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(
//   Paused(1970-01-31T00:00:00.001Z, 4d71a50b-f26e-4bb3-947e-f2b9c539c917),
//   Resumed(4000-11-03T20:17:29.116Z, f0349fae-86df-44e4-b754-fc445595153c),
//   Resumed(0001-12-29T20:17:29.116Z, 61ba7496-f3e3-46f8-a138-b25483e76b20),
//   Started(+286674005-01-01T01:54:06.255Z, d26859bd-b007-44cf-bd86-c5168946dbc8),
//   Ended(0324-12-01T13:13:45.031Z, 8071c85b-f8d0-4536-a93f-36c6ec21083d),
//   Ended(0341-12-31T08:08:51.276Z, 0c477bff-3d4f-420c-b13a-e7ad4553a20e),
//   Ended(+292269054-12-31T20:14:13.584Z, 5e3eb253-9d38-4bfd-a7e1-2832bb74ad55),
//   Started(1970-01-01T00:00:00.001Z, 0a93c080-39f7-4cc0-8115-aa6d892e8542),
//   Started(1970-01-01T00:00:00Z, 547d8c6f-4d53-4f33-8570-e6765ad66090),
//   Ended(0000-12-30T00:00:00Z, aede2f25-bae3-4ea0-9744-a5d54b20370b)
// )
```

😱 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, bf0c97b0-a461-4752-8060-060806b268d5),
//   Ended(2019-11-26T12:01:00Z, bf0c97b0-a461-4752-8060-060806b268d5)
// )
```

---

## 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,721d9d5a-0b06-4cdd-8e1e-8f0bdb4fe95f)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,721d9d5a-0b06-4cdd-8e1e-8f0bdb4fe95f)
```

---

## 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,41e893c0-29b9-461a-8ea7-32481d8d640d)
// Started(2018-12-20T01:00:00Z,a797016c-68ba-45f6-b6d1-37743491af63)
// Started(2018-12-20T01:00:00Z,6067b22f-5c67-438c-80c5-9c884841072a)
// Ended(2018-12-20T01:30:00Z,41e893c0-29b9-461a-8ea7-32481d8d640d)
// Ended(2018-12-20T01:30:00Z,6067b22f-5c67-438c-80c5-9c884841072a)
// Ended(2018-12-20T01:30:00Z,a797016c-68ba-45f6-b6d1-37743491af63)
// Started(2018-12-20T01:30:00Z,758d81d5-f914-4649-924e-527d0ae5808b)
// Started(2018-12-20T01:30:00Z,b960f986-8ac8-4616-afbe-7537bce55cfc)
// Ended(2018-12-20T02:00:00Z,b960f986-8ac8-4616-afbe-7537bce55cfc)
// Ended(2018-12-20T02:00:00Z,758d81d5-f914-4649-924e-527d0ae5808b)
```

???

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,a4507240-7304-4111-bb97-92c0e147608c)
// Started(2018-12-20T02:00:00Z,a4ba15f0-f249-4c9b-bc13-71289933e6b7)
// Started(2018-12-20T02:00:00Z,47c86cba-4187-47ce-8fff-04e9426d0de7)
// Paused(2018-12-20T02:00:20Z,47c86cba-4187-47ce-8fff-04e9426d0de7)
// Resumed(2018-12-20T02:00:25Z,47c86cba-4187-47ce-8fff-04e9426d0de7)
// Ended(2018-12-20T02:20:25Z,47c86cba-4187-47ce-8fff-04e9426d0de7)
// Started(2018-12-20T02:20:25Z,deb4652e-aaa3-40c8-ae1f-9bd0af572ee6)
// Paused(2018-12-20T02:20:45Z,deb4652e-aaa3-40c8-ae1f-9bd0af572ee6)
// Resumed(2018-12-20T02:20:50Z,deb4652e-aaa3-40c8-ae1f-9bd0af572ee6)
// Ended(2018-12-20T02:30:00Z,a4ba15f0-f249-4c9b-bc13-71289933e6b7)
// Ended(2018-12-20T02:30:00Z,a4507240-7304-4111-bb97-92c0e147608c)
// Ended(2018-12-20T02:40:50Z,deb4652e-aaa3-40c8-ae1f-9bd0af572ee6)
```

---

## 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, 9361909c-3306-4509-b8d1-c0550ede25af),
//       Ended(2018-12-20T03:30:00Z, 9361909c-3306-4509-b8d1-c0550ede25af)
//     )
//   )
// )
```

---

## 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?