4This is an implementation of Cloud Haskell, as described in
5/Towards Haskell in the Cloud/ by Jeff Epstein, Andrew Black, and Simon
7<http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/>),
8although some of the details are different. The precise message passing
9semantics are based on /A unified semantics for future Erlang/ by Hans
10Svensson, Lars-Åke Fredlund and Clara Benac Earle.
12For a detailed description of the package and other reference materials,
13please see the distributed-process wiki page on github:
14<https://github.com/haskell-distributed/distributed-process/wiki>.
17module Control.Distributed.Process
25 , liftIO -- Reexported for convenience
41 -- * Unsafe messaging variants
46 -- * Advanced messaging
72 -- * Process management
81 , ProcessTerminationException(..)
82 , ProcessRegistrationException(..)
91 -- * Monitoring and linking
103 , MonitorRef -- opaque
104 , ProcessLinkException(..)
105 , NodeLinkException(..)
106 , PortLinkException(..)
107 , ProcessMonitorNotification(..)
108 , NodeMonitorNotification(..)
109 , PortMonitorNotification(..)
126 , registerRemoteAsync
127 , reregisterRemoteAsync
128 , unregisterRemoteAsync
133 -- * Exception handling
151 -- * Local versions of 'spawn'
160#if ! MIN_VERSION_base(4,6,0)
161import Prelude hiding (catch)
164import Control.Monad.IO.Class (liftIO)
165import Control.Applicative ((<$>))
166import Control.Monad.Reader (ask)
167import Control.Concurrent.MVar
173import Control.Distributed.Static
179import Control.Distributed.Process.Internal.Types
184 , ProcessMonitorNotification(..)
185 , NodeMonitorNotification(..)
186 , PortMonitorNotification(..)
187 , ProcessLinkException(..)
188 , NodeLinkException(..)
189 , PortLinkException(..)
190 , ProcessRegistrationException(..)
199 , LocalProcess(processNode)
202import Control.Distributed.Process.Serializable (Serializable)
203import Control.Distributed.Process.Internal.Primitives
217 -- Advanced messaging
243 -- Process management
245 , ProcessTerminationException(..)
258 -- Monitoring and linking
278 , registerRemoteAsync
279 , reregisterRemoteAsync
280 , unregisterRemoteAsync
286 -- Exception handling
305import Control.Distributed.Process.Node (forkProcess)
306import Control.Distributed.Process.Internal.Spawn
307 ( -- Spawning Processes/Channels
315import Control.Exception (SomeException, throw)
319-- 1. 'send' never fails. If you want to know that the remote process received
320-- your message, you will need to send an explicit acknowledgement. If you
321-- want to know when the remote process failed, you will need to monitor
322-- that remote process.
324-- 2. 'send' may block (when the system TCP buffers are full, while we are
325-- trying to establish a connection to the remote endpoint, etc.) but its
326-- return does not imply that the remote process received the message (much
327-- less processed it). When 'send' targets a local process, it is dispatched
328-- via the node controller however, in which cases it will /not/ block. This
329-- isn't part of the semantics, so it should be ok. The ordering is maintained
330-- because the ctrlChannel still has FIFO semantics with regards interactions
331-- between two disparate forkIO threads.
333-- 3. Message delivery is reliable and ordered. That means that if process A
334-- sends messages m1, m2, m3 to process B, B will either arrive all three
335-- messages in order (m1, m2, m3) or a prefix thereof; messages will not be
336-- 'missing' (m1, m3) or reordered (m1, m3, m2)
338-- In order to guarantee (3), we stipulate that
340-- 3a. We do not garbage collect connections because Network.Transport provides
341-- ordering guarantees only *per connection*.
343-- 3b. Once a connection breaks, we have no way of knowing which messages
344-- arrived and which did not; hence, once a connection fails, we assume the
345-- remote process to be forever unreachable. Otherwise we might sent m1 and
346-- m2, get notified of the broken connection, reconnect, send m3, but only
349-- 3c. As a consequence of (3b) we should not reuse PIDs. If a process dies,
350-- we consider it forever unreachable. Hence, new processes should get new
351-- IDs or they too would be considered unreachable.
353-- Main reference for Cloud Haskell is
355-- [1] "Towards Haskell in the Cloud", Jeff Epstein, Andrew Black and Simon
357-- http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/remote.pdf
359-- The precise semantics for message passing is based on
361-- [2] "A Unified Semantics for Future Erlang", Hans Svensson, Lars-Ake Fredlund
362-- and Clara Benac Earle (not freely available online, unfortunately)
364-- Some pointers to related documentation about Erlang, for comparison and
367-- [3] "Programming Distributed Erlang Applications: Pitfalls and Recipes",
368-- Hans Svensson and Lars-Ake Fredlund
369-- http://man.lupaworld.com/content/develop/p37-svensson.pdf
370-- [4] The Erlang manual, sections "Message Sending" and "Send"
371-- http://www.erlang.org/doc/reference_manual/processes.html#id82409
372-- http://www.erlang.org/doc/reference_manual/expressions.html#send
373-- [5] Questions "Is the order of message reception guaranteed?" and
374-- "If I send a message, is it guaranteed to reach the receiver?" of
376-- http://www.erlang.org/faq/academic.html
377-- [6] "Delivery of Messages", post on erlang-questions
378-- http://erlang.org/pipermail/erlang-questions/2012-February/064767.html
380--------------------------------------------------------------------------------
381-- Local versions of spawn --
382--------------------------------------------------------------------------------
384-- | Spawn a process on the local node
385spawnLocal :: Process () -> Process ProcessId
387 node <- processNode <$> ask
388 liftIO $ forkProcess node proc
390-- | Create a new typed channel, spawn a process on the local node, passing it
391-- the receive port, and return the send port
392spawnChannelLocal :: Serializable a
393 => (ReceivePort a -> Process ())
394 -> Process (SendPort a)
395spawnChannelLocal proc = do
396 node <- processNode <$> ask
399 _ <- forkProcess node $ do
400 -- It is important that we allocate the new channel in the new process,
401 -- because otherwise it will be associated with the wrong process ID
402 (sport, rport) <- newChan
403 liftIO $ putMVar mvar sport
407-- | Local version of 'call'. Running a process in this way isolates it from
408-- messages sent to the caller process, and also allows silently dropping late
409-- or duplicate messages sent to the isolated process after it exits.
410-- Silently dropping messages may not always be the best approach.
411callLocal :: Process a -> Process a
412callLocal proc = mask $ \release -> do
413 mv <- liftIO newEmptyMVar :: Process (MVar (Either SomeException a))
414 child <- spawnLocal $ try (release proc) >>= liftIO . putMVar mv
415 rs <- liftIO (takeMVar mv) `onException`
416 (kill child "exception in parent process" >> liftIO (takeMVar mv))
417 either throw return rs
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