How to test Elixir GenServers

Co-authored with Sofiia Yurkevska | Originally published on Freshcode

Summary

This technical guide tackles one of the most common challenges in Elixir development: properly designing and testing GenServers. The article emphasizes that effective GenServer testing starts with proper design principles, advocating for thin coordinators that delegate business logic to external modules rather than embedding complex logic within the process itself.

Key Ideas:

• Design-First Approach: GenServers should act as coordinators, not business logic containers—following single responsibility principle and avoiding OOP-style object treatment
• When to Use GenServers: Clear criteria for when GenServer complexity is justified versus simpler struct-and-function approaches
• Testing Strategies: Two main approaches—isolated callback testing for simple state transitions and live GenServer testing for complex interactions with timeouts and external services
• Explicit Contracts: Using behavior-based adapters and dependency injection to create testable interfaces with external services
• Practical Implementation: Real-world examples showing stub implementations, failing scenarios, and proper mocking strategies

Read the full article on Freshcode.