Cloud Haskell Platform

This is the Cloud Haskell Platform. Cloud Haskell is a set of libraries that bring Erlang-style concurrency and distribution to Haskell programs. This project is an implementation of that distributed computing interface, where processes communicate with one another through explicit message passing rather than shared memory.

Originally described by the joint Towards Haskell in the Cloud paper, Cloud Haskell has be re-written from the ground up and supports a rich and growing number of features for

  • building concurrent applications using asynchronous message passing
  • building distributed computing applications
  • building fault tolerant systems
  • running Cloud Haskell nodes on various network transports
  • working with several network transport implementations (and more in the pipeline)
  • supporting static values (required for remote communication)

There is a presentation on Cloud Haskell and this reimplementation, which is worth reading in conjunction with the documentation and wiki pages on this website..

Cloud Haskell comprises the following components, some of which are complete, others experimental. There are three main parts:

The core libraries

  • distributed-process: Base concurrency and distribution support. It provides a number of primitives known from Erlang like link and monitor.
  • distributed-static: Support for static values
  • rank1dynamic: Like Data.Dynamic and Data.Typeable but supporting polymorphic values

The platform libraries

The network layer

One of Cloud Haskell’s goals is to separate the transport layer from the process layer, so that the transport backend is entirely independent. In fact other projects can and do reuse the transport layer, even if they don’t use or have their own process layer (see e.g. HdpH).

Abstracting over the transport layer allows different protocols for message passing, including TCP/IP, UDP, MPI, CCI, ZeroMQ, SSH, MVars, Unix pipes, and more. Each of these transports provides its own implementation of the Network.Transport API and provide a means of creating new connections for use within Control.Distributed.Process.

The following diagram shows dependencies between the various subsystems, in an application using Cloud Haskell, where arrows represent explicit directional dependencies.


+------------------------------------------------------------+
|                        Application                         |
+------------------------------------------------------------+
             |                               |
             V                               V
+-------------------------+   +------------------------------+
|      Cloud Haskell      |<--|    Cloud Haskell Backend     |
|  (distributed-process)  |   | (distributed-process-...)    |
+-------------------------+   +------------------------------+
             |           ______/             |
             V           V                   V
+-------------------------+   +------------------------------+
|   Transport Interface   |<--|   Transport Implementation   |
|   (network-transport)   |   |   (network-transport-...)    |
+-------------------------+   +------------------------------+
                                             |
                                             V
                              +------------------------------+
                              | Haskell/C Transport Library  |
                              +------------------------------+

In this diagram, the various nodes roughly correspond to specific modules:

Cloud Haskell                : Control.Distributed.Process
Cloud Haskell                : Control.Distributed.Process.*
Transport Interface          : Network.Transport
Transport Implementation     : Network.Transport.*

An application is built using the primitives provided by the Cloud Haskell layer, provided by the Control.Distributed.Process module, which defines abstractions such as nodes and processes.

The application also depends on a Cloud Haskell Backend, which provides functions to allow the initialisation of the transport layer using whatever topology might be appropriate to the application.

It is, of course, possible to create new Cloud Haskell nodes by using a Network Transport Backend such as Network.Transport.TCP directly.

The Cloud Haskell interface and backend make use of the Transport interface provided by the Network.Transport module. This also serves as an interface for the Network.Transport.* module, which provides a specific implementation for this transport, and may, for example, be based on some external library written in Haskell or C.

Network Transport Abstraction Layer

Cloud Haskell’s generic network-transport API is entirely independent of the concurrency and messaging passing capabilities of the process layer. Cloud Haskell applications are built using the primitives provided by the process layer (i.e., distributed-process), which provides abstractions such as nodes and processes. Applications must also depend on a Cloud Haskell backend, which provides functions to allow the initialisation of the transport layer using whatever topology might be appropriate to the application.

Network.Transport is a network abstraction layer geared towards specific classes of applications, offering the following high level concepts:

  • Nodes in the network are represented by EndPoints. These are heavyweight stateful objects.
  • Each EndPoint has an EndPointAddress.
  • Connections can be established from one EndPoint to another using the EndPointAddress of the remote end.
  • The EndPointAddress can be serialised and sent over the network, whereas EndPoints and connections cannot.
  • Connections between EndPoints are unidirectional and lightweight.
  • Outgoing messages are sent via a Connection object that represents the sending end of the connection.
  • Incoming messages for all of the incoming connections on an EndPoint are collected via a shared receive queue.
  • In addition to incoming messages, EndPoints are notified of other Events such as new connections or broken connections.

