From 0d9fd548577206908f1827f3c0687816aae8ac6f Mon Sep 17 00:00:00 2001
From: Jacek Sieka <jacek@status.im>
Date: Thu, 18 Aug 2022 20:07:01 +0200
Subject: [PATCH] cache shuffling separately from other EpochRef data (fixes
 #2677) (#3990)

In order to avoid full replays when validating attestations hailing from
untaken forks, it's better to keep shufflings separate from `EpochRef`
and perform a lookahead on the shuffling when processing the block that
determines them.

This also helps performance in the case where REST clients are trying to
perform lookahead on attestation duties and decreases memory usage by
sharing shufflings between EpochRef instances of the same dependent
root.
---
 .../block_clearance.nim                       |   7 +
 .../block_pools_types.nim                     |  17 +-
 .../consensus_object_pools/blockchain_dag.nim | 251 ++++++++++++------
 .../consensus_object_pools/spec_cache.nim     |  84 +++---
 .../gossip_processing/batch_validation.nim    |   6 +-
 .../gossip_processing/gossip_validation.nim   |  46 ++--
 beacon_chain/rpc/rest_validator_api.nim       |  51 ++--
 beacon_chain/validators/action_tracker.nim    |   4 +-
 beacon_chain/validators/message_router.nim    |   6 +-
 beacon_chain/validators/validator_duties.nim  |  49 ++--
 tests/test_blockchain_dag.nim                 |  19 +-
 11 files changed, 319 insertions(+), 221 deletions(-)

diff --git a/beacon_chain/consensus_object_pools/block_clearance.nim b/beacon_chain/consensus_object_pools/block_clearance.nim
index 15f8dbbcf..c05f0e996 100644
--- a/beacon_chain/consensus_object_pools/block_clearance.nim
+++ b/beacon_chain/consensus_object_pools/block_clearance.nim
@@ -88,6 +88,13 @@ proc addResolvedHeadBlock(
   # Update light client data
   dag.processNewBlockForLightClient(state, trustedBlock, parent.bid)
 
+  # Pre-heat the shuffling cache with the shuffling caused by this block - this
+  # is useful for attestation duty lookahead, REST API queries and attestation
+  # validation of untaken forks (in case of instability / multiple heads)
+  if dag.findShufflingRef(blockRef.bid, blockRef.slot.epoch + 1).isNone:
+    dag.putShufflingRef(
+      ShufflingRef.init(state, cache, blockRef.slot.epoch + 1))
+
   if not blockVerified:
     dag.optimisticRoots.incl blockRoot
 
diff --git a/beacon_chain/consensus_object_pools/block_pools_types.nim b/beacon_chain/consensus_object_pools/block_pools_types.nim
index ed2f3f01c..a6e2639b0 100644
--- a/beacon_chain/consensus_object_pools/block_pools_types.nim
+++ b/beacon_chain/consensus_object_pools/block_pools_types.nim
@@ -189,6 +189,8 @@ type
 
     cfg*: RuntimeConfig
 
+    shufflingRefs*: array[16, ShufflingRef]
+
     epochRefs*: array[32, EpochRef]
       ## Cached information about a particular epoch ending with the given
       ## block - we limit the number of held EpochRefs to put a cap on
@@ -243,8 +245,18 @@ type
     epoch*: Epoch
     bid*: BlockId
 
+  ShufflingRef* = ref object
+    ## Attestation committee shuffling that determines when validators have
+    ## duties - determined with one epoch of look-ahead - the dependent root is
+    ## the block root that is the last to affect the shuffling outcome.
+    epoch*: Epoch
+    attester_dependent_root*: Eth2Digest
+      ## Root of the block that determined the shuffling - this is the last
+      ## block that was applied in (epoch - 2).
+
+    shuffled_active_validator_indices*: seq[ValidatorIndex]
+
   EpochRef* = ref object
-    dag*: ChainDAGRef
     key*: EpochKey
 
     eth1_data*: Eth1Data
@@ -255,8 +267,7 @@ type
     beacon_proposers*: array[SLOTS_PER_EPOCH, Option[ValidatorIndex]]
     proposer_dependent_root*: Eth2Digest
 
-    shuffled_active_validator_indices*: seq[ValidatorIndex]
-    attester_dependent_root*: Eth2Digest
+    shufflingRef*: ShufflingRef
 
     # balances, as used in fork choice
     effective_balances_bytes*: seq[byte]
diff --git a/beacon_chain/consensus_object_pools/blockchain_dag.nim b/beacon_chain/consensus_object_pools/blockchain_dag.nim
index 5c2315f59..b93635ee7 100644
--- a/beacon_chain/consensus_object_pools/blockchain_dag.nim
+++ b/beacon_chain/consensus_object_pools/blockchain_dag.nim
@@ -139,14 +139,6 @@ func validatorKey*(
   ## non-head branch)!
   dag.db.immutableValidators.load(index)
 
-func validatorKey*(
-    epochRef: EpochRef, index: ValidatorIndex or uint64): Option[CookedPubKey] =
-  ## Returns the validator pubkey for the index, assuming it's been observed
-  ## at any point in time - this function may return pubkeys for indicies that
-  ## are not (yet) part of the head state (if the key has been observed on a
-  ## non-head branch)!
-  validatorKey(epochRef.dag, index)
-
 template is_merge_transition_complete(
     stateParam: ForkedHashedBeaconState): bool =
   withState(stateParam):
@@ -155,57 +147,6 @@ template is_merge_transition_complete(
     else:
       false
 
-func init*(
-    T: type EpochRef, dag: ChainDAGRef, state: ForkedHashedBeaconState,
-    cache: var StateCache): T =
-  let
-    epoch = state.get_current_epoch()
-    proposer_dependent_root = withState(state): state.proposer_dependent_root
-    attester_dependent_root = withState(state): state.attester_dependent_root
-    epochRef = EpochRef(
-      dag: dag, # This gives access to the validator pubkeys through an EpochRef
-      key: dag.epochAncestor(state.latest_block_id, epoch),
-
-      eth1_data:
-        getStateField(state, eth1_data),
-      eth1_deposit_index:
-        getStateField(state, eth1_deposit_index),
-
-      checkpoints:
-        FinalityCheckpoints(
-          justified: getStateField(state, current_justified_checkpoint),
-          finalized: getStateField(state, finalized_checkpoint)),
-
-      # beacon_proposers: Separately filled below
-      proposer_dependent_root: proposer_dependent_root,
-
-      shuffled_active_validator_indices:
-        cache.get_shuffled_active_validator_indices(state, epoch),
-      attester_dependent_root: attester_dependent_root,
-    )
-    epochStart = epoch.start_slot()
-
-  for i in 0'u64..<SLOTS_PER_EPOCH:
-    epochRef.beacon_proposers[i] =
-      get_beacon_proposer_index(state, cache, epochStart + i)
-
-  # When fork choice runs, it will need the effective balance of the justified
-  # checkpoint - we pre-load the balances here to avoid rewinding the justified
-  # state later and compress them because not all checkpoints end up being used
-  # for fork choice - specially during long periods of non-finalization
-  proc snappyEncode(inp: openArray[byte]): seq[byte] =
-    try:
-      snappy.encode(inp)
-    except CatchableError as err:
-      raiseAssert err.msg
-
-  epochRef.effective_balances_bytes =
-    snappyEncode(SSZ.encode(
-      List[Gwei, Limit VALIDATOR_REGISTRY_LIMIT](
-        get_effective_balances(getStateField(state, validators).asSeq, epoch))))
-
-  epochRef
-
 func effective_balances*(epochRef: EpochRef): seq[Gwei] =
   try:
     SSZ.decode(snappy.decode(epochRef.effective_balances_bytes, uint32.high),
@@ -346,17 +287,146 @@ func epochAncestor*(dag: ChainDAGRef, bid: BlockId, epoch: Epoch): EpochKey =
 
