Queueing

Flow-control mechanism: downstream signals upstream to slow producers rather than overwhelm consumers; bounded queues vs unbounded; backpressure vs load shedding (internal vs boundary); credit-based flow control; pull-based streams; anti-patterns (unbounded queues, retries defeating backpressure)

Symptom → diagnosis → action playbook; three analytical lenses (Little's Law, utilisation curve, percentile arithmetic); capacity planning workflow; antipatterns (sizing for average, running at high utilisation, unbounded queues); when performance tuning doesn't apply