diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 0e2e106..73df27f 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         cache_nonce: [ 1 ]
-        nim_version: [ 1.6.14 ]
+        nim_version: [ 1.6.18 ]
         platform:
           - {
             icon: 🐧,
diff --git a/datastore/datastore.nim b/datastore/datastore.nim
index d7fa45f..f664ec0 100644
--- a/datastore/datastore.nim
+++ b/datastore/datastore.nim
@@ -1,4 +1,5 @@
 import pkg/chronos
+import pkg/questionable
 import pkg/questionable/results
 import pkg/upraises
 
@@ -12,6 +13,9 @@ push: {.upraises: [].}
 
 type
   BatchEntry* = tuple[key: Key, data: seq[byte]]
+  Function*[T, U] = proc(value: T): U {.raises: [CatchableError], gcsafe, closure.}
+  Modify* = Function[?seq[byte], Future[?seq[byte]]]
+  ModifyGet* = Function[?seq[byte], Future[(?seq[byte], seq[byte])]]
 
 method has*(self: Datastore, key: Key): Future[?!bool] {.base, locks: "unknown".} =
   raiseAssert("Not implemented!")
@@ -42,3 +46,34 @@ method query*(
 
 proc contains*(self: Datastore, key: Key): Future[bool] {.async.} =
   return (await self.has(key)) |? false
+
+method modify*(self: Datastore, key: Key, fn: Modify): Future[?!void] {.base, locks: "unknown".} =
+  ## Concurrently safe way of modifying the value associated with the `key`.
+  ##
+  ## Same as `modifyGet`, but this takes `fn` that doesn't produce any auxillary value.
+  ##
+
+  raiseAssert("Not implemented!")
+
+method modifyGet*(self: Datastore, 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.
+  ##
+  ## 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.
+  ##
+  ## Table below presents four possibilities of execution. `curr` represents a value passed to `fn`,
+  ## while `fn(curr)` represents a value returned by calling `fn` (auxillary value is omitted for clarity).
+  ##
+  ## | curr    | fn(curr) | action                       |
+  ## |---------|----------|------------------------------|
+  ## | none    | none     | no action                    |
+  ## | none    | some(v)  | insert v                     |
+  ## | some(u) | none     | delete u                     |
+  ## | some(u) | some(v)  | replace u with v (if u != v) |
+  ##
+  ## Note that `fn` can be called multiple times (when concurrent modification was detected). In such case
+  ## only the last auxillary value is returned.
+  ##
+
+  raiseAssert("Not implemented!")
\ No newline at end of file
diff --git a/datastore/defaultimpl.nim b/datastore/defaultimpl.nim
new file mode 100644
index 0000000..1562263
--- /dev/null
+++ b/datastore/defaultimpl.nim
@@ -0,0 +1,63 @@
+import pkg/chronos
+import pkg/questionable
+import pkg/questionable/results
+
+import ./datastore
+
+proc defaultModifyGetImpl*(
+  self: Datastore,
+  lock: AsyncLock,
+  key: Key,
+  fn: ModifyGet
+  ): Future[?!seq[byte]] {.async.} =
+  # Default implementation, serializes all modify operations using provided lock
+  #
+
+  await lock.acquire()
+
+  try:
+    without data =? await self.get(key), err:
+      if not (err of DatastoreKeyNotFound):
+        return failure(err)
+
+    let maybeCurrentData =
+      if data.len == 0:
+        seq[byte].none
+      else:
+        data.some
+
+    var
+      maybeNewData: ?seq[byte]
+      aux: seq[byte]
+
+    try:
+      (maybeNewData, aux) = await fn(maybeCurrentData)
+    except CatchableError as err:
+      return failure(err)
+
+    if newData =? maybeNewData:
+      if err =? (await self.put(key, newData)).errorOption:
+        return failure(err)
+    elif currentData =? maybeCurrentData:
+      if err =? (await self.delete(key)).errorOption:
+        return failure(err)
+
+    return aux.success
+  finally:
+    lock.release()
+
+proc defaultModifyImpl*(
+  self: Datastore,
+  lock: AsyncLock,
+  key: Key,
+  fn: Modify
+  ): 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.defaultModifyGetImpl(lock, key, wrappedFn)).errorOption:
+    return failure(err)
+  else:
+    return success()
diff --git a/datastore/fsds.nim b/datastore/fsds.nim
index 0be95c9..2a58a99 100644
--- a/datastore/fsds.nim
+++ b/datastore/fsds.nim
@@ -1,5 +1,6 @@
 import std/os
 import std/options
+import std/tables
 import std/strutils
 
 import pkg/chronos
@@ -8,6 +9,7 @@ import pkg/questionable/results
 from pkg/stew/results as stewResults import get, isErr
 import pkg/upraises
 