   EpochKey(epoch: epoch, bid: bsi.bid)
 
+func findShufflingRef*(
+    dag: ChainDAGRef, bid: BlockId, epoch: Epoch): Opt[ShufflingRef] =
+  ## Lookup a shuffling in the cache, returning `none` if it's not present - see
+  ## `getShufflingRef` for a version that creates a new instance if it's missing
+  let
+    dependent_slot = if epoch > 2: (epoch - 1).start_slot() - 1 else: Slot(0)
+    dependent_bsi = dag.atSlot(bid, dependent_slot).valueOr:
+      return Opt.none(ShufflingRef)
+
+  for s in dag.shufflingRefs:
+    if s == nil: continue
+    if s.epoch == epoch and dependent_bsi.bid.root == s.attester_dependent_root:
+      return Opt.some s
+  Opt.none(ShufflingRef)
+
+func putShufflingRef*(dag: ChainDAGRef, shufflingRef: ShufflingRef) =
+  ## Store shuffling in the cache
+  if shufflingRef.epoch < dag.finalizedHead.slot.epoch():
+    # Only cache epoch information for unfinalized blocks - earlier states
+    # are seldomly used (ie RPC), so no need to cache
+    return
+
+  # Because we put a cap on the number of shufflingRef we store, we want to
+  # prune the least useful state - for now, we'll assume that to be the
+  # oldest shufflingRef we know about.
+  var
+    oldest = 0
+  for x in 0..<dag.shufflingRefs.len:
+    let candidate = dag.shufflingRefs[x]
+    if candidate == nil:
+      oldest = x
+      break
+    if candidate.epoch < dag.shufflingRefs[oldest].epoch:
+      oldest = x
+
+  dag.shufflingRefs[oldest] = shufflingRef
+
 func findEpochRef*(
     dag: ChainDAGRef, bid: BlockId, epoch: Epoch): Opt[EpochRef] =
-  ## Look for an existing cached EpochRef, but unlike `getEpochRef`, don't
-  ## try to create one by recreating the epoch state
+  ## Lookup an EpochRef in the cache, returning `none` if it's not present - see
+  ## `getEpochRef` for a version that creates a new instance if it's missing
   let ancestor = dag.epochAncestor(bid, epoch)
 
-  for i in 0..<dag.epochRefs.len:
-    if dag.epochRefs[i] != nil and dag.epochRefs[i].key == ancestor:
-      return ok(dag.epochRefs[i])
+  for e in dag.epochRefs:
+    if e == nil: continue
+    if e.key == ancestor:
+      return Opt.some e
 
-  err()
+  Opt.none(EpochRef)
+
+func putEpochRef(dag: ChainDAGRef, epochRef: EpochRef) =
+  if epochRef.epoch < dag.finalizedHead.slot.epoch():
+    # Only cache epoch information for unfinalized blocks - earlier states
+    # are seldomly used (ie RPC), so no need to cache
+    return
+
+  # Because we put a cap on the number of epochRefs we store, we want to
+  # prune the least useful state - for now, we'll assume that to be the
+  # oldest epochRef we know about.
+
+  var
+    oldest = 0
+  for x in 0..<dag.epochRefs.len:
+    let candidate = dag.epochRefs[x]
+    if candidate == nil:
+      oldest = x
+      break
+    if candidate.key.epoch < dag.epochRefs[oldest].epoch:
+      oldest = x
+
+  dag.epochRefs[oldest] = epochRef
+
+func init*(
+    T: type ShufflingRef, state: ForkedHashedBeaconState,
+    cache: var StateCache, epoch: Epoch): T =
+  let
+    dependent_epoch =
+      if epoch < 1: Epoch(0) else: epoch - 1
+    attester_dependent_root =
+      withState(state): state.dependent_root(dependent_epoch)
+
+  ShufflingRef(
+    epoch: epoch,
+    attester_dependent_root: attester_dependent_root,
+    shuffled_active_validator_indices:
+      cache.get_shuffled_active_validator_indices(state, epoch),
+  )
+
+func init*(
+    T: type EpochRef, dag: ChainDAGRef, state: ForkedHashedBeaconState,
+    cache: var StateCache): T =
+  let
+    epoch = state.get_current_epoch()
+    proposer_dependent_root = withState(state): state.proposer_dependent_root
+    shufflingRef = dag.findShufflingRef(state.latest_block_id, epoch).valueOr:
+      let tmp = ShufflingRef.init(state, cache, epoch)
+      dag.putShufflingRef(tmp)
+      tmp
+
+    attester_dependent_root = withState(state): state.attester_dependent_root
+    epochRef = EpochRef(
+      key: dag.epochAncestor(state.latest_block_id, epoch),
+
+      eth1_data:
+        getStateField(state, eth1_data),
+      eth1_deposit_index:
+        getStateField(state, eth1_deposit_index),
+
+      checkpoints:
+        FinalityCheckpoints(
+          justified: getStateField(state, current_justified_checkpoint),
+          finalized: getStateField(state, finalized_checkpoint)),
+
+      # beacon_proposers: Separately filled below
+      proposer_dependent_root: proposer_dependent_root,
+
+      shufflingRef: shufflingRef
+    )
+    epochStart = epoch.start_slot()
+
+  for i in 0'u64..<SLOTS_PER_EPOCH:
+    epochRef.beacon_proposers[i] =
+      get_beacon_proposer_index(state, cache, epochStart + i)
+
+  # When fork choice runs, it will need the effective balance of the justified
+  # checkpoint - we pre-load the balances here to avoid rewinding the justified
+  # state later and compress them because not all checkpoints end up being used
+  # for fork choice - specially during long periods of non-finalization
+  proc snappyEncode(inp: openArray[byte]): seq[byte] =
+    try:
+      snappy.encode(inp)
+    except CatchableError as err:
+      raiseAssert err.msg
+
+  epochRef.effective_balances_bytes =
+    snappyEncode(SSZ.encode(
+      List[Gwei, Limit VALIDATOR_REGISTRY_LIMIT](
+        get_effective_balances(getStateField(state, validators).asSeq, epoch))))
+
+  epochRef
 
