How to reconnect to RabbitMQ?

Question

My python script constantly has to send messages to RabbitMQ once it receives one from another data source. The frequency in which the python script sends them can vary, say, 1 minute - 30 minutes.

Here's how I establish a connection to RabbitMQ:

  rabt_conn = pika.BlockingConnection(pika.ConnectionParameters("some_host"))
  channel = rbt_conn.channel()

I just got an exception

pika.exceptions.ConnectionClosed

How can I reconnect to it? What's the best way? Is there any "strategy"? Is there an ability to send pings to keep a connection alive or set timeout?

Any pointers will be appreciated.

Solution 1

RabbitMQ uses heartbeats to detect and close "dead" connections and to prevent network devices (firewalls etc.) from terminating "idle" connections. From version 3.5.5 on, the default timeout is set to 60 seconds (previously it was ~10 minutes). From the docs:

Heartbeat frames are sent about every timeout / 2 seconds. After two missed heartbeats, the peer is considered to be unreachable.

The problem with Pika's BlockingConnection is that it is unable to respond to heartbeats until some API call is made (for example, channel.basic_publish(), connection.sleep(), etc).

The approaches I found so far:

Increase or deactivate the timeout

RabbitMQ negotiates the timeout with the client when establishing the connection. In theory, it should be possible to override the server default value with a bigger one using the heartbeat_interval argument, but the current Pika version (0.10.0) uses the min value between those offered by the server and the client. This issue is fixed on current master.

On the other hand, is possible to deactivate the heartbeat functionality completely by setting the heartbeat_interval argument to 0, which may well drive you into new issues (firewalls dropping connections, etc)

Reconnecting

Expanding on @itsafire's answer, you can write your own publisher class, letting you reconnect when required. An example naive implementation:

import logging
import json
import pika

class Publisher:
    EXCHANGE='my_exchange'
    TYPE='topic'
    ROUTING_KEY = 'some_routing_key'

    def __init__(self, host, virtual_host, username, password):
        self._params = pika.connection.ConnectionParameters(
            host=host,
            virtual_host=virtual_host,
            credentials=pika.credentials.PlainCredentials(username, password))
        self._conn = None
        self._channel = None

    def connect(self):
        if not self._conn or self._conn.is_closed:
            self._conn = pika.BlockingConnection(self._params)
            self._channel = self._conn.channel()
            self._channel.exchange_declare(exchange=self.EXCHANGE,
                                           type=self.TYPE)

    def _publish(self, msg):
        self._channel.basic_publish(exchange=self.EXCHANGE,
                                    routing_key=self.ROUTING_KEY,
                                    body=json.dumps(msg).encode())
        logging.debug('message sent: %s', msg)

    def publish(self, msg):
        """Publish msg, reconnecting if necessary."""

        try:
            self._publish(msg)
        except pika.exceptions.ConnectionClosed:
            logging.debug('reconnecting to queue')
            self.connect()
            self._publish(msg)

    def close(self):
        if self._conn and self._conn.is_open:
            logging.debug('closing queue connection')
            self._conn.close()

Other possibilities

Other possibilities which I yet didn't explore:

• Using an asynchronous adapter for publishing
• Keeping your RabbitMQ connection and your "publish" code on a background thread, which calls periodically connection.sleep() to responde to server heartbeats.

Solution 2

Dead simple: some pattern like this.

import time

while True:
    try:
        communication_handles = connect_pika()
        do_your_stuff(communication_handles)
    except pika.exceptions.ConnectionClosed:
        print 'oops. lost connection. trying to reconnect.'
        # avoid rapid reconnection on longer RMQ server outage
        time.sleep(0.5) 

You will probably have to re-factor your code, but basically it is about catching the exception, mitigate the problem and continue doing your stuff. The communication_handles contain all the pika elements like channels, queues and whatever that your stuff needs to communicate with RabbitMQ via pika.