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
48 -- * Advanced messaging
74 -- * Process management
83 , ProcessTerminationException(..)
84 , ProcessRegistrationException(..)
93 -- * Monitoring and linking
105 , MonitorRef -- opaque
106 , ProcessLinkException(..)
107 , NodeLinkException(..)
108 , PortLinkException(..)
109 , ProcessMonitorNotification(..)
110 , NodeMonitorNotification(..)
111 , PortMonitorNotification(..)
128 , registerRemoteAsync
129 , reregisterRemoteAsync
130 , unregisterRemoteAsync
135 -- * Exception handling
153 -- * Local versions of 'spawn'
162import Control.Monad.IO.Class (liftIO)
163import Control.Applicative
164import Control.Monad.Reader (ask)
165import Control.Concurrent (killThread)
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)
202import Control.Distributed.Process.Serializable (Serializable)
203import 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.Types
310 , withValidLocalState
312import Control.Distributed.Process.Internal.Spawn
313 ( -- Spawning Processes/Channels
321import qualified Control.Monad.Catch as Catch
323#if MIN_VERSION_base(4,6,0)
326import Prelude hiding (catch)
328import qualified Control.Exception as Exception (onException)
329import Data.Accessor ((^.))
330import Data.Foldable (forM_)
335-- 1. 'send' never fails. If you want to know that the remote process received
336-- your message, you will need to send an explicit acknowledgement. If you
337-- want to know when the remote process failed, you will need to monitor
338-- that remote process.
340-- 2. 'send' may block (when the system TCP buffers are full, while we are
341-- trying to establish a connection to the remote endpoint, etc.) but its
342-- return does not imply that the remote process received the message (much
343-- less processed it). When 'send' targets a local process, it is dispatched
344-- via the node controller however, in which cases it will /not/ block. This
345-- isn't part of the semantics, so it should be ok. The ordering is maintained
346-- because the ctrlChannel still has FIFO semantics with regards interactions
347-- between two disparate forkIO threads.
349-- 3. Message delivery is reliable and ordered. That means that if process A
350-- sends messages m1, m2, m3 to process B, B will either arrive all three
351-- messages in order (m1, m2, m3) or a prefix thereof; messages will not be
352-- 'missing' (m1, m3) or reordered (m1, m3, m2)
354-- In order to guarantee (3), we stipulate that
356-- 3a. We do not garbage collect connections because Network.Transport provides
357-- ordering guarantees only *per connection*.
359-- 3b. Once a connection breaks, we have no way of knowing which messages
360-- arrived and which did not; hence, once a connection fails, we assume the
361-- remote process to be forever unreachable. Otherwise we might sent m1 and
362-- m2, get notified of the broken connection, reconnect, send m3, but only
365-- 3c. As a consequence of (3b) we should not reuse PIDs. If a process dies,
366-- we consider it forever unreachable. Hence, new processes should get new
367-- IDs or they too would be considered unreachable.
369-- Main reference for Cloud Haskell is
371-- [1] "Towards Haskell in the Cloud", Jeff Epstein, Andrew Black and Simon
373-- http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/remote.pdf
375-- The precise semantics for message passing is based on
377-- [2] "A Unified Semantics for Future Erlang", Hans Svensson, Lars-Ake Fredlund
378-- and Clara Benac Earle (not freely available online, unfortunately)
380-- Some pointers to related documentation about Erlang, for comparison and
383-- [3] "Programming Distributed Erlang Applications: Pitfalls and Recipes",
384-- Hans Svensson and Lars-Ake Fredlund
385-- http://man.lupaworld.com/content/develop/p37-svensson.pdf
386-- [4] The Erlang manual, sections "Message Sending" and "Send"
387-- http://www.erlang.org/doc/reference_manual/processes.html#id82409
388-- http://www.erlang.org/doc/reference_manual/expressions.html#send
389-- [5] Questions "Is the order of message reception guaranteed?" and
390-- "If I send a message, is it guaranteed to reach the receiver?" of
392-- http://www.erlang.org/faq/academic.html
393-- [6] "Delivery of Messages", post on erlang-questions
394-- http://erlang.org/pipermail/erlang-questions/2012-February/064767.html
396--------------------------------------------------------------------------------
397-- Local versions of spawn --
398--------------------------------------------------------------------------------
400-- | Spawn a process on the local node
401spawnLocal :: Process () -> Process ProcessId
403 node <- processNode <$> ask
404 liftIO $ forkProcess node proc
406-- | Create a new typed channel, spawn a process on the local node, passing it
407-- the receive port, and return the send port
408spawnChannelLocal :: Serializable a
409 => (ReceivePort a -> Process ())
410 -> Process (SendPort a)
411spawnChannelLocal proc = do
412 node <- processNode <$> ask
415 _ <- forkProcess node $ do
416 -- It is important that we allocate the new channel in the new process,
417 -- because otherwise it will be associated with the wrong process ID
418 (sport, rport) <- newChan
419 liftIO $ putMVar mvar sport
423-- | Local version of 'call'. Running a process in this way isolates it from
424-- messages sent to the caller process, and also allows silently dropping late
425-- or duplicate messages sent to the isolated process after it exits.
426-- Silently dropping messages may not always be the best approach.
427callLocal :: Process a -> Process a
428callLocal proc = Catch.mask $ \release -> do
429 mv <- liftIO newEmptyMVar :: Process (MVar (Either Catch.SomeException a))
430 child <- spawnLocal $ Catch.try (release proc) >>= liftIO . putMVar mv
433 rs <- Exception.onException (takeMVar mv) $ Catch.uninterruptibleMask_ $
434 -- Exceptions need to be prevented from interrupting the clean up or
435 -- the original exception which caused entering the handler could be
436 -- forgotten. For instance, this could have a problematic effect
437 -- when the original exception was meant to kill the thread and the
438 -- second exception doesn't (like the exception thrown by
439 -- 'System.Timeout.timeout').
440 do mchildThreadId <- withValidLocalState (processNode lproc) $
441 \vst -> return $ fmap processThread $
442 vst ^. localProcessWithId (processLocalId child)
443 forM_ mchildThreadId killThread
445 either Catch.throwM return rs
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