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Imagine you have one wire and you want to send a message from a machine A to another machine B, and wait for an answer from B (a "request/response transaction" in the following). Now imagine you have many threads on A which can make this kind of transaction "simultaneously", then you have to be cautious, otherwise you might get the wrong responses for your transactions. One solution is to serialize the transactions: the threads on A will have to wait for their turn to make their request/response transaction.

Another solution is to map a thread requesting a transaction on A, to a thread handling it on B. This allows for several transactions to be in flight at the same time on the same wire. This is what this module does: on A lives a multiplexer and threads call its transaction() method to initiate the request/response. On B, process_transaction() of a demultiplexer will get called by worker threads. All this in a multi-threaded way on a single wire. The module takes care of managing the interleaving requests/responses.

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# Author: David Decotigny, Oct 1, 2008
#  @brief Multiplexer for parallel transactions over a single data
#  channel.  This is like a pipe on which we provide a multithreaded
#  request/response messaging system. This system allows multiple
#  threads to issue several requests in parallel: they are treated in
#  parallel on the receiving side and the responses are sent back to
#  their respective requesting thread. The exceptions are correctly
#  transferred: the _trace member of the exception object will
#  indicate the traceback (as text).
#
#  The basic synopsis is:
#    - call Mux::transaction(*args, **kwds) from a "sender" thread
#    - the transaction is sent to the Demux via the channel
#    - DeMux::process_transaction(*args, **kwds) gets called by another thread,
#      on the other side of the channel (in general in
#      another process/machine)
#    - the result/exception of process_transaction() is sent back to the
#      sender via the channel
#    - Mux::transaction() on the sender returns or raises the exception
#      raised by DeMux::process_transaction()
#
#  The user code of this module has to override the
#  DeMux::process_transaction() method for the whole system to be
#  useful.
#
#  This module makes the following assumptions on the channel used to
#  transmit the requests/responses:
#  - the channel is bidirectional: both parties can send and receive data on it
#  - the channel can transmit arbitrary serializable python objects
#  - the channel consists of 2 endpoints having the same API: one
#    endpoint for the Multiplexer, one endpoint for the DeMultiplexer
#  - the endpoints of the channel have the following methods:
#      fileno(): return a file descriptor suitable for select/poll of data
#                ready to be received in non-blocking mode (at least for the
#                first byte)
#      send(data): send the given python data to the receiving party
#      data = recv(): wait for python data from the sending party and return it
#      close(): close the endpoint in both send/receive directions
#  - send() is multithread-safe
#
#  To achieve parallel handling of "simultaneous" requests, the
#  demultiplexer handles each request in a separate thread: either the
#  threads are created on demand (nworkers = None), or a pool of
#  pre-allocated threads is used (nworkers = integer). To manage the
#  interleaving of the transactions, each transaction has its own ID,
#  the "xid".

import sys, os, threading, Queue, itertools, traceback, select, struct
import cPickle as pickle # Only for SimpleChannelEndpoint

__all__ = ["Mux", "DeMux", "ChannelPair"]


## Magic token to mark the end of job submission by the DeMux
SENTINEL = "QUIT"
def is_sentinel(obj):
    """Predicate True when a DeMux worker thread receives a
    "terminate" order from the DeMux"""
    return type(obj) is str and obj == SENTINEL


class ReceiverThread(threading.Thread):
    """Generic wrapper class to wait for data from a channel:
    handle_message() is called for each data received. Provides a
    stop() method to stop receiving the data. This is a thread
    object: call start() to start it"""
    def __init__(self, channel, *args, **kwds):
        """
        \param channel is a Channel endpoint (fileno/recv/close
        methods expected)
        """
        threading.Thread.__init__(self, *args, **kwds)
        self._channel = channel
        self.__terms  = os.pipe()

        self._recv = channel.recv
        self._send = channel.send

    def run(self):
        """
        Wait for either a call to stop() or for a data to be available
        on the channel and then call handle_message. And loop over.
        """
        # Initialize poll()
        fd        = self._channel.fileno()
        waitset   = select.poll()
        eventmask = select.POLLIN | select.POLLERR \
                    | select.POLLHUP | select.POLLPRI
        waitset.register(fd, eventmask)
        waitset.register(self.__terms[0], eventmask)

