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(1970-01-01T00:00:00Z, 54946a63-31ab-4d9c-8096-a7f5d25983d3),
//   Ended(1969-12-31T23:59:59.059Z, 8699a537-a2cd-497f-9c37-c8b90aef89fa),
//   Ended(1579-09-16T23:59:59.059Z, f9c1b338-5a73-49e3-9a56-94854d426038),
//   Paused(3800-10-28T11:02:42.628Z, d674c7e9-471d-4594-8710-4067379b573f),
//   Started(1969-12-31T23:59:59.999Z, e8178bb4-b055-486e-8f34-e3cc1ab181f0),
//   Paused(1958-06-16T02:46:25.742Z, b5a3b32c-26c5-4241-abe0-da8dc4612cd5),
//   Paused(1970-01-01T00:00:00.001Z, 0b7a7936-4461-424c-bfae-9c410b82452f),
//   Ended(1970-01-31T00:00:00.001Z, bf11438b-ce4b-4072-819c-9ee07ef44be5),
//   Paused(1739-12-20T03:58:44.257Z, d095d96e-b550-475e-8522-7a4ed5f8c3ca),
//   Ended(1324-12-20T00:00:00Z, eb5fa352-762e-4a19-9a00-9cd187e359fb)
// )
```

😱 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, f0e4aad6-a11b-4b10-aa65-24236495e59c),
//   Ended(2019-11-26T12:01:00Z, f0e4aad6-a11b-4b10-aa65-24236495e59c)
// )
```

---

## 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,452f1cb1-5b5b-4c45-a8fe-3cbc60675d84)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,452f1cb1-5b5b-4c45-a8fe-3cbc60675d84)
```

---

## 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,a160b00b-12a3-4f7e-9d58-5886582459ca)
// Started(2018-12-20T01:00:00Z,571da93f-ac3d-4f9e-b94a-c15d14f1a47d)
// Started(2018-12-20T01:00:00Z,433f442f-70c8-4064-9ceb-044055b70667)
// Ended(2018-12-20T01:30:00Z,433f442f-70c8-4064-9ceb-044055b70667)
// Ended(2018-12-20T01:30:00Z,571da93f-ac3d-4f9e-b94a-c15d14f1a47d)
// Ended(2018-12-20T01:30:00Z,a160b00b-12a3-4f7e-9d58-5886582459ca)
// Started(2018-12-20T01:30:00Z,ae740b1a-85c1-4f17-a236-714f464a4760)
// Started(2018-12-20T01:30:00Z,59a77519-3f63-4c1a-9f2d-958e5ad29deb)
// Ended(2018-12-20T02:00:00Z,59a77519-3f63-4c1a-9f2d-958e5ad29deb)
// Ended(2018-12-20T02:00:00Z,ae740b1a-85c1-4f17-a236-714f464a4760)
```

???

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,3c07cff7-0cff-4cbe-862f-c60b30385bf2)
// Started(2018-12-20T02:00:00Z,a9c4c076-fca6-4489-8d04-8730d702ef0b)
// Started(2018-12-20T02:00:00Z,13e60401-f82b-4d36-9fdc-3741cf3eb156)
// Paused(2018-12-20T02:00:20Z,3c07cff7-0cff-4cbe-862f-c60b30385bf2)
// Resumed(2018-12-20T02:00:25Z,3c07cff7-0cff-4cbe-862f-c60b30385bf2)
// Ended(2018-12-20T02:20:25Z,3c07cff7-0cff-4cbe-862f-c60b30385bf2)
// Started(2018-12-20T02:20:25Z,605c0f6f-3b8a-4bfe-9c64-0c40f710dd87)
// Paused(2018-12-20T02:20:45Z,605c0f6f-3b8a-4bfe-9c64-0c40f710dd87)
// Resumed(2018-12-20T02:20:50Z,605c0f6f-3b8a-4bfe-9c64-0c40f710dd87)
// Ended(2018-12-20T02:30:00Z,a9c4c076-fca6-4489-8d04-8730d702ef0b)
// Ended(2018-12-20T02:30:00Z,13e60401-f82b-4d36-9fdc-3741cf3eb156)
// Ended(2018-12-20T02:40:50Z,605c0f6f-3b8a-4bfe-9c64-0c40f710dd87)
```

---

## 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, 447c38e0-8eab-4369-8bf5-dafa7a241852),
//       Ended(2018-12-20T03:30:00Z, 447c38e0-8eab-4369-8bf5-dafa7a241852)
//     )
//   )
// )
```

---

## 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?