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(1970-01-01T23:59:59.059Z, 2f6a9050-a458-499e-b1e1-a4f01d256a5b),
//   Started(1384-12-10T11:58:17.456Z, 42f18fd4-bc45-469d-9cd2-d34c597a3cbc),
//   Ended(+46017796-01-01T00:00:00Z, 558f6ef7-eaa8-4389-8373-38f91d16a21b),
//   Started(1969-12-31T23:59:59.059Z, bb282e54-1292-4c49-9a0e-dfae86f61498),
//   Started(4000-11-03T20:17:29.116Z, 0dd503d5-bc97-4bb8-b4f2-3e7576951deb),
//   Ended(+160853648-01-01T00:57:26.853Z, d0211393-830f-4bb7-8b1b-cb0c64dec091),
//   Ended(0079-08-07T04:36:06.271Z, 902ad631-d0a6-4e92-b3c2-a4b915969117),
//   Resumed(1969-12-31T23:59:59.999Z, d528d608-7864-432d-91df-a5c5ed6fa376),
//   Resumed(3522-06-08T00:00:00Z, 9e17c88e-f0a4-499d-aac3-8d520e070446),
//   Paused(2428-12-26T00:00:00Z, 4ddfc224-6e09-4f05-8094-daf05a3589e8)
// )
```

😱 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, 807451b9-bf65-43dc-8e50-345cfa02fc86),
//   Ended(2019-11-26T12:01:00Z, 807451b9-bf65-43dc-8e50-345cfa02fc86)
// )
```

---

## 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,ccd56f23-8de8-4698-b1c1-92d3355c5579)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,ccd56f23-8de8-4698-b1c1-92d3355c5579)
```

---

## 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,b4c8c61d-cf07-42a1-b428-a219a8d31b52)
// Started(2018-12-20T01:00:00Z,d7ea5c06-6bfe-4c82-9dee-502854f6d5a4)
// Started(2018-12-20T01:00:00Z,5183d3d7-b89f-4d2c-81ec-36ad226560ce)
// Ended(2018-12-20T01:30:00Z,d7ea5c06-6bfe-4c82-9dee-502854f6d5a4)
// Ended(2018-12-20T01:30:00Z,b4c8c61d-cf07-42a1-b428-a219a8d31b52)
// Ended(2018-12-20T01:30:00Z,5183d3d7-b89f-4d2c-81ec-36ad226560ce)
// Started(2018-12-20T01:30:00Z,99ad0ff2-464b-41c4-a592-f32f1ad1dede)
// Started(2018-12-20T01:30:00Z,f04b506a-47fb-4c5a-9656-7f0b721ec402)
// Ended(2018-12-20T02:00:00Z,f04b506a-47fb-4c5a-9656-7f0b721ec402)
// Ended(2018-12-20T02:00:00Z,99ad0ff2-464b-41c4-a592-f32f1ad1dede)
```

???

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,a0afa669-14e9-4764-b671-03c13e70a98c)
// Started(2018-12-20T02:00:00Z,e9b58936-8962-454c-94dc-3bc4833e6a5b)
// Started(2018-12-20T02:00:00Z,65217cd0-1270-41ad-b97e-f36fd061df09)
// Paused(2018-12-20T02:00:20Z,a0afa669-14e9-4764-b671-03c13e70a98c)
// Resumed(2018-12-20T02:00:25Z,a0afa669-14e9-4764-b671-03c13e70a98c)
// Ended(2018-12-20T02:20:25Z,a0afa669-14e9-4764-b671-03c13e70a98c)
// Started(2018-12-20T02:20:25Z,8899d29a-52ac-4761-a339-a0d5eee61e6d)
// Paused(2018-12-20T02:20:45Z,8899d29a-52ac-4761-a339-a0d5eee61e6d)
// Resumed(2018-12-20T02:20:50Z,8899d29a-52ac-4761-a339-a0d5eee61e6d)
// Ended(2018-12-20T02:30:00Z,e9b58936-8962-454c-94dc-3bc4833e6a5b)
// Ended(2018-12-20T02:30:00Z,65217cd0-1270-41ad-b97e-f36fd061df09)
// Ended(2018-12-20T02:40:50Z,8899d29a-52ac-4761-a339-a0d5eee61e6d)
```

---

## 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, fe47e12a-c134-4c59-b9fc-55b028232918),
//       Ended(2018-12-20T03:30:00Z, fe47e12a-c134-4c59-b9fc-55b028232918)
//     )
//   )
// )
```

---

## 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?