        while 1:
            exit_loop = False
            for fd_, evt in waitset.poll():
                if fd_ != fd:
                    # Received sthg on the __terms pipe
                    exit_loop = True
                    break
                if evt != select.POLLIN:
                    # Receive something on the channel, but not a normal
                    # data (probably a HUP)
                    exit_loop = True
                    break

            if exit_loop:
                break

            # Error while receiving => term thread
            data = self._recv()

            # Call handle_message (dump the exceptions, but ignore them)
            try:
                self.handle_message(data)
            except:
                traceback.print_exc()
        # End while

    def handle_message(self, message):
        """Method to override: called each time a message is received"""
        raise NotImplementedError("Children classes expected to override it")

    def stop(self):
        """Stop receiving data. Waits until the thread is
        terminated. DO NOT CALL THIS from inside handle_message()"""
        os.write(self.__terms[1], "TERMINATION")
        self._channel.close()
        self.join()


class Mux(ReceiverThread):
    """Thread that multiplexes calls to the transaction() method on
    the given channel"""
    
    def __init__(self, channel):
        """
        \param channel is a Channel endpoint (fileno/recv/close
        methods expected)
        """
        ReceiverThread.__init__(self, channel)
        self.__lock  = threading.Lock()
        self.__waitq = dict()
        self.__idgen = itertools.count(42)

    def transaction(self, *args, **kwds):
        """Call this method to send the given args on the wire and
        wait for a response"""
        evt = threading.Event(self.__lock)

        # Allocate a transaction ID
        self.__lock.acquire()
        try:
            xid = self.__idgen.next()
            assert xid not in self.__waitq
            self.__waitq[xid] = [evt, None] # If except: means MUX stopped
        except AttributeError:
            raise EOFError("MUX has been stopped.")
        finally:
            self.__lock.release()

        # Send the request
        self._send((xid, args, kwds))

        # Wait for the answer
        evt.wait()

        # Return the answer/raise the exception to the caller
        self.__lock.acquire()
        try:
            # Retrieve the result
            try:
                result = self.__waitq[xid][1]
            except (AttributeError, IndexError):
                raise EOFError("MUX has been stopped.")
            except:
                print "EX", self.__waitq

            # Work done
            del self.__waitq[xid]

            # Reformat the result
            xid_, result_ = result
            assert xid_ == xid, \
                   "Expected txn id %s != received (%s)" % (xid, xid_)
            
            status, details = result_
            if status == "OK":
                return details
            elif status == "EXCEPTION":
                raise details
            else:
                raise RuntimeError("Invalid status %s !" % repr(status))
            return result
        finally:
            self.__lock.release()

    def run(self):
        """Listen to the messages coming from the endpoint and
        dispatch them to the threads which sent them"""
        try:
            ReceiverThread.run(self)
        except:
            traceback.print_exc()
        
        # If we're here, it means that a stop has been requested:
        # unblock _all_ the waiting caller threads and force them
        # to fail in transaction()
        self.__lock.acquire()
        try:
            for xid, slot in self.__waitq.iteritems():
                del slot[1] # Force IndexError on the waiting threads
                slot[0].set()
            del self.__waitq # Force AttributeError on next transaction()
        finally:
            self.__lock.release()

    def handle_message(self, msg):
        """Needed by the ReceiverThread object: dispatch the messages
        to the caller threads"""
        xid, result = msg
        self.__lock.acquire()
        try:
            slot = self.__waitq[xid]
            slot[1] = msg
            slot[0].set() # wake up the caller thread
        finally:
            self.__lock.release()


class DeMux(ReceiverThread):
    """Thread that demultiplexes transactions coming from a
    multiplexer, and calls process_transaction() for each of them. The
    transactions are processed in parallel in different worker
    threads. The worker threads are either consisting in a pool of
    threads (when nworkers is not None), or are created on-demand when
    requests arrive (when nworkers is None)"""
    
    __lock     = None # Lock object
    __workq    = None # Queue object or None (in on-demand mode)
    __nworkers = None # Specified size of the pool of threads
    __workers  = None # Either a list of threads (pool) or a dict xid->thread
                      # (in on-demand mode)
    
    def __init__(self, channel, nworkers = None):
        """
        \param channel is a Channel endpoint (fileno/recv/close
        methods expected)
        \param nworkers (integer) number of threads in the pool able
        to process the transaction requests, or None when threads have
        to be created on demand
        """
        ReceiverThread.__init__(self, channel)
        self.__nworkers = nworkers
        self.__lock     = threading.Lock()
        if nworkers is not None:
            self.__workers = []
            self.__workq   = Queue.Queue()
            for idworker in range(nworkers):
                thr = threading.Thread(target=self._pool_work)
                self.__workers.append(thr)
                thr.start()
        else:
            self.__workers = dict()

    def handle_message(self, msg):
        """Required by ReceiverThread"""
        xid, args, kwds = msg
        if self.__nworkers is not None:
            # In pool mode: send the job to the pool
            self.__workq.put((xid, args, kwds))
        else:
            # In on-demand mode: spawn a new thread to do the job
            thr = threading.Thread(target=self._do_process_transaction,
                                   args=(xid,)+args, kwargs=kwds)

            # Register the thread for this transaction
            self.__lock.acquire()
            try:
                self.__workers[xid] = thr
            finally:
                self.__lock.release()

            try:
                thr.start()
            except:
                # Oops, cannot start worker...
                self.__lock.acquire()
                try:
                    del self.__workers[xid]
                finally:
                    self.__lock.release()

                # Sending exception back to sender
                ex = sys.exc_info()[1]
                if ex is not None:
                    ex._trace = traceback.format_exc()
                else:
                    ex = sys.exc_info()[0]
                self._send((xid, ("EXCEPTION", ex)))
                
    def _pool_work(self):
        """Method run by the pool worker threads in pool mode"""
        while 1:
            # Simply consume the jobs from the queue until we get the
            # sentinel token
            data = self.__workq.get()
            if is_sentinel(data):
                break

            xid, args, kwds = data
            # Will raise exception ONLY when connection problems:
            self._do_process_transaction(xid, *args, **kwds)

    def _do_process_transaction(self, xid, *args, **kwds):
        """Method run by the worker threads to process one transaction"""
        # Call process_transaction and prepare the result to send
        result = None
        try:
            result = ("OK", self.process_transaction(*args, **kwds))
        except Exception, ex:
            ex._trace = traceback.format_exc
            result = ("EXCEPTION", ex)
        except:
            ex = sys.exc_info()[1]
            if ex is not None:
                ex._trace = traceback.format_exc()
            else:
                ex = sys.exc_info()[0]
            result = ("EXCEPTION", ex)
        finally:
            if result is None:
                ex = RuntimeError("Unexpected error !")
                result = ("EXCEPTION", ex)

        # Send response
        self._send((xid, result))

        # Unregister the thread in on-demand mode
        if self.__nworkers is None:
            self.__lock.acquire()
            try:
                # In on-demand mode: unregister the thread for this transaction
                del self.__workers[xid]
            finally:
                self.__lock.release()

    def process_transaction(self, *args, **kwds):
        """Implement this method in order to generate a response from
        the given transaction arguments"""
        raise NotImplementedError("Children must implement this method")

    def stop(self):
        """Stop the worker threads and close the channel"""
        ReceiverThread.stop(self)

        #
        # No lock because the listening thread is dead already (no new
        # thread)
        #

        # Clearing job queue
        if self.__workq is not None:
            while 1:
                try:
                    self.__workq.get_nowait()
                except Queue.Empty:
                    break

        # Stopping workers
        if self.__nworkers is not None:
            for i in range(self.__nworkers):
                self.__workq.put(SENTINEL)
            for thr in self.__workers:
                thr.join()
        else:
            while self.__workers:
                xid, thr = self.__workers.popitem()
                thr.join()


class SimpleChannelEndpoint:
    """Construct a channel compliant with the channel specifications
    from a pair of r/w file descriptors"""
    SZI = struct.calcsize('I')
    
    def __init__(self, fd_r, fd_w):
        """
        \param r,w The read-write file descriptors used for this endpoint
        """
        self._fd_r  = fd_r
        self._fd_w  = fd_w
        self._wlock = threading.Lock() # send() has to be thread-safe

    def fileno(self):
        """Return a file descriptor suitable for select/poll of data
        ready to be received in non-blocking mode (at least for the
        first byte)"""
        return self._fd_r

