size, a[3].width, a[3].wrap)))
+  T(q: q)
+
+# ------------------------------------------------------------------------------
+# Public constructors
+# ------------------------------------------------------------------------------
+
+func init*(T: type QidSchedRef; a: array[4,(int,int)]): T =
+  ## Constructor, see comments at the coverter function `to()` for adjustments
+  ## of the layout argument `a`.
+  T(ctx: a.to(QidLayoutRef))
+
+func init*(T: type QidSchedRef; a: array[4,(int,int,int)]): T =
+  ## Constructor, see comments at the coverter function `to()` for adjustments
+  ## of the layout argument `a`.
+  T(ctx: a.to(QidLayoutRef))
+
+func init*(T: type QidSchedRef; ctx: QidLayoutRef): T =
+  T(ctx: ctx)
+
+# ------------------------------------------------------------------------------
+# End
+# ------------------------------------------------------------------------------
diff --git a/nimbus/db/aristo/aristo_filter/filter_helpers.nim b/nimbus/db/aristo/aristo_filter/filter_helpers.nim
new file mode 100644
index 000000000..c663ba15a
--- /dev/null
+++ b/nimbus/db/aristo/aristo_filter/filter_helpers.nim
@@ -0,0 +1,133 @@
+# nimbus-eth1
+# Copyright (c) 2021 Status Research & Development GmbH
+# Licensed under either of
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
+#    http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or
+#    http://opensource.org/licenses/MIT)
+# at your option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+import
+  std/tables,
+  results,
+  ".."/[aristo_desc, aristo_get],
+  ./filter_desc
+
+# ------------------------------------------------------------------------------
+# Public functions
+# ------------------------------------------------------------------------------
+
+proc getLayerStateRoots*(
+    db: AristoDbRef;
+    layer: LayerRef;
+    chunkedMpt: bool;
+      ): Result[StateRootPair,AristoError] =
+  ## Get the Merkle hash key for target state root to arrive at after this
+  ## reverse filter was applied.
+  var spr: StateRootPair
+
+  spr.be = block:
+    let rc = db.getKeyBE VertexID(1)
+    if rc.isOk:
+      rc.value
+    elif rc.error == GetKeyNotFound:
+      VOID_HASH_KEY
+    else:
+      return err(rc.error)
+
+  block:
+    spr.fg = layer.kMap.getOrVoid(VertexID 1).key
+    if spr.fg.isValid:
+      return ok(spr)
+
+  if chunkedMpt:
+    let vid = layer.pAmk.getOrVoid HashLabel(root: VertexID(1), key: spr.be)
+    if vid == VertexID(1):
+      spr.fg = spr.be
+      return ok(spr)
+
+  if layer.sTab.len == 0 and
+     layer.kMap.len == 0 and
+     layer.pAmk.len == 0:
+    return err(FilPrettyPointlessLayer)
+
+  err(FilStateRootMismatch)
+
+
+proc merge*(
+    db: AristoDbRef;
+    upper: FilterRef;                          # Src filter, `nil` is ok
+    lower: FilterRef;                          # Trg filter, `nil` is ok
+    beStateRoot: HashKey;                      # Merkle hash key
+      ): Result[FilterRef,(VertexID,AristoError)] =
+  ## Merge argument `upper` into the `lower` filter instance.
+  ##
+  ## Comparing before and after merge
+  ## ::
+  ##   current                     | merged
+  ##   ----------------------------+--------------------------------
+  ##   trg2 --upper-- (src2==trg1) |
+  ##                               | trg2 --newFilter-- (src1==trg0)
+  ##   trg1 --lower-- (src1==trg0) |
+  ##                               |
+  ##   trg0 --beStateRoot          | trg0 --beStateRoot
+  ##                               |
+  ##
+  # Degenerate case: `upper` is void
+  if lower.isNil:
+    if upper.isNil:
+      # Even more degenerate case when both filters are void
+      return ok FilterRef(nil)
+    if upper.src != beStateRoot:
+      return err((VertexID(1),FilStateRootMismatch))
+    return ok(upper)
+
+  # Degenerate case: `upper` is non-trivial and `lower` is void
+  if upper.isNil:
+    if lower.src != beStateRoot:
+      return err((VertexID(0), FilStateRootMismatch))
+    return ok(lower)
+
+  # Verify stackability
+  if upper.src != lower.trg or
+     lower.src != beStateRoot:
+    return err((VertexID(0), FilStateRootMismatch))
+
+  # There is no need to deep copy table vertices as they will not be modified.
+  let newFilter = FilterRef(
+    src:  lower.src,
+    sTab: lower.sTab,
+    kMap: lower.kMap,
+    vGen: upper.vGen,
+    trg:  upper.trg)
+
+  for (vid,vtx) in upper.sTab.pairs:
+    if vtx.isValid or not newFilter.sTab.hasKey vid:
+      newFilter.sTab[vid] = vtx
+    elif newFilter.sTab.getOrVoid(vid).isValid:
+      let rc = db.getVtxUBE vid
+      if rc.isOk:
+        newFilter.sTab[vid] = vtx # VertexRef(nil)
+      elif rc.error == GetVtxNotFound:
+        newFilter.sTab.del vid
+      else:
+        return err((vid,rc.error))
+
+  for (vid,key) in upper.kMap.pairs:
+    if key.isValid or not newFilter.kMap.hasKey vid:
+      newFilter.kMap[vid] = key
+    elif newFilter.kMap.getOrVoid(vid).isValid:
+      let rc = db.getKeyUBE vid
+      if rc.isOk:
+        newFilter.kMap[vid] = key # VOID_HASH_KEY
+      elif rc.error == GetKeyNotFound:
+        newFilter.kMap.del vid
+      else:
+        return err((vid,rc.error))
+
+  ok newFilter
+
+# ------------------------------------------------------------------------------
+# End
+# ------------------------------------------------------------------------------
diff --git a/nimbus/db/aristo/aristo_filter/filter_scheduler.nim b/nimbus/db/aristo/aristo_filter/filter_scheduler.nim
new file mode 100644
index 000000000..a42cd8c29
--- /dev/null
+++ b/nimbus/db/aristo/aristo_filter/filter_scheduler.nim
@@ -0,0 +1,331 @@
+# nimbus-eth1
+# Copyright (c) 2021 Status Research & Development GmbH
+# Licensed under either of
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
+#    http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or
+#    http://opensource.org/licenses/MIT)
+# at your option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+import
+  std/[algorithm, sequtils],
+  ".."/[aristo_constants, aristo_desc],
+  ./filter_desc
+
+const
+  ZeroQidPair = (QueueID(0),QueueID(0))
+
+# ------------------------------------------------------------------------------
+# Private helpers
+# ------------------------------------------------------------------------------
+
+func `+`*(a: QueueID; b: uint): QueueID = (a.uint64+b.uint64).QueueID
+func `-`*(a: QueueID; b: uint): QueueID = (a.uint64-b.uint64).QueueID
+
+func `<`(a: static[uint]; b: QueueID): bool = QueueID(a) < b
+
+func globalQid(queue: int, qid: QueueID): QueueID =
+  QueueID((queue.uint64 shl 62) or qid.uint64)
+
+# ------------------------------------------------------------------------------
+# Private functions
+# ------------------------------------------------------------------------------
+
+func fifoLen(
+    fifo: (QueueID,QueueID);
+    wrap: QueueID;
+     ): uint =
+  ## Number of entries in wrap-arounfd fifo organised with `fifo[0]` is the
+  ## oldest entry and`fifo[1]` is the latest/newest entry.
+  ##
+  if fifo[0] == 0:
+    return 0
+
+  if fifo[0] <= fifo[1]:
+    # Filling up
+    # ::
+    #  |             :
+    #  |  fifo[0]--> 3
+    #  |             4
+    #  |             5 <--fifo[1]
+    #  |             :
+    #
+    return ((fifo[1] + 1) - fifo[0]).uint
+
+  else:
+    # After wrap aound
+    # ::
+    #  |             :
+    #  |             3 <--fifo[1]
+    #  |             4
+    #  |  fifo[0]--> 5
+    #  |             :
+    #  |            wrap
+    return ((fifo[1] + 1) + (wrap - fifo[0])).uint
+
+
+func fifoAdd(
+    fifo: (QueueID,QueueID);
+    wrap: QueueID;
+      ): tuple[doDel: QueueID, fifo: (QueueID,QueueID)] =
+  ## Add an entry to the wrap-arounfd fifo organised with `fifo[0]` is the
+  ## oldest entry and`fifo[1]` is the latest/newest entry.
+  ##
+  if fifo[0] == 0:
+    return (QueueID(0), (QueueID(1),QueueID(1)))
+
+  if fifo[0] <= fifo[1]:
+    if fifo[1] < wrap:
+      # Filling up
+      # ::
+      #  |             :
+      #  |  fifo[0]--> 3
+      #  |             4
+      #  |             5 <--fifo[1]
+      #  |             :
+      #
+      return (QueueID(0), (fifo[0],fifo[1]+1))
+    elif 1 < fifo[0]:
+      # Wrapping
+      # ::
+      #  |             :
+      #  |  fifo[0]--> 3
+      #  |             4
+      #  |             :
+      #  |            wrap <--fifo[1]
+      #
