NATS - Python3 Client for Asyncio

An asyncio Python client for the NATS messaging system.

Supported platforms

Should be compatible with at least Python +3.6.

Installing

pip install asyncio-nats-client

Starting from v0.9.0 release, you can also optionally install NKEYS in order to use the new NATS v2.0 auth features:

pip install asyncio-nats-client[nkeys]

Basic Usage

import asyncio
from nats.aio.client import Client as NATS
from nats.aio.errors import ErrConnectionClosed, ErrTimeout, ErrNoServers

async def run(loop):
    nc = NATS()

    await nc.connect("demo.nats.io:4222", loop=loop)

    async def message_handler(msg):
        subject = msg.subject
        reply = msg.reply
        data = msg.data.decode()
        print("Received a message on '{subject} {reply}': {data}".format(
            subject=subject, reply=reply, data=data))

    # Simple publisher and async subscriber via coroutine.
    sid = await nc.subscribe("foo", cb=message_handler)

    # Stop receiving after 2 messages.
    await nc.auto_unsubscribe(sid, 2)
    await nc.publish("foo", b'Hello')
    await nc.publish("foo", b'World')
    await nc.publish("foo", b'!!!!!')

    async def help_request(msg):
        subject = msg.subject
        reply = msg.reply
        data = msg.data.decode()
        print("Received a message on '{subject} {reply}': {data}".format(
            subject=subject, reply=reply, data=data))
        await nc.publish(reply, b'I can help')

    # Use queue named 'workers' for distributing requests
    # among subscribers.
    sid = await nc.subscribe("help", "workers", help_request)

    # Send a request and expect a single response
    # and trigger timeout if not faster than 1 second.
    try:
        response = await nc.request("help", b'help me', timeout=1)
        print("Received response: {message}".format(
            message=response.data.decode()))
    except ErrTimeout:
        print("Request timed out")

    # Remove interest in subscription.
    await nc.unsubscribe(sid)

    # Terminate connection to NATS.
    await nc.close()

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(run(loop))
    loop.close()

Wildcard Subscriptions

import asyncio
from nats.aio.client import Client as NATS
from nats.aio.errors import ErrConnectionClosed, ErrTimeout, ErrNoServers


async def run(loop):
    nc = NATS()

    await nc.connect("nats://127.0.0.1:4222", loop=loop)

    async def message_handler(msg):
        subject = msg.subject
        reply = msg.reply
        data = msg.data.decode()
        print("Received a message on '{subject} {reply}': {data}".format(
            subject=subject, reply=reply, data=data))

    # "*" matches any token, at any level of the subject.
    await nc.subscribe("foo.*.baz", cb=message_handler)
    await nc.subscribe("foo.bar.*", cb=message_handler)

    # ">" matches any length of the tail of a subject, and can only be the last token
    # E.g. 'foo.>' will match 'foo.bar', 'foo.bar.baz', 'foo.foo.bar.bax.22'
    await nc.subscribe("foo.>", cb=message_handler)

    # Matches all of the above.
    await nc.publish("foo.bar.baz", b'Hello World')

    # Gracefully close the connection.
    await nc.drain()

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(run(loop))
    loop.close()

Advanced Usage

import asyncio
from nats.aio.client import Client as NATS
from nats.aio.errors import ErrTimeout, ErrNoServers

async def run(loop):
    nc = NATS()

    try:
        # Setting explicit list of servers in a cluster.
        await nc.connect(servers=["nats://127.0.0.1:4222", "nats://127.0.0.1:4223", "nats://127.0.0.1:4224"], loop=loop)
    except ErrNoServers as e:
        print(e)
        return

    async def message_handler(msg):
        subject = msg.subject
        reply = msg.reply
        data = msg.data.decode()
        for i in range(0, 20):
            await nc.publish(reply, "i={i}".format(i=i).encode())

    await nc.subscribe("help.>", cb=message_handler)

    async def request_handler(msg):
        subject = msg.subject
        reply = msg.reply
        data = msg.data.decode()
        print("Received a message on '{subject} {reply}': {data}".format(
            subject=subject, reply=reply, data=data))

    # Signal the server to stop sending messages after we got 10 already.
    await nc.request(
        "help.please", b'help', expected=10, cb=request_handler)

    try:
        # Flush connection to server, returns when all messages have been processed.
        # It raises a timeout if roundtrip takes longer than 1 second.
        await nc.flush(1)
    except ErrTimeout:
        print("Flush timeout")

    await asyncio.sleep(1, loop=loop)

