Control.Concurrent.Supervisor.Tutorial

Use threads-supervisor if you want the "poor-man's Erlang supervisors". threads-supervisor is an IO-based library with minimal dependencies which does only one thing: It provides you a Supervisor entity you can use to monitor your forked computations. If one of the managed threads dies, you can decide if and how to restart it. This gives you:

• Protection against silent exceptions which might terminate your workers.
• A simple but powerful way of structure your program into a supervision tree, where the leaves are the worker threads, and the nodes can be other supervisors being monitored.
• A disaster recovery mechanism.

You can install the threads-supervisor library by running: > $ cabal install threads-supervisor

Introduction

Who worked with Haskell's concurrency primitives would be surely familiar with the forkIO function, which allow us to fork an IO computation in a separate green thread. forkIO is great, but is also very low level, and has a couple of subtleties, as you can read from this passage of the documentation:

The newly created thread has an exception handler that discards the exceptions BlockedIndefinitelyOnMVar,BlockedIndefinitelyOnSTM, and ThreadKilled, and passes all other exceptions to the uncaught exception handler.

To mitigate this, we have a couple of libraries available, for example http://hackage.haskell.org/package/async and http://hackage.haskell.org/package/slave-thread.

But what about if I do not want to take explicit action, but instead specifying upfront how to react to disaster, and leave the library work out the details? This is what this library aims to do.

Different type of jobs

In this example, let's create four different threads:

job1 :: IO ()
job1 = do
  threadDelay 5000000
  fail "Dead"

This job will die after five seconds.

job2 :: ThreadId -> IO ()
job2 tid = do
  threadDelay 3000000
  killThread tid

This other job instead, we have waited three seconds, and then kill a target thread, generating an asynchronous exception.

job3 :: IO ()
job3 = do
  threadDelay 5000000
  error "Oh boy, I'm good as dead"

This guy is very similar to the first one, except for the fact error is used instead of fail.

job4 :: IO ()
job4 = threadDelay 7000000

job4 is what we wish for all our passing cross computation: smooth sailing.

These jobs represent a significant pool of our everyday computations in the IO monad

Creating a SupervisorSpec

Here I discuss the creation of a SupervisorSpec

Creating a Supervisor

Here I discuss the creation of a Supervisor from a SupervisionSpec

Supervising and choosing a RestartStrategy

Here I discuss how you can supervise other threads.

Wrapping up

I hope that you are now convinced that this library can be of some use to you!

main :: IO ()
main = bracketOnError (do
  supSpec <- newSupervisor
  sup <- supervise supSpec
  j1 <- forkSupervised sup OneForOne job1
  _ <- forkSupervised sup OneForOne (job2 j1)
  _ <- forkSupervised sup OneForOne job3
  _ <- forkSupervised sup OneForOne job4
  _ <- forkIO (go (eventStream sup))
  return sup) shutdownSupervisor (\_ -> threadDelay 10000000000)
  where
   go eS = do
     newE <- atomically $ readTBQueue eS
     print newE
     go eS