pyzmq/docs/source/howto/eventloop.md

(eventloop)=

# Eventloops and PyZMQ

As of pyzmq 17, integrating pyzmq with eventloops should work without any pre-configuration.
Due to the use of an edge-triggered file descriptor,
this has been known to have issues, so please report problems with eventloop integration.

(asyncio)=

## AsyncIO

PyZMQ 15 adds support for {mod}`asyncio` via {mod}`zmq.asyncio`, containing a Socket subclass
that returns {py:class}`asyncio.Future` objects for use in {py:mod}`asyncio` coroutines.
To use this API, import {class}`zmq.asyncio.Context`.
Sockets created by this Context will return Futures from any would-be blocking method.

```python
import asyncio
import zmq
from zmq.asyncio import Context

ctx = Context.instance()


async def recv():
    s = ctx.socket(zmq.SUB)
    s.connect("tcp://127.0.0.1:5555")
    s.subscribe(b"")
    while True:
        msg = await s.recv_multipart()
        print("received", msg)
    s.close()
```

````{note}
In PyZMQ \< 17, an additional step is needed to register the zmq poller prior to starting any async code:

```python
import zmq.asyncio
zmq.asyncio.install()

ctx = zmq.asyncio.Context()
```

This step is no longer needed in pyzmq 17.

````

## Tornado IOLoop

[Tornado] includes an eventloop for handing poll events on filedescriptors and
native sockets. We have included a small part of Tornado (specifically its
{mod}`.ioloop`), and adapted its {class}`IOStream` class into {class}`.ZMQStream` for
handling poll events on ØMQ sockets. A ZMQStream object works much like a Socket object,
but instead of calling {meth}`~.Socket.recv` directly, you register a callback with
{meth}`~.ZMQStream.on_recv`. Callbacks can also be registered for send events
with {meth}`~.ZMQStream.on_send`.

(futures)=

### Futures and coroutines

```{note}
With recent Python (3.6) and tornado (5),
there's no reason to use {mod}`zmq.eventloop.future`
instead of the strictly-more-compatible {mod}`zmq.asyncio`.
```

PyZMQ 15 adds {mod}`zmq.eventloop.future`, containing a Socket subclass
that returns {class}`~.tornado.concurrent.Future` objects for use in {mod}`tornado` coroutines.
To use this API, import {class}`zmq.eventloop.future.Context`.
Sockets created by this Context will return Futures from any would-be blocking method.

```python
from tornado import gen, ioloop
import zmq
from zmq.eventloop.future import Context

ctx = Context.instance()


@gen.coroutine
def recv():
    s = ctx.socket(zmq.SUB)
    s.connect("tcp://127.0.0.1:5555")
    s.subscribe(b"")
    while True:
        msg = yield s.recv_multipart()
        print("received", msg)
    s.close()
```

### {class}`ZMQStream`

{class}`ZMQStream` objects let you register callbacks to handle messages as they arrive,
for use with the tornado eventloop.

#### {meth}`send`

ZMQStream objects do have {meth}`~.ZMQStream.send` and {meth}`~.ZMQStream.send_multipart`
methods, which behaves the same way as {meth}`.Socket.send`, but instead of sending right
away, the {class}`.IOLoop` will wait until socket is able to send (for instance if `HWM`
is met, or a `REQ/REP` pattern prohibits sending at a certain point). Messages sent via
send will also be passed to the callback registered with {meth}`~.ZMQStream.on_send` after
sending.

#### {meth}`on_recv`

{meth}`.ZMQStream.on_recv` is the primary method for using a ZMQStream. It registers a
callback to fire with messages as they are received, which will *always* be multipart,
even if its length is 1. You can easily use this to build things like an echo socket:

```python
s = ctx.socket(zmq.REP)
s.bind("tcp://localhost:12345")
stream = ZMQStream(s)


def echo(msg):
    stream.send_multipart(msg)


stream.on_recv(echo)
ioloop.IOLoop.instance().start()
```

on_recv can also take a `copy` flag, just like {meth}`.Socket.recv`. If `copy=False`, then
callbacks registered with on_recv will receive tracked {class}`.Frame` objects instead of
bytes.

```{note}
A callback must be registered using either {meth}`.ZMQStream.on_recv` or
{meth}`.ZMQStream.on_recv_stream` before any data will be received on the
underlying socket.  This allows you to temporarily pause processing on a
socket by setting both callbacks to None.  Processing can later be resumed
by restoring either callback.
```