+import ./defaultimpl
 import ./datastore
 
 export datastore
@@ -19,6 +21,7 @@ type
     root*: string
     ignoreProtected: bool
     depth: int
+    locks: TableRef[Key, AsyncLock]
 
 proc validDepth*(self: FSDatastore, key: Key): bool =
   key.len <= self.depth
@@ -217,6 +220,30 @@ method query*(
   iter.next = next
   return success iter
 
+method modifyGet*(
+  self: FSDatastore,
+  key: Key,
+  fn: ModifyGet): Future[?!seq[byte]] {.async.} =
+  var lock: AsyncLock
+  try:
+    lock = self.locks.mgetOrPut(key, newAsyncLock())
+    return await defaultModifyGetImpl(self, lock, key, fn)
+  finally:
+    if not lock.locked:
+      self.locks.del(key)
+
+method modify*(
+  self: FSDatastore,
+  key: Key,
+  fn: Modify): Future[?!void] {.async.} =
+  var lock: AsyncLock
+  try:
+    lock = self.locks.mgetOrPut(key, newAsyncLock())
+    return await defaultModifyImpl(self, lock, key, fn)
+  finally:
+    if not lock.locked:
+      self.locks.del(key)
+
 proc new*(
   T: type FSDatastore,
   root: string,
@@ -235,4 +262,6 @@ proc new*(
   success T(
     root: root,
     ignoreProtected: ignoreProtected,
-    depth: depth)
+    depth: depth,
+    locks: newTable[Key, AsyncLock]()
+    )
diff --git a/datastore/mountedds.nim b/datastore/mountedds.nim
index dbe74df..9102e49 100644
--- a/datastore/mountedds.nim
+++ b/datastore/mountedds.nim
@@ -119,6 +119,26 @@ method put*(
 
   return success()
 
+method modifyGet*(
+  self: MountedDatastore,
+  key: Key,
+  fn: ModifyGet): Future[?!seq[byte]] {.async.} =
+
+  without mounted =? self.dispatch(key), error:
+    return failure(error)
+
+  return await mounted.store.store.modifyGet(mounted.relative, fn)
+
+method modify*(
+  self: MountedDatastore,
+  key: Key,
+  fn: Modify): Future[?!void] {.async.} =
+
+  without mounted =? self.dispatch(key), error:
+    return failure(error)
+
+  return await mounted.store.store.modify(mounted.relative, fn)
+
 method close*(self: MountedDatastore): Future[?!void] {.async.} =
   for s in self.stores.values:
     discard await s.store.close()
diff --git a/datastore/sql/sqliteds.nim b/datastore/sql/sqliteds.nim
index aa63274..ff440e4 100644
--- a/datastore/sql/sqliteds.nim
+++ b/datastore/sql/sqliteds.nim
@@ -29,6 +29,106 @@ proc `readOnly=`*(self: SQLiteDatastore): bool
 proc timestamp*(t = epochTime()): int64 =
   (t * 1_000_000).int64
 
+const initVersion* = 0.int64
+
+method modifyGet*(self: SQLiteDatastore, key: Key, fn: ModifyGet): Future[?!seq[byte]] {.async.} =
+  var
+    retriesLeft = 100 # allows reasonable concurrency, avoids infinite loop
+    aux: seq[byte]
+
+  while retriesLeft > 0:
+    var
+      currentData: seq[byte]
+      currentVersion: int64
+
+    proc onData(s: RawStmtPtr) =
+      currentData = dataCol(s, GetVersionedStmtDataCol)()
+      currentVersion = versionCol(s, GetVersionedStmtVersionCol)()
+
+    if err =? self.db.getVersionedStmt.query((key.id), onData).errorOption:
+      return failure(err)
+
+    let maybeCurrentData = if currentData.len > 0: some(currentData) else: seq[byte].none
+    var maybeNewData: ?seq[byte]
+
+    try:
+      (maybeNewData, aux) = await fn(maybeCurrentData)
+    except CatchableError as err:
+      return failure(err)
+
+    if maybeCurrentData == maybeNewData:
+      # no need to change currently stored value
+      break
+
+    if err =? self.db.beginStmt.exec().errorOption:
+      return failure(err)
+    if currentData =? maybeCurrentData and newData =? maybeNewData:
+      let updateParams = (
+        newData,
+        currentVersion + 1,
+        timestamp(),
+        key.id,
+        currentVersion
+      )
+      if err =? (self.db.updateVersionedStmt.exec(updateParams)).errorOption:
+        return failure(err)
+    elif currentData =? maybeCurrentData:
+      let deleteParams = (
+        key.id,
+        currentVersion
+      )
+      if err =? (self.db.deleteVersionedStmt.exec(deleteParams)).errorOption:
+        return failure(err)
+    elif newData =? maybeNewData:
+      let insertParams = (
+        key.id,
+        newData,
+        initVersion,
+        timestamp()
+      )
+      if err =? (self.db.insertVersionedStmt.exec(insertParams)).errorOption:
+        return failure(err)
+
+    var changes = 0.int64
+    proc onChangesResult(s: RawStmtPtr) =
+      changes = changesCol(s, 0)()
+
+    if err =? self.db.getChangesStmt.query((), onChangesResult).errorOption:
+      if err =? self.db.rollbackStmt.exec().errorOption:
+        return failure(err)
+      return failure(err)
+
+    if changes == 1:
+      if err =? self.db.endStmt.exec().errorOption:
+        return failure(err)
+      break
+    elif changes == 0:
+      # race condition detected
+      if err =? self.db.rollbackStmt.exec().errorOption:
+        return failure(err)
+      retriesLeft.dec
+    else:
+      if err =? self.db.rollbackStmt.exec().errorOption:
+        return failure(err)
+      return failure("Unexpected number of changes, expected either 0 or 1, was " & $changes)
+
+  if retriesLeft == 0:
+    return failure("Retry limit exceeded")
+
+  return success(aux)
+
+
+method modify*(self: SQLiteDatastore, key: Key, fn: Modify): 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 has*(self: SQLiteDatastore, key: Key): Future[?!bool] {.async.} =
   var
     exists = false
@@ -81,14 +181,14 @@ method get*(self: SQLiteDatastore, key: Key): Future[?!seq[byte]] {.async.} =
   return success bytes
 
 method put*(self: SQLiteDatastore, key: Key, data: seq[byte]): Future[?!void] {.async.} =
-  return self.db.putStmt.exec((key.id, data, timestamp()))
+  return self.db.putStmt.exec((key.id, data, initVersion, timestamp()))
 
 method put*(self: SQLiteDatastore, batch: seq[BatchEntry]): Future[?!void] {.async.} =
   if err =? self.db.beginStmt.exec().errorOption:
     return failure err
 
   for entry in batch:
-    if err =? self.db.putStmt.exec((entry.key.id, entry.data, timestamp())).errorOption:
+    if err =? self.db.putStmt.exec((entry.key.id, entry.data, initVersion, timestamp())).errorOption:
       if err =? self.db.rollbackStmt.exec().errorOption:
         return failure err
 
diff --git a/datastore/sql/sqlitedsdb.nim b/datastore/sql/sqlitedsdb.nim
index 503dea4..f35e087 100644
--- a/datastore/sql/sqlitedsdb.nim
+++ b/datastore/sql/sqlitedsdb.nim
@@ -1,4 +1,5 @@
 import std/os
+import std/strformat
 
 import pkg/questionable
 import pkg/questionable/results
@@ -10,6 +11,7 @@ export sqliteutils
 
 type
   BoundIdCol* = proc (): string {.closure, gcsafe, upraises: [].}
+  BoundVersionCol* = proc (): int64 {.closure, gcsafe, upraises: [].}
   BoundDataCol* = proc (): seq[byte] {.closure, gcsafe, upraises: [].}
   BoundTimestampCol* = proc (): int64 {.closure, gcsafe, upraises: [].}
 
