distributed-process-client-server- The Cloud Haskell Application Platform

Copyright(c) Tim Watson 2012 - 2017
LicenseBSD3 (see the file LICENSE)
MaintainerTim Watson <watson.timothy@gmail.com>
Portabilitynon-portable (requires concurrency)
Safe HaskellNone




This module provides a high(er) level API for building complex Process implementations by abstracting out the management of the process' mailbox, reply/response handling, timeouts, process hiberation, error handling and shutdown/stop procedures. It is modelled along similar lines to OTP's gen_server API - http://www.erlang.org/doc/man/gen_server.html.

In particular, a managed process will interoperate cleanly with the supervisor API in distributed-process-supervision.

API Overview For The Impatient

Once started, a managed process will consume messages from its mailbox and pass them on to user defined handlers based on the types received (mapped to those accepted by the handlers) and optionally by also evaluating user supplied predicates to determine which handler(s) should run. Each handler returns a ProcessAction which specifies how we should proceed. If none of the handlers is able to process a message (because their types are incompatible), then the unhandledMessagePolicy will be applied.

The ProcessAction type defines the ways in which our process can respond to its inputs, whether by continuing to read incoming messages, setting an optional timeout, sleeping for a while, or stopping. The optional timeout behaves a little differently to the other process actions: If no messages are received within the specified time span, a user defined timeoutHandler will be called in order to determine the next action.

The ProcessDefinition type also defines a shutdownHandler, which is called whenever the process exits, whether because a callback has returned stop as the next action, or as the result of unhandled exit signal or similar asynchronous exceptions thrown in (or to) the process itself.

The handlers are split into groups: apiHandlers, infoHandlers, and extHandlers.

Seriously, TL;DR

Use serve for a process that sits reading its mailbox and generally behaves as you'd expect. Use pserve and PrioritisedProcessDefinition for a server that manages its mailbox more comprehensively and handles errors a bit differently. Both use the same client API.

DO NOT mask in handler code, unless you can guarantee it won't be long running and absolutely won't block kill signals from a supervisor.

Do look at the various API offerings, as there are several, at different levels of abstraction.

Managed Process Mailboxes

Managed processes come in two flavours, with different runtime characteristics and (to some extent) semantics. These flavours are differentiated by the way in which they handle the server process mailbox - all client interactions remain the same.

The vanilla managed process mailbox, provided by the serve API, is roughly akin to a tail recursive listen function that calls a list of passed in matchers. We might naively implement it roughly like this:

loop :: stateT -> [(stateT -> Message -> Maybe stateT)] -> Process ()
loop state handlers = do
  st2 <- receiveWait $ map (\d -> handleMessage (d state)) handlers
  case st2 of
    Nothing -> {- we're done serving -} return ()
    Just s2 -> loop s2 handlers

Obviously all the details have been ellided, but this is the essential premise behind a managed process loop. The process keeps reading from its mailbox indefinitely, until either a handler instructs it to stop, or an asynchronous exception (or exit signal - in the form of an async ProcessExitException) terminates it. This kind of mailbox has fairly intuitive runtime characteristics compared to a plain server process (i.e. one implemented without the use of this library): messages will pile up in its mailbox whilst handlers are running, and each handler will be checked against the mailbox based on the type of messages it recognises. We can potentially end up scanning a very large mailbox trying to match each handler, which can be a performance bottleneck depending on expected traffic patterns.

For most simple server processes, this technique works well and is easy to reason about a use. See the sections on error and exit handling later on for more details about serve based managed processes.

Prioritised Mailboxes

A prioritised mailbox serves two purposes. The first of these is to allow a managed process author to specify that certain classes of message should be prioritised by the server loop. This is achieved by draining the real process mailbox into an internal priority queue, and running the server's handlers repeatedly over its contents, which are dequeued in priority order. The obvious consequence of this approach leads to the second purpose (or the accidental side effect, depending on your point of view) of a prioritised mailbox, which is that we avoid scanning a large mailbox when searching for messages that match the handlers we anticipate running most frequently (or those messages that we deem most important).