 func loadStateCache(
     dag: ChainDAGRef, cache: var StateCache, bid: BlockId, epoch: Epoch) =
@@ -368,12 +438,14 @@ func loadStateCache(
     block:
       let epoch = e
       if epoch notin cache.shuffled_active_validator_indices:
+        let shufflingRef = dag.findShufflingRef(bid, epoch)
+        if shufflingRef.isSome():
+          cache.shuffled_active_validator_indices[epoch] =
+            shufflingRef[][].shuffled_active_validator_indices
         let epochRef = dag.findEpochRef(bid, epoch)
         if epochRef.isSome():
-          cache.shuffled_active_validator_indices[epoch] =
-            epochRef[].shuffled_active_validator_indices
           let start_slot = epoch.start_slot()
-          for i, idx in epochRef[].beacon_proposers:
+          for i, idx in epochRef[][].beacon_proposers:
             cache.beacon_proposer_indices[start_slot + i] = idx
 
   load(epoch)
@@ -992,25 +1064,7 @@ func getEpochRef*(
   var epochRef = dag.findEpochRef(bid, epoch)
   if epochRef.isErr:
     let res = EpochRef.init(dag, state, cache)
-    if epoch >= dag.finalizedHead.slot.epoch():
-      # Only cache epoch information for unfinalized blocks - earlier states
-      # are seldomly used (ie RPC), so no need to cache
-
-      # Because we put a cap on the number of epochRefs we store, we want to
-      # prune the least useful state - for now, we'll assume that to be the
-      # oldest epochRef we know about.
-
-      var
-        oldest = 0
-      for x in 0..<dag.epochRefs.len:
-        let candidate = dag.epochRefs[x]
-        if candidate == nil:
-          oldest = x
-          break
-        if candidate.key.epoch < dag.epochRefs[oldest].epoch:
-          oldest = x
-
-      dag.epochRefs[oldest] = res
+    dag.putEpochRef(res)
     res
   else:
     epochRef.get()
@@ -1071,6 +1125,24 @@ proc getFinalizedEpochRef*(dag: ChainDAGRef): EpochRef =
     dag.finalizedHead.blck, dag.finalizedHead.slot.epoch, false).expect(
       "getEpochRef for finalized head should always succeed")
 
+proc getShufflingRef*(
+    dag: ChainDAGRef, blck: BlockRef, epoch: Epoch,
+    preFinalized: bool): Opt[ShufflingRef] =
+  ## Return the shuffling in the given history and epoch - this potentially is
+  ## faster than returning a full EpochRef because the shuffling is determined
+  ## an epoch in advance and therefore is less sensitive to reorgs
+  let shufflingRef = dag.findShufflingRef(blck.bid, epoch)
+  if shufflingRef.isNone:
+    # TODO here, we could check the existing cached states and see if any one
+    # has the right dependent root - unlike EpochRef, we don't need an _exact_
+    # epoch match
+    let epochRef = dag.getEpochRef(blck, epoch, preFinalized).valueOr:
+      return Opt.none ShufflingRef
+    dag.putShufflingRef(epochRef.shufflingRef)
+    Opt.some epochRef.shufflingRef
+  else:
+    shufflingRef
+
 func stateCheckpoint*(dag: ChainDAGRef, bsi: BlockSlotId): BlockSlotId =
   ## The first ancestor BlockSlot that is a state checkpoint
   var bsi = bsi
@@ -1601,6 +1673,11 @@ proc pruneStateCachesDAG*(dag: ChainDAGRef) =
       if dag.epochRefs[i] != nil and
           dag.epochRefs[i].epoch < dag.finalizedHead.slot.epoch:
         dag.epochRefs[i] = nil
+    for i in 0..<dag.shufflingRefs.len:
+      if dag.shufflingRefs[i] != nil and
+          dag.shufflingRefs[i].epoch < dag.finalizedHead.slot.epoch:
+        dag.shufflingRefs[i] = nil
+
   let epochRefPruneTick = Moment.now()
 
   dag.lastPrunePoint = dag.finalizedHead.toBlockSlotId().expect("not nil")
diff --git a/beacon_chain/consensus_object_pools/spec_cache.nim b/beacon_chain/consensus_object_pools/spec_cache.nim
index c0e6b6810..73639a498 100644
--- a/beacon_chain/consensus_object_pools/spec_cache.nim
+++ b/beacon_chain/consensus_object_pools/spec_cache.nim
@@ -25,71 +25,71 @@ export
 logScope: topics = "spec_cache"
 
 # Spec functions implemented based on cached values instead of the full state
-func count_active_validators*(epochInfo: EpochRef): uint64 =
-  epochInfo.shuffled_active_validator_indices.lenu64
+func count_active_validators*(shufflingRef: ShufflingRef): uint64 =
+  shufflingRef.shuffled_active_validator_indices.lenu64
 
 # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.2/specs/phase0/beacon-chain.md#get_committee_count_per_slot
-func get_committee_count_per_slot*(epochInfo: EpochRef): uint64 =
-  get_committee_count_per_slot(count_active_validators(epochInfo))
+func get_committee_count_per_slot*(shufflingRef: ShufflingRef): uint64 =
+  get_committee_count_per_slot(count_active_validators(shufflingRef))
 
-iterator get_committee_indices*(epochRef: EpochRef): CommitteeIndex =
-  let committees_per_slot = get_committee_count_per_slot(epochRef)
+iterator get_committee_indices*(shufflingRef: ShufflingRef): CommitteeIndex =
+  let committees_per_slot = get_committee_count_per_slot(shufflingRef)
   for committee_index in get_committee_indices(committees_per_slot):
     yield committee_index
 
-func get_committee_index*(epochRef: EpochRef, index: uint64):
+func get_committee_index*(shufflingRef: ShufflingRef, index: uint64):
     Result[CommitteeIndex, cstring] =
-  check_attestation_index(index, get_committee_count_per_slot(epochRef))
+  check_attestation_index(index, get_committee_count_per_slot(shufflingRef))
 
 # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.2/specs/phase0/beacon-chain.md#get_beacon_committee
 iterator get_beacon_committee*(
-    epochRef: EpochRef, slot: Slot, committee_index: CommitteeIndex):
+    shufflingRef: ShufflingRef, slot: Slot, committee_index: CommitteeIndex):
     (int, ValidatorIndex) =
   ## Return the beacon committee at ``slot`` for ``index``.
-  doAssert slot.epoch == epochRef.epoch
-  let committees_per_slot = get_committee_count_per_slot(epochRef)
+  doAssert slot.epoch == shufflingRef.epoch
+  let committees_per_slot = get_committee_count_per_slot(shufflingRef)
   for index_in_committee, idx in compute_committee(
-    epochRef.shuffled_active_validator_indices,
+    shufflingRef.shuffled_active_validator_indices,
     (slot mod SLOTS_PER_EPOCH) * committees_per_slot + committee_index.asUInt64,
     committees_per_slot * SLOTS_PER_EPOCH
   ): yield (index_in_committee, idx)
 
 # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.2/specs/phase0/beacon-chain.md#get_beacon_committee
 func get_beacon_committee*(
-    epochRef: EpochRef, slot: Slot, committee_index: CommitteeIndex):
+    shufflingRef: ShufflingRef, slot: Slot, committee_index: CommitteeIndex):
     seq[ValidatorIndex] =
   ## Return the beacon committee at ``slot`` for ``index``.
-  doAssert slot.epoch == epochRef.epoch
-  let committees_per_slot = get_committee_count_per_slot(epochRef)
+  doAssert slot.epoch == shufflingRef.epoch
+  let committees_per_slot = get_committee_count_per_slot(shufflingRef)
   compute_committee(
-    epochRef.shuffled_active_validator_indices,
+    shufflingRef.shuffled_active_validator_indices,
     (slot mod SLOTS_PER_EPOCH) * committees_per_slot + committee_index.asUInt64,
     committees_per_slot * SLOTS_PER_EPOCH
   )
 
 # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.2/specs/phase0/beacon-chain.md#get_beacon_committee
 func get_beacon_committee_len*(
-    epochRef: EpochRef, slot: Slot, committee_index: CommitteeIndex): uint64 =
+    shufflingRef: ShufflingRef, slot: Slot, committee_index: CommitteeIndex): uint64 =
   ## Return the number of members in the beacon committee at ``slot`` for ``index``.
-  doAssert slot.epoch == epochRef.epoch
-  let committees_per_slot = get_committee_count_per_slot(epochRef)
+  doAssert slot.epoch == shufflingRef.epoch
+  let committees_per_slot = get_committee_count_per_slot(shufflingRef)
   compute_committee_len(
-    count_active_validators(epochRef),
+    count_active_validators(shufflingRef),
     (slot mod SLOTS_PER_EPOCH) * committees_per_slot + committee_index.asUInt64,
     committees_per_slot * SLOTS_PER_EPOCH
   )
 
 # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.2/specs/phase0/beacon-chain.md#get_attesting_indices
-iterator get_attesting_indices*(epochRef: EpochRef,
+iterator get_attesting_indices*(shufflingRef: ShufflingRef,
                                 slot: Slot,
                                 committee_index: CommitteeIndex,
                                 bits: CommitteeValidatorsBits):
                                   ValidatorIndex =
-  if bits.lenu64 != get_beacon_committee_len(epochRef, slot, committee_index):
+  if bits.lenu64 != get_beacon_committee_len(shufflingRef, slot, committee_index):
     trace "get_attesting_indices: inconsistent aggregation and committee length"
   else:
     for index_in_committee, validator_index in get_beacon_committee(
-        epochRef, slot, committee_index):
+        shufflingRef, slot, committee_index):
       if bits[index_in_committee]:
         yield validator_index
 
@@ -111,13 +111,13 @@ iterator get_attesting_indices*(
           warn "Invalid attestation target"
           doAssert strictVerification notin dag.updateFlags
           break
-      epochRef =
-        dag.getEpochRef(target.blck, target.slot.epoch, false).valueOr:
-          warn "Attestation `EpochRef` not found"
+      shufflingRef =
+        dag.getShufflingRef(target.blck, target.slot.epoch, false).valueOr:
+          warn "Attestation shuffling not found"
           doAssert strictVerification notin dag.updateFlags
           break
 
-      committeesPerSlot = get_committee_count_per_slot(epochRef)
+      committeesPerSlot = get_committee_count_per_slot(shufflingRef)
       committeeIndex =
         CommitteeIndex.init(attestation.data.index, committeesPerSlot).valueOr:
           warn "Unexpected committee index in block attestation"
@@ -125,10 +125,10 @@ iterator get_attesting_indices*(
           break
 
     for validator in get_attesting_indices(
-        epochRef, slot, committeeIndex, attestation.aggregation_bits):
+        shufflingRef, slot, committeeIndex, attestation.aggregation_bits):
       yield validator
 
-func get_attesting_indices_one*(epochRef: EpochRef,
+func get_attesting_indices_one*(shufflingRef: ShufflingRef,
                                 slot: Slot,
                                 committee_index: CommitteeIndex,
                                 bits: CommitteeValidatorsBits):
@@ -137,25 +137,25 @@ func get_attesting_indices_one*(epochRef: EpochRef,
   # if only one validator index is set
   var res = none(ValidatorIndex)
   for validator_index in get_attesting_indices(
-      epochRef, slot, committee_index, bits):
+      shufflingRef, slot, committee_index, bits):
     if res.isSome(): return none(ValidatorIndex)
     res = some(validator_index)
   res
 
 # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.2/specs/phase0/beacon-chain.md#get_attesting_indices
-func get_attesting_indices*(epochRef: EpochRef,
+func get_attesting_indices*(shufflingRef: ShufflingRef,
                             slot: Slot,
                             committee_index: CommitteeIndex,
                             bits: CommitteeValidatorsBits):
                               seq[ValidatorIndex] =
   # TODO sequtils2 mapIt
-  for idx in get_attesting_indices(epochRef, slot, committee_index, bits):
+  for idx in get_attesting_indices(shufflingRef, slot, committee_index, bits):
     result.add(idx)
 
 # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.2/specs/phase0/beacon-chain.md#is_valid_indexed_attestation
 proc is_valid_indexed_attestation*(
     fork: Fork, genesis_validators_root: Eth2Digest,
-    epochRef: EpochRef,
+    dag: ChainDAGRef, shufflingRef: EpochRef,
     attestation: SomeAttestation, flags: UpdateFlags): Result[void, cstring] =
   # This is a variation on `is_valid_indexed_attestation` that works directly
   # with an attestation instead of first constructing an `IndexedAttestation`
@@ -171,8 +171,8 @@ proc is_valid_indexed_attestation*(
     var
       pubkeys = newSeqOfCap[CookedPubKey](sigs)
     for index in get_attesting_indices(
-        epochRef, attestation.data, attestation.aggregation_bits):
-      pubkeys.add(epochRef.validatorKey(index).get())
+        shufflingRef, attestation.data, attestation.aggregation_bits):
+      pubkeys.add(dag.validatorKey(index).get())
 
     if not verify_attestation_signature(
         fork, genesis_validators_root, attestation.data,
@@ -209,23 +209,23 @@ func makeAttestationData*(
 
 # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/validator.md#validator-assignments
 iterator get_committee_assignments*(
-    epochRef: EpochRef, validator_indices: HashSet[ValidatorIndex]):
+    shufflingRef: ShufflingRef, validator_indices: HashSet[ValidatorIndex]):
     tuple[committee_index: CommitteeIndex,
       subnet_id: SubnetId, slot: Slot] =
   let
-    committees_per_slot = get_committee_count_per_slot(epochRef)
-    epoch = epochRef.epoch
+    committees_per_slot = get_committee_count_per_slot(shufflingRef)
+    epoch = shufflingRef.epoch
 
   for slot in epoch.slots():
     for committee_index in get_committee_indices(committees_per_slot):
-      if anyIt(get_beacon_committee(epochRef, slot, committee_index), it in validator_indices):
+      if anyIt(get_beacon_committee(shufflingRef, slot, committee_index), it in validator_indices):
         yield (
           committee_index,
           compute_subnet_for_attestation(committees_per_slot, slot, committee_index),
           slot)
 
-func is_aggregator*(epochRef: EpochRef, slot: Slot, index: CommitteeIndex,
-    slot_signature: ValidatorSig): bool =
+func is_aggregator*(shufflingRef: ShufflingRef, slot: Slot,
+  index: CommitteeIndex, slot_signature: ValidatorSig): bool =
   let
-    committee_len = get_beacon_committee_len(epochRef, slot, index)
+    committee_len = get_beacon_committee_len(shufflingRef, slot, index)
   return is_aggregator(committee_len, slot_signature)
diff --git a/beacon_chain/gossip_processing/batch_validation.nim b/beacon_chain/gossip_processing/batch_validation.nim
index 19463380e..b9c99e60d 100644
--- a/beacon_chain/gossip_processing/batch_validation.nim
+++ b/beacon_chain/gossip_processing/batch_validation.nim
@@ -339,7 +339,7 @@ proc scheduleAggregateChecks*(
       batchCrypto: ref BatchCrypto,
       fork: Fork, genesis_validators_root: Eth2Digest,
       signedAggregateAndProof: SignedAggregateAndProof,
-      epochRef: EpochRef,
+      dag: ChainDAGRef,
       attesting_indices: openArray[ValidatorIndex]
      ): Result[tuple[
         aggregatorFut, slotFut, aggregateFut: Future[BatchResult],
@@ -364,13 +364,13 @@ proc scheduleAggregateChecks*(
   # Do the eager steps first to avoid polluting batches with needlessly
   let
     aggregatorKey =
-      epochRef.validatorKey(aggregate_and_proof.aggregator_index).orReturnErr(
+      dag.validatorKey(aggregate_and_proof.aggregator_index).orReturnErr(
         "SignedAggregateAndProof: invalid aggregator index")
     aggregatorSig = signedAggregateAndProof.signature.load().orReturnErr(
       "aggregateAndProof: invalid proof signature")
     slotSig = aggregate_and_proof.selection_proof.load().orReturnErr(
       "aggregateAndProof: invalid selection signature")
-    aggregateKey = ? aggregateAll(epochRef.dag, attesting_indices)
+    aggregateKey = ? aggregateAll(dag, attesting_indices)
     aggregateSig = aggregate.signature.load().orReturnErr(
       "aggregateAndProof: invalid aggregate signature")
 
diff --git a/beacon_chain/gossip_processing/gossip_validation.nim b/beacon_chain/gossip_processing/gossip_validation.nim
index e737af62a..354b62a7b 100644
--- a/beacon_chain/gossip_processing/gossip_validation.nim
+++ b/beacon_chain/gossip_processing/gossip_validation.nim
@@ -142,11 +142,11 @@ func check_aggregation_count(
   ok()
 
 func check_attestation_subnet(
-    epochRef: EpochRef, slot: Slot, committee_index: CommitteeIndex,
+    shufflingRef: ShufflingRef, slot: Slot, committee_index: CommitteeIndex,
     subnet_id: SubnetId): Result[void, ValidationError] =
   let
     expectedSubnet = compute_subnet_for_attestation(
-      get_committee_count_per_slot(epochRef), slot, committee_index)
+      get_committee_count_per_slot(shufflingRef), slot, committee_index)
 
   if expectedSubnet != subnet_id:
     return errReject("Attestation not on the correct subnet")
@@ -447,18 +447,17 @@ proc validateAttestation*(
   # compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)) ==
   # store.finalized_checkpoint.root
   let
-    epochRef = block:
-      let tmp = pool.dag.getEpochRef(target.blck, target.slot.epoch, false)
-      if isErr(tmp): # shouldn't happen since we verified target
-        warn "No EpochRef for attestation",
+    shufflingRef =
+      pool.dag.getShufflingRef(target.blck, target.slot.epoch, false).valueOr:
+        # Target is verified - shouldn't happen
+        warn "No shuffling for attestation - report bug",
           attestation = shortLog(attestation), target = shortLog(target)
-        return errIgnore("Attestation:  no EpochRef")
-      tmp.get()
+        return errIgnore("Attestation: no shuffling")
 
   # [REJECT] The committee index is within the expected range -- i.e.
   # data.index < get_committee_count_per_slot(state, data.target.epoch).
   let committee_index = block:
-    let idx = epochRef.get_committee_index(attestation.data.index)
+    let idx = shufflingRef.get_committee_index(attestation.data.index)
     if idx.isErr():
       return checkedReject("Attestation: committee index not within expected range")
     idx.get()
@@ -471,7 +470,7 @@ proc validateAttestation*(
   # committee information for the signature check.
   block:
     let v = check_attestation_subnet(
-      epochRef, attestation.data.slot, committee_index, subnet_id) # [REJECT]
+      shufflingRef, attestation.data.slot, committee_index, subnet_id) # [REJECT]
     if v.isErr():
       return err(v.error)
 
@@ -483,7 +482,7 @@ proc validateAttestation*(
   # epoch matches its target and attestation.data.target.root is an ancestor of
   # attestation.data.beacon_block_root.
   if not (attestation.aggregation_bits.lenu64 == get_beacon_committee_len(
-      epochRef, attestation.data.slot, committee_index)):
+      shufflingRef, attestation.data.slot, committee_index)):
     return checkedReject(
       "Attestation: number of aggregation bits and committee size mismatch")
 
@@ -492,7 +491,7 @@ proc validateAttestation*(
     genesis_validators_root =
       getStateField(pool.dag.headState, genesis_validators_root)
     attesting_index = get_attesting_indices_one(
-      epochRef, slot, committee_index, attestation.aggregation_bits)
+      shufflingRef, slot, committee_index, attestation.aggregation_bits)
 
   # The number of aggregation bits matches the committee size, which ensures
   # this condition holds.
@@ -509,7 +508,7 @@ proc validateAttestation*(
         attestation.data.target.epoch:
     return errIgnore("Attestation: Validator has already voted in epoch")
 
