RabbitMQ tutorial - Routing

Routing

(using the Objective-C 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 built a simple logging system. We were able to broadcast log messages to many receivers.

In this tutorial we're going to add a feature to it - we're going to make it possible to subscribe only to a subset of the messages. For example, we will be able to direct only critical error messages to the log file (to save disk space), while still being able to print all of the log messages on the console.

Bindings

In previous examples we were already creating bindings. You may recall code like:

[q bind:exchange];

A binding is a relationship between an exchange and a queue. This can be simply read as: the queue is interested in messages from this exchange.

Bindings can take an extra routingKey parameter. To avoid the confusion with an RMQExchange publish: parameter we're going to call it a binding key. This is how we could create a binding with a key:

[q bind:exchange routingKey:@"black"];

The meaning of a binding key depends on the exchange type. The fanout exchanges, which we used previously, simply ignored its value.

Direct exchange

Our logging system from the previous tutorial broadcasts all messages to all consumers. We want to extend that to allow filtering messages based on their severity. For example we may want the script which is writing log messages to the disk to only receive critical errors, and not waste disk space on warning or info log messages.

We were using a fanout exchange, which doesn't give us much flexibility - it's only capable of mindless broadcasting.

We will use a direct exchange instead. The routing algorithm behind a direct exchange is simple - a message goes to the queues whose binding key exactly matches the routing key of the message.

To illustrate that, consider the following setup:

In this setup, we can see the direct exchange X with two queues bound to it. The first queue is bound with binding key orange, and the second has two bindings, one with binding key black and the other one with green.

In such a setup a message published to the exchange with a routing key orange will be routed to queue Q1. Messages with a routing key of black or green will go to Q2. All other messages will be discarded.

Multiple bindings

It is perfectly legal to bind multiple queues with the same binding key. In our example we could add a binding between X and Q1 with binding key black. In that case, the direct exchange will behave like fanout and will broadcast the message to all the matching queues. A message with routing key black will be delivered to both Q1 and Q2.

Emitting logs

We'll use this model for our logging system. Instead of fanout we'll send messages to a direct exchange. We will supply the log severity as a routing key. That way the receiving method will be able to select the severity it wants to receive. Let's focus on emitting logs first.

As always, we need to create an exchange first:

[ch direct:@"logs"];

And we're ready to send a message:

RMQExchange *x = [ch direct:@"logs"];
[x publish:[msg dataUsingEncoding:NSUTF8StringEncoding] routingKey:severity];

To simplify things we will assume that 'severity' can be one of info, warning, or error.

Subscribing

Receiving messages will work just like in the previous tutorial, with one exception - we're going to create a new binding for each severity we're interested in.

RMQQueue *q = [ch queue:@"" options:RMQQueueDeclareExclusive];

NSArray *severities = @[@"error", @"warning", @"info"];
for (NSString *severity in severities) {
    [q bind:x routingKey:severity];
}

Putting it all together

The code for the emitLogDirect method:

- (void)emitLogDirect:(NSString *)msg severity:(NSString *)severity {
    RMQConnection *conn = [[RMQConnection alloc] initWithDelegate:[RMQConnectionDelegateLogger new]];
    [conn start];

    id<RMQChannel> ch = [conn createChannel];
    RMQExchange *x    = [ch direct:@"direct_logs"];

    [x publish:[msg dataUsingEncoding:NSUTF8StringEncoding] routingKey:severity];
    NSLog(@"Sent '%@'", msg);

    [conn close];
}

The code for receiveLogsDirect:

- (void)receiveLogsDirect {
    RMQConnection *conn = [[RMQConnection alloc] initWithDelegate:[RMQConnectionDelegateLogger new]];
    [conn start];

    id<RMQChannel> ch = [conn createChannel];
    RMQExchange *x    = [ch direct:@"direct_logs"];
    RMQQueue *q       = [ch queue:@"" options:RMQQueueDeclareExclusive];

    NSArray *severities = @[@"error", @"warning", @"info"];
    for (NSString *severity in severities) {
        [q bind:x routingKey:severity];
    }

    NSLog(@"Waiting for logs.");

    [q subscribe:^(RMQMessage * _Nonnull message) {
        NSLog(@"%@:%@", message.routingKey, [[NSString alloc] initWithData:message.body encoding:NSUTF8StringEncoding]);
    }];
}

To emit an error log message just call:

[self emitLogDirect:@"Hi there!" severity:@"error"];

(source code)

Move on to tutorial 5 to find out how to listen for messages based on a pattern.