    def send(self, data):
        """send the given python data to the receiving party"""
        sdata = pickle.dumps(data)
        sdata = struct.pack('I', len(sdata)) + sdata
        self._wlock.acquire()
        try:
            os.write(self._fd_w, sdata)
        finally:
            self._wlock.release()

    def recv(self):
        """wait for python data from the sending party and return it"""
        (expected,) = struct.unpack('I', os.read(self._fd_r, self.SZI))
        sdata = ""
        while 1:
            sdata += os.read(self._fd_r, expected - len(sdata))
            assert len(sdata) <= expected
            if len(sdata) == expected:
                break
        return pickle.loads(sdata)

    def close(self):
        """close the endpoint in both send/receive directions"""
        self._wlock.acquire()
        try:
            os.close(self._fd_w)
        finally:
            self._wlock.release()

        os.close(self._fd_r)


def ChannelPair():
    """Very simple function returning a connected pair of channels"""
    r1, w2 = os.pipe()
    r2, w1 = os.pipe()
    return ( SimpleChannelEndpoint(r1, w1), SimpleChannelEndpoint(r2, w2) )


def _test():
    """
    Some tests
    """
    import time, thread

    c1, c2 = ChannelPair()
    mux = Mux(c1)

    class MyDeMux(DeMux):
        """A demultiplexer in which each transaction is a call to sleep()"""
        def process_transaction(self, message_before, duration, message_after):
            """One trasaction is just a call to sleep"""
            print "[%d] BEGIN: %s (sleep %fs)" % (thread.get_ident(),
                                                  message_before, duration)
            time.sleep(duration)
            print "[%d] END: %s" % (thread.get_ident(), message_after)

    class Submitter(threading.Thread):
        """A thread that submits 3 transactions to the mux object"""
        def run(self):
            """Submit 3 transactions and stop"""
            mux.transaction("msg1", 3, "msg2")
            mux.transaction("msg3", 2, "msg4")
            mux.transaction("msg5", 1, "msg6")
            try:
                mux.transaction("msgE", -1, "msgEE")
            except IOError, ex:
                print "Got expected exception from the DeMux: %s" % repr(ex)
 
    demux = MyDeMux(c2, 100)
    # demux = MyDeMux(c2)

    # Starting mux/demux
    mux.start()
    demux.start()

    # Starting as many threads that run transactions as possible
    children = []
    for i in range(700):
        thr = Submitter()
        try:
            thr.start()
            children.append(thr)
        except:
            break

    print "Started %d submission threads" % len(children)

    # Waiting for the children
    for thr in children:
        try:
            thr.join()
        except KeyboardInterrupt:
            print "User interruption."
            break

    # Stopping mux/demux
    mux.stop()
    demux.stop()

    print "Bye."


if __name__ == "__main__":
    _test()

This recipe has been tested on Linux with python 2.5.2 and 2.6b3.

It defines 2 classes: Mux and DeMux. Instances of Mux present a "transaction()" method that, when called, will result in the "process_transaction()" method of a DeMux instance to be called. The result/exception of process_transaction() on DeMux will be sent back to transaction() on the Mux side.

The Mux and DeMux are connected to one another via a single bidirectional data link, such as a pair of pipes for example. The assumptions regarding the channel are described at the beginning of the source. This module provides a simple example of such a channel, based on Unix pipes, but this is by no means a reference implementation.

What this module provides is a way for Mux::transaction() to be called simultaneously from different threads: the request/responses will be interleaved and this module is here to manage this. As a result, DeMux::process_transaction() will be called simultaneously by different threads too. The threads on the DeMux side are either created on-the-fly for each request (when its ctor nworkers is None), or taken from a pre-allocated pool of threads. This module has its own implementation of a pool of threads (when nworkers is an integer). The implementation of the pool of threads is ad-hoc, self-contained in that module. The exceptions raised in process_transaction() are correctly transmitted back to the sender.

The only thing that the user needs to do is override the process_transaction() method of DeMux.

1 comment

david decotigny (author) 13 years, 2 months ago  # | flag

It is trivial to use the threadpool API of recipe #576519 to replace the ad-hoc thread pool implementation: use the apply_async() method of the pool.