@@ -19,8 +21,13 @@ type
   ContainsStmt* = SQLiteStmt[(string), void]
   DeleteStmt* = SQLiteStmt[(string), void]
   GetStmt* = SQLiteStmt[(string), void]
-  PutStmt* = SQLiteStmt[(string, seq[byte], int64), void]
+  PutStmt* = SQLiteStmt[(string, seq[byte], int64, int64), void]
   QueryStmt* = SQLiteStmt[(string), void]
+  GetVersionedStmt* = SQLiteStmt[(string), void]
+  InsertVersionedStmt* = SQLiteStmt[(string, seq[byte], int64, int64), void]
+  UpdateVersionedStmt* = SQLiteStmt[(seq[byte], int64, int64, string, int64), void]
+  DeleteVersionedStmt* = SQLiteStmt[(string, int64), void]
+  GetChangesStmt* = NoParamsStmt
   BeginStmt* = NoParamsStmt
   EndStmt* = NoParamsStmt
   RollbackStmt* = NoParamsStmt
@@ -34,6 +41,11 @@ type
     getDataCol*: BoundDataCol
     getStmt*: GetStmt
     putStmt*: PutStmt
+    getVersionedStmt*: GetVersionedStmt
+    updateVersionedStmt*: UpdateVersionedStmt
+    insertVersionedStmt*: InsertVersionedStmt
+    deleteVersionedStmt*: DeleteVersionedStmt
+    getChangesStmt*: GetChangesStmt
     beginStmt*: BeginStmt
     endStmt*: EndStmt
     rollbackStmt*: RollbackStmt
@@ -44,10 +56,12 @@ const
 
   IdColName* = "id"
   DataColName* = "data"
+  VersionColName* = "version"
   TimestampColName* = "timestamp"
 
   IdColType = "TEXT"
   DataColType = "BLOB"
+  VersionColType = "INTEGER"
   TimestampColType = "INTEGER"
 
   Memory* = ":memory:"
@@ -69,6 +83,7 @@ const
     CREATE TABLE IF NOT EXISTS """ & TableName & """ (
       """ & IdColName & """ """ & IdColType & """ NOT NULL PRIMARY KEY,
       """ & DataColName & """ """ & DataColType & """,
+      """ & VersionColName & """ """ & VersionColType & """ NOT NULL,
       """ & TimestampColName & """ """ & TimestampColType & """ NOT NULL
     ) WITHOUT ROWID;
   """
@@ -89,8 +104,9 @@ const
     REPLACE INTO """ & TableName & """ (
       """ & IdColName & """,
       """ & DataColName & """,
+      """ & VersionColName & """,
       """ & TimestampColName & """
-    ) VALUES (?, ?, ?)
+    ) VALUES (?, ?, ?, ?)
   """
 
   QueryStmtIdStr* = """
@@ -119,6 +135,43 @@ const
     ORDER BY """ & IdColName & """ DESC
   """
 
+  GetVersionedStmtStr* = fmt"""
+    SELECT {DataColName}, {VersionColName} FROM {TableName}
+    WHERE {IdColName} = ?
+  """
+
+  GetVersionedStmtDataCol* = 0
+  GetVersionedStmtVersionCol* = 1
+
+  InsertVersionedStmtStr* = fmt"""
+    INSERT INTO {TableName}
+    (
+      {IdColName},
+      {DataColName},
+      {VersionColName},
+      {TimestampColName}
+    )
+    VALUES (?, ?, ?, ?)
+  """
+
+  UpdateVersionedStmtStr* = fmt"""
+    UPDATE {TableName}
+    SET
+      {DataColName} = ?,
+      {VersionColName} = ?,
+      {TimestampColName} = ?
+    WHERE {IdColName} = ? AND {VersionColName} = ?
+  """
+
+  DeleteVersionedStmtStr* = fmt"""
+    DELETE FROM {TableName}
+    WHERE {IdColName} = ? AND {VersionColName} = ?
+  """
+
+  GetChangesStmtStr* = fmt"""
+    SELECT changes()
+  """
+
   BeginTransactionStr* = """
     BEGIN;
   """
@@ -197,6 +250,21 @@ proc timestampCol*(
   return proc (): int64 =
     sqlite3_column_int64(s, index.cint)
 
