Control.Distributed.Process.Internal.Primitives

Cloud Haskell primitives

We define these in a separate module so that we don't have to rely on the closure combinators

Basic messaging

send :: Serializable a => ProcessId -> a -> Process ()

Send a message

expect :: forall a. Serializable a => Process a

Wait for a message of a specific type

Channels

newChan :: Serializable a => Process (SendPort a, ReceivePort a)

Create a new typed channel

sendChan :: Serializable a => SendPort a -> a -> Process ()

Send a message on a typed channel

receiveChan :: Serializable a => ReceivePort a -> Process a

Wait for a message on a typed channel

mergePortsBiased :: Serializable a => [ReceivePort a] -> Process (ReceivePort a)

Merge a list of typed channels.

The result port is left-biased: if there are messages available on more than one port, the first available message is returned.

mergePortsRR :: Serializable a => [ReceivePort a] -> Process (ReceivePort a)

Like mergePortsBiased, but with a round-robin scheduler (rather than left-biased)

Unsafe messaging variants

unsafeSend :: Serializable a => ProcessId -> a -> Process ()

Unsafe variant of send. This function makes no attempt to serialize and (in the case when the destination process resides on the same local node) therefore ensure that the payload is fully evaluated before it is delivered.

unsafeSendChan :: Serializable a => SendPort a -> a -> Process ()

Send a message on a typed channel. This function makes no attempt to serialize and (in the case when the ReceivePort resides on the same local node) therefore ensure that the payload is fully evaluated before it is delivered.

unsafeNSend :: Serializable a => String -> a -> Process ()

Named send to a process in the local registry (asynchronous). This function makes no attempt to serialize and (in the case when the destination process resides on the same local node) therefore ensure that the payload is fully evaluated before it is delivered.

Advanced messaging

data Match b

Opaque type used in receiveWait and receiveTimeout

receiveWait :: [Match b] -> Process b

Test the matches in order against each message in the queue

receiveTimeout :: Int -> [Match b] -> Process (Maybe b)

Like receiveWait but with a timeout.

If the timeout is zero do a non-blocking check for matching messages. A non-zero timeout is applied only when waiting for incoming messages (that is, after we have checked the messages that are already in the mailbox).

match :: forall a b. Serializable a => (a -> Process b) -> Match b

Match against any message of the right type

matchIf :: forall a b. Serializable a => (a -> Bool) -> (a -> Process b) -> Match b

Match against any message of the right type that satisfies a predicate

matchUnknown :: Process b -> Match b

Remove any message from the queue

matchAny :: forall b. (Message -> Process b) -> Match b

Match against an arbitrary message. matchAny removes the first available message from the process mailbox. To handle arbitrary raw messages once removed from the mailbox, see handleMessage and unwrapMessage.

matchAnyIf :: forall a b. Serializable a => (a -> Bool) -> (Message -> Process b) -> Match b

Match against an arbitrary message. Intended for use with handleMessage and unwrapMessage, this function only removes a message from the process mailbox, if the supplied condition matches. The success (or failure) of runtime type checks deferred to handleMessage and friends is irrelevant here, i.e., if the condition evaluates to True then the message will be removed from the process mailbox and decoded, but that does not guarantee that an expression passed to handleMessage will pass the runtime type checks and therefore be evaluated.

matchChan :: ReceivePort a -> (a -> Process b) -> Match b

Match on a typed channel

matchSTM :: STM a -> (a -> Process b) -> Match b

Match on an arbitrary STM action.

This rather unusaul match primitive allows us to compose arbitrary STM actions with checks against our process' mailbox and/or any typed channel ReceivePorts we may hold.

This allows us to process multiple input streams along with our mailbox, in just the same way that matchChan supports checking both the mailbox and an arbitrary set of typed channels in one atomic transaction.

