Three replicas of one application against one PostgreSQL database, proving
duplicate @Scheduled execution and then removing it with ShedLock: 24
executions where 8 were due, then 7 for 7 ticks.
Also measured: @SchedulerLock without @EnableSchedulerLock does nothing and
warns about nothing; spring.task.scheduling.pool.size=1 does not starve a
fixedRate job but delays it and fires 35 of 40 executions in a burst; and a
node whose clock is 40 seconds fast takes a live lock unless the provider
uses usingDbTime().
Every @Async behaviour that surprises people, asserted by a test and captured
to docs/output/: the self-invocation trap, what CGLIB cannot override, the
IllegalArgumentException a plain return type throws, the unbounded queue that
makes max-size decoration, spring.task.execution.propagate-context (new in
Boot 4.1.0), the two Executor beans that leave @Async on an unpooled
SimpleAsyncTaskExecutor, and JEP 491 measured on JDK 21 against JDK 25.
