Cats-effect 1.0.0-RC2 is out, and among various improvements, it brought to us some goodies in cats.effect.concurrent, exported from fs2 and Monix with a number of changes:

• Ref - pure mutable reference
• Deferred - a purely functional alternative to Promise
• Semaphore - an access control tool
• MVar - a mutable location that can be empty, useful as synchronization and communication channel

This post will focus on Ref and to show what interesting techniques it enables.

Problem:

I want to figure out which log entries belong to which request.

Also, being a lazy person, I want to get away with as little work as possible. In particular that means:

• I don’t want to pass extra function parameters / implicits (solutions like Logger.takingImplicit of scala-logging won’t cut)
• I don’t want to pollute my domain signatures with something like ReaderT[Task, RequestId, A] instead of plain Tasks.
• I don’t want to manually insert request ID to every call to log function. I want to set it once and be done with it.

Java SLF4J API already support all this functionality in form of MDC. However, the existing implementations use a ThreadLocal variable, which doesn’t work if you’re not creating a separate thread per each request - which I do not.

Solution: