diff --git a/datastore/concurrentds.nim b/datastore/concurrentds.nim
index da4170c..a0f96c4 100644
--- a/datastore/concurrentds.nim
+++ b/datastore/concurrentds.nim
@@ -13,22 +13,17 @@ export key, query, types, datastore
 push: {.upraises: [].}
 
 type
-  Function*[T, U] = proc(value: T): U {.upraises: [], gcsafe, closure.}
+  Function*[T, U] = proc(value: T): U {.upraises: [CatchableError], gcsafe, closure.}
   Modify* = Function[?seq[byte], ?seq[byte]]
+  ModifyGet* = Function[?seq[byte], (?seq[byte], seq[byte])]
   ModifyAsync* = Function[?seq[byte], Future[?seq[byte]]]
+  ModifyGetAsync* = Function[?seq[byte], Future[(?seq[byte], seq[byte])]]
 
 method modify*(self: ConcurrentDatastore, key: Key, fn: Modify): Future[?!void] {.base, locks: "unknown".} =
   ## Concurrently safe way of modifying the value associated with the `key`.
   ##
-  ## This method first reads a value stored under the `key`, if such value exists it's wrapped into `some`
-  ## and passed as the only arg to the `fn`, otherwise `none` is passed.
-  ##
-  ## When `fn` returns `some`, returned value is put into the store, but only if it's different than
-  ## the existing value, otherwise nothing happens.
-  ## When `fn` returns `none` existing value is deleted from the store, if no value existed before
-  ## nothing happens.
-  ##
-  ## Note that `fn` can be called multiple times (when concurrent modify of the value was detected).
+  ## Same as `modifyGet` with `fn: ModifyGetAsync` argument, but this takes non-async `fn` that doesn't
+  ## produce any auxillary value.
   ##
 
   raiseAssert("Not implemented!")
@@ -36,6 +31,25 @@ method modify*(self: ConcurrentDatastore, key: Key, fn: Modify): Future[?!void]
 method modify*(self: ConcurrentDatastore, key: Key, fn: ModifyAsync): Future[?!void] {.base, locks: "unknown".} =
   ## Concurrently safe way of modifying the value associated with the `key`.
   ##
+  ## Same as `modifyGet` with `fn: ModifyGetAsync` argument, but this takes `fn` that doesn't produce
+  ## any auxillary value.
+  ##
+
+  raiseAssert("Not implemented!")
+
+method modifyGet*(self: ConcurrentDatastore, key: Key, fn: ModifyGet): Future[?!seq[byte]] {.base, locks: "unknown".} =
+  ## Concurrently safe way of updating value associated with the `key`. Returns auxillary value on
+  ## successful update.
+  ##
+  ## Same as `modifyGet` with `fn: ModifyGetAsync` argument, but this takes non-async `fn`.
+  ##
+
+  raiseAssert("Not implemented!")
+
+method modifyGet*(self: ConcurrentDatastore, key: Key, fn: ModifyGetAsync): Future[?!seq[byte]] {.base, locks: "unknown".} =
+  ## Concurrently safe way of updating value associated with the `key`. Returns auxillary value on
+  ## successful update.
+  ##
   ## This method first reads a value stored under the `key`, if such value exists it's wrapped into `some`
   ## and passed as the only arg to the `fn`, otherwise `none` is passed.
   ##
@@ -44,7 +58,8 @@ method modify*(self: ConcurrentDatastore, key: Key, fn: ModifyAsync): Future[?!v
   ## When `fn` returns `none` existing value is deleted from the store, if no value existed before
   ## nothing happens.
   ##
-  ## Note that `fn` can be called multiple times (when concurrent modify of the value was detected).
+  ## Note that `fn` can be called multiple times (when concurrent modify of the value was detected). Only the
+  ## last auxillary value is returned.
   ##
 
   raiseAssert("Not implemented!")
diff --git a/datastore/sql/sqliteds.nim b/datastore/sql/sqliteds.nim
index 41a6a71..cb48e95 100644
--- a/datastore/sql/sqliteds.nim
+++ b/datastore/sql/sqliteds.nim
@@ -31,9 +31,10 @@ proc timestamp*(t = epochTime()): int64 =
 
 const initVersion* = 0.int64
 
-method modify*(self: SQLiteDatastore, key: Key, fn: ModifyAsync): Future[?!void] {.async.} =
+method modifyGet*(self: SQLiteDatastore, key: Key, fn: ModifyGetAsync): Future[?!seq[byte]] {.async.} =
   var
     retriesLeft = 100 # allows reasonable concurrency, avoids infinite loop
+    aux: seq[byte]
 
   while retriesLeft > 0:
     var
@@ -51,7 +52,7 @@ method modify*(self: SQLiteDatastore, key: Key, fn: ModifyAsync): Future[?!void]
     var maybeNewData: ?seq[byte]
 
     try:
-      maybeNewData = await fn(maybeCurrentData)
+      (maybeNewData, aux) = await fn(maybeCurrentData)
     except CatchableError as err:
       return failure("Error running modify function: " & err.msg)
 