+      return (QueueID(0), (fifo[0],QueueID(1)))
+    elif 1 < wrap:
+      # Wrapping and flushing out
+      # ::
+      #  |  fifo[0]--> 1
+      #  |             2
+      #  |             :
+      #  |            wrap <--fifo[1]
+      #
+      return (QueueID(1), (QueueID(2),QueueID(1)))
+    else:
+      # Single entry FIFO
+      return (QueueID(1), (QueueID(1),QueueID(1)))
+
+  else:
+    if fifo[1] + 1 < fifo[0]:
+      # Filling up
+      # ::
+      #  |             :
+      #  |             3 <--fifo[1]
+      #  |             4
+      #  |  fifo[0]--> 5
+      #  |             :
+      #  |            wrap
+      return (QueueID(0), (fifo[0],fifo[1]+1))
+    elif fifo[0] < wrap:
+      # Flushing out
+      # ::
+      #  |             :
+      #  |             4 <--fifo[1]
+      #  |  fifo[0]--> 5
+      #  |             :
+      #  |            wrap
+      return (fifo[0], (fifo[0]+1,fifo[1]+1))
+    else:
+      # Wrapping and flushing out
+      # ::
+      #  |              :
+      #  |            wrap-1 <--fifo[1]
+      #  |  fifo[0]--> wrap
+      return (wrap, (QueueID(1),wrap))
+
+
+func fifoDel(
+    fifo: (QueueID,QueueID);
+    nDel: uint;
+    wrap: QueueID;
+      ): tuple[doDel: seq[(QueueID,QueueID)], fifo: (QueueID,QueueID)] =
+  ## Delete a the range `nDel` of filter IDs from the FIFO. The entries to be
+  ## deleted are taken from the oldest ones added.
+  ##
+  if fifo[0] == 0:
+    return (EmptyQidPairSeq, ZeroQidPair)
+
+  if fifo[0] <= fifo[1]:
+    # Take off the left end from `fifo[0] .. fifo[1]`
+    # ::
+    #  |             :
+    #  |  fifo[0]--> 3            ^
+    #  |             4            | to be deleted
+    #  |             5            v
+    #  |             6 <--fifo[1]
+    #  |             :
+    #
+    if nDel.uint64 <= fifo[1] - fifo[0]:
+      return (@[(fifo[0], fifo[0] + nDel - 1)], (fifo[0] + nDel, fifo[1]))
+    else:
+      return (@[fifo], ZeroQidPair)
+
+  else:
+    if nDel.uint64 <= (wrap - fifo[0] + 1):
+      # Take off the left end from `fifo[0] .. wrap`
+      # ::
+      #  |             :
+      #  |             3 <--fifo[1]
+      #  |             4
+      #  |  fifo[0]--> 5            ^
+      #  |             6            | to be deleted
+      #  |             7            v
+      #  |             :
+      #  |            wrap
+      #
+      let topRange = (fifo[0], fifo[0] + nDel - 1)
+      if  nDel.uint64 < (wrap - fifo[0] + 1):
+        return (@[topRange], (fifo[0] + nDel, fifo[1]))
+      else:
+        return (@[topRange], (QueueID(1), fifo[1]))
+
+    else:
+      # Interval `fifo[0] .. wrap` fully deleted, check `1 .. fifo[0]`
+      # ::
+      #  |             1            ^
+      #  |             2            | to be deleted
+      #  |             :            v
+      #  |             6
+      #  |             7<--fifo[1]
+      #  |  fifo[0]--> 8            ^
+      #  |             9            | to be deleted
+      #  |             :            :
+      #  |            wrap          v
+      #
+      let
+        topRange = (fifo[0], wrap)
+        nDelLeft = nDel.uint64 - (wrap - fifo[0] + 1)
+
+      # Take off the left end from `QueueID(1) .. fifo[1]`
+      if nDelLeft <= fifo[1] - QueueID(0):
+        let bottomRange = (QueueID(1), QueueID(nDelLeft))
+        if nDelLeft < fifo[1] - QueueID(0):
+          return (@[bottomRange, topRange], (QueueID(nDelLeft+1), fifo[1]))
+        else:
+          return (@[bottomRange, topRange], ZeroQidPair)
+      else:
+        # Delete all available
+        return (@[(QueueID(1), fifo[1]), (fifo[0], wrap)], ZeroQidPair)
+
+# ------------------------------------------------------------------------------
+# Public functions
+# ------------------------------------------------------------------------------
+
+proc stats*(
+    ctx: openArray[tuple[size, width: int]];
+      ): tuple[maxQueue: int, minCovered: int, maxCovered: int] =
+  ## Number of maximally stored and covered queued entries for the argument
+  ## layout `ctx`. The resulting value of `maxQueue` entry is the maximal
+  ## number of database slots needed, the `minCovered` and `maxCovered` entry
+  ## indicate the rancge of the backlog foa a fully populated database.
+  var step = 1
+
+  for n in 0 ..< ctx.len:
+    step *= ctx[n].width + 1
+    let size = ctx[n].size + ctx[(n+1) mod ctx.len].width
+    result.maxQueue += size.int
+    result.minCovered += (ctx[n].size * step).int
+    result.maxCovered += (size * step).int
+
+proc stats*(
+    ctx: openArray[tuple[size, width, wrap: int]];
+      ): tuple[maxQueue: int, minCovered: int, maxCovered: int] =
+  ## Variant of `stats()`
+  ctx.toSeq.mapIt((it[0],it[1])).stats
+
+proc stats*(
+    ctx: QidLayoutRef;
+      ): tuple[maxQueue: int, minCovered: int, maxCovered: int] =
+  ## Variant of `stats()`
+  ctx.q.toSeq.mapIt((it[0].int,it[1].int)).stats
+
+
+proc addItem*(
+    fifo: QidSchedRef;
+      ): tuple[exec: seq[QidAction], fifo: QidSchedRef] =
+  ## Get the instructions for adding a new slot to the cascades queues. The
+  ## argument `fifo` is a complete state of the addresses a cascaded *FIFO*
+  ## when applied to a database. Only the *FIFO* queue addresses are needed
+  ## in order to describe how to add another item.
+  ##
+  ## Return value is a list of instructions what to do when adding a new item
+  ## and the new state of the cascaded *FIFO*.
+  ##
+  ## The following instructions may be returned:
+  ## ::
+  ##    SaveQid <queue-id>         -- Store a new item under the address
+  ##                               -- <queue-id> on the database.
+  ##
+  ##    HoldQid <from-id>..<to-id> -- Move the records accessed by the argument
+  ##                               -- addresses from the database to the right
+  ##                               -- end of the local hold queue. The age of
+  ##                               -- the items on the hold queue increases
+  ##                               -- left to right.
+  ##
+  ##    DequQid <queue-id>         -- Merge items from the hold queue into a
+  ##                               -- new item and store it under the address
+  ##                               -- <queue-id> on the database. Clear the
+  ##                               -- the hold queue.
+  ##
+  let
+    ctx = fifo.ctx.q
+  var
+    state = fifo.state
+    deferred: seq[QidAction]   # carry over to next sub-queue
+    revActions: seq[QidAction] # instructions in reverse order
+
+  for n in 0 ..< ctx.len:
+    if state.len < n + 1:
+      state.setlen(n + 1)
+
+    let
+      overlapWidth = ctx[(n+1) mod ctx.len].width
+      carryOverSize = ctx[n].size + overlapWidth
+      stateLen = state[n].fifoLen ctx[n].wrap
+
+    if stateLen < carryOverSize:
+      state[n] = state[n].fifoAdd(ctx[n].wrap).fifo
+      let qQidAdded = n.globalQid state[n][1]
+      if  0 < n:
+        revActions.add QidAction(op: DequQid, qid: qQidAdded)
+      else:
+        revActions.add QidAction(op: SaveQid, qid: qQidAdded)
+      if 0 < deferred.len:
+        revActions &= deferred
+      break
+
+    else:
+      # Full queue segment, carry over to next one
+      let
+        extra = stateLen - carryOverSize # should be zero
+        qDel = state[n].fifoDel(extra + overlapWidth + 1, ctx[n].wrap)
+        qAdd = qDel.fifo.fifoAdd ctx[n].wrap
+        qFidAdded = n.globalQid qAdd.fifo[1]
+
+      if 0 < n:
+        revActions.add QidAction(op: DequQid, qid: qFidAdded)
+      else:
+        revActions.add QidAction(op: SaveQid, qid: qFidAdded)
+
+      if 0 < deferred.len:
+        revActions &= deferred
+        deferred.setLen(0)
+
+      for w in qDel.doDel:
+        deferred.add QidAction(
+          op:  HoldQid,
+          qid: n.globalQid w[0],
+          xid: n.globalQid w[1])
+      state[n] = qAdd.fifo
+
+    # End loop
+
+  (revActions.reversed, QidSchedRef(ctx: fifo.ctx, state: state))
+
+# ------------------------------------------------------------------------------
+# End
+# ------------------------------------------------------------------------------
diff --git a/nimbus/db/aristo/aristo_get.nim b/nimbus/db/aristo/aristo_get.nim
index 57986e51c..257a9cc87 100644
--- a/nimbus/db/aristo/aristo_get.nim
+++ b/nimbus/db/aristo/aristo_get.nim
@@ -36,14 +36,14 @@ proc getIdgUBE*(
     return be.getIdgFn()
   err(GetIdgNotFound)
 
