Testing and verifying concurrent systems is hard due to their non-deterministic nature — verifying behavior that changes with each execution is difficult. Race conditions thrive in the non-deterministic world of thread scheduling. Even more challenging is verifying timeliness constraints, i.e. ensuring that operations complete within specified deadlines or that service guarantees are maintained under load. Traditional testing approaches struggle with concurrency, and mocking strategies often fail to capture the subtle interactions between threads, time, and shared state that cause real production failures.
The io-sim Haskell library, developed
by Well-Typed in partnership with engineers from IOG and Quviq, offers a
compelling solution to this problem. The library provides a pure simulation environment
for IO computations, enabling deterministic execution of concurrent code with
accurate time simulation and detailed execution traces. Unlike other testing
approaches, with io-sim one is able to write highly concurrent, real-time
systems and verify their timeliness constraints in a deterministic manner, by
accurately simulating GHC’s runtime system (e.g. asynchronous exceptions,
timeouts & delays, etc.).
This blog post introduces and explores io-sim through a practical example:
debugging an elevator controller that violates its response time requirements.