+proc versionCol*(
+  s: RawStmtPtr,
+  index: int): BoundVersionCol =
+
+  checkColMetadata(s, index, VersionColName)
+
+  return proc (): int64 =
+    sqlite3_column_int64(s, index.cint)
+
+proc changesCol*(
+  s: RawStmtPtr,
+  index: int): BoundVersionCol =
+  return proc (): int64 =
+    sqlite3_column_int64(s, index.cint)
+
 proc getDBFilePath*(path: string): ?!string =
   try:
     let
@@ -217,6 +285,11 @@ proc close*(self: SQLiteDsDb) =
   self.beginStmt.dispose
   self.endStmt.dispose
   self.rollbackStmt.dispose
+  self.getVersionedStmt.dispose
+  self.updateVersionedStmt.dispose
+  self.insertVersionedStmt.dispose
+  self.deleteVersionedStmt.dispose
+  self.getChangesStmt.dispose
 
   if not RawStmtPtr(self.deleteStmt).isNil:
     self.deleteStmt.dispose
@@ -266,6 +339,11 @@ proc open*(
     deleteStmt: DeleteStmt
     getStmt: GetStmt
     putStmt: PutStmt
+    getVersionedStmt: GetVersionedStmt
+    updateVersionedStmt: UpdateVersionedStmt
+    insertVersionedStmt: InsertVersionedStmt
+    deleteVersionedStmt: DeleteVersionedStmt
+    getChangesStmt: GetChangesStmt
     beginStmt: BeginStmt
     endStmt: EndStmt
     rollbackStmt: RollbackStmt
@@ -279,6 +357,18 @@ proc open*(
     putStmt = ? PutStmt.prepare(
       env.val, PutStmtStr, SQLITE_PREPARE_PERSISTENT)
 
+    insertVersionedStmt = ? InsertVersionedStmt.prepare(
+      env.val, InsertVersionedStmtStr, SQLITE_PREPARE_PERSISTENT)
+
+    updateVersionedStmt = ? UpdateVersionedStmt.prepare(
+      env.val, UpdateVersionedStmtStr, SQLITE_PREPARE_PERSISTENT)
+
+    deleteVersionedStmt = ? DeleteVersionedStmt.prepare(
+      env.val, DeleteVersionedStmtStr, SQLITE_PREPARE_PERSISTENT)
+
+    getChangesStmt = ? GetChangesStmt.prepare(
+      env.val, GetChangesStmtStr, SQLITE_PREPARE_PERSISTENT)
+
   beginStmt = ? BeginStmt.prepare(
     env.val, BeginTransactionStr, SQLITE_PREPARE_PERSISTENT)
 
@@ -294,6 +384,9 @@ proc open*(
   getStmt = ? GetStmt.prepare(
     env.val, GetStmtStr, SQLITE_PREPARE_PERSISTENT)
 
+  getVersionedStmt = ? GetVersionedStmt.prepare(
+    env.val, GetVersionedStmtStr, SQLITE_PREPARE_PERSISTENT)
+
   # if a readOnly/existing database does not satisfy the expected schema
   # `pepare()` will fail and `new` will return an error with message
   # "SQL logic error"
@@ -310,6 +403,11 @@ proc open*(
     getStmt: getStmt,
     getDataCol: getDataCol,
     putStmt: putStmt,
+    getVersionedStmt: getVersionedStmt,
+    updateVersionedStmt: updateVersionedStmt,
+    insertVersionedStmt: insertVersionedStmt,
+    deleteVersionedStmt: deleteVersionedStmt,
+    getChangesStmt: getChangesStmt,
     beginStmt: beginStmt,
     endStmt: endStmt,
     rollbackStmt: rollbackStmt)
diff --git a/datastore/tieredds.nim b/datastore/tieredds.nim
index 4eca23b..0bc0044 100644
--- a/datastore/tieredds.nim
+++ b/datastore/tieredds.nim
@@ -116,6 +116,37 @@ method put*(
 
   return success()
 
