Where should concurrency be limited?

Goal & mental model verified

Concurrency coordinates overlapping tasks. Visibility and atomicity are different: volatile can publish writes, but does not make read-modify-write increments atomic. Virtual threads make blocking I/O concurrency cheaper; they do not create CPU or database capacity.

Worked example · design exercise synthesis

Two workers read count=0 and both write 1: one increment is lost. AtomicInteger or a lock can protect this invariant. For quote calls, a semaphore can bound access to a scarce insurer connection.

Engineering decision synthesis

Use virtual threads for many blocking I/O tasks when dependencies fit. Bound downstream concurrency; use workload-appropriate executors for CPU work and decide timeout/cancellation ownership.

Pitfall & diagnosis synthesis

A shared mutable singleton remains shared under virtual threads. Avoid pooling virtual threads. Pinning guidance is JDK-dependent; older synchronized pinning advice should not be repeated universally for newer JDKs.

Improve & validate synthesis

Load-test with slow dependencies. Record pool wait, in-flight calls, CPU and tail latency; stop increasing concurrency when the constrained resource saturates.