Cloud//Cloudflare//Workers//V8 isolate

A lightweight sandbox within the V8 JavaScript engine. Each Worker request gets its own isolate, an independent execution context with its own memory, but sharing the same V8 engine process.

A lightweight sandbox within the V8 JavaScript engine. Each Worker request gets its own isolate, an independent execution context with its own memory, but sharing the same V8 engine process.

Not a container. Not a VM. Not a Docker process. Just a V8 sandbox: starts in microseconds, not milliseconds.

Single-threaded: V8 is single-threaded, so each isolate uses one thread. A PoP runs many isolates in parallel across multiple V8 engine instances to handle concurrency.

Why Workers are so limited: isolates share resources on the same engine. If one isolate hogs the CPU, others in the same engine stall. That's why CPU time is capped, to keep every request fair and the PoP responsive.

The tradeoff vs Lambda containers: isolates are faster to start (near-zero cold start) and lighter, but they can't do heavy compute or access the filesystem. Containers are heavier but more capable.

No VM, no Docker, no containerization, no OS: just V8. That's why it's so fast AND so limited.