There are several consequences to this approach. One is that we do quite a bit more work to manage the process mailbox behind the scenes, therefore we have additional space overhead to consider (although we are also reducing the size of the mailbox, so there is some counter balance here). The other is that if we do not see the anticipated traffic patterns at runtime, then we might spend more time attempting to prioritise infrequent messages than we would have done simply receiving them! We do however, gain a degree of safety with regards message loss that the serve based vanilla mailbox cannot offer. See the sections on error and exit handling later on for more details about these.

A Prioritised pserve loop maintains its internal state - including the user defined server state - in an IORef, ensuring it is held consistently between executions, even in the face of unhandled exceptions.

Defining Prioritised Process Definitions

A PrioritisedProcessDefintion combines the usual ProcessDefintion - containing the cast/call API, error, termination and info handlers - with a list of Priority entries, which are used at runtime to prioritise the server's inputs. Note that it is only messages which are prioritised; The server's various handlers are still evaluated in the order in which they are specified in the ProcessDefinition.

Prioritisation does not guarantee that a prioritised message/type will be processed before other traffic - indeed doing so in a multi-threaded runtime would be very hard - but in the absence of races between multiple processes, if two messages are both present in the process' own mailbox, they will be applied to the ProcessDefinition's handlers in priority order.

A prioritised process should probably be configured with a Priority list to be useful. Creating a prioritised process without any priorities could be a potential waste of computational resources, and it is worth thinking carefully about whether or not prioritisation is truly necessary in your design before choosing to use it.

Using a prioritised process is as simple as calling pserve instead of serve, and passing an initialised PrioritisedProcessDefinition.

The Cast and Call Protocols

Deliberate interactions with a managed process usually falls into one of two categories. A cast interaction involves a client sending a message asynchronously and the server handling this input. No reply is sent to the client. On the other hand, a call is a remote procedure call, where the client sends a message and waits for a reply from the server.

All expressions given to apiHandlers have to conform to the cast or call protocol. The protocol (messaging) implementation is hidden from the user; API functions for creating user defined apiHandlers are given instead, which take expressions (i.e., a function or lambda expression) and create the appropriate Dispatcher for handling the cast (or call).

These cast and call protocols are for dealing with expected inputs. They will usually form the explicit public API for the process, and be exposed by providing module level functions that defer to the cast or call client API, giving the process author an opportunity to enforce the correct input and response types. For example:

{- Ask the server to add two numbers -}
add :: ProcessId -> Double -> Double -> Double
add pid x y = call pid (Add x y)

Note here that the return type from the call is inferred and will not be enforced by the type system. If the server sent a different type back in the reply, then the caller might be blocked indefinitely! In fact, the result of mis-matching the expected return type (in the client facing API) with the actual type returned by the server is more severe in practise. The underlying types that implement the call protocol carry information about the expected return type. If there is a mismatch between the input and output types that the client API uses and those which the server declares it can handle, then the message will be considered unroutable - no handler will be executed against it and the unhandled message policy will be applied. You should, therefore, take great care to align these types since the default unhandled message policy is to terminate the server! That might seem pretty extreme, but you can alter the unhandled message policy and/or use the various overloaded versions of the call API in order to detect errors on the server such as this.

The cost of potential type mismatches between the client and server is the main disadvantage of this looser coupling between them. This mechanism does however, allow servers to handle a variety of messages without specifying the entire protocol to be supported in excruciating detail. For that, we would want session types, which are beyond the scope of this library.