-  let pubkey = epochRef.validatorKey(validator_index)
+  let pubkey = pool.dag.validatorKey(validator_index)
   if pubkey.isNone():
     # can't happen, in theory, because we checked the aggregator index above
     return errIgnore("Attestation: cannot find validator pubkey")
@@ -635,18 +634,17 @@ proc validateAggregate*(
     return errIgnore("Aggregate already covered")
 
   let
-    epochRef = block:
-      let tmp = pool.dag.getEpochRef(target.blck, target.slot.epoch, false)
-      if tmp.isErr: # shouldn't happen since we verified target
-        warn "No EpochRef for attestation - report bug",
+    shufflingRef =
+      pool.dag.getShufflingRef(target.blck, target.slot.epoch, false).valueOr:
+        # Target is verified - shouldn't happen
+        warn "No shuffling for attestation - report bug",
           aggregate = shortLog(aggregate), target = shortLog(target)
-        return errIgnore("Aggregate: no EpochRef")
-      tmp.get()
+        return errIgnore("Aggregate: no shuffling")
 
   # [REJECT] The committee index is within the expected range -- i.e.
   # data.index < get_committee_count_per_slot(state, data.target.epoch).
   let committee_index = block:
-    let idx = epochRef.get_committee_index(aggregate.data.index)
+    let idx = shufflingRef.get_committee_index(aggregate.data.index)
     if idx.isErr():
       return checkedReject("Attestation: committee index not within expected range")
     idx.get()
@@ -655,7 +653,7 @@ proc validateAggregate*(
   # aggregator for the slot -- i.e. is_aggregator(state, aggregate.data.slot,
   # aggregate.data.index, aggregate_and_proof.selection_proof) returns True.
   if not is_aggregator(
-      epochRef, slot, committee_index, aggregate_and_proof.selection_proof):
+      shufflingRef, slot, committee_index, aggregate_and_proof.selection_proof):
     return checkedReject("Aggregate: incorrect aggregator")
 
   # [REJECT] The aggregator's validator index is within the committee -- i.e.
@@ -667,7 +665,7 @@ proc validateAggregate*(
       return checkedReject("Aggregate: invalid aggregator index")
 
   if aggregator_index notin
-      get_beacon_committee(epochRef, slot, committee_index):
+      get_beacon_committee(shufflingRef, slot, committee_index):
     return checkedReject("Aggregate: aggregator's validator index not in committee")
 
   # 1. [REJECT] The aggregate_and_proof.selection_proof is a valid signature of the
@@ -682,14 +680,14 @@ proc validateAggregate*(
     genesis_validators_root =
       getStateField(pool.dag.headState, genesis_validators_root)
     attesting_indices = get_attesting_indices(
-      epochRef, slot, committee_index, aggregate.aggregation_bits)
+      shufflingRef, slot, committee_index, aggregate.aggregation_bits)
 
   let
     sig = if checkSignature:
       let deferredCrypto = batchCrypto
                     .scheduleAggregateChecks(
                       fork, genesis_validators_root,
-                      signedAggregateAndProof, epochRef, attesting_indices
+                      signedAggregateAndProof, pool.dag, attesting_indices
                     )
       if deferredCrypto.isErr():
         return checkedReject(deferredCrypto.error)
diff --git a/beacon_chain/rpc/rest_validator_api.nim b/beacon_chain/rpc/rest_validator_api.nim
index f8e72a849..adb5a8d9d 100644
--- a/beacon_chain/rpc/rest_validator_api.nim
+++ b/beacon_chain/rpc/rest_validator_api.nim
@@ -68,7 +68,7 @@ proc installValidatorApiHandlers*(router: var RestRouter, node: BeaconNode) =
         if res.isErr():
           return RestApiResponse.jsonError(Http503, BeaconNodeInSyncError)
         res.get()
-    let epochRef = node.dag.getEpochRef(qhead, qepoch, true).valueOr:
+    let shufflingRef = node.dag.getShufflingRef(qhead, qepoch, true).valueOr:
       return RestApiResponse.jsonError(Http400, PrunedStateError)
 
     let duties =
@@ -76,13 +76,15 @@ proc installValidatorApiHandlers*(router: var RestRouter, node: BeaconNode) =
         var res: seq[RestAttesterDuty]
 
         let
-          committees_per_slot = get_committee_count_per_slot(epochRef)
+          committees_per_slot = get_committee_count_per_slot(shufflingRef)
         for committee_index in get_committee_indices(committees_per_slot):
           for slot in qepoch.slots():
-            let committee = get_beacon_committee(epochRef, slot, committee_index)
+            let
+              committee =
+                get_beacon_committee(shufflingRef, slot, committee_index)
             for index_in_committee, validator_index in committee:
               if validator_index in indexList:
-                let validator_key = epochRef.validatorKey(validator_index)
+                let validator_key = node.dag.validatorKey(validator_index)
                 if validator_key.isSome():
                   res.add(
                     RestAttesterDuty(
@@ -105,7 +107,7 @@ proc installValidatorApiHandlers*(router: var RestRouter, node: BeaconNode) =
         none[bool]()
 
     return RestApiResponse.jsonResponseWRoot(
-      duties, epochRef.attester_dependent_root, optimistic)
+      duties, shufflingRef.attester_dependent_root, optimistic)
 
   # https://ethereum.github.io/beacon-APIs/#/Validator/getProposerDuties
   router.api(MethodGet, "/eth/v1/validator/duties/proposer/{epoch}") do (
@@ -144,7 +146,7 @@ proc installValidatorApiHandlers*(router: var RestRouter, node: BeaconNode) =
           if bp.isSome():
             res.add(
               RestProposerDuty(
-                pubkey: epochRef.validatorKey(bp.get()).get().toPubKey(),
+                pubkey: node.dag.validatorKey(bp.get()).get().toPubKey(),
                 validator_index: bp.get(),
                 slot: qepoch.start_slot() + i
               )
@@ -566,17 +568,16 @@ proc installValidatorApiHandlers*(router: var RestRouter, node: BeaconNode) =
       wallEpoch = wallSlot.epoch
       head = node.dag.head
 
-    var currentEpoch, nextEpoch: Option[EpochRef]
-    template getAndCacheEpochRef(epochRefVar: var Option[EpochRef],
-                                 epoch: Epoch): EpochRef =
-      if epochRefVar.isNone:
-        epochRefVar = block:
-          let epochRef = node.dag.getEpochRef(head, epoch, true)
-          if isErr(epochRef):
+    var currentEpoch, nextEpoch: Opt[ShufflingRef]
+    template getAndCacheShufflingRef(shufflingRefVar: var Opt[ShufflingRef],
+                                     epoch: Epoch): ShufflingRef =
+      if shufflingRefVar.isNone:
+        shufflingRefVar = block:
+          let tmp = node.dag.getShufflingRef(head, epoch, true).valueOr:
             return RestApiResponse.jsonError(Http400, PrunedStateError)
-          some epochRef.get()
+          Opt.some tmp
 