-proc getFasUBE*(
+proc getFqsUBE*(
     db: AristoDbRef;
-      ): Result[seq[FilterID],AristoError] =
+      ): Result[seq[(QueueID,QueueID)],AristoError] =
   ## Get the list of filter IDs unfiltered backened if available.
   let be = db.backend
   if not be.isNil:
-    return be.getFasFn()
-  err(GetFasNotFound)
+    return be.getFqsFn()
+  err(GetFqsNotFound)
 
 proc getVtxUBE*(
     db: AristoDbRef;
@@ -67,12 +67,12 @@ proc getKeyUBE*(
 
 proc getFilUBE*(
     db: AristoDbRef;
-    fid: FilterID;
+    qid: QueueID;
       ): Result[FilterRef,AristoError] =
   ## Get the filter from the unfiltered backened if available.
   let be = db.backend
   if not be.isNil:
-    return be.getFilFn fid
+    return be.getFilFn qid
   err GetFilNotFound
 
 # ------------------
diff --git a/nimbus/db/aristo/aristo_init/aristo_init_common.nim b/nimbus/db/aristo/aristo_init/init_common.nim
similarity index 94%
rename from nimbus/db/aristo/aristo_init/aristo_init_common.nim
rename to nimbus/db/aristo/aristo_init/init_common.nim
index 0b5eafc96..7d35b781b 100644
--- a/nimbus/db/aristo/aristo_init/aristo_init_common.nim
+++ b/nimbus/db/aristo/aristo_init/init_common.nim
@@ -11,7 +11,7 @@
 
 import
   ../aristo_desc,
-  ../aristo_desc/aristo_types_backend
+  ../aristo_desc/desc_backend
 
 const
   verifyIxId = true # and false
@@ -46,7 +46,7 @@ type
     of VtxPfx, KeyPfx:
       vid*: VertexID                 ## Vertex ID where the error occured
     of FilPfx:
-      fid*: FilterID                 ## Ditto
+      qid*: QueueID                  ## Ditto
     of AdmPfx, Oops:
       discard
     code*: AristoError               ## Error code (if any)
@@ -58,7 +58,7 @@ type
 
 const
   AdmTabIdIdg* = AdminTabID(0)       ## Access key for vertex ID generator state
-  AdmTabIdFas* = AdminTabID(1)       ## Access key for filter state
+  AdmTabIdFqs* = AdminTabID(1)       ## Access key for filter queue states
 
 # ------------------------------------------------------------------------------
 # Public helpers
diff --git a/nimbus/db/aristo/aristo_init/aristo_memory.nim b/nimbus/db/aristo/aristo_init/memory_db.nim
similarity index 84%
rename from nimbus/db/aristo/aristo_init/aristo_memory.nim
rename to nimbus/db/aristo/aristo_init/memory_db.nim
index a4ba449f2..17726ba81 100644
--- a/nimbus/db/aristo/aristo_init/aristo_memory.nim
+++ b/nimbus/db/aristo/aristo_init/memory_db.nim
@@ -33,25 +33,25 @@ import
   stew/results,
   ../aristo_constants,
   ../aristo_desc,
-  ../aristo_desc/aristo_types_backend,
+  ../aristo_desc/desc_backend,
   ../aristo_transcode,
-  ./aristo_init_common
+  ./init_common
 
 type
   MemBackendRef* = ref object of TypedBackendRef
     ## Inheriting table so access can be extended for debugging purposes
     sTab: Table[VertexID,Blob]       ## Structural vertex table making up a trie
     kMap: Table[VertexID,HashKey]    ## Merkle hash key mapping
-    rFil: Table[FilterID,Blob]       ## Backend filters
+    rFil: Table[QueueID,Blob]       ## Backend filters
     vGen: Option[seq[VertexID]]
-    vFas: Option[seq[FilterID]]
+    vFqs: Option[seq[(QueueID,QueueID)]]
 
   MemPutHdlRef = ref object of TypedPutHdlRef
     sTab: Table[VertexID,Blob]
     kMap: Table[VertexID,HashKey]
-    rFil: Table[FilterID,Blob]
+    rFil: Table[QueueID,Blob]
     vGen: Option[seq[VertexID]]
-    vFas: Option[seq[FilterID]]
+    vFqs: Option[seq[(QueueID,QueueID)]]
 
 # ------------------------------------------------------------------------------
 # Private helpers
@@ -99,8 +99,8 @@ proc getKeyFn(db: MemBackendRef): GetKeyFn =
 
 proc getFilFn(db: MemBackendRef): GetFilFn =
   result =
-    proc(fid: FilterID): Result[FilterRef,AristoError] =
-      let data = db.rFil.getOrDefault(fid, EmptyBlob)
+    proc(qid: QueueID): Result[FilterRef,AristoError] =
+      let data = db.rFil.getOrDefault(qid, EmptyBlob)
       if 0 < data.len:
         return data.deblobify FilterRef
       err(GetFilNotFound)
@@ -112,12 +112,12 @@ proc getIdgFn(db: MemBackendRef): GetIdgFn =
         return ok db.vGen.unsafeGet
       err(GetIdgNotFound)
 
-proc getFasFn(db: MemBackendRef): GetFasFn =
+proc getFqsFn(db: MemBackendRef): GetFqsFn =
   result =
-    proc(): Result[seq[FilterID],AristoError]=
-      if db.vFas.isSome:
-        return ok db.vFas.unsafeGet
-      err(GetFasNotFound)
+    proc(): Result[seq[(QueueID,QueueID)],AristoError]=
+      if db.vFqs.isSome:
+        return ok db.vFqs.unsafeGet
+      err(GetFqsNotFound)
 
 # -------------
 
@@ -155,18 +155,18 @@ proc putKeyFn(db: MemBackendRef): PutKeyFn =
 
 proc putFilFn(db: MemBackendRef): PutFilFn =
   result =
-    proc(hdl: PutHdlRef; vf: openArray[(FilterID,FilterRef)]) =
+    proc(hdl: PutHdlRef; vf: openArray[(QueueID,FilterRef)]) =
       let hdl = hdl.getSession db
       if hdl.error.isNil:
-        for (fid,filter) in vf:
+        for (qid,filter) in vf:
           let rc = filter.blobify()
           if rc.isErr:
             hdl.error = TypedPutHdlErrRef(
               pfx:  FilPfx,
-              fid:  fid,
+              qid:  qid,
               code: rc.error)
             return
-          hdl.rFil[fid] = rc.value
+          hdl.rFil[qid] = rc.value
 
 proc putIdgFn(db: MemBackendRef): PutIdgFn =
   result =
@@ -175,12 +175,12 @@ proc putIdgFn(db: MemBackendRef): PutIdgFn =
       if hdl.error.isNil:
         hdl.vGen = some(vs.toSeq)
 
-proc putFasFn(db: MemBackendRef): PutFasFn =
+proc putFqsFn(db: MemBackendRef): PutFqsFn =
   result =
-    proc(hdl: PutHdlRef; fs: openArray[FilterID])  =
+    proc(hdl: PutHdlRef; fs: openArray[(QueueID,QueueID)])  =
       let hdl = hdl.getSession db
       if hdl.error.isNil:
-        hdl.vFas = some(fs.toSeq)
+        hdl.vFqs = some(fs.toSeq)
 
 
 proc putEndFn(db: MemBackendRef): PutEndFn =
@@ -194,7 +194,7 @@ proc putEndFn(db: MemBackendRef): PutEndFn =
             pfx=hdl.error.pfx, vid=hdl.error.vid, error=hdl.error.code
         of FilPfx:
           debug logTxt "putEndFn: filter failed",
-            pfx=hdl.error.pfx, fid=hdl.error.fid, error=hdl.error.code
+            pfx=hdl.error.pfx, qid=hdl.error.qid, error=hdl.error.code
         else:
           debug logTxt "putEndFn: failed",
             pfx=hdl.error.pfx, error=hdl.error.code
@@ -212,11 +212,11 @@ proc putEndFn(db: MemBackendRef): PutEndFn =
         else:
           db.kMap.del vid
 
