RabbitMQ is an event bus composed of mainly 3 concepts:
The system uses AMP 0-9-1 protocol to interact with RabbitMQ.
RabbitMQ provides an Administration Panel to have a comprehensive overview of the cluster status. The Panel is available on 15672 port of the cluster.
(while working locally it is available at the address http://localhost:15672/)
Exchanges are the entrypoints to RabbitMQ, when publishing, a producer is interacting with an exchange.
There are 3 types of exchanges:
- Direct exchanges, which route messages to a single
queue/consumer/exchange based on the Routing Key - Fanout exchanges, which broadcast messages to all subscribed
queue/consumer/exchange - Topic exchanges, which route messages to multiple
queue/consumer/exchange based on the Routing Key allowing route key regex filtering
Since the system utilizes mostly the topic exchange, we suggest reading more about them in the official documentation and tutorials.
When a message enters an exchange it is immediately broadcasted to all subscribed queue/consumer/exchange if any.
If there are no subscribers, the message is simply lost.
Queues are buffers where messages can be stored waiting to be consumed.
In the most common configuration, an exchange is bounded to a queue.
In this way, messages published to the exchange are not thrown away on the fly; but persisted in a queue waiting to be processed.
By default queues do not have a limit (message number or bytes). For more information about Queue's length limit read the official documentation.
When a Route or Transformer pulls a message, it becomes responsible to tell RabbitMQ how the message process went.
It has a timeout (configure consumer_timout docs) to give a response about the message. After the timeout RabbitMQ flags the consumer as non-responsive, disconnects it and sends the messages he was responsible for to other consumers.
There are 3 types of aknowledgement:
-
ack
deletes the message from the system, since it has been successfully processed -
reject
deletes the message from the system syem because a permanent error occurred.
If a dead letter exchange is configured, all rejected messages will forwarded to that exchange -
nack(negative ack)
There was a retriable error in the process, therefore the message is requeued to further retry.
Bindings are the glue between exchanges and queues.
More technically a bind creates a rule to tell an exchange when to forward a message to a specific queue.
The most basic bind simply tells the queue to broadcast all messages to a queue.
When declaring a bind it is possible to specify a Routing Key to tell the exchange to broadcast only those messages routed with that Routing Key.
An exchange can have multiple bindings with a queue.
To know more about binding we suggest reading the official tutorials.
Routing Keys are a very important concept in RabbitMQ. As the name suggest, by configuring a bind to allow only messages flagged with a specific HEADER to be forwarded to the target queue.
When bounded with a topic exchange, it is also possible to use wildcards in the routing key, allowing more powerful routing logic.
In the above example, messages published with a routing key will be routed so:
| Message Routing Key | Destination Queue |
|---|---|
| an.orange.yay | Q1 |
| a.orange | unroutable |
| a.nice.rabbit | Q2 |
| lazy | Q2 |
| lazy.guy.singing | Q2 |
Topic exchange is powerful and can behave like other exchanges.
When a queue is bound with "#" (hash) binding key - it will receive all the messages, regardless of the routing key - like in fanout exchange.
When special characters "*" (star) and "#" (hash) aren't used in bindings, the topic exchange will behave just like a direct one.
Read more info about Routing Keys in the tutorials
References: