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
47 -- * Advanced messaging
73 -- * Process management
82 , ProcessTerminationException(..)
83 , ProcessRegistrationException(..)
92 -- * Monitoring and linking
104 , MonitorRef -- opaque
105 , ProcessLinkException(..)
106 , NodeLinkException(..)
107 , PortLinkException(..)
108 , ProcessMonitorNotification(..)
109 , NodeMonitorNotification(..)
110 , PortMonitorNotification(..)
127 , registerRemoteAsync
128 , reregisterRemoteAsync
129 , unregisterRemoteAsync
134 -- * Exception handling
152 -- * Local versions of 'spawn'
161#if ! MIN_VERSION_base(4,6,0)
162import Prelude hiding (catch)
165import Control.Monad.IO.Class (liftIO)
166import Control.Applicative ((<$>))
167import Control.Monad.Reader (ask)
168import Control.Concurrent.MVar
174import Control.Distributed.Static
180import Control.Distributed.Process.Internal.Types
185 , ProcessMonitorNotification(..)
186 , NodeMonitorNotification(..)
187 , PortMonitorNotification(..)
188 , ProcessLinkException(..)
189 , NodeLinkException(..)
190 , PortLinkException(..)
191 , ProcessRegistrationException(..)
200 , LocalProcess(processNode)
203import Control.Distributed.Process.Serializable (Serializable)
204import Control.Distributed.Process.Internal.Primitives
219 -- Advanced messaging
245 -- Process management
247 , ProcessTerminationException(..)
260 -- Monitoring and linking
280 , registerRemoteAsync
281 , reregisterRemoteAsync
282 , unregisterRemoteAsync
288 -- Exception handling
307import Control.Distributed.Process.Node (forkProcess)
308import Control.Distributed.Process.Internal.Spawn
309 ( -- Spawning Processes/Channels
317import Control.Exception (SomeException, throw)
321-- 1. 'send' never fails. If you want to know that the remote process received
322-- your message, you will need to send an explicit acknowledgement. If you
323-- want to know when the remote process failed, you will need to monitor
324-- that remote process.
326-- 2. 'send' may block (when the system TCP buffers are full, while we are
327-- trying to establish a connection to the remote endpoint, etc.) but its
328-- return does not imply that the remote process received the message (much
329-- less processed it). When 'send' targets a local process, it is dispatched
330-- via the node controller however, in which cases it will /not/ block. This
331-- isn't part of the semantics, so it should be ok. The ordering is maintained
332-- because the ctrlChannel still has FIFO semantics with regards interactions
333-- between two disparate forkIO threads.
335-- 3. Message delivery is reliable and ordered. That means that if process A
336-- sends messages m1, m2, m3 to process B, B will either arrive all three
337-- messages in order (m1, m2, m3) or a prefix thereof; messages will not be
338-- 'missing' (m1, m3) or reordered (m1, m3, m2)
340-- In order to guarantee (3), we stipulate that
342-- 3a. We do not garbage collect connections because Network.Transport provides
343-- ordering guarantees only *per connection*.
345-- 3b. Once a connection breaks, we have no way of knowing which messages
346-- arrived and which did not; hence, once a connection fails, we assume the
347-- remote process to be forever unreachable. Otherwise we might sent m1 and
348-- m2, get notified of the broken connection, reconnect, send m3, but only
351-- 3c. As a consequence of (3b) we should not reuse PIDs. If a process dies,
352-- we consider it forever unreachable. Hence, new processes should get new
353-- IDs or they too would be considered unreachable.
355-- Main reference for Cloud Haskell is
357-- [1] "Towards Haskell in the Cloud", Jeff Epstein, Andrew Black and Simon
359-- http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/remote.pdf
361-- The precise semantics for message passing is based on
363-- [2] "A Unified Semantics for Future Erlang", Hans Svensson, Lars-Ake Fredlund
364-- and Clara Benac Earle (not freely available online, unfortunately)
366-- Some pointers to related documentation about Erlang, for comparison and
369-- [3] "Programming Distributed Erlang Applications: Pitfalls and Recipes",
370-- Hans Svensson and Lars-Ake Fredlund
371-- http://man.lupaworld.com/content/develop/p37-svensson.pdf
372-- [4] The Erlang manual, sections "Message Sending" and "Send"
373-- http://www.erlang.org/doc/reference_manual/processes.html#id82409
374-- http://www.erlang.org/doc/reference_manual/expressions.html#send
375-- [5] Questions "Is the order of message reception guaranteed?" and
376-- "If I send a message, is it guaranteed to reach the receiver?" of
378-- http://www.erlang.org/faq/academic.html
379-- [6] "Delivery of Messages", post on erlang-questions
380-- http://erlang.org/pipermail/erlang-questions/2012-February/064767.html
382--------------------------------------------------------------------------------
383-- Local versions of spawn --
384--------------------------------------------------------------------------------
386-- | Spawn a process on the local node
387spawnLocal :: Process () -> Process ProcessId
389 node <- processNode <$> ask
390 liftIO $ forkProcess node proc
392-- | Create a new typed channel, spawn a process on the local node, passing it
393-- the receive port, and return the send port
394spawnChannelLocal :: Serializable a
395 => (ReceivePort a -> Process ())
396 -> Process (SendPort a)
397spawnChannelLocal proc = do
398 node <- processNode <$> ask
401 _ <- forkProcess node $ do
402 -- It is important that we allocate the new channel in the new process,
403 -- because otherwise it will be associated with the wrong process ID
404 (sport, rport) <- newChan
405 liftIO $ putMVar mvar sport
409-- | Local version of 'call'. Running a process in this way isolates it from
410-- messages sent to the caller process, and also allows silently dropping late
411-- or duplicate messages sent to the isolated process after it exits.
412-- Silently dropping messages may not always be the best approach.
413callLocal :: Process a -> Process a
414callLocal proc = mask $ \release -> do
415 mv <- liftIO newEmptyMVar :: Process (MVar (Either SomeException a))
416 child <- spawnLocal $ try (release proc) >>= liftIO . putMVar mv
417 rs <- liftIO (takeMVar mv) `onException`
418 (kill child "exception in parent process" >> liftIO (takeMVar mv))
419 either throw return rs
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