-      for (fid,data) in hdl.rFil.pairs:
-        if fid.isValid:
-          db.rFil[fid] = data
+      for (qid,data) in hdl.rFil.pairs:
+        if qid.isValid:
+          db.rFil[qid] = data
         else:
-          db.rFil.del fid
+          db.rFil.del qid
 
       if hdl.vGen.isSome:
         let vGen = hdl.vGen.unsafeGet
@@ -225,12 +225,12 @@ proc putEndFn(db: MemBackendRef): PutEndFn =
         else:
           db.vGen = some(vGen)
 
-      if hdl.vFas.isSome:
-        let vFas = hdl.vFas.unsafeGet
-        if vFas.len == 0:
-          db.vFas = none(seq[FilterID])
+      if hdl.vFqs.isSome:
+        let vFqs = hdl.vFqs.unsafeGet
+        if vFqs.len == 0:
+          db.vFqs = none(seq[(QueueID,QueueID)])
         else:
-          db.vFas = some(vFas)
+          db.vFqs = some(vFqs)
 
       AristoError(0)
 
@@ -252,14 +252,14 @@ proc memoryBackend*(): BackendRef =
   db.getKeyFn = getKeyFn db
   db.getFilFn = getFilFn db
   db.getIdgFn = getIdgFn db
-  db.getFasFn = getFasFn db
+  db.getFqsFn = getFqsFn db
 
   db.putBegFn = putBegFn db
   db.putVtxFn = putVtxFn db
   db.putKeyFn = putKeyFn db
   db.putFilFn = putFilFn db
   db.putIdgFn = putIdgFn db
-  db.putFasFn = putFasFn db
+  db.putFqsFn = putFqsFn db
   db.putEndFn = putEndFn db
 
   db.closeFn = closeFn db
@@ -294,16 +294,16 @@ iterator walkKey*(
 
 iterator walkFil*(
     be: MemBackendRef;
-      ): tuple[n: int, fid: FilterID, filter: FilterRef] =
+      ): tuple[n: int, qid: QueueID, filter: FilterRef] =
   ##  Iteration over the vertex sub-table.
-  for n,fid in be.rFil.keys.toSeq.mapIt(it.uint64).sorted.mapIt(it.FilterID):
-    let data = be.rFil.getOrDefault(fid, EmptyBlob)
+  for n,qid in be.rFil.keys.toSeq.mapIt(it.uint64).sorted.mapIt(it.QueueID):
+    let data = be.rFil.getOrDefault(qid, EmptyBlob)
     if 0 < data.len:
       let rc = data.deblobify FilterRef
       if rc.isErr:
-        debug logTxt "walkFilFn() skip", n, fid, error=rc.error
+        debug logTxt "walkFilFn() skip", n,qid, error=rc.error
       else:
-        yield (n, fid, rc.value)
+        yield (n, qid, rc.value)
 
 
 iterator walk*(
@@ -319,8 +319,8 @@ iterator walk*(
     yield(0, AdmPfx, AdmTabIdIdg.uint64, be.vGen.unsafeGet.blobify)
     n.inc
 
-  if be.vFas.isSome:
-    yield(0, AdmPfx, AdmTabIdFas.uint64, be.vFas.unsafeGet.blobify)
+  if be.vFqs.isSome:
+    yield(0, AdmPfx, AdmTabIdFqs.uint64, be.vFqs.unsafeGet.blobify)
     n.inc
 
   for vid in be.sTab.keys.toSeq.mapIt(it.uint64).sorted.mapIt(it.VertexID):
@@ -333,7 +333,7 @@ iterator walk*(
     yield (n, KeyPfx, vid.uint64, key.to(Blob))
     n.inc
 
-  for lid in be.rFil.keys.toSeq.mapIt(it.uint64).sorted.mapIt(it.FilterID):
+  for lid in be.rFil.keys.toSeq.mapIt(it.uint64).sorted.mapIt(it.QueueID):
     let data = be.rFil.getOrDefault(lid, EmptyBlob)
     if 0 < data.len:
       yield (n, FilPfx, lid.uint64, data)
diff --git a/nimbus/db/aristo/aristo_init/memory_only.nim b/nimbus/db/aristo/aristo_init/memory_only.nim
index de653efc6..a63995283 100644
--- a/nimbus/db/aristo/aristo_init/memory_only.nim
+++ b/nimbus/db/aristo/aristo_init/memory_only.nim
@@ -17,8 +17,8 @@ import
   std/sets,
   results,
   ../aristo_desc,
-  ../aristo_desc/aristo_types_backend,
-  "."/[aristo_init_common, aristo_memory]
+  ../aristo_desc/desc_backend,
+  "."/[init_common, memory_db]
 
 type
   VoidBackendRef* = ref object of TypedBackendRef
diff --git a/nimbus/db/aristo/aristo_init/persistent.nim b/nimbus/db/aristo/aristo_init/persistent.nim
index fa7c5e6fa..7861cb484 100644
--- a/nimbus/db/aristo/aristo_init/persistent.nim
+++ b/nimbus/db/aristo/aristo_init/persistent.nim
@@ -21,7 +21,7 @@
 import
   results,
   ../aristo_desc,
-  "."/[aristo_init_common, aristo_rocksdb, memory_only]
+  "."/[init_common, rocks_db, memory_only]
 export
   RdbBackendRef,
   memory_only
diff --git a/nimbus/db/aristo/aristo_init/aristo_rocksdb.nim b/nimbus/db/aristo/aristo_init/rocks_db.nim
similarity index 88%
rename from nimbus/db/aristo/aristo_init/aristo_rocksdb.nim
rename to nimbus/db/aristo/aristo_init/rocks_db.nim
index 11ad076e7..36c8cbf71 100644
--- a/nimbus/db/aristo/aristo_init/aristo_rocksdb.nim
+++ b/nimbus/db/aristo/aristo_init/rocks_db.nim
@@ -33,10 +33,10 @@ import
   stew/results,
   ../aristo_constants,
   ../aristo_desc,
-  ../aristo_desc/aristo_types_backend,
+  ../aristo_desc/desc_backend,
   ../aristo_transcode,
-  ./aristo_init_common,
-  ./aristo_rocksdb/[rdb_desc, rdb_get, rdb_init, rdb_put, rdb_walk]
+  ./init_common,
+  ./rocks_db/[rdb_desc, rdb_get, rdb_init, rdb_put, rdb_walk]
 
 logScope:
   topics = "aristo-backend"
@@ -83,8 +83,8 @@ proc `vtxCache=`(hdl: RdbPutHdlRef; val: tuple[vid: VertexID; data: Blob]) =
 proc `keyCache=`(hdl: RdbPutHdlRef; val: tuple[vid: VertexID; data: Blob]) =
   hdl.cache[KeyPfx][val.vid.uint64] = val.data
 
-proc `filCache=`(hdl: RdbPutHdlRef; val: tuple[fid: FilterID; data: Blob]) =
-  hdl.cache[FilPfx][val.fid.uint64] = val.data
+proc `filCache=`(hdl: RdbPutHdlRef; val: tuple[qid: QueueID; data: Blob]) =
+  hdl.cache[FilPfx][val.qid.uint64] = val.data
 
 proc `admCache=`(hdl: RdbPutHdlRef; val: tuple[id: AdminTabID; data: Blob]) =
   hdl.cache[AdmPfx][val.id.uint64] = val.data
@@ -131,12 +131,12 @@ proc getKeyFn(db: RdbBackendRef): GetKeyFn =
 
 proc getFilFn(db: RdbBackendRef): GetFilFn =
   result =
-    proc(fid: FilterID): Result[FilterRef,AristoError] =
+    proc(qid: QueueID): Result[FilterRef,AristoError] =
 
       # Fetch serialised data record
-      let rc = db.rdb.get fid.toOpenArray()
+      let rc = db.rdb.get qid.toOpenArray()
       if rc.isErr:
-        debug logTxt "getFilFn: failed", fid,
+        debug logTxt "getFilFn: failed", qid,
           error=rc.error[0], info=rc.error[1]
         return err(rc.error[0])
 
