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(
//   Ended(1969-12-31T23:59:59.999Z, 46eaf54a-3b1e-490d-a78e-db4c22d36e7a),
//   Resumed(3658-10-18T23:04:08.567Z, 2f71150b-b864-43fb-8402-4351aaaefbed),
//   Paused(4000-11-03T20:17:29.116Z, 007b20c3-e015-4b9f-ac0e-abe48d8956b4),
//   Started(4000-11-03T20:17:29.116Z, 3810bfdc-1a5b-42b9-bffb-6f8dafd36666),
//   Ended(+24653963-12-31T09:14:34.193Z, 29c9cb2d-8fcb-4c97-b1b7-17f4c842b046),
//   Resumed(1970-01-01T00:00:00.001Z, b9a921cf-baad-4e59-a900-45c8b1d5680c),
//   Started(0003-12-30T05:00:00Z, 7614a15d-43e2-44e1-a6c1-1e465626b43f),
//   Paused(1975-11-12T04:30:06.570Z, 335ff44f-716d-455c-b29b-73be23e5cea4),
//   Resumed(1970-01-01T00:00:00Z, 9ff20856-f370-4b74-86eb-87b9f7574b5d),
//   Ended(2051-07-19T00:00:00Z, ba5c68e1-5743-4e5b-8d2c-3f1bbdbac1ba)
// )
```

😱 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, c94c1bf5-cff7-45a7-b67b-3b06a5de86c3),
//   Ended(2019-11-26T12:01:00Z, c94c1bf5-cff7-45a7-b67b-3b06a5de86c3)
// )
```

---

## 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,9995f395-363d-43cd-9424-fefe19ca7fa0)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,9995f395-363d-43cd-9424-fefe19ca7fa0)
```

---

## 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,367c8d38-05c7-40dc-9dcd-4e4c3f928cf4)
// Started(2018-12-20T01:00:00Z,27082b8f-bd8a-49b1-8623-2ca18a741552)
// Started(2018-12-20T01:00:00Z,222ad43c-9385-40b6-b538-c285e8b5694a)
// Ended(2018-12-20T01:30:00Z,27082b8f-bd8a-49b1-8623-2ca18a741552)
// Ended(2018-12-20T01:30:00Z,222ad43c-9385-40b6-b538-c285e8b5694a)
// Ended(2018-12-20T01:30:00Z,367c8d38-05c7-40dc-9dcd-4e4c3f928cf4)
// Started(2018-12-20T01:30:00Z,b5b7eb5e-da81-4d27-bd0c-64ac84e97898)
// Started(2018-12-20T01:30:00Z,902b353e-eca3-4cae-b83d-35f5a084ed9e)
// Ended(2018-12-20T02:00:00Z,902b353e-eca3-4cae-b83d-35f5a084ed9e)
// Ended(2018-12-20T02:00:00Z,b5b7eb5e-da81-4d27-bd0c-64ac84e97898)
```

???

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,d0f1e3ec-50a8-422d-8ead-7fd3dd5c4d52)
// Started(2018-12-20T02:00:00Z,fabf197f-7475-4d35-8549-fd28df2dfa97)
// Started(2018-12-20T02:00:00Z,462ad39b-18d3-4a66-b1cb-1313f1e21558)
// Paused(2018-12-20T02:00:20Z,462ad39b-18d3-4a66-b1cb-1313f1e21558)
// Resumed(2018-12-20T02:00:25Z,462ad39b-18d3-4a66-b1cb-1313f1e21558)
// Ended(2018-12-20T02:20:25Z,462ad39b-18d3-4a66-b1cb-1313f1e21558)
// Started(2018-12-20T02:20:25Z,724f3cbd-d2f6-43eb-9003-ff2b5d1fa913)
// Paused(2018-12-20T02:20:45Z,724f3cbd-d2f6-43eb-9003-ff2b5d1fa913)
// Resumed(2018-12-20T02:20:50Z,724f3cbd-d2f6-43eb-9003-ff2b5d1fa913)
// Ended(2018-12-20T02:30:00Z,fabf197f-7475-4d35-8549-fd28df2dfa97)
// Ended(2018-12-20T02:30:00Z,d0f1e3ec-50a8-422d-8ead-7fd3dd5c4d52)
// Ended(2018-12-20T02:40:50Z,724f3cbd-d2f6-43eb-9003-ff2b5d1fa913)
```

---

## 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, 61b42889-bb89-425c-9295-02bf8be39c1b),
//       Ended(2018-12-20T03:30:00Z, 61b42889-bb89-425c-9295-02bf8be39c1b)
//     )
//   )
// )
```

---

## 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?