FS2 - Fake it till you make it

class: center, middle, inverse

# Fake it till you make it with FS2

James Collier @JRCCollier

![FS2 Logo](./assets/fs2-logo.png)

---

## Playback Session

Telemetry events will be emitted from devices.

```scala
import java.time.Instant
import java.util.UUID

sealed trait PlaybackEvent {
  def sessionId: UUID
}
final case class Started(timestamp: Instant, sessionId: UUID) extends PlaybackEvent
final case class Paused(timestamp: Instant, sessionId: UUID) extends PlaybackEvent
final case class Resumed(timestamp: Instant, sessionId: UUID) extends PlaybackEvent
final case class Ended(timestamp: Instant, sessionId: UUID) extends PlaybackEvent
```

---

## 1000ft view

![device to cloud](./assets/device-to-cloud.png)

---
class: inverse

## The catch

😱 it's going to be a while till devices are instrumented!

The show must go on!

Need to develop while application teams are instrumenting.

---

## Fake it till we instrument it

We have the ~~technology~~ schema, we can build the events.

![gen to cloud](./assets/gen-to-cloud.png)

---

## Let's just use scalacheck!

```scala
import org.scalacheck.ScalacheckShapeless._
import org.scalacheck.{Arbitrary, Gen}

private implicit val arbInstant: Arbitrary[Instant] =
    Arbitrary(Gen.calendar.map(_.toInstant))
val arbPlaybackEvt = implicitly[Arbitrary[PlaybackEvent]]
```

Now let's generate some events...

```scala
List.fill(10)(arbPlaybackEvt.arbitrary.sample).flatten
// res0: List[PlaybackEvent] = List(
//   Started(1969-12-31T23:59:59.999Z, 9721ddaf-e493-45e3-ad77-426c8486830e),
//   Ended(+292269054-01-01T00:00:00.001Z, bc3e8982-08ea-48f8-b486-a79f45ac4aba),
//   Resumed(4000-11-03T00:00:00Z, 04ae83da-f1c3-4136-bbf7-30266c452d69),
//   Started(2009-12-30T04:03:09.691Z, ec997827-5f1c-41e7-bf9c-6c9a297d5c04),
//   Paused(1490-11-23T19:35:43.358Z, 1089f4c9-a521-4fc0-bc2d-f60e6e9eeda9),
//   Ended(+292269054-01-01T12:03:33.892Z, d78a0b10-6c30-43df-9ac4-0539dda38e2f),
//   Started(+148943484-12-31T01:36:22.260Z, 649f3f19-c0e3-433e-a774-b556d4f7faaf),
//   Resumed(+7887036-01-01T00:00:00.001Z, f3e50717-0151-45c2-9688-704bc7170a80),
//   Paused(+292269054-01-01T00:00:00.001Z, 4d7252df-8e35-4605-8081-f98e4827fe27),
//   Resumed(0956-01-30T07:41:08.081Z, 7b489a98-e020-49f1-b2bd-47de165ade2c)
// )
```

😱 not useful!

???

* timestamp is nonsense.
* each event has it's own session.
* not a representative lifecycle.

---

## Requirements

* Sessions modelled.
* Representative delays between events.
* Configure the number of concurrent sessions.

Maybe this can be modelled as streams?

Step in FS2:

![FS2 Logo](./assets/fs2-logo.png)

```scala
import cats.effect._
import fs2.Stream
import scala.concurrent.ExecutionContext
import scala.concurrent.duration._
```

---

## Model a playback scenario

```scala
def session: Stream[IO, PlaybackEvent] = for {
  now <- Stream.emit(Instant.parse("2019-11-26T12:00:00Z"))
  sessionId <- Stream.eval(IO(UUID.randomUUID()))
  s <- Stream.emits(List(
    Started(now, sessionId),
    Ended(now.plusSeconds(60), sessionId)
  ))
} yield s
```

```scala
session.compile.toList.unsafeRunSync()
// res2: List[PlaybackEvent] = List(
//   Started(2019-11-26T12:00:00Z, 1d2de628-e653-45a6-86ea-abffbb0643b8),
//   Ended(2019-11-26T12:01:00Z, 1d2de628-e653-45a6-86ea-abffbb0643b8)
// )
```

---

## Nap between events 💤💤💤

Useful to model the progress of time between emitting
events for a session.

```scala
def nowS: Stream[IO, Instant] =
  Stream.eval(timer.clock.realTime(MILLISECONDS).map(Instant.ofEpochMilli))

def idealSession: Stream[IO, PlaybackEvent] = for {
  sessionId <- Stream.eval(IO(UUID.randomUUID()))
  s <- nowS.map(Started(_, sessionId)) ++
    nowS.map(Ended(_, sessionId)).delayBy(30.minutes)
} yield s
```

```scala
idealSession
  .evalMap(event => IO(println(event)))
  .compile
  .drain
  .unsafeRunAsyncAndForget()
// Started(2018-12-20T00:00:00Z,f4cecdcf-5d3e-4138-8a0b-7a5715ca8fc2)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,f4cecdcf-5d3e-4138-8a0b-7a5715ca8fc2)
```

---

## A stream of sessions

![Session streams](./assets/session-streams.png)

???

Par join to choose how many concurrent sessions to have.

