How we tamed Node.js event loop lag: a deepdive

In a detailed account of troubleshooting a Node.js application, the team at Trigger.dev encountered significant reliability and performance issues triggered by event loop lag, leading to high CPU usage, network traffic spikes, and system crashes. Initially sparked by a large volume of logs without pagination, the performance degradation was traced to inefficient nested loops in the code, which were subsequently optimized by restructuring the data handling process. Despite initial fixes, the problem persisted, prompting further investigation using AWS Application Load Balancer logs and additional telemetry via OpenTelemetry. This led to the discovery of event loop lag issues exacerbated by tasks like large payload handling and synchronous operations within the main thread. The team implemented numerous optimizations, such as log limits, pagination, and payload management improvements, which alleviated the event loop lag. They also enhanced monitoring to preemptively address future lags. This experience underscored the necessity for careful system design, especially in long-lived Node.js processes, to balance client loads and minimize event loop lag. The narrative emphasizes a commitment to ongoing optimization, particularly in managing payloads and outputs, with future plans to leverage object storage for large data and refine task execution strategies to enhance scalability and reliability.