Quick Start

This is a quick-start guide that covers a few common use-cases and contains pointers to more in-depth documentation for the curious.

Local Broker

To publish and consume messages locally can be a useful way to learn about the library, and is also helpful during development of your application or service.

To install the message broker on Fedora:

$ sudo dnf install rabbitmq-server

RabbitMQ is also available in EPEL7, although it is quite old and the library is not regularly tested against it. You can also install the broker from RabbitMQ directly if you are not using Fedora.

Next, it’s recommended that you enable the management interface:

$ sudo rabbitmq-plugins enable rabbitmq_management

This provides an HTTP interface and API, available at http://localhost:15672/ by default. The “guest” user with the password “guest” is created by default.

Finally, start the broker:

$ sudo systemctl start rabbitmq-server

You should now be able to consume messages with the following Python script:

from fedora_messaging import api, config

config.conf.setup_logging()
api.consume(lambda message: print(message))

To learn more about consuming messages, check out the Consumers documentation.

You can publish messages with:

from fedora_messaging import api, config

config.conf.setup_logging()
api.publish(api.Message(topic="hello", body={"Hello": "world!"}))

To learn more about publishing messages, check out the Publishing documentation.

Fedora’s Public Broker

Fedora’s message broker has a publicly accessible virtual host located at amqps://rabbitmq.fedoraproject.org/%2Fpublic_pubsub. This virtual host mirrors all messages published to the restricted /pubsub virtual host and allows anyone to consume messages being published by the various Fedora services.

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 if they have no consumers 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"

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

[client_properties]
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"]

[exchanges."amq.topic"]
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.
[queues.00000000-0000-0000-0000-000000000000]
durable = false
auto_delete = true
exclusive = true
arguments = {}

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

[consumer_config]
example_key = "for my consumer"

[qos]
prefetch_size = 0
prefetch_count = 25

[log_config]
version = 1
disable_existing_loggers = true

[log_config.formatters.simple]
format = "[%(levelname)s %(name)s] %(message)s"

[log_config.handlers.console]
class = "logging.StreamHandler"
formatter = "simple"
stream = "ext://sys.stdout"

[log_config.loggers.fedora_messaging]
level = "INFO"
propagate = false
handlers = ["console"]

[log_config.loggers.twisted]
level = "INFO"
propagate = false
handlers = ["console"]

[log_config.loggers.pika]
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:
#[log_config.loggers.example_printer]
#level = "INFO"
#propagate = false
#handlers = ["console"]

[log_config.root]
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

Warning

Do not skip the step above. This is important because if there are multiple consumers on a queue the broker delivers messages to them in a round-robin fashion. In other words, you’ll only get some of the messages being sent.

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

Alternatively, you can start a Python shell and use the API:

$ FEDORA_MESSAGING_CONF=my_config.toml python
>>> from fedora_messaging import api, config
>>> config.conf.setup_logging()
>>> api.consume(lambda message: print(message))

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.

Fedora’s Restricted Broker

Connecting the Fedora’s private virtual host requires working with the Fedora infrastructure team. The current process and configuration for this is documented in the infrastructure team’s development guide.