Handling Unexpected/Info Messages

An explicit protocol for communicating with the process can be configured using cast and call, but it is not possible to prevent other kinds of messages from being sent to the process mailbox. When any message arrives for which there are no handlers able to process its content, the UnhandledMessagePolicy will be applied. Sometimes it is desirable to process incoming messages which aren't part of the protocol, rather than let the policy deal with them. This is particularly true when incoming messages are important to the process, but their point of origin is outside the author's control. Handling signals such as ProcessMonitorNotification is a typical example of this:

handleInfo_ (\(ProcessMonitorNotification _ _ r) -> say $ show r >> continue_)
Handling Process State

The ProcessDefinition is parameterised by the type of state it maintains. A process that has no state will have the type ProcessDefinition () and can be bootstrapped by evaluating statelessProcess.

All call/cast handlers come in two flavours, those which take the process state as an input and those which do not. Handlers that ignore the process state have to return a function that takes the state and returns the required action. Versions of the various action generating functions ending in an underscore are provided to simplify this:

  statelessProcess {
      apiHandlers = [
        handleCall_   (\(n :: Int) -> return (n * 2))
      , handleCastIf_ (\(c :: String, _ :: Delay) -> c == "timeout")
                      (\("timeout", (d :: Delay)) -> timeoutAfter_ d)
    , timeoutHandler = \_ _ -> stop $ ExitOther "timeout"
Avoiding Side Effects

If you wish to only write side-effect free code in your server definition, then there is an explicit API for doing so. Instead of using the handler definition functions in this module, import the pure server module instead, which provides a StateT based monad for building referentially transparent callbacks.

See Control.Distributed.Process.ManagedProcess.Server.Restricted for details and API documentation.

Handling Errors

Error handling appears in several contexts and process definitions can hook into these with relative ease. Catching exceptions inside handle functions is no different to ordinary exception handling in monadic code.

  handleCall (\x y ->
               catch (hereBeDragons x y)
                     (\(e :: SmaugTheTerribleException) ->
                          return (Left (show e))))

The caveats mentioned in Control.Distributed.Process.Extras about exit signal handling are very important here - it is strongly advised that you do not catch exceptions of type ProcessExitException unless you plan to re-throw them again.

Structured Exit Handling

Because Control.Distributed.Process.ProcessExitException is a ubiquitous signalling mechanism in Cloud Haskell, it is treated unlike other asynchronous exceptions. The ProcessDefinition exitHandlers field accepts a list of handlers that, for a specific exit reason, can decide how the process should respond. If none of these handlers matches the type of reason then the process will exit. with DiedException why. In addition, a private exit handler is installed for exit signals where (reason :: ExitReason) == ExitShutdown, which is an of exit signal used explicitly by supervision APIs. This behaviour, which cannot be overriden, is to gracefully shut down the process, calling the shutdownHandler as usual, before stopping with reason given as the final outcome.

Example: handling custom data is ProcessExitException

handleExit  (\state from (sigExit :: SomeExitData) -> continue s)

Under some circumstances, handling exit signals is perfectly legitimate. Handling of other forms of asynchronous exception (e.g., exceptions not generated by an exit signal) is not supported by this API. Cloud Haskell's primitives for exception handling will work normally in managed process callbacks, but you are strongly advised against swallowing exceptions in general, or masking, unless you have carefully considered the consequences.

Different Mailbox Types and Exceptions: Message Loss

Neither the vanilla nor the prioritised mailbox implementations will allow you to handle arbitrary asynchronous exceptions outside of your handler code. The way in which the two mailboxes handle unexpected asynchronous exceptions differs significantly however. The first consideration pertains to potential message loss.

Consider a plain Cloud Haskell expression such as the following:

  catch (receiveWait [ match ((m :: SomeType) -> doSomething m) ])
        ((e :: SomeCustomAsyncException) -> handleExFrom e pid)

It is entirely possible that receiveWait will succeed in matching a message of type SomeType from the mailbox and removing it, to be handed to the supplied expression doSomething. Should an asynchronous exception arrive at this moment in time, though the handler might run and allow the server to recover, the message will be permanently lost.

The mailbox exposed by serve operates in exactly this way, and as such it is advisible to avoid swallowing asynchronous exceptions, since doing so can introduce the possibility of unexpected message loss.

