We analyzed the design and testing of a distributed message queuing system called Courier, developed by Datadog. The team aimed to tackle challenges in analyzing complex systems by applying formal modeling and lightweight simulations techniques, which they used to verify the correctness properties of Courier before its implementation. Formal modeling involved using TLA+ (Temporal Logic with Allen's Intervals), a high-level specification language that allows for precise definition of system specifications, capturing both structure and behavior, and verifying system properties against those definitions. Lightweight simulations were used to describe statistical properties under various real-world conditions and workloads, providing insights into performance characteristics. The team also applied chaos testing to validate the system's robustness in failure scenarios. After implementing Courier, they performed performance testing to optimize its design, including adding a sequencer process to order messages for delivery. The TLA+ model was updated to reflect this change, but it still did not guarantee perfect alignment with the implementation due to subtle differences. Deterministic simulations, which involve executing an actual implementation of a system in tightly controlled environments, are considered for future use to verify every release and cover a large number of test cases.