Fedora’s Message Broker

Once you’ve got an application that publishes or consumes messages, you need to get it connected to Fedora’s message broker. The message broker is located at amqps://rabbitmq.fedoraproject.org/.

External Connections

Fedora allows anyone on the Internet to create queues and consume from them in the public /public_pubsub AMQP virtual host. This virtual host mirrors all messages published to the restricted /pubsub virtual host.

These public queues have some restrictions applied to them. Firstly, they are limited to about 50 megabytes in size, so if your application cannot handle the message throughput messages will be automatically discarded once you hit this limit. Secondly, queues that are set to be durable (in other words, not exclusive or auto-deleted) are automatically deleted after approximately an hour.

If you need more robust guarantees about message delivery, or if you need to publish messages into Fedora’s message broker, contact the Fedora Infrastructure team about getting access to the private virtual host.

Getting Connected

The public virtual host still requires users to authenticate when connecting, so a public user has been created and its private key and x509 certificate are distributed with fedora-messaging.

If fedora-messaging was installed via RPM, they should be in /etc/fedora-messaging/ along with a configuration file called fedora.toml. If it’s been installed via pip, it’s easiest to get the key, certificate, and the CA certificate from the upstream git repository and start with the following configuration file:

# A basic configuration for Fedora's message broker, using the example callback
# which simply prints messages to standard output.
# This file is in the TOML format.
amqp_url = "amqps://fedora:@rabbitmq.fedoraproject.org/%2Fpublic_pubsub"
callback = "fedora_messaging.example:printer"

ca_cert = "/etc/fedora-messaging/cacert.pem"
keyfile = "/etc/fedora-messaging/fedora-key.pem"
certfile = "/etc/fedora-messaging/fedora-cert.pem"

app = "Example Application"
# Some suggested extra fields:
# URL of the project that provides this consumer
app_url = "https://github.com/fedora-infra/fedora-messaging"
# Contact emails for the maintainer(s) of the consumer - in case the
# broker admin needs to contact them, for e.g.
app_contacts_email = ["jcline@fedoraproject.org"]

type = "topic"
durable = true
auto_delete = false
arguments = {}

# Queue names *must* be in the normal UUID format: run "uuidgen" and use the
# output as your queue name. If your queue is not exclusive, anyone can connect
# and consume from it, causing you to miss messages, so do not share your queue
# name. Any queues that are not auto-deleted on disconnect are garbage-collected
# after approximately one hour.
# If you require a stronger guarantee about delivery, please talk to Fedora's
# Infrastructure team.
durable = false
auto_delete = true
exclusive = true
arguments = {}

queue = "00000000-0000-0000-0000-000000000000"
exchange = "amq.topic"
routing_keys = ["#"]  # Set this to the specific topics you are interested in.

example_key = "for my consumer"

prefetch_size = 0
prefetch_count = 25

version = 1
disable_existing_loggers = true

format = "[%(levelname)s %(name)s] %(message)s"

class = "logging.StreamHandler"
formatter = "simple"
stream = "ext://sys.stdout"

level = "INFO"
propagate = false
handlers = ["console"]

level = "INFO"
propagate = false
handlers = ["console"]

level = "WARNING"
propagate = false
handlers = ["console"]

# If your consumer sets up a logger, you must add a configuration for it
# here in order for the messages to show up. e.g. if it set up a logger
# called 'example_printer', you could do:
#level = "INFO"
#propagate = false
#handlers = ["console"]

level = "ERROR"
handlers = ["console"]

Assuming the /etc/fedora-messaging/fedora.toml, /etc/fedora-messaging/cacert.pem, /etc/fedora-messaging/fedora-key.pem, and /etc/fedora-messaging/fedora-cert.pem files exist, the following command will create a configuration file called my_config.toml with a unique queue name for your consumer:

$ sed -e "s/[0-9a-f]\{8\}-[0-9a-f]\{4\}-[0-9a-f]\{4\}-[0-9a-f]\{4\}-[0-9a-f]\{12\}/$(uuidgen)/g" \
    /etc/fedora-messaging/fedora.toml > my_config.toml

Run a quick test to make sure you can connect to the broker. The configuration file comes with an example consumer which simply prints the message to standard output:

$ fedora-messaging --conf my_config.toml consume

If all goes well, you’ll see a log entry similar to:

Successfully registered AMQP consumer Consumer(queue=af0f78d2-159e-4279-b404-7b8c1b4649cc, callback=<function printer at 0x7f9a59e077b8>)

This will be followed by the messages being sent inside Fedora’s Infrastructure. All that’s left to do is change the callback in the configuration to use your consumer callback and adjusting the routing keys in your bindings to receive only the messages your consumer is interested in.