@@ -114,13 +115,35 @@ method modify*(self: SQLiteDatastore, key: Key, fn: ModifyAsync): Future[?!void]
   if retriesLeft == 0:
     return failure("Retry limit exceeded")
 
-  return success()
+  return success(aux)
 
-method modify*(self: SQLiteDatastore, key: Key, fn: Modify): Future[?!void] {.async.} =
-  proc wrappedFn(maybeValue: ?seq[byte]): Future[(?seq[byte])] {.async.} =
+method modifyGet*(self: SQLiteDatastore, key: Key, fn: ModifyGet): Future[?!seq[byte]] {.async.} =
+  proc wrappedFn(maybeValue: ?seq[byte]): Future[(?seq[byte], seq[byte])] {.async.} =
     return fn(maybeValue)
 
-  return await self.modify(key, wrappedFn)
+  return await self.modifyGet(key, wrappedFn)
+
+method modify*(self: SQLiteDatastore, key: Key, fn: ModifyAsync): Future[?!void] {.async.} =
+  proc wrappedFn(maybeValue: ?seq[byte]): Future[(?seq[byte], seq[byte])] {.async.} =
+    let res = await fn(maybeValue)
+    let ignoredAux = newSeq[byte]()
+    return (res, ignoredAux)
+
+  if err =? (await self.modifyGet(key, wrappedFn)).errorOption:
+    return failure(err)
+  else:
+    return success()
+
+method modify*(self: SQLiteDatastore, key: Key, fn: Modify): Future[?!void] {.async.} =
+  proc wrappedFn(maybeValue: ?seq[byte]): Future[(?seq[byte], seq[byte])] {.async.} =
+    let maybeNewValue = fn(maybeValue)
+    let ignoredAux = newSeq[byte]()
+    return (maybeNewValue, ignoredAux)
+
+  if err =? (await self.modifyGet(key, wrappedFn)).errorOption:
+    return failure(err)
+  else:
+    return success()
 
 method has*(self: SQLiteDatastore, key: Key): Future[?!bool] {.async.} =
   var
diff --git a/tests/datastore/concurrentdstests.nim b/tests/datastore/concurrentdstests.nim
index 9900072..11b6a7c 100644
--- a/tests/datastore/concurrentdstests.nim
+++ b/tests/datastore/concurrentdstests.nim
@@ -2,6 +2,7 @@ import std/options
 import std/sugar
 import std/random
 import std/sequtils
+import std/strutils
 
 import pkg/asynctest
 import pkg/chronos
@@ -79,18 +80,45 @@ proc concurrentStoreTests*(
   test "should put value":
     (await ds.delete(key)).tryGet()
 
-    (await ds.modify(key, (_: ?seq[byte]) => @(123.uint64.toBytes).some)).tryGet
+    proc returningSomeValue(_: ?seq[byte]): ?seq[byte] =
+      return @(123.uint64.toBytes).some
+
+    (await ds.modify(key, returningSomeValue)).tryGet
 
     let finalValue = uint64.fromBytes((await ds.get(key)).tryGet)
 
     check finalValue.int == 123
 
   test "should delete value":
-    let key = Key.init(Key.random).tryGet
     (await ds.put(key, @(0.uint64.toBytes))).tryGet
 
-    (await ds.modify(key, (_: ?seq[byte]) => seq[byte].none)).tryGet
+    proc returningNone(_: ?seq[byte]): ?seq[byte] =
+      return seq[byte].none
+
+    (await ds.modify(key, returningNone)).tryGet
 
     let hasKey = (await ds.has(key)).tryGet
 
     check not hasKey
+
+  test "should return correct auxillary value":
+    proc returningAux(_: ?seq[byte]): (?seq[byte], seq[byte]) =
+      return (seq[byte].none, @[byte 123])
+
+    let result = await ds.modifyGet(key, returningAux)
+
+    check:
+      result == success(@[byte 123])
+
+  test "should propagate exception as failure":
+    proc throwing(a: ?seq[byte]): ?seq[byte] =
+      raise newException(CatchableError, "some error msg")
+
+    let result = await ds.modify(key, throwing)
+
+    if err =? result.errorOption:
+      check:
+        err.msg.contains("some error msg")
+    else:
+      # result was not an error
+      fail()
\ No newline at end of file