Scaling to 1 million concurrent sandboxes in seconds

Modal rebuilt its sandbox platform to meet growing demand from AI agents and reinforcement learning workloads, claiming it can now run millions of concurrent sandboxes and create tens of thousands per second, including one million sandboxes in under a minute. The redesign replaces centralized coordination and strongly consistent databases in the critical creation path with horizontally scalable scheduling servers that use cached worker-state data, directly request container creation from workers, and rely on asynchronous Redis streams and durable metadata storage outside the latency-sensitive path. Modal argues that conventional systems such as Kubernetes and its prior Postgres-based architecture face scaling constraints from serialized scheduling, centralized state, and operations proportional to the number of containers or nodes. Developing the new system required extensive backend, worker-management, runtime, networking, observability, and feature rewrites, including changes to mitigate Linux kernel networking contention during large startup bursts. Benchmark results showed median sandbox startup times below half a second, though the company notes a longer latency tail under extreme concurrent starts and plans further container-startup optimizations; the platform is currently available as a beta opt-in before wider deployment.

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