The prioritised mailbox exposed by pserve on the other hand, does not suffer this scenario. Whilst the mailbox is drained into the internal priority queue, asynchronous exceptions are masked, and only once the queue has been updated are they removed. In addition, it is possible to peek at the priority queue without removing a message, thereby ensuring that should the handler fail or an asynchronous exception arrive whilst processing the message, we can resume handling our message immediately upon recovering from the exception. This behaviour allows the process to guarantee against message loss, whilst avoiding masking within handlers, which is generally bad form (and can potentially lead to zombie processes, when supervised servers refuse to respond to kill signals whilst stuck in a long running handler).

Also note that a process' internal state is subject to the same semantics, such that the arrival of an asynchronous exception (including exit signals!) can lead to handlers (especially exit and shutdown handlers) running with a stale version of their state. For this reason - since we cannot guarantee an up to date state in the presence of these semantics - a shutdown handler for a serve loop will always have its state passed as LastKnown stateT.

Different Mailbox Types and Exceptions: Error Recovery And Shutdown

If any asynchronous exception goes unhandled by a vanilla process, the server will immediately exit without running the user supplied shutdownHandler. It is very important to note that in Cloud Haskell, link failures generate asynchronous exceptions in the target and these will NOT be caught by the serve API and will therefore cause the process to exit /without running the termination handler/ callback. If your termination handler is set up to do important work (such as resource cleanup) then you should avoid linking you process and use monitors instead. If your code absolutely must run its termination handlers in the face of any unhandled (async) exception, consider using a prioritised mailbox, which handles this. Alternatively, consider arranging your processes in a supervision tree, and using a shutdown strategy to ensure that siblings terminate cleanly (based off a supervisor's ordered shutdown signal) in order to ensure cleanup code can run reliably.

As mentioned above, a prioritised mailbox behaves differently in the face of unhandled asynchronous exceptions. Whilst pserve still offers no means for handling arbitrary async exceptions outside your handlers - and you should avoid handling them within, to the maximum extent possible - it does execute its receiving process in such a way that any unhandled exception will be caught and rethrown. Because of this, and the fact that a prioritised process manages its internal state in an IORef, shutdown handlers are guaranteed to run even in the face of async exceptions. These are run with the latest version of the server state available, given as CleanShutdown stateT when the process is terminating normally (i.e. for reasons ExitNormal or ExitShutdown), and LastKnown stateT when an exception terminated the server process abruptly. The latter acknowledges that we cannot guarantee the exception did not interrupt us after the last handler ran and returned an updated state, but prior to storing the update.

Although shutdown handlers are run even in the face of unhandled exceptions (and prior to re-throwing, when there is one present), they are not run in a masked state. In fact, exceptions are explicitly unmasked prior to executing a handler, therefore it is possible for a shutdown handler to terminate abruptly. Once again, supervision hierarchies are a better way to ensure consistent cleanup occurs when valued resources are held by a process.

Filters, pre-processing, and safe handlers

A prioritised process can take advantage of filters, which enable the server to pre-process messages, reject them (based on the message itself, or the server's state), and mark classes of message as requiring safe handling.

Assuming a PrioritisedProcessDefinition that holds its state as an Int, here are some simple applications of filters:

  let rejectUnchecked =
       rejectApi Foo :: Int -> P.Message String String -> Process (Filter Int)

    filters = [
      store  (+1)
    , ensure (>0)

    , check $ api_ (\(s :: String) -> return $ "checked-" `isInfixOf` s) rejectUnchecked
    , check $ info (\_ (_ :: MonitorRef, _ :: ProcessId) -> return False) $ reject Foo
    , refuse ((> 10) :: Int -> Bool)

We can store/update our state, ensure our state is in a valid condition, check api and info messages, and refuse messages using simple predicates. Messages cannot be modified by filters, not can reply data.

