IOLocal

IOLocal provides a handy way of manipulating a context on different scopes. In some scenarios, IOLocal can be considered as an alternative to cats.data.Kleisli.

IOLocal should not be treated as Ref, since the former abides different laws.

Once a fiber is forked, for example by Spawn[F].start, the forked fiber manipulates the copy of the parent's context. For example, two forked fibers will never see each other's modifications to the same IOLocal, each fiber will only see its own modifications.

Operations on IOLocal are visible to the fiber

┌────────────┐               ┌────────────┐               ┌────────────┐
│  Fiber A   │ update(_ + 1) │  Fiber A   │ update(_ + 1) │  Fiber A   │
│ (local 42) │──────────────►│ (local 43) │──────────────►│ (local 44) │
└────────────┘               └────────────┘               └────────────┘

import cats.effect.{IO, IOLocal}
import scala.concurrent.duration._

def inc(idx: Int, local: IOLocal[Int]): IO[Unit] =
  local.update(_ + 1) >> local.get.flatMap(current => IO.println(s"update $idx: $current"))

for {
  local   <- IOLocal(42)
  _       <- inc(1, local)
  _       <- inc(2, local)
  current <- local.get
  _       <- IO.println(s"fiber A: $current")
} yield ()

// output:
// update 1: 43
// update 2: 44
// fiber A: 44

---

A forked fiber operates on a copy of the parent IOLocal

A forked fiber (i.e. via Spawn[F].start) operates on a copy of the parent IOLocal. Hence, the children operations are not reflected on the parent context.

                      ┌────────────┐               ┌────────────┐
               fork   │  Fiber B   │ update(_ - 1) │  Fiber B   │
               ┌─────►│ (local 42) │──────────────►│ (local 41) │
               │      └────────────┘               └────────────┘
┌────────────┐─┘                                   ┌────────────┐
│  Fiber A   │                                     │  Fiber A   │
│ (local 42) │────────────────────────────────────►│ (local 42) │
└────────────┘─┐                                   └────────────┘
               │      ┌────────────┐               ┌────────────┐
               │ fork │  Fiber C   │ update(_ + 1) │  Fiber C   │
               └─────►│ (local 42) │──────────────►│ (local 43) │
                      └────────────┘               └────────────┘

def update(name: String, local: IOLocal[Int], f: Int => Int): IO[Unit] =
  local.update(f) >> local.get.flatMap(current => IO.println(s"$name: $current"))
    
for {
  local   <- IOLocal(42)
  fiberA  <- update("fiber B", local, _ - 1).start
  fiberB  <- update("fiber C", local, _ + 1).start
  _       <- fiberA.joinWithNever
  _       <- fiberB.joinWithNever
  current <- local.get
  _       <- IO.println(s"fiber A: $current")
} yield ()

// output:
// fiber B: 41
// fiber C: 43
// fiber A: 42

---

Parent operations on IOLocal are invisible to children

                      ┌────────────┐               ┌────────────┐
                 fork │  Fiber B   │ update(_ + 1) │  Fiber B   │
               ┌─────►│ (local 42) │──────────────►│ (local 43) │
               │      └────────────┘               └────────────┘
┌────────────┐─┘                                   ┌────────────┐
│  Fiber A   │        update(_ - 1)                │  Fiber A   │
│ (local 42) │────────────────────────────────────►│ (local 41) │
└────────────┘─┐                                   └────────────┘
               │      ┌────────────┐               ┌────────────┐
               │ fork │  Fiber C   │ update(_ + 2) │  Fiber C   │
               └─────►│ (local 42) │──────────────►│ (local 44) │
                      └────────────┘               └────────────┘

---

Example: TraceId propagation

The IOLocal can be used for the propagation of a TraceId:

import cats.Monad
import cats.effect.{IO, IOLocal, Sync, Resource}
import cats.effect.std.{Console, Random}
import cats.syntax.flatMap._
import cats.syntax.functor._

case class TraceId(value: String)

object TraceId {
  def gen[F[_]: Sync]: F[TraceId] =
    Random.scalaUtilRandom[F].flatMap(_.nextString(8)).map(TraceId(_))
}

trait TraceIdScope[F[_]] {
  def get: F[TraceId]
  def scope(traceId: TraceId): Resource[F, Unit]
}

object TraceIdScope {
  
  def apply[F[_]](implicit ev: TraceIdScope[F]): TraceIdScope[F] = ev
  
  def fromIOLocal: IO[TraceIdScope[IO]] =
    for {
      local <- IOLocal(TraceId("global"))
    } yield new TraceIdScope[IO] {
      def get: IO[TraceId] =
        local.get

      def scope(traceId: TraceId): Resource[IO, Unit] =
        Resource.make(local.getAndSet(traceId))(previous => local.set(previous)).void
    }
    
}

def service[F[_]: Sync: Console: TraceIdScope]: F[String] =
  for {
    traceId <- TraceId.gen[F]
    result <- TraceIdScope[F].scope(traceId).use(_ => callRemote[F])
  } yield result
  
def callRemote[F[_]: Monad: Console: TraceIdScope]: F[String] =
  for {
    traceId <- TraceIdScope[F].get
    _ <- Console[F].println(s"Processing request. TraceId: ${traceId}")
  } yield "some response"
  
TraceIdScope.fromIOLocal.flatMap { implicit traceIdScope: TraceIdScope[IO] =>
  service[IO]
}