#### {meth}`on_recv_stream`

{meth}`.ZMQStream.on_recv_stream` is just like on_recv above, but the callback will be
passed both the message and the stream, rather than just the message.  This is meant to make
it easier to use a single callback with multiple streams.

```python
s1 = ctx.socket(zmq.REP)
s1.bind("tcp://localhost:12345")
stream1 = ZMQStream(s1)

s2 = ctx.socket(zmq.REP)
s2.bind("tcp://localhost:54321")
stream2 = ZMQStream(s2)


def echo(stream, msg):
    stream.send_multipart(msg)


stream1.on_recv_stream(echo)
stream2.on_recv_stream(echo)

ioloop.IOLoop.instance().start()
```

#### {meth}`flush`

Sometimes with an eventloop, there can be multiple events ready on a single iteration of
the loop. The {meth}`~.ZMQStream.flush` method allows developers to pull messages off of
the queue to enforce some priority over the event loop ordering. flush pulls any pending
events off of the queue. You can specify to flush only recv events, only send events, or
any events, and you can specify a limit for how many events to flush in order to prevent
starvation.

### {func}`install()`

```{note}
If you are using pyzmq \< 17, there is an additional step
to tell tornado to use the zmq poller instead of its default.
{func}`.ioloop.install` is no longer needed for pyzmq ≥ 17.
```

With PyZMQ's ioloop, you can use zmq sockets in any tornado application.  You can tell tornado to use zmq's poller by calling the {func}`.ioloop.install` function:

```python
from zmq.eventloop import ioloop

ioloop.install()
```

You can also do the same thing by requesting the global instance from pyzmq:

```python
from zmq.eventloop.ioloop import IOLoop

loop = IOLoop.current()
```

This configures tornado's {class}`tornado.ioloop.IOLoop` to use zmq's poller,
and registers the current instance.

Either `install()` or retrieving the zmq instance must be done before the global * instance is registered, else there will be a conflict.

It is possible to use PyZMQ sockets with tornado *without* registering as the global instance,
but it is less convenient. First, you must instruct the tornado IOLoop to use the zmq poller:

```python
from zmq.eventloop.ioloop import ZMQIOLoop

loop = ZMQIOLoop()
```

Then, when you instantiate tornado and ZMQStream objects, you must pass the `io_loop`
argument to ensure that they use this loop, instead of the global instance.

This is especially useful for writing tests, such as this:

```python
from tornado.testing import AsyncTestCase
from zmq.eventloop.ioloop import ZMQIOLoop
from zmq.eventloop.zmqstream import ZMQStream


class TestZMQBridge(AsyncTestCase):
    # Use a ZMQ-compatible I/O loop so that we can use `ZMQStream`.
    def get_new_ioloop(self):
        return ZMQIOLoop()
```

You can also manually install this IOLoop as the global tornado instance, with:

```python
from zmq.eventloop.ioloop import ZMQIOLoop

loop = ZMQIOLoop()
loop.install()
```

(zmq-green)=

## PyZMQ and gevent

PyZMQ ≥ 2.2.0.1 ships with a [gevent](https://www.gevent.org/) compatible API as {mod}`zmq.green`.
To use it, simply:

```python
import zmq.green as zmq
```

Then write your code as normal.

Socket.send/recv and zmq.Poller are gevent-aware.

In PyZMQ ≥ 2.2.0.2, green.device and green.eventloop should be gevent-friendly as well.

```{note}
The green device does *not* release the GIL, unlike the true device in zmq.core.
```

zmq.green.eventloop includes minimally patched IOLoop/ZMQStream in order to use the gevent-enabled Poller,
so you should be able to use the ZMQStream interface in gevent apps as well,
though using two eventloops simultaneously (tornado + gevent) is not recommended.

```{warning}
There is a [known issue](https://github.com/zeromq/pyzmq/issues/229) in gevent ≤ 1.0 or libevent,
which can cause zeromq socket events to be missed.
PyZMQ works around this by adding a timeout so it will not wait forever for gevent to notice events.
The only known solution for this is to use gevent ≥ 1.0, which is currently at 1.0b3,
and does not exhibit this behavior.
```

```{seealso}
zmq.green examples [on GitHub](https://github.com/zeromq/pyzmq/tree/HEAD/examples/gevent).
```

{mod}`zmq.green` began as [gevent_zeromq](https://github.com/tmc/gevent-zeromq),
merged into the pyzmq project.

[tornado]: https://tornadoweb.org