---

## Concurrent sessions

```scala
val numConcurrent = 3

Stream.emit(idealSession)
  .repeat
  .take(5) // limit to 5 for example
  .parJoin(numConcurrent)
  .evalMap(event => IO(println(event)))
  .compile
  .drain
  .unsafeRunAsyncAndForget()
```

```scala
testContext.tick(1.hour)
// Started(2018-12-20T01:00:00Z,9b622c58-544e-42b2-8677-698fa2feb7db)
// Started(2018-12-20T01:00:00Z,7a62fa52-75b7-4eb5-b40b-65ff31e122eb)
// Started(2018-12-20T01:00:00Z,612549ed-e5b0-4996-ae5d-6dac50360119)
// Ended(2018-12-20T01:30:00Z,9b622c58-544e-42b2-8677-698fa2feb7db)
// Ended(2018-12-20T01:30:00Z,7a62fa52-75b7-4eb5-b40b-65ff31e122eb)
// Started(2018-12-20T01:30:00Z,c0d3744e-d56e-4c60-82f1-3f35312a155d)
// Ended(2018-12-20T01:30:00Z,612549ed-e5b0-4996-ae5d-6dac50360119)
// Started(2018-12-20T01:30:00Z,0cb9ce57-5322-4720-a94b-9d66d924bff7)
// Ended(2018-12-20T02:00:00Z,0cb9ce57-5322-4720-a94b-9d66d924bff7)
// Ended(2018-12-20T02:00:00Z,c0d3744e-d56e-4c60-82f1-3f35312a155d)
```

???

Sessions are repeated forever.

---

## Add another scenario!

```scala
def pauseSession: Stream[IO, PlaybackEvent] = for {
  sessionId <- Stream.eval(IO(UUID.randomUUID()))
  s <- nowS.map(Started(_, sessionId)) ++
    nowS.map(Paused(_, sessionId)).delayBy(20.seconds) ++
    nowS.map(Resumed(_, sessionId)).delayBy(5.seconds) ++
    nowS.map(Ended(_, sessionId)).delayBy(20.minutes)
} yield s
```

---

## Running multiple scenarios

```scala
Stream.emits(List(idealSession, pauseSession))
  .repeat
  .take(4) // limit to 4 for example
  .parJoin(numConcurrent)
  .evalMap(event => IO(println(event)))
  .compile
  .drain
  .unsafeRunAsyncAndForget()
```

```scala
testContext.tick(1.hour)
// Started(2018-12-20T02:00:00Z,eef52acd-7a7a-41e1-8cbc-7d9f0c984f82)
// Started(2018-12-20T02:00:00Z,a73a8620-e135-489e-81ac-d12a479bfa3c)
// Started(2018-12-20T02:00:00Z,6e34906a-55da-4ad0-b80a-f04aa35a986f)
// Paused(2018-12-20T02:00:20Z,a73a8620-e135-489e-81ac-d12a479bfa3c)
// Resumed(2018-12-20T02:00:25Z,a73a8620-e135-489e-81ac-d12a479bfa3c)
// Ended(2018-12-20T02:20:25Z,a73a8620-e135-489e-81ac-d12a479bfa3c)
// Started(2018-12-20T02:20:25Z,23245650-f313-48cb-81c6-d8955046d0af)
// Paused(2018-12-20T02:20:45Z,23245650-f313-48cb-81c6-d8955046d0af)
// Resumed(2018-12-20T02:20:50Z,23245650-f313-48cb-81c6-d8955046d0af)
// Ended(2018-12-20T02:30:00Z,eef52acd-7a7a-41e1-8cbc-7d9f0c984f82)
// Ended(2018-12-20T02:30:00Z,6e34906a-55da-4ad0-b80a-f04aa35a986f)
// Ended(2018-12-20T02:40:50Z,23245650-f313-48cb-81c6-d8955046d0af)
```

---

## Testing

Can use `cats.effect.laws.util.TestContext` to simulate time.

```scala
val f = idealSession
  .compile
  .toList
  .unsafeToFuture()
```

```scala
f.value
// res9: Option[scala.util.Try[List[PlaybackEvent]]] = None
```

```scala
testContext.tick(1.hour)
f.value
// res11: Option[scala.util.Try[List[PlaybackEvent]]] = Some(
//   Success(
//     List(
//       Started(2018-12-20T03:00:00Z, 47b031cf-3aa9-43ee-b5fb-18d94e14f47f),
//       Ended(2018-12-20T03:30:00Z, 47b031cf-3aa9-43ee-b5fb-18d94e14f47f)
//     )
//   )
// )
```

---

## Counting sessions

```scala
val f2 = Stream.emit(idealSession)
  .repeat
  .parJoin(1000)
  .interruptAfter(1.hour)
  .compile
  .toList
  // count sessions
  .map(_.groupBy(_.sessionId).keySet.size)
  .unsafeToFuture()
```

`idealSession` lasts 30 minutes, so after 1 hour there should be ~2000 sessions.

```scala
testContext.tick(1.hour)
f2.value
// res13: Option[scala.util.Try[Int]] = Some(Success(2000))
```
👌

---

## Further reading and links

* [FS2](https://github.com/functional-streams-for-scala/fs2) by [@mpilquist](https://github.com/mpilquist)
* [cats-effect](https://github.com/typelevel/cats-effect)

## Thanks!

* [mdoc](https://github.com/scalameta/mdoc) used to evaluate scala examples

## Questions?