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(+126277065-12-31T13:44:47.070Z, 4a4eeb1d-9a63-4ae7-8155-d36232cf74d1),
//   Started(2645-04-11T18:10:53.400Z, c4bcbd33-316b-4b49-8383-8aade7cb1a36),
//   Paused(1689-01-16T23:59:59.059Z, 32d2bcc1-08e1-48a6-847d-fbdda2c8630c),
//   Ended(+292269054-01-01T09:48:33.156Z, ef603ae0-6673-42ec-a23f-745a3a8c8f31),
//   Resumed(+117080944-05-01T06:57:10.672Z, 5f56377a-46c9-479d-87cd-5976b6552289),
//   Started(1389-03-18T18:28:07.055Z, 1165c2c1-3e6c-4f93-976b-537a50024d64),
//   Started(1050-04-07T17:36:58.706Z, 6a99ef6a-3cbc-4ebe-a1f6-df4ef015e4c5),
//   Started(4000-11-03T20:17:29.116Z, ff10e7d0-4b35-4e16-a4a7-8002cb83327b),
//   Started(0487-06-16T06:15:34.635Z, 55605a8d-0a96-440d-9659-50bac163b50a),
//   Ended(1273-06-26T23:59:59.059Z, 816239b2-8478-4c3a-90c5-4a94b4191a87)
// )
```

😱 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, 58ec5fd1-a340-4f55-91b3-1245c4347174),
//   Ended(2019-11-26T12:01:00Z, 58ec5fd1-a340-4f55-91b3-1245c4347174)
// )
```

---

## 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,52b1827f-e086-4918-8012-d8c13f7a11ce)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,52b1827f-e086-4918-8012-d8c13f7a11ce)
```

---

## 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,546051d6-03ee-4ca1-a8dc-d32a8f187c0b)
// Started(2018-12-20T01:00:00Z,36ad5f8e-5f18-44c1-8935-70bec227b117)
// Started(2018-12-20T01:00:00Z,38c20b28-d5f8-4b66-8040-6215f461b7db)
// Ended(2018-12-20T01:30:00Z,38c20b28-d5f8-4b66-8040-6215f461b7db)
// Ended(2018-12-20T01:30:00Z,36ad5f8e-5f18-44c1-8935-70bec227b117)
// Ended(2018-12-20T01:30:00Z,546051d6-03ee-4ca1-a8dc-d32a8f187c0b)
// Started(2018-12-20T01:30:00Z,476dc9db-67bf-4cfd-8c73-fcfe6a675bf1)
// Started(2018-12-20T01:30:00Z,124267b2-a700-46cd-acb5-0e3a248a0ff4)
// Ended(2018-12-20T02:00:00Z,476dc9db-67bf-4cfd-8c73-fcfe6a675bf1)
// Ended(2018-12-20T02:00:00Z,124267b2-a700-46cd-acb5-0e3a248a0ff4)
```

???

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,f5d39ef3-87cf-421d-87e9-02c20bc0fb6c)
// Started(2018-12-20T02:00:00Z,35b4f438-a3ac-4f00-8e52-6bedf5e83450)
// Started(2018-12-20T02:00:00Z,5ea62418-53f1-4625-afe0-962f79735efc)
// Paused(2018-12-20T02:00:20Z,35b4f438-a3ac-4f00-8e52-6bedf5e83450)
// Resumed(2018-12-20T02:00:25Z,35b4f438-a3ac-4f00-8e52-6bedf5e83450)
// Ended(2018-12-20T02:20:25Z,35b4f438-a3ac-4f00-8e52-6bedf5e83450)
// Started(2018-12-20T02:20:25Z,e5f625e9-91b4-4222-8ef1-e74d02618ceb)
// Paused(2018-12-20T02:20:45Z,e5f625e9-91b4-4222-8ef1-e74d02618ceb)
// Resumed(2018-12-20T02:20:50Z,e5f625e9-91b4-4222-8ef1-e74d02618ceb)
// Ended(2018-12-20T02:30:00Z,f5d39ef3-87cf-421d-87e9-02c20bc0fb6c)
// Ended(2018-12-20T02:30:00Z,5ea62418-53f1-4625-afe0-962f79735efc)
// Ended(2018-12-20T02:40:50Z,e5f625e9-91b4-4222-8ef1-e74d02618ceb)
```

---

## 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, 67434a2f-1cf0-4702-a947-e2b81ef60587),
//       Ended(2018-12-20T03:30:00Z, 67434a2f-1cf0-4702-a947-e2b81ef60587)
//     )
//   )
// )
```

---

## 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?