This design was heavily influenced by the design of the Common Communication Interface (CCI). Important design goals are:

  • Connections should be lightweight: it should be no problem to create thousands of connections between endpoints.
  • Error handling is explicit: every function declares as part of its type which errors it can return (no exceptions are thrown)
  • Error handling is “abstract”: errors that originate from implementation specific problems (such as “no more sockets” in the TCP implementation) get mapped to generic errors (“insufficient resources”) at the Transport level.

For the purposes of most Cloud Haskell applications, it is sufficient to know enough about the Network.Transport API to instantiate a backend with the required configuration and pass the returned opaque handle to the Node API in order to establish a new, connected, running node. More involved setups are, of course, possible; The simplest use of the API is thus

main :: IO
main = do
  Right transport <- createTransport "127.0.0.1" "10080" defaultTCPParameters
  node1 <- newLocalNode transport initRemoteTable

Here we can see that the application depends explicitly on the defaultTCPParameters and createTransport functions from Network.Transport.TCP, but little else. The application can make use of other Network.Transport APIs if required, but for the most part this is irrelevant and the application will interact with Cloud Haskell through the Process Layer and Platform.

For more details about Network.Transport please see the wiki page.

Concurrency and Distribution

The Process Layer is where Cloud Haskell’s support for concurrency and distributed programming are exposed to application developers. This layer deals explicitly with

The core of Cloud Haskell’s concurrency and distribution support resides in the distributed-process library. As well as the APIs necessary for starting nodes and forking processes on them, we find all the basic primitives required to

  • spawn processes locally and remotely
  • send and receive messages, optionally using typed channels
  • monitor and/or link to processes, channels and other nodes

Most of this is easy enough to follow in the haddock documentation and the various tutorials. Here we focus on the essential concepts behind the process layer.

A concurrent process is somewhat like a Haskell thread - in fact it is a forkIO thread - but one that can send and receive messages through its process mailbox. Each process can send messages asynchronously to other processes, and can receive messages synchronously from its own mailbox. The conceptual difference between threads and processes is that the latter do not share state, but communicate only via message passing.

Code that is executed in this manner must run in the Process monad. Our process will look like any other monad code, plus we provide and instance of MonadIO for Process, so you can liftIO to make IO actions available.

Processes reside on nodes, which in our implementation map directly to the Control.Distributed.Processes.Node module. Given a configured Network.Transport backend, starting a new node is fairly simple:

newLocalNode :: Transport -> IO LocalNode

Once this function returns, the node will be up and running and able to interact with other nodes and host processes. It is possible to start more than one node in the same running program, though if you do this they will continue to send messages to one another using the supplied Network.Transport backend.

Given a new node, there are two primitives for starting a new process.

forkProcess :: LocalNode -> Process () -> IO ProcessId
runProcess  :: LocalNode -> Process () -> IO ()

Once we’ve spawned some processes, they can communicate with one another using the messaging primitives provided by [distributed-processes][distributed-processes], which are well documented in the haddocks.

What is Serializable

Processes can send data if the type implements the Serializable typeclass, which is done indirectly by implementing Binary and deriving Typeable. Implementations are already provided for primitives and some commonly used data structures. As programmers, we see the messages in nice high-level form (e.g., Int, String, Ping, Pong, etc), however these data have to be encoded in order to be sent over a communications channel.

Not all types are Serializable, for example concurrency primitives such as MVar and TVar are meaningless outside the context of threads with a shared memory. Cloud Haskell programs remain free to use these constructs within processes or within processes on the same machine though. If you want to pass data between processes using ordinary concurrency primitives such as STM then you’re free to do so. Processes spawned locally can share types such as TMVar just as normal Haskell threads would.

Typed Channels

Channels provides an alternative to message transmission with send and expect. While send and expect allow us to transmit messages of any Serializable type, channels require a uniform type. Channels work like a distributed equivalent of Haskell’s Control.Concurrent.Chan, however they have distinct ends: a single receiving port and a corollary send port.

Channels provide a nice alternative to bare send and receive, which is a bit un-Haskell-ish, since our process’ message queue can contain messages of multiple types, forcing us to undertake dynamic type checking at runtime.

We create channels with a call to newChan, and send/receive on them using the {send,receive}Chan primitives:

channelsDemo :: Process ()
channelsDemo = do
    (sp, rp) <- newChan :: Process (SendPort String, ReceivePort String)

    -- send on a channel
    spawnLocal $ sendChan sp "hello!"

    -- receive on a channel
    m <- receiveChan rp
    say $ show m

Channels are particularly useful when you are sending a message that needs a response, because we know exactly where to look for the reply.

