Event Based Stateful Flows

Event based stateful flows are defined in terms of events generated upon receiving a stream element or command.

They are well suited to model entity where the frequency of state changes is high (possibly with small deltas). Furthermore they are a natural fit for event sourced solutions.


A StatefulFlowLogic.EventBased[State, Ev, In, Command] is a kind of logic which treats the state as an events log: inputs/commands generate events which are persisted and used to update the state.

The output type of an event based stateful flow is its event type, this means that a flow using an event based logic will produce as output the events generated while processing the inputs.

In order to adapt the counter example to use an event based stateful flow we need to start by defining the state, event and command models:

source/** The state model */
case class CounterState(total: Int)

/** The event occurring when the counter is incremented */
case class CounterIncrementedEvent(c: Int)

/** The command used to query the flow for its current counter value */
case class GetCounterCommand(replyTo: ActorRef[StatusReply[Int]])

Now that we have our models we can instantiate a logic operating on it:

sourceimport StatefulFlowLogic._
val logic = EventBased[CounterState, CounterIncrementedEvent, CounterSample, GetCounterCommand](
  () => CounterState(0),
  (state, sample) => {
      s"deployment:${sample.deploymentId} entrance:${sample.entranceId} - " +
        s"timestamp:${sample.timestamp} counter:${state.total + sample.entrances}"
  (state, ev) => {
    val newTotal = state.total + ev.c
  (state, command) => {
    command.replyTo ! StatusReply.success(state.total)

We do that by invoking StatefulFlowLogic.EventBased specifying the following 4 parameters:

  1. initial state: the state value to use for the first instantiation of the flow (i.e. when no state can be recovered by the backend)
  2. input handler: the function used to handle stream inputs by generating events (possibly inspecting the current state)
  3. state update function: the function used to update the state by applying a single event
  4. command handler: the function used to handle commands by generating events (possibly inspecting the current state)

The 2 handler functions for inputs and commands expects a result of type StatefulFlowLogic.EventBased.ProcessingResult[Ev]. This types serves as a representation of:

  1. The events the input/command generates
  2. The side effect we want to be performed before the state is updated with generated events
  3. The side effect we want to be performed after the state has been successfully updated with the generated events

Thanks to the native side effect support it becomes easy to write pipelines of micro-services with at-least-once semantic (use Kafka as a backbone, read from topicUpstream and emit to topicDownstream inside a before side effect. If the process fails while emitting, we have a guarantee that upon restart the stream will resume processing where it left off, hence repeating the side effects.)

Similarly you can use after side effects to model at-most-once scenarios (if the system fails after the state has been modified, there is no guarantee that the logic will produce the same side effects upon restart).

***note Both before and after side effects are evaluated as part of the stream. This means that the flow will not produce any output until both side effects groups have been completed successfully. Similarly if any side effect fails (i.e. Future.failed) the stream will be failed as well. ***


A StatefulFlowBackend.EventBased[State, Ev, _] is a kind of backend compatible with event based logics with the same state and event types.

Spekka Stateful ships with an in-memory implementation useful for testing and quick prototyping: InMemoryStatefulFlowBackend.EventBased[State, Ev].

We can create a backend for our example with the following code:

sourceval backend = InMemoryStatefulFlowBackend.EventBased[CounterState, CounterIncrementedEvent]()


Now that we have defined both the logic and the backend, we can obtain a StatefulFlowProps object describing our flow:

sourceval flowProps = logic.propsForBackend(backend)

We now register the flow for the 2 different usages we have planned: counters by deployment and counters by entrances:

sourceval byDeploymentFlowBuilder = registry.registerStatefulFlowSync("byDeployment", flowProps)
val byEntranceFlowBuilder = registry.registerStatefulFlowSync("byEntrance", flowProps)

Now that we have registered the flows, we can use the corresponding StatefulFlowBuilder to instantiate a flow for a specific entity:

sourceimport PartitionTree._
val totalByEntranceFlow = Partition
  .treeBuilder[CounterSample, Offset]
  .build { case (entrance: EntranceId) :@: (deployment: DeploymentId) :@: KNil =>
      .via(printingFlow(s"deployment:${deployment.id} entrance:${entrance.id}"))

val totalByDeploymentFlow = Partition
  .treeBuilder[CounterSample, Offset]
  .build { case deployment :@: KNil =>
      .via(printingFlow(s"deployment:${deployment.id} total"))

You can find the full example here: StatefulFlowEventBasedExample.scala.