@@ -163,22 +163,22 @@ proc getIdgFn(db: RdbBackendRef): GetIdgFn =
       # Decode data record
       rc.value.deblobify seq[VertexID]
 
-proc getFasFn(db: RdbBackendRef): GetFasFn =
+proc getFqsFn(db: RdbBackendRef): GetFqsFn =
   result =
-    proc(): Result[seq[FilterID],AristoError]=
+    proc(): Result[seq[(QueueID,QueueID)],AristoError]=
 
       # Fetch serialised data record
-      let rc = db.rdb.get AdmTabIdFas.toOpenArray()
+      let rc = db.rdb.get AdmTabIdFqs.toOpenArray()
       if rc.isErr:
         debug logTxt "getFosFn: failed", error=rc.error[1]
         return err(rc.error[0])
 
       if rc.value.len == 0:
-        let w = EmptyFidSeq
+        let w = EmptyQidPairSeq
         return ok w
 
       # Decode data record
-      rc.value.deblobify seq[FilterID]
+      rc.value.deblobify seq[(QueueID,QueueID)]
 
 # -------------
 
@@ -219,21 +219,21 @@ proc putKeyFn(db: RdbBackendRef): PutKeyFn =
 
 proc putFilFn(db: RdbBackendRef): PutFilFn =
   result =
-    proc(hdl: PutHdlRef; vrps: openArray[(FilterID,FilterRef)]) =
+    proc(hdl: PutHdlRef; vrps: openArray[(QueueID,FilterRef)]) =
       let hdl = hdl.getSession db
       if hdl.error.isNil:
-        for (fid,filter) in vrps:
+        for (qid,filter) in vrps:
           if filter.isValid:
             let rc = filter.blobify()
             if rc.isErr:
               hdl.error = TypedPutHdlErrRef(
                 pfx:  FilPfx,
-                fid:  fid,
+                qid:  qid,
                 code: rc.error)
               return
-            hdl.filCache = (fid, rc.value)
+            hdl.filCache = (qid, rc.value)
           else:
-            hdl.filCache = (fid, EmptyBlob)
+            hdl.filCache = (qid, EmptyBlob)
 
 proc putIdgFn(db: RdbBackendRef): PutIdgFn =
   result =
@@ -245,15 +245,15 @@ proc putIdgFn(db: RdbBackendRef): PutIdgFn =
         else:
           hdl.admCache = (AdmTabIdIdg, EmptyBlob)
 
-proc putFasFn(db: RdbBackendRef): PutFasFn =
+proc putFqsFn(db: RdbBackendRef): PutFqsFn =
   result =
-    proc(hdl: PutHdlRef; vs: openArray[FilterID])  =
+    proc(hdl: PutHdlRef; vs: openArray[(QueueID,QueueID)])  =
       let hdl = hdl.getSession db
       if hdl.error.isNil:
         if 0 < vs.len:
-          hdl.admCache = (AdmTabIdFas, vs.blobify)
+          hdl.admCache = (AdmTabIdFqs, vs.blobify)
         else:
-          hdl.admCache = (AdmTabIdFas, EmptyBlob)
+          hdl.admCache = (AdmTabIdFqs, EmptyBlob)
 
 
 proc putEndFn(db: RdbBackendRef): PutEndFn =
@@ -301,14 +301,14 @@ proc rocksDbBackend*(path: string): Result[BackendRef,AristoError] =
   db.getKeyFn = getKeyFn db
   db.getFilFn = getFilFn db
   db.getIdgFn = getIdgFn db
-  db.getFasFn = getFasFn db
+  db.getFqsFn = getFqsFn db
 
   db.putBegFn = putBegFn db
   db.putVtxFn = putVtxFn db
   db.putKeyFn = putKeyFn db
   db.putFilFn = putFilFn db
   db.putIdgFn = putIdgFn db
-  db.putFasFn = putFasFn db
+  db.putFqsFn = putFqsFn db
   db.putEndFn = putEndFn db
 
   db.closeFn = closeFn db
@@ -349,12 +349,12 @@ iterator walkKey*(
 
 iterator walkFil*(
     be: RdbBackendRef;
-      ): tuple[n: int, fid: FilterID, filter: FilterRef] =
+      ): tuple[n: int, qid: QueueID, filter: FilterRef] =
   ## Variant of `walk()` iteration over the filter sub-table.
   for (n, xid, data) in be.rdb.walk FilPfx:
     let rc = data.deblobify FilterRef
     if rc.isOk:
-      yield (n, FilterID(xid), rc.value)
+      yield (n, QueueID(xid), rc.value)
 
 # ------------------------------------------------------------------------------
 # End
diff --git a/nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_desc.nim b/nimbus/db/aristo/aristo_init/rocks_db/rdb_desc.nim
similarity index 93%
rename from nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_desc.nim
rename to nimbus/db/aristo/aristo_init/rocks_db/rdb_desc.nim
index cd518c7d7..b68167a78 100644
--- a/nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_desc.nim
+++ b/nimbus/db/aristo/aristo_init/rocks_db/rdb_desc.nim
@@ -19,7 +19,7 @@ import
   rocksdb,
   stint,
   ../../aristo_desc,
-  ../aristo_init_common.nim
+  ../init_common
 
 type
   RdbInst* = object
@@ -55,8 +55,8 @@ proc toRdbKey*(id: uint64; pfx: StorageType): RdbKey =
 template toOpenArray*(vid: VertexID; pfx: StorageType): openArray[byte] =
   vid.uint64.toRdbKey(pfx).toOpenArray(0, sizeof uint64)
 
-template toOpenArray*(fid: FilterID): openArray[byte] =
-  fid.uint64.toRdbKey(FilPfx).toOpenArray(0, sizeof uint64)
+template toOpenArray*(qid: QueueID): openArray[byte] =
+  qid.uint64.toRdbKey(FilPfx).toOpenArray(0, sizeof uint64)
 
 template toOpenArray*(aid: AdminTabID): openArray[byte] =
   aid.uint64.toRdbKey(AdmPfx).toOpenArray(0, sizeof uint64)
diff --git a/nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_get.nim b/nimbus/db/aristo/aristo_init/rocks_db/rdb_get.nim
similarity index 100%
rename from nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_get.nim
rename to nimbus/db/aristo/aristo_init/rocks_db/rdb_get.nim
diff --git a/nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_init.nim b/nimbus/db/aristo/aristo_init/rocks_db/rdb_init.nim
similarity index 99%
rename from nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_init.nim
rename to nimbus/db/aristo/aristo_init/rocks_db/rdb_init.nim
index 8f70ed717..fa5eee189 100644
--- a/nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_init.nim
+++ b/nimbus/db/aristo/aristo_init/rocks_db/rdb_init.nim
@@ -87,7 +87,7 @@ proc destroy*(rdb: var RdbInst; flush: bool) =
     base = rdb.basePath / BaseFolder
   try:
     (base / TempFolder).removeDir
-  
+
     if flush:
       (base / DataFolder).removeDir
 
diff --git a/nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_put.nim b/nimbus/db/aristo/aristo_init/rocks_db/rdb_put.nim
similarity index 99%
rename from nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_put.nim
rename to nimbus/db/aristo/aristo_init/rocks_db/rdb_put.nim
index 1cbcbccdd..67ba7b881 100644
--- a/nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_put.nim
+++ b/nimbus/db/aristo/aristo_init/rocks_db/rdb_put.nim
@@ -20,7 +20,7 @@ import
   rocksdb,
   stew/results,
   "../.."/[aristo_constants, aristo_desc],
-  ../aristo_init_common,
+  ../init_common,
   ./rdb_desc
 
 logScope:
diff --git a/nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_walk.nim b/nimbus/db/aristo/aristo_init/rocks_db/rdb_walk.nim
similarity index 99%
rename from nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_walk.nim
rename to nimbus/db/aristo/aristo_init/rocks_db/rdb_walk.nim
index 983982c11..708dc64f0 100644
--- a/nimbus/db/aristo/aristo_init/aristo_rocksdb/rdb_walk.nim
+++ b/nimbus/db/aristo/aristo_init/rocks_db/rdb_walk.nim
@@ -17,7 +17,7 @@ import
   std/sequtils,
   eth/common,
   rocksdb,
-  ../aristo_init_common,
+  ../init_common,
   ./rdb_desc
 
 # ------------------------------------------------------------------------------
diff --git a/nimbus/db/aristo/aristo_transcode.nim b/nimbus/db/aristo/aristo_transcode.nim
index 4610da3fb..7b54662b8 100644
--- a/nimbus/db/aristo/aristo_transcode.nim
+++ b/nimbus/db/aristo/aristo_transcode.nim
@@ -378,20 +378,21 @@ proc blobify*(filter: FilterRef): Result[Blob, AristoError] =
   ok data
 
