A simple message-oriented middleware library built for Python IPC across machine boundaries
Project description
PyMQ
PyMQ is a simple message-oriented middleware library for implementing Python IPC across machine boundaries. The API enables different styles of remoting via Pub/Sub, Queues, and synchronous RPC.
With PyMQ, developers can integrate Python applications running on different machines in a loosely coupled way over existing transport mechanisms. PyMQ currently provides a Redis backend, and an in-memory backend for testing. The API is extensible and other transports can be plugged in.
Using PyMQ
Initialize PyMQ
The core module manages a global eventbus instance that provides the remoting primitives. The default Redis
implementation uses an event loop over a pubsub object. The global eventbus is initialized via pymq.init
and by
passing a provider factory.
import pymq
from pymq.provider.redis import RedisConfig
# starts a new thread with a Redis event loop
pymq.init(RedisConfig())
# main application control loop
pymq.shutdown()
This will create an eventbus instance on top of a local Redis server.
Pub/Sub
Pub/Sub allows asynchronous event-based communication. Event classes are used to transport state and identify channels.
import pymq
# common code
class MyEvent:
pass
# subscribe code
@pymq.subscriber
def on_event(event: MyEvent):
print('event received')
# publisher code
pymq.publish(MyEvent())
Queues
Queues are straight forward, as they are compatible to the Python queue.Queue
specification.
import pymq
queue = pymq.queue('my_queue')
queue.put('obj')
print(queue.get()) # outputs 'obj'
RPC
Server code
import pymq
@pymq.remote('product_remote')
def product(a: int, b: int) -> int: # pymq relies on type hints for marshalling
return a * b
Client code
import pymq
product = pymq.stub('product_remote')
product(2, 4) # 8
With a shared code-base, methods can also be exposed and called by passing the callable. For example,
import pymq
# common code
class Remote:
def echo(self, param) -> None:
return 'echo: ' + param
# server
obj = Remote()
pymq.expose(obj.echo)
# client
echo = pymq.stub(Remote.echo)
echo('pymq') # "echo: pymq"
If there are multiple providers of the same object, then a stub can be initialized with multi=True
to get a list of
results. It may be useful to use a timeout in this case.
remote = pymq.stub('remote_method', multi=True, timeout=2)
result = remote() # result will be a list containing the results of all invocations of available remote objects
Known Limitations
- JSON serialization relies heavily on type hints. Sending complex types without type hints will cause type errors.
- There is currently no support for polymorphism with JSON serialization
- Pattern-based topic matching does not work for the in-memory eventbus
- Subscriptions by foreign components to RPC channels will cause issues in multi-call scenarios
- Using the
pymq
singleton in multiprocessing scenarios may not work as expected because the module holds a Thread in a global variable. A workaround is to re-start the bus by callingshutdown()
andinit()
in the forked Process.
Background
Originally part of the Symmetry project, was extracted as a standalone library.
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