Unpacking sandbox startup latency: why started ≠ ready | Modal Blog

This post introduces Readiness Probes for Modal Sandboxes, a mechanism to define and wait for application-level initialization to complete after container startup. It elaborates on the full sandbox startup lifecycle (Created, Scheduled, Started, Ready, In Use), highlighting the gap between 'Started' and 'Ready' which is often overlooked by benchmarks. The post details how Readiness Probes, implemented as TCP port checks or shell commands, integrate with warm pools to ensure sandboxes are truly ready before being handed out, reducing perceived user latency. It also introduces a new 'ready' event to the sandbox timeline in the dashboard for improved observability.