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, bb85db28-8d04-4c65-b869-8ce4ab78e4bd),
//   Ended(0004-11-01T20:17:29.116Z, 988adfe5-6905-41c6-9917-111889731997),
//   Started(3565-10-22T23:30:54.842Z, 38ab4ab3-2096-404b-92b0-59fa40f3db30),
//   Ended(1970-01-01T23:59:59.059Z, 47718958-f60b-496f-ba1a-272fefd8f45c),
//   Resumed(1969-12-31T23:59:59.999Z, 20846bfe-841e-4935-b1bb-49faaa0e9d24),
//   Paused(1970-01-01T00:00:00.001Z, d78d18c5-a537-4711-a91a-3fe8d99b497f),
//   Resumed(1970-01-01T00:00:00Z, 1b4df831-942d-4f94-bf51-afc2b5a9c07b),
//   Started(2214-06-01T18:00:15.272Z, 6b9442d0-4b41-4f26-a2eb-9ef7c3f87ec1),
//   Resumed(3279-10-22T23:59:59.059Z, 056e8c52-cbd2-4683-91a9-1e44641ed006),
//   Resumed(2986-02-25T06:53:00.286Z, 69f148ea-372a-4303-9ab0-f05f97204458)
// )
```

😱 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, 4601c355-079f-4980-a4b0-e6e52f65624c),
//   Ended(2019-11-26T12:01:00Z, 4601c355-079f-4980-a4b0-e6e52f65624c)
// )
```

---

## 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,b63cc99f-a373-4001-97ed-415bdce999b4)
testContext.tick(1.hour)
// Ended(2018-12-20T00:30:00Z,b63cc99f-a373-4001-97ed-415bdce999b4)
```

---

## 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,cffdb936-c255-4366-99ae-725641c2f520)
// Started(2018-12-20T01:00:00Z,ca319457-1f66-465e-bf7e-a7f017cb2726)
// Started(2018-12-20T01:00:00Z,ec337c83-08d5-4d42-867e-73f11387030f)
// Ended(2018-12-20T01:30:00Z,ec337c83-08d5-4d42-867e-73f11387030f)
// Ended(2018-12-20T01:30:00Z,ca319457-1f66-465e-bf7e-a7f017cb2726)
// Ended(2018-12-20T01:30:00Z,cffdb936-c255-4366-99ae-725641c2f520)
// Started(2018-12-20T01:30:00Z,28df084f-3816-4e93-bdaa-609cd0355c40)
// Started(2018-12-20T01:30:00Z,b353632c-1c64-40e9-b7d9-01ce6f58048e)
// Ended(2018-12-20T02:00:00Z,b353632c-1c64-40e9-b7d9-01ce6f58048e)
// Ended(2018-12-20T02:00:00Z,28df084f-3816-4e93-bdaa-609cd0355c40)
```

???

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,9bc1903e-c25d-4dc2-8b7a-46fe0e2e9a30)
// Started(2018-12-20T02:00:00Z,5ecfd51e-2ece-47d4-8811-3abf16990008)
// Started(2018-12-20T02:00:00Z,142ef96c-9aa2-4d61-af87-433a28368aa1)
// Paused(2018-12-20T02:00:20Z,5ecfd51e-2ece-47d4-8811-3abf16990008)
// Resumed(2018-12-20T02:00:25Z,5ecfd51e-2ece-47d4-8811-3abf16990008)
// Ended(2018-12-20T02:20:25Z,5ecfd51e-2ece-47d4-8811-3abf16990008)
// Started(2018-12-20T02:20:25Z,2bfb516e-949e-4403-a42d-e5c36a282742)
// Paused(2018-12-20T02:20:45Z,2bfb516e-949e-4403-a42d-e5c36a282742)
// Resumed(2018-12-20T02:20:50Z,2bfb516e-949e-4403-a42d-e5c36a282742)
// Ended(2018-12-20T02:30:00Z,9bc1903e-c25d-4dc2-8b7a-46fe0e2e9a30)
// Ended(2018-12-20T02:30:00Z,142ef96c-9aa2-4d61-af87-433a28368aa1)
// Ended(2018-12-20T02:40:50Z,2bfb516e-949e-4403-a42d-e5c36a282742)
```

---

## 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, 1c47db09-0c79-4ca2-b5ff-44ab1a00440d),
//       Ended(2018-12-20T03:30:00Z, 1c47db09-0c79-4ca2-b5ff-44ab1a00440d)
//     )
//   )
// )
```

---

## 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?