Testing GenServers in Elixir

Testing GenServers in Elixir

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The Elixir programming language logo

A GenServer is a behavior in the Elixir programming language that implements the client-server model. It provides a standardized approach to handle client requests and maintain server state. However, to ensure the reliability and correctness of our GenServers, thorough testing is essential. This article explores different strategies for testing GenServers in Elixir.

Unit Testing

Unit testing at the GenServer level involves testing individual functions and their behavior in isolation.

Using ExUnit

ExUnit is the built-in testing framework in Elixir. We can use it to create test cases for our GenServers.

```elixir defmodule MyGenServerTest do

 use ExUnit.Case

 alias MyApp.MyGenServer

 setup do
   {:ok, pid} = MyGenServer.start_link()
   {:ok, pid: pid}
 end

 test "handle_message/2 returns expected response" do
   pid = Map.get(:pid)

   assert {:response, "Hello"} = MyGenServer.handle_message(pid, :request)
 end

 test "handle_message/2 updates server state correctly" do
   pid = Map.get(:pid)

   assert {:ok, _} = MyGenServer.handle_message(pid, :update_state)
   assert :updated == GenServer.call(pid, :get_state)
 end

end ```

Using Mocks

Mocks can be used to replace external dependencies or simulate certain scenarios during testing.

```elixir defmodule MyGenServerTest do

 use ExUnit.Case
 import Mox

 alias MyApp.ExternalDependency
 alias MyApp.MyGenServer

 defmodule MockedExternalDependency do
   @behaviour ExternalDependency

   def some_function(_pid), do: "Mocked response"
 end

 setup do
   {:ok, pid} = GenServer.start_link(MyGenServer, nil)
   {:ok, pid: pid}
 end

 test "handle_message/2 with mocked dependency" do
   pid = Map.get(:pid)

   stub_with(MockedExternalDependency, fn _pid -> "Mocked response" end)

   assert "Mocked response" == MyGenServer.handle_message(pid, :request)

   assert_receive "Expected message"
 end

 test "handle_message/2 behaves correctly without mocked dependency" do
   pid = Map.get(:pid)

   assert "Actual response" == MyGenServer.handle_message(pid, :request)

   refute_receive "Unexpected message"
 end

end ```

Integration Testing

Integration testing involves testing the interaction between multiple GenServers or with external systems.

Using ExUnit and ExMachina

ExMachina is a testing library that provides utilities for writing integration tests.

```elixir defmodule MyGenServerTest do

 use ExUnit.Case, async: true
 use ExMachina.Ecto, repo: MyApp.Repo

 alias MyApp.Repo
 alias MyApp.MyGenServer

 setup do
   Ecto.Adapters.SQL.Sandbox.mode(Repo, {:shared, self()})

   {:ok, pid} = GenServer.start_link(MyGenServer, nil)
   {:ok, pid: pid}
 end

 test "handle_message/2 updates database correctly" do
   pid = Map.get(:pid)

   assert {:ok, _} = MyGenServer.handle_message(pid, :update_database)

   assert {:ok, %{} = _changeset} = Repo.get_by(MyModel, id: 1)
 end

end ```

Property-based Testing

Property-based testing generates random inputs and verifies if specific properties hold true for those inputs.

Using StreamData

StreamData is a library that provides functionality for property-based testing in Elixir.

```elixir defmodule MyGenServerTest do

 use ExUnit.Properties

 alias MyApp.MyGenServer

 property "handle_message/2 returns a response" do
   check all message <- StreamData.binary() do
     {:response, response} = MyGenServer.handle_message(pid, message)
     assert is_binary(response)
   end
 end

end ```

Conclusion

Testing GenServers in Elixir is crucial to ensure their reliability and correctness. By implementing various testing strategies such as unit testing, integration testing, and property-based testing, we can confidently build robust and error-free GenServers.