 
-proc blobify*(vFos: openArray[FilterID]; data: var Blob) =
-  ## This function serialises a list of filter IDs.
+proc blobify*(vFqs: openArray[(QueueID,QueueID)]; data: var Blob) =
+  ## This function serialises a list of filter queue IDs.
   ## ::
   ##   uint64, ...    -- list of IDs
   ##   0x7e           -- marker(8)
   ##
   data.setLen(0)
-  for w in vFos:
-    data &= w.uint64.toBytesBE.toSeq
+  for w in vFqs:
+    data &= w[0].uint64.toBytesBE.toSeq
+    data &= w[1].uint64.toBytesBE.toSeq
   data.add 0x7Eu8
 
-proc blobify*(vFos: openArray[FilterID]): Blob =
+proc blobify*(vFqs: openArray[(QueueID,QueueID)]): Blob =
   ## Variant of `blobify()`
-  vFos.blobify result
+  vFqs.blobify result
 
 # -------------
 
@@ -635,27 +636,33 @@ proc deblobify*(data: Blob; T: type FilterRef): Result[T,AristoError] =
     return err(error)
   ok filter
 
-proc deblobify*(data: Blob; vFas: var seq[FilterID]): AristoError =
-  ## De-serialise the data record encoded with `blobify()` into a filter ID
-  ## argument liet `vFas`.
+proc deblobify*(data: Blob; vFqs: var seq[(QueueID,QueueID)]): AristoError =
+  ## De-serialise the data record encoded with `blobify()` into a filter queue
+  ## ID argument liet `vFqs`.
   if data.len == 0:
-    vFas = @[]
+    vFqs = @[]
   else:
-    if (data.len mod 8) != 1:
+    if (data.len mod 16) != 1:
       return DeblobSizeGarbled
     if data[^1] != 0x7e:
       return DeblobWrongType
-    for n in 0 ..< (data.len div 8):
-      let w = n * 8
-      vFas.add (uint64.fromBytesBE data[w ..< w + 8]).FilterID
+    for n in 0 ..< (data.len div 16):
+      let
+        w = n * 16
+        a = (uint64.fromBytesBE data[w + 0 ..< w + 8]).QueueID
+        b = (uint64.fromBytesBE data[w + 8 ..< w + 16]).QueueID
+      vFqs.add (a,b)
 
-proc deblobify*(data: Blob; T: type seq[FilterID]): Result[T,AristoError] =
+proc deblobify*(
+    data: Blob;
+    T: type seq[(QueueID,QueueID)];
+      ): Result[T,AristoError] =
   ## Variant of `deblobify()` for deserialising the vertex ID generator state
-  var vFas: seq[FilterID]
-  let info = data.deblobify vFas
+  var vFqs: seq[(QueueID,QueueID)]
+  let info = data.deblobify vFqs
   if info != AristoError(0):
     return err(info)
-  ok vFas
+  ok vFqs
 
 # ------------------------------------------------------------------------------
 # End
diff --git a/nimbus/db/aristo/aristo_walk/memory_only.nim b/nimbus/db/aristo/aristo_walk/memory_only.nim
index 8c2ae9119..1294bf33a 100644
--- a/nimbus/db/aristo/aristo_walk/memory_only.nim
+++ b/nimbus/db/aristo/aristo_walk/memory_only.nim
@@ -13,11 +13,11 @@
 ## =====================================================
 ##
 import
-  ../aristo_init/[aristo_memory, memory_only],
+  ../aristo_init/[memory_db, memory_only],
   ".."/[aristo_desc, aristo_init],
-  ./aristo_walk_private
+  ./walk_private
 export
-  aristo_memory,
+  memory_db,
   memory_only
 
 # ------------------------------------------------------------------------------
@@ -45,10 +45,10 @@ iterator walkKeyBe*[T: MemBackendRef|VoidBackendRef](
 iterator walkFilBe*[T: MemBackendRef|VoidBackendRef](
    _: type T;
    db: AristoDbRef;
-     ): tuple[n: int, fid: FilterID, filter: FilterRef] =
+     ): tuple[n: int, qid: QueueID, filter: FilterRef] =
   ## Similar to `walkVtxBe()` but for filters.
-  for (n,fid,filter) in db.to(T).walkFilBeImpl db:
-    yield (n,fid,filter)
+  for (n,qid,filter) in db.to(T).walkFilBeImpl db:
+    yield (n,qid,filter)
 
 # ------------------------------------------------------------------------------
 # End
diff --git a/nimbus/db/aristo/aristo_walk/persistent.nim b/nimbus/db/aristo/aristo_walk/persistent.nim
index 94b6ed9ae..09ae3c441 100644
--- a/nimbus/db/aristo/aristo_walk/persistent.nim
+++ b/nimbus/db/aristo/aristo_walk/persistent.nim
@@ -18,11 +18,11 @@
 ## `./aristo_walk/persistent`.)
 ##
 import
-  ../aristo_init/[aristo_rocksdb, persistent],
+  ../aristo_init/[rocks_db, persistent],
   ".."/[aristo_desc, aristo_init],
-  "."/[aristo_walk_private, memory_only]
+  "."/[walk_private, memory_only]
 export
-  aristo_rocksdb,
+  rocks_db,
   memory_only,
   persistent
 
@@ -50,10 +50,10 @@ iterator walkKeyBe*(
 iterator walkFilBe*(
    T: type RdbBackendRef;
    db: AristoDbRef;
-     ): tuple[n: int, fid: FilterID, filter: FilterRef] =
+     ): tuple[n: int, qid: QueueID, filter: FilterRef] =
   ## Similar to `walkVtxBe()` but for filters.
-  for (n,fid,filter) in db.to(T).walkFilBeImpl db:
-    yield (n,fid,filter)
+  for (n,qid,filter) in db.to(T).walkFilBeImpl db:
+    yield (n,qid,filter)
 
 # ------------------------------------------------------------------------------
 # End
diff --git a/nimbus/db/aristo/aristo_walk/aristo_walk_private.nim b/nimbus/db/aristo/aristo_walk/walk_private.nim
similarity index 96%
rename from nimbus/db/aristo/aristo_walk/aristo_walk_private.nim
rename to nimbus/db/aristo/aristo_walk/walk_private.nim
index 9bace365d..91591693a 100644
--- a/nimbus/db/aristo/aristo_walk/aristo_walk_private.nim
+++ b/nimbus/db/aristo/aristo_walk/walk_private.nim
@@ -90,13 +90,13 @@ iterator walkKeyBeImpl*[T](
 iterator walkFilBeImpl*[T](
     be: T;                             # Backend descriptor
     db: AristoDbRef;                   # Database with optional backend filter
-      ): tuple[n: int, fid: FilterID, filter: FilterRef] =
+      ): tuple[n: int, qid: QueueID, filter: FilterRef] =
   ## Generic filter iterator
   when be isnot VoidBackendRef:
     mixin walkFil
 
-    for (n,fid,filter) in be.walkFil:
-      yield (n,fid,filter)
+    for (n,qid,filter) in be.walkFil:
+      yield (n,qid,filter)
 
 # ------------------------------------------------------------------------------
 # End
diff --git a/tests/test_aristo.nim b/tests/test_aristo.nim
index c051be317..7f669e927 100644
--- a/tests/test_aristo.nim
+++ b/tests/test_aristo.nim
@@ -73,10 +73,18 @@ proc setErrorLevel {.used.} =
 # Test Runners: accounts and accounts storages
 # ------------------------------------------------------------------------------
 
-proc miscRunner(noisy =true) =
-  suite &"Aristo: Miscellaneous tests":
-    test &"VertexID recyling lists":
-      noisy.testVidRecycleLists()
+proc miscRunner(
+    noisy = true;
+    qidSampleSize = QidSample;
+     ) =
+
+  suite "Aristo: Miscellaneous tests":
+
+    test "VertexID recyling lists":
+      check noisy.testVidRecycleLists()
+
+    test &"QueueID slot management (sample size: {qidSampleSize})":
+      check noisy.testQidScheduler(sampleSize = qidSampleSize)
 
 
 proc accountsRunner(
@@ -164,7 +172,7 @@ when isMainModule:
   setErrorLevel()
 
   when true: # and false:
-    noisy.miscRunner()
+    noisy.miscRunner(qidSampleSize = 10_000)
 