Note there are no ordering guarnatees with respect to these disparate input sources.

matchMessage :: (Message -> Process Message) -> Match Message

Match against any message, regardless of the underlying (contained) type

matchMessageIf :: (Message -> Bool) -> (Message -> Process Message) -> Match Message

Match against any message (regardless of underlying type) that satisfies a predicate

isEncoded :: Message -> Bool

internal use only.

wrapMessage :: Serializable a => a -> Message

Wrap a Serializable value in a Message. Note that Messages are Serializable - like the datum they contain - but also note, deserialising such a Message will yield a Message, not the type within it! To obtain the wrapped datum, use unwrapMessage or handleMessage with a specific type.

do
   self <- getSelfPid
   send self (wrapMessage "blah")
   Nothing  <- expectTimeout 1000000 :: Process (Maybe String)
   (Just m) <- expectTimeout 1000000 :: Process (Maybe Message)
   (Just "blah") <- unwrapMessage m :: Process (Maybe String)

unsafeWrapMessage :: Serializable a => a -> Message

This is the unsafe variant of wrapMessage. See Control.Distributed.Process.UnsafePrimitives for details.

unwrapMessage :: forall m a. (Monad m, Serializable a) => Message -> m (Maybe a)

Attempt to unwrap a raw Message. If the type of the decoded message payload matches the expected type, the value will be returned with Just, otherwise Nothing indicates the types do not match.

This expression, for example, will evaluate to Nothing > unwrapMessage (wrapMessage "foobar") :: Process (Maybe Int)

Whereas this expression, will yield Just "foo" > unwrapMessage (wrapMessage "foo") :: Process (Maybe String)

handleMessage :: forall m a b. (Monad m, Serializable a) => Message -> (a -> m b) -> m (Maybe b)

Attempt to handle a raw Message. If the type of the message matches the type of the first argument to the supplied expression, then the message will be decoded and the expression evaluated against its value. If this runtime type checking fails however, Nothing will be returned to indicate the fact. If the check succeeds and evaluation proceeds, the resulting value with be wrapped with Just.

Intended for use in catchesExit and matchAny primitives.

handleMessageIf :: forall m a b. (Monad m, Serializable a) => Message -> (a -> Bool) -> (a -> m b) -> m (Maybe b)

Conditionally handle a raw Message. If the predicate (a -> Bool) evaluates to True, invokes the supplied handler, other returns Nothing to indicate failure. See handleMessage for further information about runtime type checking.

handleMessage_ :: forall m a. (Monad m, Serializable a) => Message -> (a -> m ()) -> m ()

As handleMessage but ignores result, which is useful if you don't care whether or not the handler succeeded.

handleMessageIf_ :: forall m a. (Monad m, Serializable a) => Message -> (a -> Bool) -> (a -> m ()) -> m ()

Conditional version of handleMessage_.

forward :: Message -> ProcessId -> Process ()

Forward a raw Message to the given ProcessId.

delegate :: ProcessId -> (Message -> Bool) -> Process ()

Receives messages and forwards them to pid if p msg == True.

relay :: ProcessId -> Process ()

A straight relay that forwards all messages to the supplied pid.

proxy :: Serializable a => ProcessId -> (a -> Process Bool) -> Process ()

Proxies pid and forwards messages whenever proc evaluates to True. Unlike delegate the predicate proc runs in the Process monad, allowing for richer proxy behaviour. If proc returns False or the runtime type check fails, no action is taken and the proxy process will continue running.

Process management

terminate :: Process a

Terminate immediately (throws a ProcessTerminationException)

data ProcessTerminationException

Thrown by terminate

Constructors

ProcessTerminationException

Instances

Show ProcessTerminationException

Exception ProcessTerminationException

Typeable * ProcessTerminationException

die :: Serializable a => a -> Process b

Die immediately - throws a ProcessExitException with the given reason.

kill :: ProcessId -> String -> Process ()

Forceful request to kill a process. Where exit provides an exception that can be caught and handled, kill throws an unexposed exception type which cannot be handled explicitly (by type).

exit :: Serializable a => ProcessId -> a -> Process ()

Graceful request to exit a process. Throws ProcessExitException with the supplied reason encoded as a message. Any exit signal raised in this manner can be handled using the catchExit family of functions.

catchExit :: forall a b. (Show a, Serializable a) => Process b -> (ProcessId -> a -> Process b) -> Process b

Catches ProcessExitException. The handler will not be applied unless its type matches the encoded data stored in the exception (see the reason argument given to the exit primitive). If the handler cannot be applied, the exception will be re-thrown.

To handle ProcessExitException without regard for reason, see catch. To handle multiple reasons of differing types, see catchesExit.

catchesExit :: Process b -> [ProcessId -> Message -> Process (Maybe b)] -> Process b

Lift catches (almost).

As ProcessExitException stores the exit reason as a typed, encoded message, a handler must accept inputs of the expected type. In order to handle a list of potentially different handlers (and therefore input types), a handler passed to catchesExit must accept Message and return Maybe (i.e., Just p if it handled the exit reason, otherwise Nothing).

