Matthew Treinish a38d3dcc6d

Add documentation on howto consume events from firehose.o.o

This commit adds documentation on how to use mqtt clients to subscribe
to events on firehose.o.o. This will hopefully provide enough context
for people who wish to consume any events get started working with it.

Change-Id: I5e5f088d00d1c6f5bded4613fbbe1dad1c10fff3

2016-08-05 18:23:03 -04:00

4.4 KiB

Raw Blame History

title: Firehose

Firehose

The unified message bus for Infra services.

At a Glance

Hosts

firehose*.openstack.org

Puppet

https://git.openstack.org/cgit/openstack-infra/puppet-mosquitto
https://git.openstack.org/cgit/openstack-infra/puppet-germqtt
modules/openstack_project/manifests/firehose.pp

Projects

Overview

The firehose is an infra run MQTT broker that is a place for any infra run service to publish events to. The concept behind it is that if anything needs to consume an event from an infra run service we should have a single place to go for consuming them.

firehose.openstack.org hosts an instance of Mosquitto to be the MQTT broker and also locally runs an instance of germqtt to publish the gerrit event stream over MQTT.

Connection Info

firehose.openstack.org has 2 open ports for MQTT traffic:

1883 - The default MQTT port

80 - Uses websockets for the MQTT communication

Topics

Topics at a top level are set based on the name of the service publishing the messages. The higher levels are specified by the publisher. For example:

gerrit/openstack-infra/germqtt/comment-added

is a typical message topic on firehose. The top level 'gerrit' specifies the service the message is from, and the rest of the message comes from germqtt (the daemon used for publishing the gerrit events)

MQTT topics are hierarchical and you can filter your subscription on part of the hierarchy. [1]

Client Usage

There is no outside access to publishing messages to the firehose available, however anyone is able to subscribe to any topic services publish to. To interact with the firehose you need to use the MQTT protocol. The specific contents of the payload are dictated by the service publishing the messages. So this section only covers how to subscribe and receive the messages not how to consume the content received.

Available Clients

The MQTT community wiki maintains a page that lists available client bindings for many languages here: https://github.com/mqtt/mqtt.github.io/wiki/libraries For python using the paho-mqtt library is recommended

CLI Example

The mosquitto project also provides both a CLI publisher and subscriber client that can be used to easily subscribe to any topic and receive the messages. On debian based distributions these are included in the mosquitto-clients package. For example, to subscribe to every topic on the firehose you would run:

mosquitto_sub -h firehose.openstack.org --topic '#'

You can adjust the value of the topic parameter to make what you're subscribing to more specific.

Websocket Example

In addition to using the raw MQTT protocol firehose.o.o provides a websocket interface on port 80 that MQTT traffic can go through. This is especially useful for web applications that intend to consume any events from MQTT. To see an example of this in action you can try: http://mitsuruog.github.io/what-mqtt/ (the source is available here: https://github.com/mitsuruog/what-mqtt) and use that to subscribe to any topics on firehose.openstack.org.

Another advantage of using websockets over port 80 is that it's much more firewall friendly, especially in environments that are more locked down. If you would like to consume events from the firehose and are concerned about a firewall blocking your access, the websocket interface is a good choice.

You can also use the paho-mqtt python library to subscribe to mqtt over websockets fairly easily. For example this script will subscribe to all topics on the firehose and print it to STDOUT

import paho.mqtt.client as mqtt


def on_connect(client, userdata, flags, rc):
    print("Connected with result code " + str(rc))
        client.subscribe('#')

def on_message(client, userdata, msg):
    print(msg.topic+" "+str(msg.payload))

# Create a websockets client
client = mqtt.Client(transport="websockets")
client.on_connect = on_connect
client.on_message = on_message

# Connect to the firehose
client.connect('firehose.openstack.org', port=80)
# Listen forever
client.loop_forever()

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