graphql-engine

Managed resources

Motivation

Most resources need to be explicitly freed after being acquired. In most programming languages, there’s an idiomatic way of ensuring that this is done properly, even in the case of an error. C++ famously uses RAII (Resource Acquisition Is Initialization), where the constructor of a type acquires the resource and its destructor releases it. Python uses the with statement.

In Haskell, an idiomatic way of doing this is with withX functions, that take a callback as an argument, such as withFile: the function acquires the resource, calls the associated computation, and frees the resource.

withFoo :: FooArgs -> (Foo -> IO a) -> IO a
withFoo args callback = do
  foo <- acquireFoo args
  callback foo `finally` releaseFoo foo
  -- or: bracket (acquireFoo args) releaseFoo callback

However, this becomes quickly unwieldy when trying to acquire several resources, like we do in the initialization of the engine:

main = do
  withLoggers \loggers -> do
    withPGPool \pgPool -> do
      withMSSQLPool \mssqlPool -> do
        withMetadataConnection \metadataDB -> do
          runEngine loggers pgPool mssqlPool metadataDB

But an astute reader might have noticed that this is a staircase pattern in which each line just binds a newly acquired resource to a name… This is a monad!

Managed

The Managed monad is defined in Control.Monad.Managed. It is defined (roughly) as such:

newtype Managed a = Managed (forall r . (a -> IO r) -> IO r)

With it, the above block of code can be rewritten as:

main = runManaged do
  loggers    <- managed withLoggers
  pgPool     <- managed withPGPool
  mssqlPool  <- managed withMSSQLPool
  metadataDB <- managed withMetadataConnection
  runEngine loggers pgPool mssqlPool metadataDB

In theory, we could run our entire engine in Managed instead of IO, since Managed provides MonadIO, and we’d be able to acquire all of our resources easily! However…

Limitations

Managed has two major limitations that prevent us from easily doing this. First and foremost: it is limited to IO callbacks. If we happen to be operating in ReaderT HandlerCtx (ReaderT AppEnv Managed), we can’t use a with function that operates in that same monad: the definition of managed restricts us to IO callbacks.

This is, of course, the reason why MonadBaseControl IO and MonadUnliftIO exist: they provide the ability to un-lift and re-lift computations, to allow IO callbacks. But, and that’s the second thorn in our side, Managed does not provide an instance of either of those two classes.

This proves to be a major blocker for us:

• we do rely on being able to give callbacks in complex monads
• we do rely on having access to MonadBaseControl IO for some of the libraries we use

Hence the need for something a bit more sophisticated.

ManagedT

This is why our code contains a transformer definition of Managed: ManagedT. It is defined (roughly) as such:

newtype ManagedT m a = ManagedT (forall r. (a -> m r) -> m r)

Instead of being limited to IO, this version allows us to allocate resources in the underlying monad, whichever it might be. While it still doesn’t give us a MonadBaseControl IO instance, we can still run everything in the underlying monad, that is more complex than IO, and lift the result back up to ManagedT. For instance, in the initialization, we do something akin to the following:

main = do
  appEnv <- getAppEnv
  runAppM appEnv do
    -- this block is in `AppM`, which is on top of `IO`
    lowerManagedT do
      -- this block is in `ManagedT AppM`
      foo <- allocate acquireFoo releaseFoo
      -- we run the engine in the original `AppM`, *without ManagedT*
      lift $ runEngine foo

This is a practical compromise: our resources are still handed by a clean monadic construct, but since that monad sits at the top of the stack, we can run our actual computations in the underlying stack and its full complexity, without being limited to IO.

The only major downside is that we have to stick a ManagedT at the top of the stack whenever we need to resource allocation, and at time of writing we have to do that twice in the initialisation code of the engine. But that’s a small price to pay.

An additional motivating example

The best example of how we make use of ManagedT can be found in Control.Concurrent.Extended. This small internal library provides a wrapper around Control.Immortal that allows us to use the ThreadId as an acquired resource and to gracefully shutdown the immortal respawning thread when the surrounding ManagedT context ends. And, due to using ManagedT at the top of the stack, we can spawn threads in any underlying monad without having to do any manual unlifting / lifting. See for instance forkManagedT, which is defined (more or less) as follows:

forkManagedT
  :: MonadBaseControl IO m
  => m Void
  -> ManagedT m Immortal.Thread

The new immortal thread will run an action in m, and the surrounding ManagedT context will manage the lifetime of the threadId: when it exits, Control.Immortal.stop will be called on the thread.

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