Testing GenServers with Erlang Trace

When working on a messaging system with RabbitMQ for my upcoming workshop at ElixirConf, I ran into a common Elixir testing challenge--testing that a GenServer received a message. Thanks to lots of Googling and some help from a friend, I learned you can test that a GenServer received a message with the help of Erlang tracing.

The Problem

ExUnit provides an assert_receive/3, but that only allows you to check the mailbox of the current process, i.e. the process running the test. So, how can we check that a GenServer running in our application received a certain message? This is the issue I encountered when testing our RabbitMQ consumer GenServer.

We have a GenServer that runs when the application starts up and consumes messages from a RabbitMQ queue. How can we set an expectation that the consumer does in fact receive and process a given message sent by a publisher to that queue?

We can do exactly that with the help of ExUnit's start_supervised/2 callback and Erlang's trace/3 function.

Introducing Erlang Trace

Erlang's trace/3 function is pretty powerful. It allows us to attach a trace to a specified process. What does this mean? If we trace a given process, we are telling Erlang to send a message to a calling process (in our case, the test), whenever the trace process receives a message. Sneaky!

We'll use ExUnit's start_supervised/2 function to start our GenServer and capture its PID. Then, we'll use trace/3 to ensure that whenever the GenServer PID receives a message, our test gets notified. With that in place, we can assert that the test received a message from the trace, thereby testing that our GenServer received a message.

Let's do it!

Step 1. Start the GenServer with start_supervised/2

First, we'll start up our GenServer and capture its PID

Step 2: Start the trace

Next, we'll use Erlang's trace/3 function to trace the GenServer PID such that the test process receives a message whenever the GenServer PID does.

Step 3: Set the Assertion

Now we're ready to enact the code that we expect to result in our GenServer receiving a message--publishing to a RabbitMQ queue.

Publishes a message should cause our consumer GenServer to receive the :basic_consume_ok message. This will in turn send the following message to the test process which started the trace:

So, we can use ExUnit's assert_receive/3 function to assert that the test receives this message:

In this way, we can in fact test that our GenServer received a certain message.


Erlang's trace/3 function adds a powerful tool to our Elixir testing arsenal. It helps us solve a common testing problem--that of asserting that your GenServers received a certain message. Together with ExUnit's start_supervised/2 callback and assert_receive/3 function, we were able to write exactly the test we needed for our RabbitMQ messaging system.

Special Thanks

Special thanks to Steven Nuñez who turned me on to Erlang's trace/3 function and who generally gives me all of my ideas. Check out his recent post on managing RabbitMQ connections in Elixir with ExRabbitPool and learn more about working with RabbitMQ and Elixir by signing up for our ElixirConf 2020 workshop!

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Sophie DeBenedetto

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