More Than Just Bindings¶
PyZMQ is ostensibly the Python bindings for ØMQ, but the project, following Python’s ‘batteries included’ philosophy, provides more than just Python methods and objects for calling into the ØMQ C++ library.
The Core as Bindings¶
PyZMQ is currently broken up into four subpackages. First, is the Core. zmq.core
contains the actual bindings for ZeroMQ, and no extended functionality beyond the very
basic. The core modules are split, such that each basic ZeroMQ object (or function, if no
object is associated) is a separate module, e.g. zmq.core.context
contains the
Context
object, zmq.core.poll
contains a Poller
object, as well
as the select()
function, etc. ZMQ constants are, for convenience, all kept
together in zmq.core.constants
.
There are two reasons for breaking the core into submodules: recompilation and derivative projects. The monolithic PyZMQ became quite tedious to have to recompile everything for a small change to a single object. With separate files, that’s no longer necessary. The second reason has to do with Cython. PyZMQ is written in Cython, a tool for efficiently writing C-extensions for Python. By separating out our objects into individual pyx files, each with their declarations in a pxd header, other projects can write extensions in Cython and call directly to ZeroMQ at the C-level without the penalty of going through our Python objects.
Thread Safety¶
In ØMQ, Contexts are threadsafe objects, but Sockets are not. It is safe to use a
single Context (e.g. via zmq.Context.instance()
) in your entire multithreaded
application, but you should create sockets on a per-thread basis. If you share sockets
across threads, you are likely to encounter uncatchable c-level crashes of your
application unless you use judicious application of threading.Lock
, but this
approach is not recommended.
Socket Options as Attributes¶
New in version 2.1.9.
In 0MQ, socket options are set/retrieved with the set/getsockopt()
methods. With the
class-based approach in pyzmq, it would be logical to perform these operations with
simple attribute access, and this has been added in pyzmq 2.1.9. Simply assign to or
request a Socket attribute with the (case-insensitive) name of a sockopt, and it should
behave just as you would expect:
s = ctx.socket(zmq.DEALER)
s.identity = b'dealer'
s.hwm = 10
s.events
# 0
s.fd
# 16
Default Options on the Context¶
New in version 2.1.11.
Just like setting socket options as attributes on Sockets, you can do the same on Contexts. This affects the default options of any new sockets created after the assignment.
ctx = zmq.Context()
ctx.linger = 0
rep = ctx.socket(zmq.REP)
req = ctx.socket(zmq.REQ)
Socket options that do not apply to a socket (e.g. SUBSCRIBE on non-SUB sockets) will simply be ignored.
Core Extensions¶
We have extended the core functionality in two ways that appear inside the core
bindings, and are not general ØMQ features.
Builtin Serialization¶
First, we added common serialization with the builtin json
and pickle
as first-class methods to the Socket
class. 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. Unicode strings are
other objects that are not unambiguously sendable over the wire, so we include
send_string()
and recv_string()
that simply send bytes
after encoding the message (‘utf-8’ is the default).
See also
- Further information on serialization in pyzmq.
- Our Unicode discussion for more information on the trials and tribulations of working with Unicode in a C extension while supporting Python 2 and 3.
MessageTracker¶
The second extension of basic ØMQ functionality is the MessageTracker
. The
MessageTracker is an object used to track when the underlying ZeroMQ is done with a
message buffer. One of the main use cases for ØMQ in Python is the ability to perform
non-copying sends. Thanks to Python’s buffer interface, many objects (including NumPy
arrays) provide the buffer interface, and are thus directly sendable. However, as with any
asynchronous non-copying messaging system like ØMQ or MPI, it can be important to know
when the message has actually been sent, so it is safe again to edit the buffer without
worry of corrupting the message. This is what the MessageTracker is for.
The MessageTracker is a simple object, but there is a penalty to its use. Since by its
very nature, the MessageTracker must involve threadsafe communication (specifically a
builtin Queue
object), instantiating a MessageTracker takes a modest
amount of time (10s of µs), so in situations instantiating many small messages, this can
actually dominate performance. As a result, tracking is optional, via the track
flag,
which is optionally passed, always defaulting to False
, in each of the three places
where a Frame object (the pyzmq object for wrapping a segment of a message) is
instantiated: The Frame
constructor, and non-copying sends and receives.
A MessageTracker is very simple, and has just one method and one attribute. The property
MessageTracker.done
will be True
when the Frame(s) being tracked are no
longer in use by ØMQ, and MessageTracker.wait()
will block, waiting for the
Frame(s) to be released.
Note
A Frame cannot be tracked after it has been instantiated without tracking. If a
Frame is to even have the option of tracking, it must be constructed with
track=True
.
Extensions¶
So far, PyZMQ includes four extensions to core ØMQ that we found basic enough to be included in PyZMQ itself:
- zmq.log : Logging handlers for hooking Python logging up to the network
- zmq.devices : Custom devices and objects for running devices in the background
- zmq.eventloop : The Tornado event loop, adapted for use with ØMQ sockets.
- zmq.ssh : Simple tools for tunneling zeromq connections via ssh.