-      epochRefVar.get
+      shufflingRefVar.get
 
     for request in requests:
       if uint64(request.committee_index) >= uint64(MAX_COMMITTEES_PER_SLOT):
@@ -589,17 +590,19 @@ proc installValidatorApiHandlers*(router: var RestRouter, node: BeaconNode) =
       if wallSlot > request.slot + 1:
         return RestApiResponse.jsonError(Http400, SlotFromThePastError)
 
-      let epoch = request.slot.epoch
-      let epochRef = if epoch == wallEpoch:
-        currentEpoch.getAndCacheEpochRef(wallEpoch)
-      elif epoch == wallEpoch + 1:
-        nextEpoch.getAndCacheEpochRef(wallEpoch + 1)
-      else:
-        return RestApiResponse.jsonError(Http400,
-                                         SlotNotInNextWallSlotEpochError)
+      let
+        epoch = request.slot.epoch
+        shufflingRef =
+          if epoch == wallEpoch:
+            currentEpoch.getAndCacheShufflingRef(wallEpoch)
+          elif epoch == wallEpoch + 1:
+            nextEpoch.getAndCacheShufflingRef(wallEpoch + 1)
+          else:
+            return RestApiResponse.jsonError(Http400,
+                                             SlotNotInNextWallSlotEpochError)
 
       let subnet_id = compute_subnet_for_attestation(
-        get_committee_count_per_slot(epochRef), request.slot,
+        get_committee_count_per_slot(shufflingRef), request.slot,
         request.committee_index)
 
       node.actionTracker.registerDuty(
diff --git a/beacon_chain/validators/action_tracker.nim b/beacon_chain/validators/action_tracker.nim
index 8ff34fe38..6619a63c6 100644
--- a/beacon_chain/validators/action_tracker.nim
+++ b/beacon_chain/validators/action_tracker.nim
@@ -225,7 +225,7 @@ func updateActions*(
   let
     epoch = epochRef.epoch
 
-  tracker.attesterDepRoot = epochRef.attester_dependent_root
+  tracker.attesterDepRoot = epochRef.shufflingRef.attester_dependent_root
   tracker.lastCalculatedEpoch = epoch
 
   let validatorIndices = toHashSet(toSeq(tracker.knownValidators.keys()))
@@ -243,7 +243,7 @@ func updateActions*(
   static: doAssert SLOTS_PER_EPOCH <= 32
 
   for (committeeIndex, subnet_id, slot) in
-      get_committee_assignments(epochRef, validatorIndices):
+      get_committee_assignments(epochRef.shufflingRef, validatorIndices):
 
     doAssert epoch(slot) == epoch
 
diff --git a/beacon_chain/validators/message_router.nim b/beacon_chain/validators/message_router.nim
index 229d73a2d..fb516274a 100644
--- a/beacon_chain/validators/message_router.nim
+++ b/beacon_chain/validators/message_router.nim
@@ -209,19 +209,19 @@ proc routeAttestation*(
       return err(
         "Attempt to send attestation for unknown target")
 
-    epochRef = router[].dag.getEpochRef(
+    shufflingRef = router[].dag.getShufflingRef(
         target, attestation.data.target.epoch, false).valueOr:
       warn "Cannot construct EpochRef for attestation, skipping send - report bug",
         target = shortLog(target),
         attestation = shortLog(attestation)
       return
     committee_index =
-      epochRef.get_committee_index(attestation.data.index).valueOr:
+      shufflingRef.get_committee_index(attestation.data.index).valueOr:
         notice "Invalid committee index in attestation",
           attestation = shortLog(attestation)
         return err("Invalid committee index in attestation")
     subnet_id = compute_subnet_for_attestation(
-      get_committee_count_per_slot(epochRef), attestation.data.slot,
+      get_committee_count_per_slot(shufflingRef), attestation.data.slot,
       committee_index)
 
   return await router.routeAttestation(
diff --git a/beacon_chain/validators/validator_duties.nim b/beacon_chain/validators/validator_duties.nim
index d0f8b595c..ff0b74e4a 100644
--- a/beacon_chain/validators/validator_duties.nim
+++ b/beacon_chain/validators/validator_duties.nim
@@ -157,9 +157,8 @@ proc getAttachedValidator(node: BeaconNode,
     nil
 
 proc getAttachedValidator(node: BeaconNode,
-                          epochRef: EpochRef,
                           idx: ValidatorIndex): AttachedValidator =
-  let key = epochRef.validatorKey(idx)
+  let key = node.dag.validatorKey(idx)
   if key.isSome():
     let validator = node.getAttachedValidator(key.get().toPubKey())
     if validator != nil and validator.index != some(idx):
@@ -169,7 +168,7 @@ proc getAttachedValidator(node: BeaconNode,
     validator
   else:
     warn "Validator key not found",
-      idx, epoch = epochRef.epoch
+      idx, head = shortLog(node.dag.head)
     nil
 
 proc isSynced*(node: BeaconNode, head: BlockRef): bool =
@@ -875,15 +874,16 @@ proc handleAttestations(node: BeaconNode, head: BlockRef, slot: Slot) =
         warn "Cannot construct EpochRef for attestation head, report bug",
           attestationHead = shortLog(attestationHead), slot
         return
-    committees_per_slot = get_committee_count_per_slot(epochRef)
+    committees_per_slot = get_committee_count_per_slot(epochRef.shufflingRef)
     fork = node.dag.forkAtEpoch(slot.epoch)
     genesis_validators_root = node.dag.genesis_validators_root
 
   for committee_index in get_committee_indices(committees_per_slot):
-    let committee = get_beacon_committee(epochRef, slot, committee_index)
+    let committee = get_beacon_committee(
+      epochRef.shufflingRef, slot, committee_index)
 
     for index_in_committee, validator_index in committee:
-      let validator = node.getAttachedValidator(epochRef, validator_index)
+      let validator = node.getAttachedValidator(validator_index)
       if validator == nil:
         continue
 
@@ -947,16 +947,12 @@ proc createAndSendSyncCommitteeMessage(node: BeaconNode,
 
 proc handleSyncCommitteeMessages(node: BeaconNode, head: BlockRef, slot: Slot) =
   # TODO Use a view type to avoid the copy
-  var
+  let
     syncCommittee = node.dag.syncCommitteeParticipants(slot + 1)
-    epochRef = node.dag.getEpochRef(head, slot.epoch, false).valueOr:
-      warn "Cannot construct EpochRef for head, report bug",
-        attestationHead = shortLog(head), slot
-      return
 
   for subcommitteeIdx in SyncSubcommitteeIndex:
     for valIdx in syncSubcommittee(syncCommittee, subcommitteeIdx):
-      let validator = node.getAttachedValidator(epochRef, valIdx)
+      let validator = node.getAttachedValidator(valIdx)
       if isNil(validator) or validator.index.isNone():
         continue
       asyncSpawn createAndSendSyncCommitteeMessage(node, validator, slot,
@@ -1021,14 +1017,10 @@ proc handleSyncCommitteeContributions(
     fork = node.dag.forkAtEpoch(slot.epoch)
     genesis_validators_root = node.dag.genesis_validators_root
     syncCommittee = node.dag.syncCommitteeParticipants(slot + 1)
-    epochRef = node.dag.getEpochRef(head, slot.epoch, false).valueOr:
-      warn "Cannot construct EpochRef for head, report bug",
-        attestationHead = shortLog(head), slot
-      return
 
   for subcommitteeIdx in SyncSubCommitteeIndex:
     for valIdx in syncSubcommittee(syncCommittee, subcommitteeIdx):
-      let validator = node.getAttachedValidator(epochRef, valIdx)
+      let validator = node.getAttachedValidator(valIdx)
       if validator == nil:
         continue
 