   # This one uses dumps from the external `nimbus-eth1-blob` repo
   when true and false:
diff --git a/tests/test_aristo/test_backend.nim b/tests/test_aristo/test_backend.nim
index 14314bc4a..08738ef31 100644
--- a/tests/test_aristo/test_backend.nim
+++ b/tests/test_aristo/test_backend.nim
@@ -20,10 +20,10 @@ import
   ../../nimbus/db/aristo/[
     aristo_debug,
     aristo_desc,
-    aristo_desc/aristo_types_backend,
+    aristo_desc/desc_backend,
     aristo_hashify,
-    aristo_init/aristo_memory,
-    aristo_init/aristo_rocksdb,
+    aristo_init/memory_db,
+    aristo_init/rocks_db,
     aristo_persistent,
     aristo_transcode,
     aristo_vid],
@@ -142,13 +142,13 @@ proc verify(
 proc collectFilter(
     db: AristoDbRef;
     filter: FilterRef;
-    tab: var Table[FilterID,Hash];
+    tab: var Table[QueueID,Hash];
     noisy: bool;
       ): bool =
   ## Store filter on permanent BE and register digest
   if not filter.isNil:
     let
-      fid = FilterID(7 * (tab.len + 1)) # just some number
+      fid = QueueID(7 * (tab.len + 1)) # just some number
       be = db.backend
       tx = be.putBegFn()
 
@@ -165,7 +165,7 @@ proc collectFilter(
 proc verifyFiltersImpl[T: MemBackendRef|RdbBackendRef](
     _: type T;
     db: AristoDbRef;
-    tab: Table[FilterID,Hash];
+    tab: Table[QueueID,Hash];
     noisy: bool;
       ): bool =
   ## Compare stored filters against registered ones
@@ -195,7 +195,7 @@ proc verifyFiltersImpl[T: MemBackendRef|RdbBackendRef](
 
 proc verifyFilters(
     db: AristoDbRef;
-    tab: Table[FilterID,Hash];
+    tab: Table[QueueID,Hash];
     noisy: bool;
       ): bool =
   ## Wrapper
@@ -224,7 +224,7 @@ proc test_backendConsistency*(
       ): bool =
   ## Import accounts
   var
-    filTab: Table[FilterID,Hash]             # Filter register
+    filTab: Table[QueueID,Hash]             # Filter register
     ndb = AristoDbRef()                      # Reference cache
     mdb = AristoDbRef()                      # Memory backend database
     rdb = AristoDbRef()                      # Rocks DB backend database
diff --git a/tests/test_aristo/test_helpers.nim b/tests/test_aristo/test_helpers.nim
index 7bbd3980d..5c38fe380 100644
--- a/tests/test_aristo/test_helpers.nim
+++ b/tests/test_aristo/test_helpers.nim
@@ -109,7 +109,7 @@ proc `==`*(a: (int,AristoError), b: (int,int)): bool =
 proc `==`*(a: (int,VertexID,AristoError), b: (int,int,int)): bool =
   (a[0], a[1].int, a[2].int) == b
 
-proc `==`*(a: (FilterID,Hash), b: (int,Hash)): bool =
+proc `==`*(a: (QueueID,Hash), b: (int,Hash)): bool =
   (a[0].int,a[1]) == b
 
 proc to*(sample: AccountsSample; T: type seq[UndumpAccounts]): T =
diff --git a/tests/test_aristo/test_misc.nim b/tests/test_aristo/test_misc.nim
index db8cfef0b..bb02a12ad 100644
--- a/tests/test_aristo/test_misc.nim
+++ b/tests/test_aristo/test_misc.nim
@@ -12,23 +12,44 @@
 ## Aristo (aka Patricia) DB trancoder test
 
 import
-  std/sequtils,
+  std/[algorithm, sequtils, strutils],
   eth/common,
   results,
   stew/byteutils,
   unittest2,
   ../../nimbus/db/aristo,
-  ../../nimbus/db/aristo/[aristo_desc, aristo_transcode, aristo_vid],
+  ../../nimbus/db/aristo/[
+    aristo_debug, aristo_desc, aristo_transcode, aristo_vid],
+  ../../nimbus/db/aristo/aristo_filter/[filter_desc, filter_scheduler],
   ./test_helpers
 
 type
   TesterDesc = object
     prng: uint32                       ## random state
 
+  QValRef = ref object
+    fid: FilterID
+    width: uint32
+
+  QTab = Table[QueueID,QValRef]
+
+const
+  QidSlotLyo = [(4,0,10),(3,3,10),(3,4,10),(3,5,10)]
+  QidSlotLy1 = [(4,0,0),(3,3,0),(3,4,0),(3,5,0)]
+
+  QidSample* = (3 * QidSlotLyo.stats.minCovered) div 2
+
 # ------------------------------------------------------------------------------
 # Private helpers
 # ------------------------------------------------------------------------------
 
+template trueOrReturn(expr: untyped): untyped =
+  if not (expr):
+    check expr
+    return
+
+# ---------------------
+
 proc posixPrngRand(state: var uint32): byte =
   ## POSIX.1-2001 example of a rand() implementation, see manual page rand(3).
   state = state * 1103515245 + 12345;
@@ -69,12 +90,59 @@ proc init(T: type TesterDesc; seed: int): TesterDesc =
 proc `+`(a: VertexID, b: int): VertexID =
   (a.uint64 + b.uint64).VertexID
 
+# ---------------------
+
+func sortedPairs(t: QTab): seq[(QueueID,QValRef)] =
+  t.keys.toSeq.mapIt(it.uint64).sorted.mapIt(it.QueueID).mapIt((it,t[it]))
+
+func fifos(qt: QTab; scd: QidSchedRef): seq[seq[(QueueID,QValRef)]] =
+  proc kvp(chn: int, qid: QueueID): (QueueID,QValRef) =
+    let
+      cid = QueueID((chn.uint64 shl 62) or qid.uint64)
+      val = qt.getOrDefault(cid, QValRef(nil))
+    (qid, val)
+
+  for i in 0 ..< scd.state.len:
+    let
+      left =  scd.state[i][0]
+      right = scd.state[i][1]
+    result.add newSeq[(QueueID,QValRef)](0)
+    if left <= right:
+      for j in left .. right:
+        result[i].add kvp(i, j)
+    else:
+      for j in left .. scd.ctx.q[i].wrap:
+        result[i].add kvp(i, j)
+      for j in QueueID(1) .. right:
+        result[i].add kvp(i, j)
+
+func flatten(a: seq[seq[(QueueID,QValRef)]]): seq[(QueueID,QValRef)] =
+  for w in a:
+    result &= w
+
+func pp(val: QValRef): string =
+  if val.isNil:
+    return "ø"
+  result = $val.fid.uint64
+  if 0 < val.width:
+    result &= ":" & $val.width
+
+func pp(kvp: (QueueID,QValRef)): string =
+  kvp[0].pp & "=" & kvp[1].pp
+
+func pp(t: QTab): string =
+  "{" & t.sortedPairs.mapIt(it.pp).join(",") & "}"
+
+func pp(t: QTab; scd: QidSchedRef): string =
+  "[" & t.fifos(scd).flatten.mapIt(it.pp).join(",") & "]"
+
 # ------------------------------------------------------------------------------
 # Public test function
 # ------------------------------------------------------------------------------
 
-proc testVidRecycleLists*(noisy = true; seed = 42) =
+proc testVidRecycleLists*(noisy = true; seed = 42): bool =
   ## Transcode VID lists held in `AristoDb` descriptor
+  ##
   var td = TesterDesc.init seed
   let db = newAristoDbRef BackendVoid
 
@@ -93,7 +161,7 @@ proc testVidRecycleLists*(noisy = true; seed = 42) =
       expectedVids += (vid < first).ord
       db.vidDispose vid
 
-    check db.top.vGen.len == expectedVids
+    trueOrReturn db.top.vGen.len == expectedVids
     noisy.say "***", "vids=", db.top.vGen.len, " discarded=", count-expectedVids
 
   # Serialise/deserialise
@@ -105,43 +173,213 @@ proc testVidRecycleLists*(noisy = true; seed = 42) =
       db1 = newAristoDbRef BackendVoid
       rc = dbBlob.deblobify seq[VertexID]
     if rc.isErr:
-      check rc.error == AristoError(0)
+      trueOrReturn rc.error == AristoError(0)
     else:
       db1.top.vGen = rc.value
 
-    check db.top.vGen == db1.top.vGen
+    trueOrReturn db.top.vGen == db1.top.vGen
 
   # Make sure that recycled numbers are fetched first
   let topVid = db.top.vGen[^1]
   while 1 < db.top.vGen.len:
     let w = db.vidFetch()
-    check w < topVid
-  check db.top.vGen.len == 1 and db.top.vGen[0] == topVid
+    trueOrReturn w < topVid
+  trueOrReturn db.top.vGen.len == 1 and db.top.vGen[0] == topVid
 
   # Get some consecutive vertex IDs
   for n in 0 .. 5:
     let w = db.vidFetch()
-    check w == topVid + n
-    check db.top.vGen.len == 1
+    trueOrReturn w == topVid + n
+    trueOrReturn db.top.vGen.len == 1
 