A safe filter is a means to instruct the prioritised managed process loop not to dequeue the current message from the internal priority queue until a handler has successfully matched and run against it (without an exception, either synchronous or asynchronous) to completion. Messages marked thus, will remain in the priority queue even in the face of exit signals, which means that if the server process code handles and swallows them, it will begin re-processing the last message a second time.

It is important to recognise that the safe filter does not act like a transaction. There are no checkpoints, nor facilities for rolling back actions on failure. If an exit signal terminates a handler for a message marked as safe and an exit handler catches and swallows it, the handler (and all prior filters too) will be re-run in its entireity.

Special Clients: Control Channels

For advanced users and those requiring very low latency, a prioritised process definition might not be suitable, since it performs considerable work behind the scenes. There are also designs that need to segregate a process' control plane from other kinds of traffic it is expected to receive. For such use cases, a control channel may prove a better choice, since typed channels are already prioritised during the mailbox scans that the base receiveWait and receiveTimeout primitives from distribute-process provides.

In order to utilise a control channel in a server, it must be passed to the corresponding handleControlChan function (or its stateless variant). The control channel is created by evaluating newControlChan, in the same way that we create regular typed channels.

In order for clients to communicate with a server via its control channel however, they must pass a handle to a ControlPort, which can be obtained by evaluating channelControlPort on the ControlChannel. A ControlPort is Serializable, so they can alternatively be sent to other processes.

Control channel traffic will only be prioritised over other traffic if the handlers using it are present before others (e.g., handleInfo, handleCast, etc) in the process definition. It is not possible to combine prioritised processes with control channels. Attempting to do so will satisfy the compiler, but crash with a runtime error once you attempt to evaluate the prioritised server loop (i.e., pserve).

Since the primary purpose of control channels is to simplify and optimise client-server communication over a single channel, this module provides an alternate server loop in the form of chanServe. Instead of passing an initialised ProcessDefinition, this API takes an expression from a ControlChannel to ProcessDefinition, operating in the Process monad. Providing the opaque reference in this fashion is useful, since the type of messages the control channel carries will not correlate directly to the inter-process traffic we use internally.

Although control channels are intended for use as a single control plane (via chanServe), it is possible to use them as a more strictly typed communications backbone, since they do enforce absolute type safety in client code, being bound to a particular type on creation. For rpc (i.e., call) interaction however, it is not possible to have the server reply to a control channel, since they're a one way pipe. It is possible to alleviate this situation by passing a request type than contains a typed channel bound to the expected reply type, enabling client and server to match on both the input and output types as specifically as possible. Note that this still does not guarantee an agreement on types between all parties at runtime however.

An example of how to do this follows:

data Request = Request String (SendPort String)
  deriving (Typeable, Generic)
instance Binary Request where

-- note that our initial caller needs an mvar to obtain the control port...
echoServer :: MVar (ControlPort Request) -> Process ()
echoServer mv = do
  cc <- newControlChan :: Process (ControlChannel Request)
  liftIO $ putMVar mv $ channelControlPort cc
  let s = statelessProcess {
      apiHandlers = [
           handleControlChan_ cc (\(Request m sp) -> sendChan sp m >> continue_)
  serve () (statelessInit Infinity) s

echoClient :: String -> ControlPort Request -> Process String
echoClient str cp = do
  (sp, rp) <- newChan
  sendControlMessage cp $ Request str sp
  receiveChan rp
Communicating with the outside world: External (STM) Input Channels

Both client and server APIs provide a mechanism for interacting with a running server process via STM. This is primarily intended for code that runs outside of Cloud Haskell's Process monad, but can also be used as a channel for sending and/or receiving non-serializable data to or from a managed process. Obviously if you attempt to do this across a remote boundary, things will go spectacularly wrong. The APIs provided do not attempt to restrain this, or to impose any particular scheme on the programmer, therefore you're on your own when it comes to writing the STM code for reading and writing data between client and server.

For code running inside the Process monad and passing Serializable thunks, there is no real advantage to this approach, and indeed there are several serious disadvantages - none of Cloud Haskell's ordering guarantees will hold when passing data to and from server processes in this fashion, nor are there any guarantees the runtime system can make with regards interleaving between messages passed across Cloud Haskell's communication fabric vs. data shared via STM. This is true even when client(s) and server(s) reside on the same local node.

A server wishing to receive data via STM can do so using the handleExternal API. By way of example, here is a simple echo server implemented using STM:

demoExternal = do
  inChan <- liftIO newTQueueIO
  replyQ <- liftIO newTQueueIO
  let procDef = statelessProcess {
                    apiHandlers = [
                        (readTQueue inChan)
                        (\s (m :: String) -> do
                            liftIO $ atomically $ writeTQueue replyQ m
                            continue s)
  let txt = "hello 2-way stm foo"
  pid <- spawnLocal $ serve () (statelessInit Infinity) procDef
  echoTxt <- liftIO $ do
    -- firstly we write something that the server can receive
    atomically $ writeTQueue inChan txt
    -- then sit and wait for it to write something back to us
    atomically $ readTQueue replyQ

  say (show $ echoTxt == txt)

For request/reply channels such as this, a convenience based on the call API is also provided, which allows the server author to write an ordinary call handler, and the client author to utilise an API that monitors the server and does the usual stuff you'd expect an RPC style client to do. Here is another example of this in use, demonstrating the callSTM and handleCallExternal APIs in practise.

data StmServer = StmServer { serverPid  :: ProcessId
                           , writerChan :: TQueue String
                           , readerChan :: TQueue String

instance Resolvable StmServer where
  resolve = return . Just . serverPid

echoStm :: StmServer -> String -> Process (Either ExitReason String)
echoStm StmServer{..} = callSTM serverPid
                                (writeTQueue writerChan)
                                (readTQueue  readerChan)

launchEchoServer :: CallHandler () String String -> Process StmServer
launchEchoServer handler = do
  (inQ, replyQ) <- liftIO $ do
    cIn <- newTQueueIO
    cOut <- newTQueueIO
    return (cIn, cOut)

  let procDef = statelessProcess {
                  apiHandlers = [
                      (readTQueue inQ)
                      (writeTQueue replyQ)

  pid <- spawnLocal $ serve () (statelessInit Infinity) procDef
  return $ StmServer pid inQ replyQ

testExternalCall :: TestResult Bool -> Process ()
testExternalCall result = do
  let txt = "hello stm-call foo"
  srv <- launchEchoServer (\st (msg :: String) -> reply msg st)
  echoStm srv txt >>= stash result . (== Right txt)
Performance Considerations

The various server loops are fairly optimised, but there is a definite cost associated with scanning the mailbox to match on protocol messages, plus additional costs in space and time due to mapping over all available info handlers for non-protocol (i.e., neither call nor cast) messages. These are exacerbated significantly when using prioritisation, whilst using a single control channel is very fast and carries little overhead.

From the client perspective, it's important to remember that the call protocol will wait for a reply in most cases, triggering a full O(n) scan of the caller's mailbox. If the mailbox is extremely full and calls are regularly made, this may have a significant impact on the caller. The callChan family of client API functions can alleviate this, by using (and matching on) a private typed channel instead, but the server must be written to accomodate this. Similar gains can be had using a control channel and providing a typed reply channel in the request data, however the call mechanism does not support this notion, so not only are we unable to use the various reply functions, client code should also consider monitoring the server's pid and handling server failures whilst waiting on


Starting/Running server processes

data InitResult s Source #

Return type for and InitHandler expression.

type InitHandler a s = a -> Process (InitResult s) Source #

An expression used to initialise a process with its state

serve :: a -> InitHandler a s -> ProcessDefinition s -> Process () Source #

Starts the message handling loop for a managed process configured with the supplied process definition, after calling the init handler with its initial arguments. Note that this function does not return until the server exits.

