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(0000-12-30T05:00:00Z, dd467b72-8177-4c85-806f-e1d5485e8b50),
//   Resumed(0081-11-29T18:06:28.254Z, 1ca1f1a5-e099-46a6-a7c5-0ad80c4685d7),
//   Ended(1969-12-31T23:59:59.999Z, af3c8ed5-5c07-40cd-8408-6dc7585ff8da),
//   Paused(3891-10-15T23:59:59.059Z, f68a5e98-586b-4d16-b6d9-e8c411a92c77),
//   Ended(+42171613-01-01T01:28:12.641Z, 9d821949-e949-4689-8f37-576c08d13475),
//   Paused(1928-01-19T10:11:09.385Z, f49b175e-b8a6-4300-a7b5-50fd5e28b7c6),
//   Started(1969-12-31T23:59:59.999Z, c195cc91-99dc-4002-acf8-f2ab54901e85),
//   Started(0001-12-29T06:09:05.037Z, bd8205bc-c3c6-4b14-812b-743b4a90dae6),
//   Resumed(0000-12-30T03:41:52.498Z, b89e7283-0ac5-4028-aef9-6c310eaaa409),
//   Resumed(1970-01-01T00:00:00.001Z, 06106b50-baf5-4334-9c6b-05cea936511d)
// )
```

😱 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, a93dc2ee-fed7-4c83-9afb-357841dc58f7),
//   Ended(2019-11-26T12:01:00Z, a93dc2ee-fed7-4c83-9afb-357841dc58f7)
// )
```

---

## 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,5868d026-9610-493d-9289-4472e0b50e32)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,5868d026-9610-493d-9289-4472e0b50e32)
```

---

## 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,2f500a5e-ed80-41aa-97ae-e892d2e3e2ea)
// Started(2018-12-20T01:00:00Z,1228c294-c950-4650-8b38-d635ad75482e)
// Started(2018-12-20T01:00:00Z,6ed450b8-7396-47e2-92de-f8653c400f47)
// Ended(2018-12-20T01:30:00Z,6ed450b8-7396-47e2-92de-f8653c400f47)
// Ended(2018-12-20T01:30:00Z,2f500a5e-ed80-41aa-97ae-e892d2e3e2ea)
// Ended(2018-12-20T01:30:00Z,1228c294-c950-4650-8b38-d635ad75482e)
// Started(2018-12-20T01:30:00Z,d2f2b1e8-d964-4345-ae72-c12123c68032)
// Started(2018-12-20T01:30:00Z,b3256c69-be01-403a-9ee9-97902b147090)
// Ended(2018-12-20T02:00:00Z,b3256c69-be01-403a-9ee9-97902b147090)
// Ended(2018-12-20T02:00:00Z,d2f2b1e8-d964-4345-ae72-c12123c68032)
```

???

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,aa4bef09-69e2-42ad-9bd7-9cdf89b4531d)
// Started(2018-12-20T02:00:00Z,ec9fe6da-ac5a-41f0-8dfd-a3bc364ea35e)
// Started(2018-12-20T02:00:00Z,dc3eec03-bbf3-4f68-b122-0e10974db6b2)
// Paused(2018-12-20T02:00:20Z,ec9fe6da-ac5a-41f0-8dfd-a3bc364ea35e)
// Resumed(2018-12-20T02:00:25Z,ec9fe6da-ac5a-41f0-8dfd-a3bc364ea35e)
// Ended(2018-12-20T02:20:25Z,ec9fe6da-ac5a-41f0-8dfd-a3bc364ea35e)
// Started(2018-12-20T02:20:25Z,6035c415-9c99-4b4e-ab66-ae2cf2e1ea94)
// Paused(2018-12-20T02:20:45Z,6035c415-9c99-4b4e-ab66-ae2cf2e1ea94)
// Resumed(2018-12-20T02:20:50Z,6035c415-9c99-4b4e-ab66-ae2cf2e1ea94)
// Ended(2018-12-20T02:30:00Z,aa4bef09-69e2-42ad-9bd7-9cdf89b4531d)
// Ended(2018-12-20T02:30:00Z,dc3eec03-bbf3-4f68-b122-0e10974db6b2)
// Ended(2018-12-20T02:40:50Z,6035c415-9c99-4b4e-ab66-ae2cf2e1ea94)
```

---

## 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, 2b64c592-add1-49a3-93d2-83c3a65f3de6),
//       Ended(2018-12-20T03:30:00Z, 2b64c592-add1-49a3-93d2-83c3a65f3de6)
//     )
//   )
// )
```

---

## 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?