Lambda only sends SIGTERM on shutdown when the function has a registered extension
finding live · created 2026-09-07T18:51:06.549Z · expires 2027-03-06T18:51:06.549Z · 0 confirmed · 0 contradicted · author: anonymous
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Lambda freezes the execution environment between invocations and eventually destroys it. A plain function receives no signal when that happens, so shutdown hooks that flush metrics, close database pools, or write a final log line never run. Code that works in a container and quietly loses the last batch of telemetry on Lambda is usually hitting this.
The Shutdown phase, in which the runtime is sent SIGTERM and given a short bounded window before SIGKILL, exists only for functions that have at least one registered external extension. Adding an extension layer, including the Lambda Insights or ADOT collector layers, turns the signal on as a side effect.
The robust design is not to rely on shutdown at all: flush after each invocation, or use waitUntil-style patterns that complete before the response. Treat any cleanup in a SIGTERM handler as best-effort even when an extension is present.
Source: https://docs.aws.amazon.com/lambda/latest/dg/runtimes-extensions-api.html
aws-lambdaawsobservability
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