    # Drain gracefully closes the connection, allowing all subscribers to
    # handle any pending messages inflight that the server may have sent.
    await nc.drain()

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(run(loop))
    loop.close()

Clustered Usage

import asyncio
from datetime import datetime
from nats.aio.client import Client as NATS
from nats.aio.errors import ErrConnectionClosed, ErrTimeout, ErrNoServers

async def run(loop):

    nc = NATS()

    # Setup pool of servers from a NATS cluster.
    options = {
        "servers": [
            "nats://user1:pass1@127.0.0.1:4222",
            "nats://user2:pass2@127.0.0.1:4223",
            "nats://user3:pass3@127.0.0.1:4224",
        ],
        "loop": loop,
    }

    # Will try to connect to servers in order of configuration,
    # by defaults it connect to one in the pool randomly.
    options["dont_randomize"] = True

    # Optionally set reconnect wait and max reconnect attempts.
    # This example means 10 seconds total per backend.
    options["max_reconnect_attempts"] = 5
    options["reconnect_time_wait"] = 2

    async def disconnected_cb():
        print("Got disconnected!")

    async def reconnected_cb():
        # See who we are connected to on reconnect.
        print("Got reconnected to {url}".format(url=nc.connected_url.netloc))

    # Setup callbacks to be notified on disconnects and reconnects
    options["disconnected_cb"] = disconnected_cb
    options["reconnected_cb"] = reconnected_cb

    async def error_cb(e):
        print("There was an error: {}".format(e))

    async def closed_cb():
        print("Connection is closed")

    async def subscribe_handler(msg):
        print("Got message: ", msg.subject, msg.reply, msg.data)

    # Setup callbacks to be notified when there is an error
    # or connection is closed.
    options["error_cb"] = error_cb
    options["closed_cb"] = closed_cb

    try:
        await nc.connect(**options)
    except ErrNoServers as e:
        # Could not connect to any server in the cluster.
        print(e)
        return

    if nc.is_connected:
        await nc.subscribe("help.*", cb=subscribe_handler)

        max_messages = 1000
        start_time = datetime.now()
        print("Sending {} messages to NATS...".format(max_messages))

        for i in range(0, max_messages):
            try:
                await nc.publish("help.{}".format(i), b'A')
                await nc.flush(0.500)
            except ErrConnectionClosed as e:
                print("Connection closed prematurely.")
                break
            except ErrTimeout as e:
                print("Timeout occured when publishing msg i={}: {}".format(
                    i, e))

        end_time = datetime.now()
        await nc.close()
        duration = end_time - start_time
        print("Duration: {}".format(duration))

        try:
            await nc.publish("help", b"hello world")
        except ErrConnectionClosed:
            print("Can't publish since no longer connected.")

    err = nc.last_error
    if err is not None:
        print("Last Error: {}".format(err))

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(run(loop))
    loop.close()

TLS

TLS connections can be configured with an ssl context

ssl_ctx = ssl.create_default_context(purpose=ssl.Purpose.SERVER_AUTH)
ssl_ctx.load_verify_locations('ca.pem')
ssl_ctx.load_cert_chain(certfile='client-cert.pem',
                        keyfile='client-key.pem')
await nc.connect(servers=["tls://127.0.0.1:4443"], loop=loop, tls=ssl_ctx, tls_hostname="localhost")

Setting the scheme to tls in the connect URL will make the client create a default ssl context automatically:

import asyncio
import ssl
from nats.aio.client import Client as NATS

async def run(loop):
    nc = NATS()
    await nc.connect("tls://demo.nats.io:4443", loop=loop)

Note: If getting SSL certificate errors in OS X, try first installing the certifi certificate bundle. If using Python 3.7 for example, then run:

$ /Applications/Python\ 3.7/Install\ Certificates.command
 -- pip install --upgrade certifi
Collecting certifi
...
 -- removing any existing file or link
 -- creating symlink to certifi certificate bundle
 -- setting permissions
 -- update complete

Development

To run the tests:

python3 -m pipenv install
python3 -m pytest 

License

Unless otherwise noted, the NATS source files are distributed under the Apache Version 2.0 license found in the LICENSE file.