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(
//   Resumed(0000-12-30T20:17:29.116Z, 96b07b95-e04e-408e-ac34-883743104f10),
//   Ended(0547-03-24T00:00:00Z, 1272a6d7-5f31-4c02-be03-22b975f4dc59),
//   Ended(1496-10-16T08:56:59.031Z, f503992a-0881-4f73-959d-c165383b90c8),
//   Started(+227207664-10-24T15:35:06.351Z, 4c63c03a-245f-4354-b6a0-58bcf906d27c),
//   Ended(+292269052-01-01T00:00:00.001Z, c775c31f-fc77-4cad-9f8a-3328899fe5e4),
//   Started(0036-11-01T22:54:52.072Z, 21ee0c9e-2129-4888-94c6-1ccc8d4dadee),
//   Paused(1969-12-01T23:59:59.999Z, 7b778b12-3173-422b-9e3b-7594ad714af3),
//   Paused(1969-12-31T23:59:59.999Z, 2ae745e8-d8c5-413b-895f-260f680f1134),
//   Paused(+114896284-01-01T05:00:00Z, 6c3d339a-8e11-4fa6-981d-c1956c38dabe),
//   Paused(1241-01-23T23:59:59.059Z, c77882ad-0600-40f5-b7be-d1a162085d51)
// )
```

😱 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, 88df13da-7d98-45a1-aed5-43f5d23626bb),
//   Ended(2019-11-26T12:01:00Z, 88df13da-7d98-45a1-aed5-43f5d23626bb)
// )
```

---

## 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,f9dace41-bfc5-443e-8b63-58092a508341)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,f9dace41-bfc5-443e-8b63-58092a508341)
```

---

## 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,7f14d64a-864a-416f-b1b9-fc631a2e2d44)
// Started(2018-12-20T01:00:00Z,f5c77ed5-db63-4ca4-8a82-3451c2d84ea2)
// Started(2018-12-20T01:00:00Z,45500ac5-ef75-465f-aed2-b892e7fa0c4e)
// Ended(2018-12-20T01:30:00Z,7f14d64a-864a-416f-b1b9-fc631a2e2d44)
// Ended(2018-12-20T01:30:00Z,f5c77ed5-db63-4ca4-8a82-3451c2d84ea2)
// Ended(2018-12-20T01:30:00Z,45500ac5-ef75-465f-aed2-b892e7fa0c4e)
// Started(2018-12-20T01:30:00Z,2d94d5ed-a73e-4122-a81e-220d28dac6ee)
// Started(2018-12-20T01:30:00Z,9ba88dad-0aba-4779-a8be-b83ddd3d0b58)
// Ended(2018-12-20T02:00:00Z,9ba88dad-0aba-4779-a8be-b83ddd3d0b58)
// Ended(2018-12-20T02:00:00Z,2d94d5ed-a73e-4122-a81e-220d28dac6ee)
```

???

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,9f4ce565-7857-4a25-8607-79d4e0225946)
// Started(2018-12-20T02:00:00Z,78e7bc1b-78d7-418d-b160-11720bf9e1a3)
// Started(2018-12-20T02:00:00Z,0aa69bb9-8a0b-4c0c-a92b-8d7e9c0f581e)
// Paused(2018-12-20T02:00:20Z,9f4ce565-7857-4a25-8607-79d4e0225946)
// Resumed(2018-12-20T02:00:25Z,9f4ce565-7857-4a25-8607-79d4e0225946)
// Ended(2018-12-20T02:20:25Z,9f4ce565-7857-4a25-8607-79d4e0225946)
// Started(2018-12-20T02:20:25Z,b661ae68-66ce-42c0-9380-444f3f361eaf)
// Paused(2018-12-20T02:20:45Z,b661ae68-66ce-42c0-9380-444f3f361eaf)
// Resumed(2018-12-20T02:20:50Z,b661ae68-66ce-42c0-9380-444f3f361eaf)
// Ended(2018-12-20T02:30:00Z,0aa69bb9-8a0b-4c0c-a92b-8d7e9c0f581e)
// Ended(2018-12-20T02:30:00Z,78e7bc1b-78d7-418d-b160-11720bf9e1a3)
// Ended(2018-12-20T02:40:50Z,b661ae68-66ce-42c0-9380-444f3f361eaf)
```

---

## 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, a844fcde-b64e-4d00-b029-2d477b613afe),
//       Ended(2018-12-20T03:30:00Z, a844fcde-b64e-4d00-b029-2d477b613afe)
//     )
//   )
// )
```

---

## 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?