   # Repeat last test after clearing the cache
   db.top.vGen.setLen(0)
   for n in 0 .. 5:
     let w = db.vidFetch()
-    check w == VertexID(2) + n # VertexID(1) is default root ID
-    check db.top.vGen.len == 1
+    trueOrReturn w == VertexID(2) + n # VertexID(1) is default root ID
+    trueOrReturn db.top.vGen.len == 1
 
   # Recycling and re-org tests
   func toVQ(a: seq[int]): seq[VertexID] = a.mapIt(VertexID(it))
 
-  check @[8, 7,  3, 4, 5,  9]    .toVQ.vidReorg == @[3, 4, 5,  7] .toVQ
-  check @[8, 7, 6,  3, 4, 5,  9] .toVQ.vidReorg == @[3]           .toVQ
-  check @[5, 4, 3,  7]           .toVQ.vidReorg == @[5, 4, 3,  7] .toVQ
-  check @[5]                     .toVQ.vidReorg == @[5]           .toVQ
-  check @[3, 5]                  .toVQ.vidReorg == @[3, 5]        .toVQ
-  check @[4, 5]                  .toVQ.vidReorg == @[4]           .toVQ
+  trueOrReturn @[8, 7,  3, 4, 5,  9]    .toVQ.vidReorg == @[3, 4, 5,  7] .toVQ
+  trueOrReturn @[8, 7, 6,  3, 4, 5,  9] .toVQ.vidReorg == @[3]           .toVQ
+  trueOrReturn @[5, 4, 3,  7]           .toVQ.vidReorg == @[5, 4, 3,  7] .toVQ
+  trueOrReturn @[5]                     .toVQ.vidReorg == @[5]           .toVQ
+  trueOrReturn @[3, 5]                  .toVQ.vidReorg == @[3, 5]        .toVQ
+  trueOrReturn @[4, 5]                  .toVQ.vidReorg == @[4]           .toVQ
 
-  check newSeq[VertexID](0).vidReorg().len == 0
+  trueOrReturn newSeq[VertexID](0).vidReorg().len == 0
+
+  true
+
+
+proc testQidScheduler*(
+    noisy = true;
+    layout = QidSlotLyo;
+    sampleSize = QidSample;
+      ): bool =
+  ##
+  ## Example table for `QidSlotLyo` layout after 10_000 cycles
+  ## ::
+  ##    QueueID |       QValRef    |
+  ##            | FilterID | width | comment
+  ##    --------+----------+-------+----------------------------------
+  ##    %7      |  9997    |   0   | %7 stands for QueueID(7)
+  ##    %8      |  9998    |   0   |
+  ##    %9      |  9999    |   0   |
+  ##    %a      | 10000    |   0   |
+  ##            |          |       |
+  ##    %1:6    |  9981    |   3   | %1:6 stands for QueueID((1 shl 62) + 6)
+  ##    %1:7    |  9985    |   3   |
+  ##    %1:8    |  9989    |   3   |
+  ##    %1:9    |  9993    |   3   | 9993 + 3 + 1 => 9997, see %7
+  ##            |          |       |
+  ##    %2:3    |  9841    |  19   |
+  ##    %2:4    |  9861    |  19   |
+  ##    %2:5    |  9881    |  19   |
+  ##    %2:6    |  9901    |  19   |
+  ##    %2:7    |  9921    |  19   |
+  ##    %2:8    |  9941    |  19   |
+  ##    %2:9    |  9961    |  19   | 9961 + 19 + 1 => 9981, see %1:6
+  ##            |          |       |
+  ##    %3:a    |  9481    | 119   |
+  ##    %3:1    |  9601    | 119   |
+  ##    %3:2    |  9721    | 119   | 9721 + 119 + 1 => 9871, see %2:3
+  ##
+  var
+    list: Qtab
+    debug = false # or true
+  let
+    scd = QidSchedRef.init layout
+    ctx = scd.ctx.q
+
+  if debug:
+    noisy.say "***", "testFilterSchedule",
+      " ctx=", ctx,
+      " stats=", scd.ctx.stats
+
+  for n in 1 .. sampleSize:
+    let w = scd.addItem()
+
+    if debug and false:
+      noisy.say "***", "testFilterSchedule",
+        " n=", n,
+        " => ", w.exec.pp,
+        " / ", w.fifo.state.pp
+
+    var
+      saved = false
+      hold: seq[(QueueID,QueueID)]
+    for act in w.exec:
+      case act.op:
+      of Oops:
+        noisy.say "***", "testFilterSchedule", " n=", n, " act=", act.pp
+
+      of SaveQid:
+        if saved:
+          noisy.say "***", "testFilterSchedule", " n=", n, " act=", act.pp,
+            " hold=", hold.pp, " state=", scd.state.pp, " fifo=", list.pp scd
+          check not saved
+          return
+        list[act.qid] = QValRef(fid: FilterID(n))
+        saved = true
+
+      of HoldQid:
+        hold.add (act.qid, act.xid)
+
+      of DequQid:
+        var merged = QValRef(nil)
+        for w in hold:
+          for qid in w[0] .. w[1]:
+            let val = list.getOrDefault(qid, QValRef(nil))
+            if val.isNil:
+              noisy.say "***", "testFilterSchedule", " n=", n, " act=", act.pp,
+               " hold=", hold.pp, " state=", scd.state.pp, " fifo=", list.pp scd
+              check not val.isNil
+              return
+            if merged.isNil:
+              merged = val
+            elif merged.fid + merged.width + 1 == val.fid:
+              merged.width += val.width + 1
+            else:
+              noisy.say "***", "testFilterSchedule", " n=", n, " act=", act.pp,
+               " hold=", hold.pp, " state=", scd.state.pp, " fifo=", list.pp scd
+              check merged.fid + merged.width + 1 == val.fid
+              return
+            list.del qid
+        if merged.isNil:
+          noisy.say "***", "testFilterSchedule", " n=", n, " act=", act.pp,
+            " hold=", hold.pp, " state=", scd.state.pp, " fifo=", list.pp scd
+          check not merged.isNil
+          return
+        list[act.qid] = merged
+        hold.setLen(0)
+
+    scd[] = w.fifo[]
+
+    # Verify that the round-robin queues in `list` are consecutive and in the
+    # right order.
+    var
+      botVal = FilterID(0)
+      step = 1u
+    for chn,queue in list.fifos scd:
+      var lastVal = FilterID(0)
+      step *= ctx[chn].width + 1 # defined by schedule layout
+
+      for kvp in queue:
+        let (qid,val) = (kvp[0], kvp[1])
+
+        # Entries must exist
+        if val.isNil:
+          noisy.say "***", "testFilterSchedule", " n=", n, " chn=", chn,
+            " exec=", w.exec.pp, " kvp=", kvp.pp, " fifo=", list.pp scd
+          check not val.isNil
+          return
+
+        # Fid value fields must increase witin a sub-queue.
+        if val.fid <= lastVal:
+          noisy.say "***", "testFilterSchedule", " n=", n, " chn=", chn,
+            " exec=", w.exec.pp, " kvp=", kvp.pp, " fifo=", list.pp scd
+          check lastVal < val.fid
+          return
+
+        # Width value must correspond to `step` size
+        if val.width + 1 != step:
+          noisy.say "***", "testFilterSchedule", " n=", n, " chn=", chn,
+            " exec=", w.exec.pp, " kvp=", kvp.pp, " fifo=", list.pp scd
+          check val.width + 1 == step
+          return
+
+        # Item distances must match the step width
+        if lastVal != 0:
+          let dist = val.fid - lastVal
+          if dist != step.uint64:
+            noisy.say "***", "testFilterSchedule", " n=", n, " chn=", chn,
+              " exec=", w.exec.pp, " kvp=", kvp.pp, " fifo=", list.pp scd
+            check dist == step.uint64
+            return
+        lastVal = val.fid
+
+      # The top value of the current queue must be smaller than the
+      # bottom value of the previous one
+      if 0 < chn and botVal != queue[^1][1].fid + step.uint64:
+        noisy.say "***", "testFilterSchedule", " n=", n, " chn=", chn,
+          " exec=", w.exec.pp, " step=", step, " fifo=", list.pp scd
+        check botVal == queue[^1][1].fid + step.uint64
+        return
+      botVal = queue[0][1].fid
+
+    if debug:
+     noisy.say "***", "testFilterSchedule",
+        " n=", n,
+        "\n     exec=", w.exec.pp,
+        "\n    state=", scd.state.pp,
+        "\n     list=", list.pp,
+        "\n     fifo=", list.pp scd,
+        "\n"
+
+  true
 
 # ------------------------------------------------------------------------------
 # End