Concurrency
3 pieces of work and writing on this site touch concurrency.
Case studyPayment Distribution Engine30–40 min → 6–8 min per cycleDecisionHow do you stop a payment being processed twice?An idempotency key enforced by a uniqueness constraint at the point of record, not a check-then-act in application code.JournalWhen Parallel Streams Become a Production ProblemparallelStream() is one call away and looks like free concurrency. On I/O-bound work sharing a JVM-wide pool, it is a way to make unrelated code slow.