API Gateway returns 503 after one second - no Lambda request id

Question

During high load I get the following trace roughly one second after the authorizer of my API Gateway was successfully called:

"(a03fe070-b6d0-4e4c-bea1-9cc77f3f0abc) Lambda invocation failed with status: 503. Lambda request id: N/A"

Any hints what might have failed? I don't see any other traces (in called/not-called Lambda), throttling events or anything like this. Number of calls, though rather high, seems to be within the quotas.

Note that 99.9% of all calls succeed.

Answer

A 503 error code indicates a problem with the backend integration, where the API Gateway API cannot receive a response from the backend server. This error may occur in situation when the backend server is overwhelmed by excessive traffic and cannot handle new client requests.

API Gateway returns 503 after one second - no Lambda request id

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