+method modifyGet*(
+  self: TieredDatastore,
+  key: Key,
+  fn: ModifyGet): Future[?!seq[byte]] {.async.} =
+
+  let
+    pending = await allFinished(self.stores.mapIt(it.modifyGet(key, fn)))
+
+  var aux = newSeq[byte]()
+
+  for fut in pending:
+    if fut.read().isErr:
+      return fut.read()
+    else:
+      aux.add(fut.read().get)
+
+  return success(aux)
+
+method modify*(
+  self: TieredDatastore,
+  key: Key,
+  fn: Modify): Future[?!void] {.async.} =
+
+  let
+    pending = await allFinished(self.stores.mapIt(it.modify(key, fn)))
+
+  for fut in pending:
+    if fut.read().isErr: return fut.read()
+
+  return success()
+
 # method query*(
 #   self: TieredDatastore,
 #   query: ...): Future[?!(?...)] {.async.} =
diff --git a/tests/datastore/modifycommontests.nim b/tests/datastore/modifycommontests.nim
new file mode 100644
index 0000000..b7f4871
--- /dev/null
+++ b/tests/datastore/modifycommontests.nim
@@ -0,0 +1,130 @@
+import std/options
+import std/sugar
+import std/random
+import std/sequtils
+import std/strutils
+
+import pkg/asynctest
+import pkg/chronos
+import pkg/stew/endians2
+import pkg/questionable
+import pkg/questionable/results
+
+import pkg/datastore
+
+proc modifyTests*(
+  ds: Datastore,
+  key: Key,
+  multiAux: bool = false) =
+
+  randomize()
+
+  let
+    processCount = 100
+    timeout = (1 + processCount div 10).seconds
+
+  proc withRandDelay(op: Future[?!void]): Future[void] {.async: (raises: [Exception]).} =
+    await sleepAsync(rand(processCount).millis)
+
+    let errMsg = (await op).errorOption.map((err) => err.msg)
+
+    require none(string) == errMsg
+
+  proc incAsyncFn(maybeBytes: ?seq[byte]): Future[?seq[byte]] {.async.} =
+    await sleepAsync(2.millis) # allows interleaving
+    if bytes =? maybeBytes:
+      let value = uint64.fromBytes(bytes)
+      return some(@((value + 1).toBytes))
+    else:
+      return seq[byte].none
+
+  test "unsafe increment - demo":
+    # this test demonstrates non synchronized read-modify-write sequence
+    (await ds.put(key, @(0.uint64.toBytes))).tryGet
+
+    proc getIncAndPut(): Future[?!void] {.async.} =
+      without bytes =? (await ds.get(key)), err:
+        return failure(err)
+
+      let value = uint64.fromBytes(bytes)
+      await sleepAsync(2.millis) # allows interleaving
+
+      if err =? (await ds.put(key, @((value + 1).toBytes))).errorOption:
+        return failure(err)
+      else:
+        return success()
+
+    let futs = newSeqWith(processCount, withRandDelay(getIncAndPut()))
+    await allFutures(futs).wait(timeout)
+
+    let finalValue = uint64.fromBytes((await ds.get(key)).tryGet)
+
+    check finalValue.int < processCount
+
+  test "safe increment":
+    (await ds.put(key, @(0.uint64.toBytes))).tryGet
+
+    let futs = newSeqWith(processCount, withRandDelay(ds.modify(key, incAsyncFn)))
+    await allFutures(futs).wait(timeout)
+
+    let finalValue = uint64.fromBytes((await ds.get(key)).tryGet)
+
+    check finalValue.int == processCount
+
+  test "should update value":
+    (await ds.put(key, @((0.uint64).toBytes))).tryGet
+
+    (await ds.modify(key, incAsyncFn)).tryGet
+
+    let finalValue = uint64.fromBytes((await ds.get(key)).tryGet)
+
+    check finalValue.int == 1
+
+  test "should put value":
+    (await ds.delete(key)).tryGet()
+
+    proc returningSomeValue(_: ?seq[byte]): Future[?seq[byte]] {.async.} =
+      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":
+    (await ds.put(key, @(0.uint64.toBytes))).tryGet
+
+    proc returningNone(_: ?seq[byte]): Future[?seq[byte]] {.async.} =
+      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]): Future[(?seq[byte], seq[byte])] {.async.} =
+      return (seq[byte].none, @[byte 123])
+
+    let res = await ds.modifyGet(key, returningAux)
+
+    if multiAux:
+      check:
+        res.errorOption.map((err) => err.msg) == none(string)
+      for b in res |? @[]:
+        check:
+          b == 123.byte
+    else:
+      check:
+        res == success(@[byte 123])
+
+  test "should propagate exception as failure":
+    proc throwing(a: ?seq[byte]): Future[?seq[byte]] {.async.} =
+      raise newException(CatchableError, "some error msg")
+
+    let res = await ds.modify(key, throwing)
+
+    check:
+      res.errorOption.map((err) => err.msg) == some("some error msg")
diff --git a/tests/datastore/sql/testsqliteds.nim b/tests/datastore/sql/testsqliteds.nim
index c629eb0..778d7f1 100644
--- a/tests/datastore/sql/testsqliteds.nim
+++ b/tests/datastore/sql/testsqliteds.nim
@@ -11,6 +11,7 @@ import pkg/stew/byteutils
 import pkg/datastore/sql/sqliteds
 
