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.