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Add the scheduling module

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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().
This commit is contained in:
Ankur Mhatre
2026-09-01 23:28:06 +05:30
parent 243cccd4ca
commit 6af6e4a3f1
31 changed files with 1373 additions and 1 deletions
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README.md
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@@ -1,6 +1,7 @@
# spring-async-demo
Companion code for the asynchronous execution series on [ankurm.com](https://ankurm.com). Each
Companion code for the asynchronous execution and scheduling series on
[ankurm.com](https://ankurm.com). Each
directory is a self-contained Maven project for one article, with its own `pom.xml`, its own
numbered documentation chapters, and its own captured output under `docs/output/` — regenerated
by that module's `scripts/run-all.sh`, never typed by hand.
@@ -8,6 +9,7 @@ by that module's `scripts/run-all.sh`, never typed by hand.
| Module | Article | What it demonstrates |
|---|---|---|
| [`async/`](async/README.md) | [@Async in Spring Boot 4: Executors, Virtual Threads and the Self-Invocation Trap](https://ankurm.com/spring-boot-4-async-executors-virtual-threads/) | Which thread a method actually ran on, in every case where the answer is not the one you expect |
| [`scheduling/`](scheduling/README.md) | [@Scheduled, ShedLock and Distributed Cron: Scheduling That Survives Three Replicas](https://ankurm.com/spring-scheduled-shedlock-distributed-cron/) | Three replicas against one database running the same job three times, then one row and one conditional UPDATE fixing it |
## Common ground
@@ -20,6 +22,15 @@ one: the name of the thread that ran the work, returned by the code itself. Timi
fast synchronous call from an asynchronous one, which is why `@Async` failures survive so long in
production.
The two modules share a mechanism, which is why they live together: both `@Async` and ShedLock's
default `PROXY_METHOD` intercept mode are Spring AOP proxies. Every proxy limitation the `async`
module measures — self-invocation, `final` methods — applies unchanged to a
`@SchedulerLock` method, and silently produces an unlocked job rather than a synchronous one.
The `scheduling` module also needs a database. `scheduling/scripts/postgres.sh` unpacks a
throwaway PostgreSQL 14 into `target/` with no Docker and no root, which is how its transcripts
were produced; `docker-compose.yml` is there for anyone who would rather use Docker.
## Licence
MIT — see [LICENSE](LICENSE).