See maybeHandleMessage and Message for more details.

data ProcessExitException

Internal exception thrown indirectly by exit

Instances

Show ProcessExitException

Exception ProcessExitException

Typeable * ProcessExitException

getSelfPid :: Process ProcessId

Our own process ID

getSelfNode :: Process NodeId

Get the node ID of our local node

data ProcessInfo

Provide information about a running process

Constructors

ProcessInfo

Fields

infoNode :: NodeId

infoRegisteredNames :: [String]

infoMessageQueueLength :: Maybe Int

infoMonitors :: [(ProcessId, MonitorRef)]

infoLinks :: [ProcessId]

Instances

Eq ProcessInfo

Show ProcessInfo

Binary ProcessInfo

Typeable * ProcessInfo

getProcessInfo :: ProcessId -> Process (Maybe ProcessInfo)

Get information about the specified process

data NodeStats

Constructors

NodeStats

Fields

nodeStatsNode :: NodeId

nodeStatsRegisteredNames :: Int

nodeStatsMonitors :: Int

nodeStatsLinks :: Int

nodeStatsProcesses :: Int

Instances

Eq NodeStats

Show NodeStats

Binary NodeStats

Typeable * NodeStats

getNodeStats :: NodeId -> Process (Either DiedReason NodeStats)

Get statistics about the specified node

getLocalNodeStats :: Process NodeStats

Get statistics about our local node

Monitoring and linking

link :: ProcessId -> Process ()

Link to a remote process (asynchronous)

When process A links to process B (that is, process A calls link pidB) then an asynchronous exception will be thrown to process A when process B terminates (normally or abnormally), or when process A gets disconnected from process B. Although it is technically possible to catch these exceptions, chances are if you find yourself trying to do so you should probably be using monitor rather than link. In particular, code such as

link pidB   -- Link to process B
expect      -- Wait for a message from process B
unlink pidB -- Unlink again

doesn't quite do what one might expect: if process B sends a message to process A, and subsequently terminates, then process A might or might not be terminated too, depending on whether the exception is thrown before or after the unlink (i.e., this code has a race condition).

Linking is all-or-nothing: A is either linked to B, or it's not. A second call to link has no effect.

Note that link provides unidirectional linking (see spawnSupervised). Linking makes no distinction between normal and abnormal termination of the remote process.

unlink :: ProcessId -> Process ()

Remove a link

This is synchronous in the sense that once it returns you are guaranteed that no exception will be raised if the remote process dies. However, it is asynchronous in the sense that we do not wait for a response from the remote node.

monitor :: ProcessId -> Process MonitorRef

Monitor another process (asynchronous)

When process A monitors process B (that is, process A calls monitor pidB) then process A will receive a ProcessMonitorNotification when process B terminates (normally or abnormally), or when process A gets disconnected from process B. You receive this message like any other (using expect); the notification includes a reason (DiedNormal, DiedException, DiedDisconnect, etc.).

Every call to monitor returns a new monitor reference MonitorRef; if multiple monitors are set up, multiple notifications will be delivered and monitors can be disabled individually using unmonitor.

unmonitor :: MonitorRef -> Process ()

Remove a monitor

This has the same synchronous/asynchronous nature as unlink.

withMonitor :: ProcessId -> Process a -> Process a

Establishes temporary monitoring of another process.

withMonitor pid code sets up monitoring of pid for the duration of code. Note: although monitoring is no longer active when withMonitor returns, there might still be unreceived monitor messages in the queue.

Logging

say :: String -> Process ()

Log a string

say message sends a message (time, pid of the current process, message) to the process registered as logger. By default, this process simply sends the string to stderr. Individual Cloud Haskell backends might replace this with a different logger process, however.

Registry

register :: String -> ProcessId -> Process ()

Register a process with the local registry (synchronous). The name must not already be registered. The process need not be on this node. A bad registration will result in a ProcessRegistrationException

The process to be registered does not have to be local itself.

reregister :: String -> ProcessId -> Process ()

Like register, but will replace an existing registration. The name must already be registered.

unregister :: String -> Process ()

Remove a process from the local registry (asynchronous). This version will wait until a response is gotten from the management process. The name must already be registered.

whereis :: String -> Process (Maybe ProcessId)

Query the local process registry

nsend :: Serializable a => String -> a -> Process ()

Named send to a process in the local registry (asynchronous)

registerRemoteAsync :: NodeId -> String -> ProcessId -> Process ()

Register a process with a remote registry (asynchronous).