 import ../dscommontests
+import ../modifycommontests
 import ../querycommontests
 
 suite "Test Basic SQLiteDatastore":
@@ -24,6 +25,7 @@ suite "Test Basic SQLiteDatastore":
     (await ds.close()).tryGet()
 
   basicStoreTests(ds, key, bytes, otherBytes)
+  modifyTests(ds, key)
 
 suite "Test Read Only SQLiteDatastore":
   let
diff --git a/tests/datastore/sql/testsqlitedsdb.nim b/tests/datastore/sql/testsqlitedsdb.nim
index d104933..b6ff105 100644
--- a/tests/datastore/sql/testsqlitedsdb.nim
+++ b/tests/datastore/sql/testsqlitedsdb.nim
@@ -106,9 +106,9 @@ suite "Test SQLite Datastore DB operations":
 
   test "Should insert key":
     check:
-      readOnlyDb.putStmt.exec((key.id, data, timestamp())).isErr()
+      readOnlyDb.putStmt.exec((key.id, data, initVersion, timestamp())).isErr()
 
-    dsDb.putStmt.exec((key.id, data, timestamp())).tryGet()
+    dsDb.putStmt.exec((key.id, data, initVersion, timestamp())).tryGet()
 
   test "Should select key":
     let
@@ -124,9 +124,9 @@ suite "Test SQLite Datastore DB operations":
 
   test "Should update key":
     check:
-      readOnlyDb.putStmt.exec((key.id, otherData, timestamp())).isErr()
+      readOnlyDb.putStmt.exec((key.id, otherData, initVersion, timestamp())).isErr()
 
-    dsDb.putStmt.exec((key.id, otherData, timestamp())).tryGet()
+    dsDb.putStmt.exec((key.id, otherData, initVersion, timestamp())).tryGet()
 
   test "Should select updated key":
     let
diff --git a/tests/datastore/testfsds.nim b/tests/datastore/testfsds.nim
index 2633677..af9adfc 100644
--- a/tests/datastore/testfsds.nim
+++ b/tests/datastore/testfsds.nim
@@ -11,6 +11,7 @@ import pkg/stew/byteutils
 import pkg/datastore/fsds
 
 import ./dscommontests
+import ./modifycommontests
 import ./querycommontests
 
 suite "Test Basic FSDatastore":
@@ -37,6 +38,7 @@ suite "Test Basic FSDatastore":
     require(not dirExists(basePathAbs))
 
   basicStoreTests(fsStore, key, bytes, otherBytes)
+  modifyTests(fsStore, key)
 
 suite "Test Misc FSDatastore":
   let
diff --git a/tests/datastore/testmountedds.nim b/tests/datastore/testmountedds.nim
index fbe0dfd..15f91f3 100644
--- a/tests/datastore/testmountedds.nim
+++ b/tests/datastore/testmountedds.nim
@@ -12,6 +12,7 @@ import pkg/datastore/sql
 import pkg/datastore/fsds
 
 import ./dscommontests
+import ./modifycommontests
 
 suite "Test Basic Mounted Datastore":
   let
@@ -50,10 +51,12 @@ suite "Test Basic Mounted Datastore":
   suite "Mounted sql":
     let namespace = Key.init(sqlKey, key).tryGet
     basicStoreTests(mountedDs, namespace, bytes, otherBytes)
+    modifyTests(mountedDs, namespace)
 
   suite "Mounted fs":
     let namespace = Key.init(fsKey, key).tryGet
     basicStoreTests(mountedDs, namespace, bytes, otherBytes)
+    modifyTests(mountedDs, namespace)
 
 suite "Test Mounted Datastore":
 
diff --git a/tests/datastore/testtieredds.nim b/tests/datastore/testtieredds.nim
index 4ea76a7..272c852 100644
--- a/tests/datastore/testtieredds.nim
+++ b/tests/datastore/testtieredds.nim
@@ -10,6 +10,7 @@ import pkg/datastore/fsds
 import pkg/datastore/sql
 import pkg/datastore/tieredds
 
+import ./modifycommontests
 import ./dscommontests
 
 suite "Test Basic Tired Datastore":
@@ -40,6 +41,7 @@ suite "Test Basic Tired Datastore":
     require(not dirExists(rootAbs))
 
   basicStoreTests(tiredDs, key, bytes, otherBytes)
+  modifyTests(tiredDs, key, multiAux = true)
 
 suite "TieredDatastore":
   # assumes tests/test_all is run from project root, e.g. with `nimble test`