Using FS2-ES, you might want to be able to more easily inspect the history and contents of your state. A concept that appears in event-based programming is the idea of “time-travel debugging”, or the ability to go forward and back in time. Because EventState enforces a linear, event-driven access pattern, that means that we are able to store all modifications to state and replay them, giving you access to all possible states that have been achieved. If you install the fs2-es-testing module, you’ll be able to use ReplayableEventState which is an extension of EventStateTopic with special testing and debugging methods.

First, add the testing module to your project (available for ScalaJS 1.x as well):

libraryDependencies += "dev.rpeters" %% "fs2-es-testing" % <current-version>

You can create one the exact same way as other EventState implementations, either with an initial value or a stream of events to “hydrate” it with.

import cats.effect._
import cats.implicits._
import dev.rpeters.fs2.es.testing.ReplayableEventState
import scala.concurrent.ExecutionContext.global

//Allows us to do concurrent actions, not required if using IOApp
implicit val cs = IO.contextShift(global)

//Creates a new ReplayableEventState on each invocation that adds integers to state
val newState = ReplayableEventState[IO].initial[Int, Int](0)(_ + _)

From here, we can start accumulating events as normal, and it will work just like any other EventStateTopic.

Getting The Event List

For testing, you may want to know what the current event list is, so lets accumulate some events and get them back:

val eventsTest = for {
  es <- newState //Make the event state
  _ <- es.doNext(1) //Add some events
  _ <- es.doNext(2)
  _ <- es.doNext(3)
  state <- es.get //Check the current state
  events <- es.getEvents //Check the list of events
} yield (state, events)
eventsTest.flatMap { case (state, events) =>
  IO(println(s"State: $state")) >> IO(println(s"Events: $events"))
// State: 6
// Events: Chain(1, 2, 3)

You may have noticed that the events are returned as a Chain. That’s an implementation detail, and you can treat it similarly to a List or turn it into one by calling .toList as-needed. For information on how Chain works or why you would want to use it, see the Cats Chain documentation. The gist is, it works a lot like List but it performs much better in append-only scenarios.

If you don’t need the entire list of events but you just want the event count, you can call es.getEventCount.

Seeking By Index

Sometimes when debugging you might want to go “backwards” to a previous state. You can seek backwards by specifying the index of the state you would like to go to, or optionally specifying an offset to seek forwards and backwards.

The available methods for this are:

  • seekTo(n) - Seek to index n
  • seekToBeginning - Alias for seekTo(0)
  • seekBackBy(n) - Goes back n states ago.
  • seekForwardBy(n) - Goes forward n states ahead of the current state.

Seeking is a non-destructive action which means you can do it safely without destroying the current event history. If you do append a new event to the current state, it will drop all later events (if any), so be sure to save them if you want to replay them later.

val seekTest = for {
  es <- newState //Make the event state
  _ <- es.doNext(1) //Add some events
  _ <- es.doNext(2)
  _ <- es.doNext(3)
  oldState <- es.get //Check the current state
  oldEvents <- es.getEvents //Check the list of events
  newState <- es.seekTo(1) //Go to the second state, after applying the first event (1)
  sameEvents <- es.getEvents //Get the event list, to show it is non-destructive
} yield (oldState, oldEvents, newState, sameEvents)
seekTest.flatMap { case (oldState, events, newState, sameEvents) =>
  IO(println(s"Old state: $oldState")) >>
    IO(println(s"Old Events: $events")) >>
    IO(println(s"New state: $newState")) >>
    IO(println(s"Same Events: $sameEvents"))
// Old state: 6
// Old Events: Chain(1, 2, 3)
// New state: 1
// Same Events: Chain(1, 2, 3)

Resetting state

There are special reset and resetInitial methods now available that allow you to completely wipe the current state including the list of events. Calling reset allows you to go back to the first accumulated state, while resetInitial allows you to provide a new initial state to reset to.

val resetTest = for {
  es <- newState //Make the event state
  _ <- es.doNext(1) //Add some events
  _ <- es.doNext(2)
  _ <- es.doNext(3)
  latestState <- es.get //Check the current state
  resettedState <- es.reset //Reset to zero
  resettedEvents <- es.getEvents //Get events, to show it is cleared
  newInitialState <- es.resetInitial(5) //Set the first state to 5
  newInitialEvents <- es.getEvents //Check events again, which should still be empty
} yield (latestState, resettedState, resettedEvents, newInitialState, newInitialEvents)
resetTest.flatMap { case (ls, rs, re, nis, nie) =>
  IO(println(s"Latest State: $ls")) >>
    IO(println(s"Resetted State: $rs")) >>
    IO(println(s"Resetted Events: $re")) >>
    IO(println(s"New Initial State: $nis")) >>
    IO(println(s"New Initial Events: $nie"))
// Latest State: 6
// Resetted State: 0
// Resetted Events: Chain()
// New Initial State: 5
// New Initial Events: Chain()

State changes and subscriptions

Because this is based on EventStateTopic, you can subscribe to receive all state changes. You might have noticed that the methods for resetting and seeking state also return the resulting state back. Each time you call one of these methods, you will also publish the resulting state to all subscribers. This decision was made intentionally to allow for reactive debugging in environments such as ScalaJS where you might be using this as a reactive state store (think Flux/Redux).