@@ -1062,7 +1054,7 @@ proc handleProposal(node: BeaconNode, head: BlockRef, slot: Slot):
       await proposeBlock(node, validator, proposer.get(), head, slot)
 
 proc signAndSendAggregate(
-    node: BeaconNode, validator: AttachedValidator, epochRef: EpochRef,
+    node: BeaconNode, validator: AttachedValidator, shufflingRef: ShufflingRef,
     slot: Slot, committee_index: CommitteeIndex) {.async.} =
   try:
     let
@@ -1080,7 +1072,8 @@ proc signAndSendAggregate(
         res.get()
 
   # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/validator.md#aggregation-selection
-    if not is_aggregator(epochRef, slot, committee_index, selectionProof):
+    if not is_aggregator(
+        shufflingRef, slot, committee_index, selectionProof):
       return
 
     # https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.1/specs/phase0/validator.md#construct-aggregate
@@ -1119,19 +1112,19 @@ proc sendAggregatedAttestations(
   # the given slot, for which `is_aggregator` returns `true.
 
   let
-    epochRef = node.dag.getEpochRef(head, slot.epoch, false).valueOr:
+    shufflingRef = node.dag.getShufflingRef(head, slot.epoch, false).valueOr:
       warn "Cannot construct EpochRef for head, report bug",
         head = shortLog(head), slot
       return
-    committees_per_slot = get_committee_count_per_slot(epochRef)
+    committees_per_slot = get_committee_count_per_slot(shufflingRef)
 
   for committee_index in get_committee_indices(committees_per_slot):
     for _, validator_index in
-        get_beacon_committee(epochRef, slot, committee_index):
-      let validator = node.getAttachedValidator(epochRef, validator_index)
+        get_beacon_committee(shufflingRef, slot, committee_index):
+      let validator = node.getAttachedValidator(validator_index)
       if validator != nil:
         asyncSpawn signAndSendAggregate(
-          node, validator, epochRef, slot, committee_index)
+          node, validator, shufflingRef, slot, committee_index)
 
 proc updateValidatorMetrics*(node: BeaconNode) =
   # Technically, this only needs to be done on epoch transitions and if there's
@@ -1419,19 +1412,19 @@ proc registerDuties*(node: BeaconNode, wallSlot: Slot) {.async.} =
   # be getting the duties one slot at a time
   for slot in wallSlot ..< wallSlot + SUBNET_SUBSCRIPTION_LEAD_TIME_SLOTS:
     let
-      epochRef = node.dag.getEpochRef(head, slot.epoch, false).valueOr:
+      shufflingRef = node.dag.getShufflingRef(head, slot.epoch, false).valueOr:
         warn "Cannot construct EpochRef for duties - report bug",
           head = shortLog(head), slot
         return
     let
       fork = node.dag.forkAtEpoch(slot.epoch)
-      committees_per_slot = get_committee_count_per_slot(epochRef)
+      committees_per_slot = get_committee_count_per_slot(shufflingRef)
 
     for committee_index in get_committee_indices(committees_per_slot):
-      let committee = get_beacon_committee(epochRef, slot, committee_index)
+      let committee = get_beacon_committee(shufflingRef, slot, committee_index)
 
       for index_in_committee, validator_index in committee:
-        let validator = node.getAttachedValidator(epochRef, validator_index)
+        let validator = node.getAttachedValidator(validator_index)
         if validator != nil:
           let
             subnet_id = compute_subnet_for_attestation(
diff --git a/tests/test_blockchain_dag.nim b/tests/test_blockchain_dag.nim
index 8cc72cef3..a4d58aa13 100644
--- a/tests/test_blockchain_dag.nim
+++ b/tests/test_blockchain_dag.nim
@@ -74,6 +74,7 @@ suite "Block pool processing" & preset():
     let
       b2Add = dag.addHeadBlock(verifier, b2, nilPhase0Callback)
       b2Get = dag.getForkedBlock(b2.root)
+      sr = dag.findShufflingRef(b1Add[].bid, b1Add[].slot.epoch)
       er = dag.findEpochRef(b1Add[].bid, b1Add[].slot.epoch)
       validators = getStateField(dag.headState, validators).lenu64()
 
@@ -89,15 +90,20 @@ suite "Block pool processing" & preset():
       dag.getBlockIdAtSlot(b2Add[].slot).get() ==
         BlockSlotId.init(dag.genesis, b2Add[].slot)
 
-      not er.isErr()
+      sr.isSome()
+      er.isSome()
+      # er reuses shuffling ref instance
+      er[].shufflingRef == sr[]
       # Same epoch - same epochRef
       er[] == dag.findEpochRef(b2Add[].bid, b2Add[].slot.epoch)[]
       # Different epoch that was never processed
-      dag.findEpochRef(b1Add[].bid, b1Add[].slot.epoch + 1).isErr()
+      dag.findEpochRef(b1Add[].bid, b1Add[].slot.epoch + 1).isNone()
+      # ... but we know the shuffling already!
+      dag.findShufflingRef(b1Add[].bid, b1Add[].slot.epoch + 1).isSome()
 
-      er[].validatorKey(0'u64).isSome()
-      er[].validatorKey(validators - 1).isSome()
-      er[].validatorKey(validators).isNone()
+      dag.validatorKey(0'u64).isSome()
+      dag.validatorKey(validators - 1).isSome()
+      dag.validatorKey(validators).isNone()
 
     # Skip one slot to get a gap
     check:
@@ -172,6 +178,9 @@ suite "Block pool processing" & preset():
     check:
       parentBsi.bid == dag.head.parent.bid
       parentBsi.slot == nextEpochSlot
+      # Pre-heated caches
+      dag.findShufflingRef(dag.head.parent.bid, dag.head.slot.epoch).isOk()
+      dag.findShufflingRef(dag.head.parent.bid, nextEpoch).isOk()
       dag.getEpochRef(dag.head.parent, nextEpoch, true).isOk()
 
       # Getting an EpochRef should not result in states being stored