Channels can also allow message types to be simplified, as passing a ProcessId for the reply isn’t required. Channels aren’t so useful when we need to spawn a process and send a bunch a messages to it, then wait for replies however; we can’t send a ReceivePort since it is not Serializable.

ReceivePorts can be merged, so we can listen on several simultaneously. In the latest version of distributed-process, we can listen for regular messages and multiple channels at the same time, using matchChan in the list of allowed matches passed receiveWait and receiveTimeout.

Linking and monitoring

Processes can be linked to other processes, nodes or channels. Links are unidirectional, and guarantee that once the linked object dies, the linked process will also be terminated. Monitors do not cause the listening process to exit, but rather they put a ProcessMonitorNotification into the process’ mailbox. Linking and monitoring are foundational tools for supervising processes, where a top level process manages a set of children, starting, stopping and restarting them as necessary.

Stopping Processes

Because processes are implemented with forkIO we might be tempted to stop them by throwing an asynchronous exception to the process, but this is almost certainly the wrong thing to do. Firstly, processes might reside on a remote node, in which case throwing an exception is impossible. Secondly, if we send some messages to a process’ mailbox and then dispatch an exception to kill it, there is no guarantee that the subject will receive our message before being terminated by the asynchronous exception.

To terminate a process unconditionally, we use the kill primitive, which dispatches an asynchronous exception (killing the subject) safely, respecting remote calls to processes on disparate nodes and observing message ordering guarantees such that send pid "hello" >> kill pid "goodbye" behaves quite unsurprisingly, delivering the message before the kill signal.

Exit signals come in two flavours however - those that can be caught and those that cannot. Whilst a call to kill results in an un-trappable exception, a call to exit :: (Serializable a) => ProcessId -> a -> Process () will dispatch an exit signal to the specified process that can be caught. These signals are intercepted and handled by the destination process using catchExit, allowing the receiver to match on the Serializable datum tucked away in the exit signal and decide whether to oblige or not.


Rethinking the Task Layer

Towards Haskell in the Cloud describes a multi-layered architecture, in which manipulation of concurrent processes and message passing between them is managed in the process layer, whilst a higher level API described as the task layer provides additional features such as

  • automatic recovery from failures
  • data centric processing model
  • a promise (or future) abstraction, representing the result of a calculation that may or may not have yet completed

The distributed-process-task library implements parts of the task layer, but takes a very different approach to that described in the original paper and implemented by the remote package. In particular, we diverge from the original design and defer to many of the principles defined by Erlang’s Open Telecom Platform, taking in some well established Haskell concurrency design patterns along the way.

In fact, distributed-process-async does not really consider the task layer in great detail. We provide an API comparable to remote’s Promise in Control.Distributed.Process.Async. This API however, is derived from Simon Marlow’s Control.Concurrent.Async package, and is not limited to blocking queries on Async handles in the same way. Instead our API handles both blocking and non-blocking queries, polling and working with lists of Async handles. We also eschew throwing exceptions to indicate asynchronous task failures, instead handling task and connectivity failures using monitors. Users of the API need only concern themselves with the AsyncResult, which encodes the status and (possibly) outcome of the computation simply.


demoAsync :: Process ()
demoAsync = do
  -- spawning a new task is fairly easy - this one is linked
  -- so if the caller dies, the task is killed too
  hAsync :: Async String
  hAsync <- asyncLinked $ (expect >>= return) :: Process String

  -- there is a rich API of functions to query an async handle
  AsyncPending <- poll hAsync   -- not finished yet

  -- we can cancel the task if we want to
  -- cancel hAsync

  -- or cancel it and wait until it has exited
  -- cancelWait hAsync

  -- we can wait on the task and timeout if it's still busy
  Nothing <- waitTimeout (within 3 Seconds) hAsync

  -- or finally, we can block until the task is finished!
  asyncResult <- wait hAsync
  case asyncResult of
      (AsyncDone res) -> say (show res)  -- a finished task/result
      AsyncCancelled  -> say "it was cancelled!?"
      AsyncFailed (DiedException r) -> say $ "it failed: " ++ (show r)

Unlike remote’s task layer, we do not exclude IO, allowing tasks to run in the Process monad and execute arbitrary code. Providing a monadic wrapper around Async that disallows side effects is relatively simple, and we do not consider the presence of side effects a barrier to fault tolerance and automated process restarts. Erlang does not forbid IO in its processes, and yet that doesn’t render supervision trees ineffective. They key is to provide a rich enough API that stateful processes can recognise whether or not they need to provide idempotent initialisation routines.

