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(4000-11-03T20:17:29.116Z, 0db2a126-4e47-4de7-b0de-0689bb3fb5a8),
//   Ended(2882-06-25T20:13:34.617Z, 3688c612-c59d-4241-8794-dff98098af30),
//   Ended(0229-04-15T00:00:00Z, 54c9e69a-e47f-4d02-b261-e5788dbe90c0),
//   Resumed(0000-12-30T00:00:00Z, d0e42676-4cba-42b2-9165-4b3185d73dd6),
//   Ended(1969-12-31T23:59:59.999Z, 6726a3fe-12ba-4822-8f42-9e2f3f532a1b),
//   Paused(4000-11-03T00:00:00Z, 975a8773-f4fa-4bea-9912-b4b4cf5aa1d5),
//   Paused(4000-11-30T20:17:29.116Z, 9672aa2b-07c5-48a5-9cc1-818ffcb7160c),
//   Started(1127-05-23T18:41:36.654Z, 25d26ca7-120c-4391-aac3-093e432bc9ee),
//   Ended(1969-12-31T23:59:59.999Z, b3178a96-f17c-48c1-aef7-9324b58cfe18),
//   Paused(4000-11-30T20:17:29.116Z, 85e1f28a-295e-475a-b237-baed513837b8)
// )
```

😱 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, ef2171bb-4dd0-4b57-9fbb-c5631880f187),
//   Ended(2019-11-26T12:01:00Z, ef2171bb-4dd0-4b57-9fbb-c5631880f187)
// )
```

---

## 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,9577f393-0c1a-4e2a-9cab-fe48318a4715)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,9577f393-0c1a-4e2a-9cab-fe48318a4715)
```

---

## 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,39a0d3c2-36d0-408d-9352-56917c5a4924)
// Started(2018-12-20T01:00:00Z,e030f935-f049-4a1e-b1a0-06eb9342865e)
// Started(2018-12-20T01:00:00Z,c5b447c4-aac4-4439-9719-12e34c6e868c)
// Ended(2018-12-20T01:30:00Z,e030f935-f049-4a1e-b1a0-06eb9342865e)
// Ended(2018-12-20T01:30:00Z,39a0d3c2-36d0-408d-9352-56917c5a4924)
// Ended(2018-12-20T01:30:00Z,c5b447c4-aac4-4439-9719-12e34c6e868c)
// Started(2018-12-20T01:30:00Z,7b2c4c6c-252b-434a-97aa-e1d2e4c243e6)
// Started(2018-12-20T01:30:00Z,1d6a48c2-6359-4f15-a3d6-e667dca253c9)
// Ended(2018-12-20T02:00:00Z,7b2c4c6c-252b-434a-97aa-e1d2e4c243e6)
// Ended(2018-12-20T02:00:00Z,1d6a48c2-6359-4f15-a3d6-e667dca253c9)
```

???

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,add70e06-b6a1-4471-af43-455493108d7c)
// Started(2018-12-20T02:00:00Z,6821487e-2941-4726-a828-2c0cec99abb9)
// Started(2018-12-20T02:00:00Z,0980e41a-0bed-4dd4-b62e-c2480d391e24)
// Paused(2018-12-20T02:00:20Z,6821487e-2941-4726-a828-2c0cec99abb9)
// Resumed(2018-12-20T02:00:25Z,6821487e-2941-4726-a828-2c0cec99abb9)
// Ended(2018-12-20T02:20:25Z,6821487e-2941-4726-a828-2c0cec99abb9)
// Started(2018-12-20T02:20:25Z,dcf936bf-d9c4-463b-be44-ce4c29afac2f)
// Paused(2018-12-20T02:20:45Z,dcf936bf-d9c4-463b-be44-ce4c29afac2f)
// Resumed(2018-12-20T02:20:50Z,dcf936bf-d9c4-463b-be44-ce4c29afac2f)
// Ended(2018-12-20T02:30:00Z,add70e06-b6a1-4471-af43-455493108d7c)
// Ended(2018-12-20T02:30:00Z,0980e41a-0bed-4dd4-b62e-c2480d391e24)
// Ended(2018-12-20T02:40:50Z,dcf936bf-d9c4-463b-be44-ce4c29afac2f)
```

---

## 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, 403cea5a-630f-4d2f-963c-9c6a8032bbe7),
//       Ended(2018-12-20T03:30:00Z, 403cea5a-630f-4d2f-963c-9c6a8032bbe7)
//     )
//   )
// )
```

---

## 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?