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RabbitMQ tutorial - Topics

Topics​

(using the AMQP 1.0 Go client)​

info

Prerequisites​

This tutorial assumes RabbitMQ is installed and running on localhost on the standard port (5672). In case you use a different host, port or credentials, connections settings would require adjusting.

Where to get help​

If you're having trouble going through this tutorial you can contact us through GitHub Discussions or RabbitMQ community Discord.

In the previous tutorial we improved our logging system. Instead of using a fanout exchange only capable of mindless broadcasting, we used a direct one, and gained the ability to selectively receive the logs.

Although using the direct exchange improved our system, it still has limitations - it can't do routing based on multiple criteria.

In our logging system we might want to subscribe to not only logs based on severity, but also based on the source which emitted the log. You might know this concept from the syslog Unix tool, which routes logs based on both severity (info/warn/crit...) and facility (auth/cron/kern...).

That would give us a lot of flexibility - we may want to listen to just critical errors coming from 'cron' but also all logs from 'kern'.

To implement our logging system we're going to use a topic exchange. Topic exchanges implement pattern-based message routing.

The topic exchange is powerful and can behave like other exchanges:

  • When a queue is bound with binding key "*" (star) it will match any routing key.
  • When special characters "*" (star) and "#" (hash) aren't used in bindings, the topic exchange will behave just like a direct one.

Publishing​

We'll use a topic exchange named logs with routing keys that have the form "<facility>.<severity>":

go run emit_log_topic.go kern.critical "A critical kernel error"
go run emit_log_topic.go auth.info "Auth info message"

See the full emit_log_topic.go for the implementation using TopicExchangeSpecification.

Subscribing​

Receiving will show the power of topic exchanges. We'll create bindings with pattern-based routing keys:

go run receive_logs_topic.go "kern.*"
go run receive_logs_topic.go "*.critical"
go run receive_logs_topic.go "kern.critical" "auth.warn"
  • The first consumer will receive all messages from the kern facility.
  • The second will receive all messages of severity critical.
  • The third will receive only critical errors from kern and warnings from auth.

See the full receive_logs_topic.go for the implementation.

Topic exchange binding rules​

The topic exchange binding rules are:

  • "#" can substitute for zero or more words.
  • "*" can substitute for exactly one word.
  • Routing keys are separated by dots (e.g., "kern.critical").

Putting it all together​

The full code examples are available at:

The main pattern is using TopicExchangeSpecification instead of DirectExchangeSpecification, and binding queues with pattern-based routing keys using "*" and "#" wildcards.

Now we can move on to tutorial 6 to learn about the RPC (Request/Reply) pattern with RabbitMQ.