What is hot code loading?

Hot code loading (or hot code swapping) is the BEAM's ability to load a new version of a module into a running system without stopping it. The runtime can keep two versions of a module in memory at once: the old version, still running for processes mid-execution, and the new version for anything freshly called.

Processes that were running old code keep running it until they make a fully-qualified call back into their own module (e.g. ?MODULE:loop(State)), at which point they switch to the new version. This "purge on next reference" model is why long-running server loops are typically written to explicitly call back into themselves.

This capability is what let early telecom switches be patched for bug fixes without dropping active calls, and it's still used today for rolling upgrades in systems that can't tolerate downtime.

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