Testing HTTP API Clients in Elixir using Req

The content elucidates various strategies for testing HTTP API integrations in Elixir applications, specifically focusing on the Req library. It starts by illustrating the hardships of testing functions that perform external API requests due to potential flakiness and unreliability. The author suggests using mocks and stubs with explicit contracts to better define the expected behavior and mitigate the pain points. The article then describes practical examples of testing by defining contracts and stubs for different environments, employing Mox for flexible mox, using the Req plug option, and setting up Req.Test for concurrent tests. Moreover, it touches upon the concept of 'Testing with Req :plug' which leverages the well-understood Plug API in Elixir for crafting server stubs. The author explains the benefits of making contracts explicit, the drawbacks of getting contracts wrong, and the advantage of testing more production code by pushing mocks further down. Finally, the article mentions 'Req.Test' for configuring the system under test concurrently and testing the network itself by stubbing it out. The piece concludes by summarizing the benefits and trade-offs of explicit contracts, local stubs, ownership stubs, and network stubs in API client testing.