pserve :: a -> InitHandler a s -> PrioritisedProcessDefinition s -> Process () Source #

Starts the message handling loop for a prioritised managed process, configured with the supplied process definition, after calling the init handler with its initial arguments. Note that this function does not return until the server exits.

chanServe :: Serializable b => a -> InitHandler a s -> (ControlChannel b -> Process (ProcessDefinition s)) -> Process () Source #

Starts the message handling loop for a managed process, configured with a typed control channel. The caller supplied expression is evaluated with an opaque reference to the channel, which must be passed when calling handleControlChan. The meaning and behaviour of the init handler and initial arguments are the same as those given to serve. Note that this function does not return until the server exits.

runProcess :: (s -> Delay -> Process ExitReason) -> a -> InitHandler a s -> Process () Source #

Wraps any process loop and ensures that it adheres to the managed process start/stop semantics, i.e., evaluating the InitHandler with an initial state and delay will either die due to InitStop, exit silently (due to InitIgnore) or evaluate the process' loop. The supplied loop must evaluate to ExitNormal, otherwise the calling processing will die with whatever ExitReason is given.

prioritised :: ProcessDefinition s -> [DispatchPriority s] -> PrioritisedProcessDefinition s Source #

Turns a standard ProcessDefinition into a PrioritisedProcessDefinition, by virtue of the supplied list of DispatchPriority expressions.

Client interactions

Defining server processes

data ProcessDefinition s Source #

Stores the functions that determine runtime behaviour in response to incoming messages and a policy for responding to unhandled messages.




data PrioritisedProcessDefinition s Source #

A ProcessDefinition decorated with DispatchPriority for certain input domains.

data RecvTimeoutPolicy Source #

For a PrioritisedProcessDefinition, this policy determines for how long the receive loop should continue draining the process' mailbox before processing its received mail (in priority order).

If a prioritised managed process is receiving a lot of messages (into its real mailbox), the server might never get around to actually processing its inputs. This (mandatory) policy provides a guarantee that eventually (i.e., after a specified number of received messages or time interval), the server will stop removing messages from its mailbox and process those it has already received.

data Priority a Source #

Priority of a message, encoded as an Int

data DispatchPriority s Source #

Dispatcher for prioritised handlers

type ShutdownHandler s = ExitState s -> ExitReason -> Process () Source #

An expression used to handle process termination

type TimeoutHandler s = ActionHandler s Delay Source #

An expression used to handle process timeouts

data Condition s m Source #

Wraps a predicate that is used to determine whether or not a handler is valid based on some combination of the current process state, the type and/or value of the input message or both.

type Action s = Process (ProcessAction s) Source #

An action (server state transition) in the Process monad

data ProcessAction s Source #

The action taken by a process after a handler has run and its updated state. See "Control.Distributed.Process.ManagedProcess.Server.continue" "Control.Distributed.Process.ManagedProcess.Server.timeoutAfter" "Control.Distributed.Process.ManagedProcess.Server.hibernate" "Control.Distributed.Process.ManagedProcess.Server.stop" "Control.Distributed.Process.ManagedProcess.Server.stopWith"

Also see "Control.Distributed.Process.Management.Priority.act" and "Control.Distributed.Process.ManagedProcess.Priority.runAfter".

And other actions. This type should not be used directly.

type Reply b s = Process (ProcessReply b s) Source #

An action (server state transition) causing a reply to a caller, in the Process monad

data ProcessReply r s Source #

Returned from handlers for the synchronous call protocol, encapsulates the reply data and the action to take after sending the reply. A handler can return NoReply if they wish to ignore the call.

