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MDC4s

Hack-utility for setting up MDC context

  • in Slf4j
  • in such a way that it would get propagated along Cats Effect
  • without user having to manually extract some value from IOLocal just put it to some global available in methods which where we couldn't pass these vales by arguments (like every logging method from library not designed with Kleisli in mind)

Why ?

The problem (and solutions) were described in:

This attempt differs because:

  • original solution was implemented only for Monix, this one aims for both Monix Task and Cats Effect IO
    • at the time of writing this, the IO does not let you propagate context in unsafe parts of the code OOTB (like into MDCAdapter) so a decorator for a whole Async[IO] was needed
  • uses tagless final - it allows using the same approach for both Task and IO, which could make the migration of tagless final codebases from Monix to CE# slightly easier
  • provides some utilities to modify context on forking and on joining - providing the same UX when working with Monix and CE3:
    • OOTB Monix does not propagate context in Locals - when enabling it selectively .start will create a fiber without "main" Fibers state. When we enable it everywhere... .start will propagate the context, but the Fiber and the "main" Fiber will write to the same context overriding each other
    • meanwhile IOLocal would copy the state on .start but provide no way of automatically merging values in the "main" Fiber and forked one
    • integrations provide some way of automatically modifying the Local/IOLocal on .start, and combining values from both Fibers on .join
  • is cross-compiled for 2.12, 2.13 AND 3

How it works?

// Cats Effect 3 example
import cats.effect.{IO, Async}
import io.scalaland.mdc.MDC

def program[F[_]: Async](mdcF: F[MDC[F]]): F[Result]

// import integrations
import io.scalaland.mdc.cats.*
import io.scalaland.mdc.slf4j.*
import org.slf4j.spi.MDCAdapter

program(
  // configures MDC, modifies MDCAdapted in Slf4j!!!
  IOMDC.configure[MDCAdapter](
    // an example of automatic modification of MDC context on .start, identity by default
    onFork = ctx => ctx.updated("forked", "yes"),
    // an example of automatic merging of MDCs from 2 fibers .onJoin, picks current fiber by default
    onJoin = (ctx1, ctx2) =>
      ctx1.map { case (k, v) => s"current.$k" -> v } ++ ctx2.map { case (k, v) => s"forked.$k" -> v }
  )
)(
  // replaces Async[IO] with a decorator doing the context propagation to ThreadLocals and handling context updates
  IOGlobal.configuredStatePropagation
)
// Monix 3 example
import cats.effect.ConcurrentEffect
import monix.eval.Task
import io.scalaland.mdc.MDC

def program[F[_]: ConcurrentEffect](mdcF: F[MDC[F]]): F[Result]

// import integrations
import io.scalaland.mdc.monix.*
import io.scalaland.mdc.slf4j.*
import org.slf4j.spi.MDCAdapter

program(
  // configures MDC, modifies MDCAdapted in Slf4j!!!
  TaskMDC.configure[MDCAdapter](
    // an example of automatic modification of MDC context on .start, identity by default
    onFork = ctx => ctx.updated("forked", "yes"),
    // an example of automatic merging of MDCs from 2 fibers .onJoin, picks current fiber by default
    onJoin = (ctx1, ctx2) =>
      ctx1.map { case (k, v) => s"current.$k" -> v } ++ ctx2.map { case (k, v) => s"forked.$k" -> v }
  )
)(
  // replaces ConcurrentEffect[Task] with a decorator configuring the context propagation and handling context updates
  TaskGlobal.configuredStatePropagation
)

Would Cats Effect maintainers like it?

No. But they didn't let us propagate things in a sane way like Monix used to. At least not before typelevel/cats-effect#3636 is merged.

Also I want something working with any library not just a few "blessed" ones.