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(0000-12-30T05:00:00Z, f7496dc1-2d02-4c40-84a7-4ad374293d84),
//   Ended(+292269052-08-23T02:23:20.516Z, 54112d79-bf5a-44de-89cb-18ab6fd855e5),
//   Paused(1579-11-08T22:01:34.366Z, 9a18624f-92d4-4187-895b-dd6250e32e10),
//   Started(2687-02-25T17:27:35.937Z, 80931d28-871c-4c2c-bf87-7ae3fd2fba3f),
//   Started(0004-11-01T20:17:29.116Z, 45629f39-4678-491c-a7c5-d1367cf04d08),
//   Started(0000-12-30T00:00:00Z, d3275765-dde6-4dc9-91b5-097b7c8812bf),
//   Ended(+57502701-01-01T00:00:00.001Z, 1345fd86-4cb4-4a94-8b55-98404bc36d2f),
//   Ended(3354-09-07T08:07:20Z, 84d05966-bf03-4ab4-84d9-173283261b45),
//   Started(1970-01-01T00:00:00.001Z, 66e3a7d3-7cc8-4873-91c6-d2990d6ba87b),
//   Paused(0991-03-22T23:59:59.059Z, 56eb25ac-ad0a-4b0f-8b30-c17e6a9147c5)
// )
```

😱 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, 62480eba-10e9-4a0a-a598-89e2fb374683),
//   Ended(2019-11-26T12:01:00Z, 62480eba-10e9-4a0a-a598-89e2fb374683)
// )
```

---

## 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,d81d0a26-5608-41a4-8f5b-95e1c0ca55e3)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,d81d0a26-5608-41a4-8f5b-95e1c0ca55e3)
```

---

## 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,55a0103d-4085-4820-b9e7-e008f4ba18c1)
// Started(2018-12-20T01:00:00Z,298f0a5d-b88d-4d49-894a-f631bbc6afe4)
// Started(2018-12-20T01:00:00Z,e0b57cd6-6925-4960-b17a-785801f109df)
// Ended(2018-12-20T01:30:00Z,55a0103d-4085-4820-b9e7-e008f4ba18c1)
// Ended(2018-12-20T01:30:00Z,298f0a5d-b88d-4d49-894a-f631bbc6afe4)
// Ended(2018-12-20T01:30:00Z,e0b57cd6-6925-4960-b17a-785801f109df)
// Started(2018-12-20T01:30:00Z,fdc11130-4cdc-438d-8b06-5e2a936df234)
// Started(2018-12-20T01:30:00Z,04a41122-e9aa-4742-9e57-047becdbd2ed)
// Ended(2018-12-20T02:00:00Z,04a41122-e9aa-4742-9e57-047becdbd2ed)
// Ended(2018-12-20T02:00:00Z,fdc11130-4cdc-438d-8b06-5e2a936df234)
```

???

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,fc2578f4-cc06-48ae-82d2-1ffbebe0941d)
// Started(2018-12-20T02:00:00Z,aa0d1a91-b262-40b1-bf83-58c23cd6a690)
// Started(2018-12-20T02:00:00Z,70500817-8eb2-4fdb-8155-664da3d068e7)
// Paused(2018-12-20T02:00:20Z,aa0d1a91-b262-40b1-bf83-58c23cd6a690)
// Resumed(2018-12-20T02:00:25Z,aa0d1a91-b262-40b1-bf83-58c23cd6a690)
// Ended(2018-12-20T02:20:25Z,aa0d1a91-b262-40b1-bf83-58c23cd6a690)
// Started(2018-12-20T02:20:25Z,57da679d-f630-4a37-9a41-78f4a9181a8c)
// Paused(2018-12-20T02:20:45Z,57da679d-f630-4a37-9a41-78f4a9181a8c)
// Resumed(2018-12-20T02:20:50Z,57da679d-f630-4a37-9a41-78f4a9181a8c)
// Ended(2018-12-20T02:30:00Z,fc2578f4-cc06-48ae-82d2-1ffbebe0941d)
// Ended(2018-12-20T02:30:00Z,70500817-8eb2-4fdb-8155-664da3d068e7)
// Ended(2018-12-20T02:40:50Z,57da679d-f630-4a37-9a41-78f4a9181a8c)
```

---

## 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, 4a91bf8e-90c0-47d4-8f2b-624273408081),
//       Ended(2018-12-20T03:30:00Z, 4a91bf8e-90c0-47d4-8f2b-624273408081)
//     )
//   )
// )
```

---

## 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?