Serializing messages with PyZMQ

When sending messages over a network, you often need to marshall your data into bytes.

Builtin serialization

PyZMQ is primarily bindings for libzmq, but we do provide three builtin serialization methods for convenience, to help Python developers learn libzmq. Python has two primary packages for serializing objects: json and pickle, so we provide simple convenience methods for sending and receiving objects serialized with these modules. A socket has the methods send_json() and send_pyobj(), which correspond to sending an object over the wire after serializing with json and pickle respectively, and any object sent via those methods can be reconstructed with the recv_json() and recv_pyobj() methods.

These methods designed for convenience, not for performance, so developers who do want to emphasize performance should use their own serialized send/recv methods.

Using your own serialization

In general, you will want to provide your own serialization that is optimized for your application or library availability. This may include using your own preferred serialization ([msgpack], [protobuf]), or adding compression via [zlib] in the standard library, or the super fast [blosc] library.

There are two simple models for implementing your own serialization: write a function that takes the socket as an argument, or subclass Socket for use in your own apps.

For instance, pickles can often be reduced substantially in size by compressing the data. The following will send compressed pickles over the wire:

import zlib, cPickle as pickle

def send_zipped_pickle(socket, obj, flags=0, protocol=-1):
    """pickle an object, and zip the pickle before sending it"""
    p = pickle.dumps(obj, protocol)
    z = zlib.compress(p)
    return socket.send(z, flags=flags)

def recv_zipped_pickle(socket, flags=0, protocol=-1):
    """inverse of send_zipped_pickle"""
    z = socket.recv(flags)
    p = zlib.decompress(z)
    return pickle.loads(p)

A common data structure in Python is the numpy array. PyZMQ supports sending numpy arrays without copying any data, since they provide the Python buffer interface. However just the buffer is not enough information to reconstruct the array on the receiving side. Here is an example of a send/recv that allow non-copying sends/recvs of numpy arrays including the dtype/shape data necessary for reconstructing the array.

import numpy

def send_array(socket, A, flags=0, copy=True, track=False):
    """send a numpy array with metadata"""
    md = dict(
        dtype = str(A.dtype),
        shape = A.shape,
    socket.send_json(md, flags|zmq.SNDMORE)
    return socket.send(A, flags, copy=copy, track=track)

def recv_array(socket, flags=0, copy=True, track=False):
    """recv a numpy array"""
    md = socket.recv_json(flags=flags)
    msg = socket.recv(flags=flags, copy=copy, track=track)
    buf = buffer(msg)
    A = numpy.frombuffer(buf, dtype=md['dtype'])
    return A.reshape(md['shape'])
[msgpack]Message Pack serialization library
[protobuf]Google Protocol Buffers
[zlib]Python stdlib module for zip compression: zlib
[blosc]Blosc: A blocking, shuffling and loss-less (and crazy-fast) compression library