Bulkhead
A separate limit on concurrent calls to one dependency, so a slow dependency can't use up resources the rest of the service needs.
Definition
Named after the watertight compartments in a ship. A bulkhead caps how many requests to, say, a recommendations service can be in progress at once; extra calls are rejected immediately or wait briefly, and the caller falls back. Rate limits count requests per second, but slow requests pile up even at a constant rate, which is what bulkheads and other concurrency limits protect against. Every permit must be released, including on errors and timeouts.
In practice
At most 50 concurrent calls go to recommendations; when they're all busy, product pages show no recommendations instead of timing out.