The process to be registered does not have to live on the same remote node. Reply wil come in the form of a RegisterReply message

See comments in whereisRemoteAsync

reregisterRemoteAsync :: NodeId -> String -> ProcessId -> Process ()

unregisterRemoteAsync :: NodeId -> String -> Process ()

Remove a process from a remote registry (asynchronous).

Reply wil come in the form of a RegisterReply message

See comments in whereisRemoteAsync

whereisRemoteAsync :: NodeId -> String -> Process ()

Query a remote process registry (asynchronous)

Reply will come in the form of a WhereIsReply message.

There is currently no synchronous version of whereisRemoteAsync: if you implement one yourself, be sure to take into account that the remote node might die or get disconnect before it can respond (i.e. you should use monitorNode and take appropriate action when you receive a NodeMonitorNotification).

nsendRemote :: Serializable a => NodeId -> String -> a -> Process ()

Named send to a process in a remote registry (asynchronous)

Closures

unClosure :: Typeable a => Closure a -> Process a

Resolve a closure

unStatic :: Typeable a => Static a -> Process a

Resolve a static value

Exception handling

catch :: Exception e => Process a -> (e -> Process a) -> Process a

Lift catch

data Handler a

You need this when using catches

Constructors

forall e . Exception e => Handler (e -> Process a)

Instances

Functor Handler

catches :: Process a -> [Handler a] -> Process a

Lift catches

try :: Exception e => Process a -> Process (Either e a)

Lift try

mask :: ((forall a. Process a -> Process a) -> Process b) -> Process b

Lift mask

mask_ :: Process a -> Process a

Lift mask_

onException :: Process a -> Process b -> Process a

Lift onException

bracket :: Process a -> (a -> Process b) -> (a -> Process c) -> Process c

Lift bracket

bracket_ :: Process a -> Process b -> Process c -> Process c

Lift bracket_

finally :: Process a -> Process b -> Process a

Lift finally

Auxiliary API

expectTimeout :: forall a. Serializable a => Int -> Process (Maybe a)

Like expect but with a timeout

receiveChanTimeout :: Serializable a => Int -> ReceivePort a -> Process (Maybe a)

Like receiveChan but with a timeout. If the timeout is 0, do a non-blocking check for a message.

spawnAsync :: NodeId -> Closure (Process ()) -> Process SpawnRef

Asynchronous version of spawn

(spawn is defined in terms of spawnAsync and expect)

linkNode :: NodeId -> Process ()

Link to a node (asynchronous)

linkPort :: SendPort a -> Process ()

Link to a channel (asynchronous)

unlinkNode :: NodeId -> Process ()

Remove a node link

This has the same synchronous/asynchronous nature as unlink.

unlinkPort :: SendPort a -> Process ()

Remove a channel (send port) link

This has the same synchronous/asynchronous nature as unlink.

monitorNode :: NodeId -> Process MonitorRef

Monitor a node (asynchronous)

monitorPort :: forall a. Serializable a => SendPort a -> Process MonitorRef

Monitor a typed channel (asynchronous)

Reconnecting

reconnect :: ProcessId -> Process ()

Cloud Haskell provides the illusion of connection-less, reliable, ordered message passing. However, when network connections get disrupted this illusion cannot always be maintained. Once a network connection breaks (even temporarily) no further communication on that connection will be possible. For example, if process A sends a message to process B, and A is then notified (by monitor notification) that it got disconnected from B, A will not be able to send any further messages to B, unless A explicitly indicates that it is acceptable to attempt to reconnect to B using the Cloud Haskell reconnect primitive.

Importantly, when A calls reconnect it acknowledges that some messages to B might have been lost. For instance, if A sends messages m1 and m2 to B, then receives a monitor notification that its connection to B has been lost, calls reconnect and then sends m3, it is possible that B will receive m1 and m3 but not m2.

Note that reconnect does not mean reconnect now but rather /it is okay to attempt to reconnect on the next send/. In particular, if no further communication attempts are made to B then A can use reconnect to clean up its connection to B.

reconnectPort :: SendPort a -> Process ()

Reconnect to a sendport. See reconnect for more information.

Internal Exports

sendCtrlMsg

Arguments

:: Maybe NodeId

Nothing for the local node

-> ProcessSignal

Message to send

-> Process ()