The utility of preventing side effects using the type system is, however, not to be sniffed at. A substrate of the ManagedProcess API is under development that provides a safe process abstraction in which side effect free computations can be embedded, whilst reaping the benefits of the framework.

Work is also underway to provide abstractions for managing asynchronous tasks at a higher level, focussing on workload distribution and load regulation.

The kinds of task that can be performed by the async implementations in distributed-process-async are limited only by their return type: it must be Serializable - that much should’ve been obvious by now. The type of asynchronous task definitions comes in two flavours, one for local nodes which require no remote-table or static serialisation dictionary, and another for tasks you wish to execute on remote nodes.

-- | A task to be performed asynchronously.
data AsyncTask a =
    AsyncTask
      {
        asyncTask :: Process a -- ^ the task to be performed
      }
  | AsyncRemoteTask
      {
        asyncTaskDict :: Static (SerializableDict a)
          -- ^ the serializable dict required to spawn a remote process
      , asyncTaskNode :: NodeId
          -- ^ the node on which to spawn the asynchronous task
      , asyncTaskProc :: Closure (Process a)
          -- ^ the task to be performed, wrapped in a closure environment
      }

The API for Async is fairly rich, so reading the haddocks is suggested.

Managed Processes

The main idea behind a ManagedProcess is to separate the functional and non-functional aspects of an actor. By functional, we mean whatever application specific task the actor performs, and by non-functional we mean the concurrency or, more precisely, handling of the process’ mailbox and its interaction with other actors (i.e., clients).

Looking at typed channels, we noted that their insistence on a specific input domain was more haskell-ish than working with bare send and receive primitives. The Async sub-package also provides a type safe interface for receiving data, although it is limited to running a computation and waiting for its result.

The Control.Distributed.Processes.Platform.ManagedProcess API provides a number of different abstractions that can be used to achieve similar benefits in your code. It works by introducing a standard protocol between your process and the world outside, which governs how to handle request/reply processing, exit signals, timeouts, sleeping/hibernation with threadDelay and even provides hooks that terminating processes can use to clean up residual state.

The API documentation is quite extensive, so here we will simply point out the obvious differences. A process implemented with ManagedProcess can present a type safe API to its callers (and the server side code too!), although that’s not its primary benefit. For a very simplified example:

add :: ProcessId -> Double -> Double -> Process Double
add sid x y = call sid (Add x y)

divide :: ProcessId -> Double -> Double
          -> Process (Either DivByZero Double)
divide sid x y = call sid (Divide x y )

launchMathServer :: Process ProcessId
launchMathServer =
  let server = statelessProcess {
      apiHandlers = [
          handleCall_   (\(Add    x y) -> return (x + y))
        , handleCallIf_ (input (\(Divide _ y) -> y /= 0)) handleDivide
        , handleCall_   (\(Divide _ _) -> divByZero)
        ]
    }
  in spawnLocal $ start () (statelessInit Infinity) server >> return ()
  where handleDivide :: Divide -> Process (Either DivByZero Double)
        handleDivide (Divide x y) = return $ Right $ x / y

        divByZero :: Process (Either DivByZero Double)
        divByZero = return $ Left DivByZero

Apart from the types and the imports, that is a complete definition. Whilst it’s not so obvious what’s going on here, the key point is that the invocation of call in the client facing API functions handles all of the relevant waiting/blocking, converting the async result and so on. Note that the managed process does not interact with its mailbox at all, but rather just provides callback functions which take some state and either return a new state and a reply, or just a new state. The process is managed in the sense that its mailbox is under someone else’s control.

A NOTE ABOUT THE CALL API AND THAT IT WILL FAIL (WITH UNHANDLED MESSAGE) IF THE CALLER IS EXPECTING A TYPE THAT DIFFERS FROM THE ONE THE SERVER PLANS TO RETURN, SINCE THE RETURN TYPE IS ENCODED IN THE CALL-MESSAGE TYPE ITSELF.

TODO: WRITE A TEST TO PROVE THE ABOVE

TODO: ADD AN API BASED ON SESSION TYPES AS A KIND OF MANAGED PROCESS…..

In a forthcoming tutorial, we’ll look at the Control.Distributed.Process.Platform.Task API, which looks a lot like Async but manages exit signals in a single thread and makes configurable task pools and task supervision strategy part of its API.

More complex examples of the ManagedProcess API can be seen in the Managed Processes tutorial. API documentation for HEAD is available here.

Supervision Trees

TBC

Process Groups

TBC