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
71 -- * Process management
80 , ProcessTerminationException(..)
81 , ProcessRegistrationException(..)
90 -- * Monitoring and linking
102 , MonitorRef -- opaque
103 , ProcessLinkException(..)
104 , NodeLinkException(..)
105 , PortLinkException(..)
106 , ProcessMonitorNotification(..)
107 , NodeMonitorNotification(..)
108 , PortMonitorNotification(..)
125 , registerRemoteAsync
126 , reregisterRemoteAsync
127 , unregisterRemoteAsync
132 -- * Exception handling
150 -- * Local versions of 'spawn'
159#if ! MIN_VERSION_base(4,6,0)
160import Prelude hiding (catch)
163import Control.Monad.IO.Class (liftIO)
164import Control.Applicative ((<$>))
165import Control.Monad.Reader (ask)
166import Control.Concurrent.MVar
172import Control.Distributed.Static
178import Control.Distributed.Process.Internal.Types
183 , ProcessMonitorNotification(..)
184 , NodeMonitorNotification(..)
185 , PortMonitorNotification(..)
186 , ProcessLinkException(..)
187 , NodeLinkException(..)
188 , PortLinkException(..)
189 , ProcessRegistrationException(..)
198 , LocalProcess(processNode)
201import Control.Distributed.Process.Serializable (Serializable)
202import Control.Distributed.Process.Internal.Primitives
216 -- Advanced messaging
241 -- Process management
243 , ProcessTerminationException(..)
256 -- Monitoring and linking
276 , registerRemoteAsync
277 , reregisterRemoteAsync
278 , unregisterRemoteAsync
284 -- Exception handling
303import Control.Distributed.Process.Node (forkProcess)
304import Control.Distributed.Process.Internal.Spawn
305 ( -- Spawning Processes/Channels
313import Control.Exception (SomeException, throw)
317-- 1. 'send' never fails. If you want to know that the remote process received
318-- your message, you will need to send an explicit acknowledgement. If you
319-- want to know when the remote process failed, you will need to monitor
320-- that remote process.
322-- 2. 'send' may block (when the system TCP buffers are full, while we are
323-- trying to establish a connection to the remote endpoint, etc.) but its
324-- return does not imply that the remote process received the message (much
325-- less processed it). When 'send' targets a local process, it is dispatched
326-- via the node controller however, in which cases it will /not/ block. This
327-- isn't part of the semantics, so it should be ok. The ordering is maintained
328-- because the ctrlChannel still has FIFO semantics with regards interactions
329-- between two disparate forkIO threads.
331-- 3. Message delivery is reliable and ordered. That means that if process A
332-- sends messages m1, m2, m3 to process B, B will either arrive all three
333-- messages in order (m1, m2, m3) or a prefix thereof; messages will not be
334-- 'missing' (m1, m3) or reordered (m1, m3, m2)
336-- In order to guarantee (3), we stipulate that
338-- 3a. We do not garbage collect connections because Network.Transport provides
339-- ordering guarantees only *per connection*.
341-- 3b. Once a connection breaks, we have no way of knowing which messages
342-- arrived and which did not; hence, once a connection fails, we assume the
343-- remote process to be forever unreachable. Otherwise we might sent m1 and
344-- m2, get notified of the broken connection, reconnect, send m3, but only
347-- 3c. As a consequence of (3b) we should not reuse PIDs. If a process dies,
348-- we consider it forever unreachable. Hence, new processes should get new
349-- IDs or they too would be considered unreachable.
351-- Main reference for Cloud Haskell is
353-- [1] "Towards Haskell in the Cloud", Jeff Epstein, Andrew Black and Simon
355-- http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/remote.pdf
357-- The precise semantics for message passing is based on
359-- [2] "A Unified Semantics for Future Erlang", Hans Svensson, Lars-Ake Fredlund
360-- and Clara Benac Earle (not freely available online, unfortunately)
362-- Some pointers to related documentation about Erlang, for comparison and
365-- [3] "Programming Distributed Erlang Applications: Pitfalls and Recipes",
366-- Hans Svensson and Lars-Ake Fredlund
367-- http://man.lupaworld.com/content/develop/p37-svensson.pdf
368-- [4] The Erlang manual, sections "Message Sending" and "Send"
369-- http://www.erlang.org/doc/reference_manual/processes.html#id82409
370-- http://www.erlang.org/doc/reference_manual/expressions.html#send
371-- [5] Questions "Is the order of message reception guaranteed?" and
372-- "If I send a message, is it guaranteed to reach the receiver?" of
374-- http://www.erlang.org/faq/academic.html
375-- [6] "Delivery of Messages", post on erlang-questions
376-- http://erlang.org/pipermail/erlang-questions/2012-February/064767.html
378--------------------------------------------------------------------------------
379-- Local versions of spawn --
380--------------------------------------------------------------------------------
382-- | Spawn a process on the local node
383spawnLocal :: Process () -> Process ProcessId
385 node <- processNode <$> ask
386 liftIO $ forkProcess node proc
388-- | Create a new typed channel, spawn a process on the local node, passing it
389-- the receive port, and return the send port
390spawnChannelLocal :: Serializable a
391 => (ReceivePort a -> Process ())
392 -> Process (SendPort a)
393spawnChannelLocal proc = do
394 node <- processNode <$> ask
397 _ <- forkProcess node $ do
398 -- It is important that we allocate the new channel in the new process,
399 -- because otherwise it will be associated with the wrong process ID
400 (sport, rport) <- newChan
401 liftIO $ putMVar mvar sport
405-- | Local version of 'call'. Running a process in this way isolates it from
406-- messages sent to the caller process, and also allows silently dropping late
407-- or duplicate messages sent to the isolated process after it exits.
408-- Silently dropping messages may not always be the best approach.
409callLocal :: Process a -> Process a
410callLocal proc = mask $ \release -> do
411 mv <- liftIO newEmptyMVar :: Process (MVar (Either SomeException a))
412 child <- spawnLocal $ try (release proc) >>= liftIO . putMVar mv
413 rs <- liftIO (takeMVar mv) `onException`
414 (kill child "exception in parent process" >> liftIO (takeMVar mv))
415 either throw return rs
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