type ActionHandler s a = s -> a -> Action s Source #

An expression used to handle a message

type CallHandler s a b = s -> a -> Reply b s Source #

An expression used to handle a message and providing a reply

type CastHandler s a = ActionHandler s a Source #

An expression used to handle a cast message

type StatelessHandler s a = a -> s -> Action s Source #

An expression used to ignore server state during handling

type DeferredCallHandler s a b = CallRef b -> CallHandler s a b Source #

An expression used to handle a call message where the reply is deferred via the CallRef

type StatelessCallHandler s a b = CallRef b -> a -> Reply b s Source #

An expression used to handle a call message ignoring server state

type InfoHandler s a = ActionHandler s a Source #

An expression used to handle an info message

type ChannelHandler s a b = SendPort b -> ActionHandler s a Source #

An expression used to handle a channel message

type StatelessChannelHandler s a b = SendPort b -> StatelessHandler s a Source #

An expression used to handle a channel message in a stateless process

data UnhandledMessagePolicy Source #

Policy for handling unexpected messages, i.e., messages which are not sent using the call or cast APIs, and which are not handled by any of the handleInfo handlers.



stop immediately, giving ExitOther UnhandledInput as the reason

DeadLetter ProcessId

forward the message to the given recipient


log messages, then behave identically to Drop


dequeue and then drop/ignore the message

data CallRef a Source #

Wraps a consumer of the call API


Eq (CallRef a) Source # 


(==) :: CallRef a -> CallRef a -> Bool #

(/=) :: CallRef a -> CallRef a -> Bool #

Show (CallRef a) Source # 


showsPrec :: Int -> CallRef a -> ShowS #

show :: CallRef a -> String #

showList :: [CallRef a] -> ShowS #

Generic (CallRef a) Source # 

Associated Types

type Rep (CallRef a) :: * -> * #


from :: CallRef a -> Rep (CallRef a) x #

to :: Rep (CallRef a) x -> CallRef a #

Binary (CallRef a) Source # 


put :: CallRef a -> Put #

get :: Get (CallRef a) #

putList :: [CallRef a] -> Put #

NFData (CallRef a) Source # 


rnf :: CallRef a -> () #

Resolvable (CallRef a) Source # 
Routable (CallRef a) Source # 


sendTo :: (Serializable m, Resolvable (CallRef a)) => CallRef a -> m -> Process () #

unsafeSendTo :: (NFSerializable m, Resolvable (CallRef a)) => CallRef a -> m -> Process () #

type Rep (CallRef a) Source # 
type Rep (CallRef a) = D1 * (MetaData "CallRef" "Control.Distributed.Process.ManagedProcess.Internal.Types" "distributed-process-client-server-" True) (C1 * (MetaCons "CallRef" PrefixI True) (S1 * (MetaSel (Just Symbol "unCaller") NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 * (Recipient, CallId))))

data ExitState s Source #

Informs a shutdown handler of whether it is running due to a clean shutdown, or in response to an unhandled exception.


CleanShutdown s

given when an ordered shutdown is underway

LastKnown s 

isCleanShutdown :: ExitState s -> Bool Source #

True if the ExitState is CleanShutdown, otherwise False.

exitState :: ExitState s -> s Source #

Evaluates to the s state datum in the given ExitState.

defaultProcess :: ProcessDefinition s Source #

A default ProcessDefinition, with no api, info or exit handler. The default timeoutHandler simply continues, the shutdownHandler is a no-op and the unhandledMessagePolicy is Terminate.

statelessProcess :: ProcessDefinition () Source #

A basic, stateless ProcessDefinition. See defaultProcess for the default field values.

statelessInit :: Delay -> InitHandler () () Source #

A default, state unaware InitHandler that can be used with statelessProcess. This simply returns InitOk with the empty state (i.e., unit) and the given Delay.

Control channels

data ControlChannel m Source #

Provides a means for servers to listen on a separate, typed control channel, thereby segregating the channel from their regular (and potentially busy) mailbox.

data ControlPort m Source #

The writable end of a ControlChannel.

channelControlPort :: ControlChannel m -> ControlPort m Source #

Obtain an opaque expression for communicating with a ControlChannel.

Server side callbacks

Prioritised mailboxes

Low level and internal APIs & Process implementation