Ensuring Elixir Application Performance with Testing and ExUnit/

...

/

GenServer

GenServer

Get introduced to GenServers, which are usually considered the fundamental abstraction in OTP, and learn how to test GenServers.

We'll cover the following...

A GenServer is a process that holds state and optionally provides an interface to read or update that state.

OTP abstracts all of the common parts of a GenServer, like spawning the process, having a receive loop, and sending messages. What’s left to the user is to implement callbacks that implement code specific to a particular GenServer.

Initializing a GenServer

We’ll see how we can use a GenServer in the Soggy Waffle application, but let’s start with a self-contained example, which will make things easier when illustrating some concepts.

We’ll use a GenServer that provides a simple rolling average of the last N numeric measurements. We can start with a simple implementation by looking at the public API exposed by the GenServer:

Press + to interact
#file path -> example/lib/overloaded_genserver.ex
#add this code at the indicated place mentioned in comments of example/lib/
#overloaded_genserver.ex in the playground widget
def start_link(max_measurements) do
  GenServer.start_link(__MODULE__, max_measurements)
end
def add_element(pid, element) do
  GenServer.cast(pid, {:add_element, element})
end
def average(pid) do
  GenServer.call(pid, :average)
end

We have a function to start the GenServer, a function to store a new element in the GenServer, and a function to get the average of the elements that the GenServer is storing at a given time. From the public API, we know that this is going to be a GenServer (because of the calls ...