queues

One malformed message can silently wedge an entire pipeline, and it happens at every customer eventually. The DLQ answer interviewers want covers what most candidates forget: what happens *after* the message lands there.

The bug only appears under load, the code 'hasn't changed,' and the customer just got charged twice. There's one root-cause pattern behind almost every version of this incident, and a fix with a subtle race most candidates miss.

A 'simple' queue question with four traps wired in: minutes-long tasks break every default timeout, cancellation has to actually stop spend, progress needs a contract, and one bad job must never wedge the lane.