Ankur Mhatre
6af6e4a3f1
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().
76 lines
2.7 KiB
Markdown
76 lines
2.7 KiB
Markdown
[README](../README.md) · next: [The lock](02-the-lock.md)
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# 1. Three replicas, three executions
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`@Scheduled` is a per-JVM timer. It has no idea that other JVMs exist. Scale a deployment to
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three replicas and every `@Scheduled` method in the application runs three times per tick, on
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three machines, at almost the same instant.
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That is the whole problem, and it is worth seeing rather than reading. `scripts/three-replicas.sh`
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starts three copies of this application against one PostgreSQL database, lets the 3-second
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schedule tick for 30 seconds, and counts.
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From [`docs/output/three-replicas-unlocked.txt`](output/three-replicas-unlocked.txt):
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```
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-- executions per replica --
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instance_id | count
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-------------+-------
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replica-1 | 8
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replica-2 | 8
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replica-3 | 8
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-- pairs of executions that overlapped --
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overlapping_pairs
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-------------------
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8
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first | first_at | second | second_at
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-----------+--------------+-----------+--------------
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replica-1 | 22:48:54.564 | replica-3 | 22:48:54.752
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replica-1 | 22:48:57.551 | replica-3 | 22:48:57.736
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replica-1 | 22:49:00.551 | replica-3 | 22:49:00.736
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```
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Twenty-four executions where eight were due, and eight of them started while another replica was
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still inside the method — 188 milliseconds apart in the first pair, against a job that only takes
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200 milliseconds. Anything in that method which
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is not idempotent is now a support ticket: a second invoice email, a double refund, two rows
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where the unique constraint you did not add would have saved you.
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## The part that makes it hard to spot
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`ReportJob` is annotated `@SchedulerLock` in this run. Look at the end of the same transcript:
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```
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-- the lock row --
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name | locked_by | locked_at | lock_until
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------+-----------+-----------+------------
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(0 rows)
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```
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The annotation is present, the lock table exists, the application started cleanly, and nothing
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was ever locked. `@SchedulerLock` on its own is inert: it needs `@EnableSchedulerLock` to install
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the interceptor and a `LockProvider` bean to have somewhere to record the lock. In this module
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both live on `LockConfiguration`, which is `@Profile("locked")`.
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There is no warning for the missing half. That is the single most important sentence in this
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module — an application with the annotation and without the plumbing behaves exactly like an
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application with no locking at all, and it looks locked in code review.
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## Reproducing it
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```bash
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scripts/postgres.sh start # a throwaway PostgreSQL 14, no Docker, no root
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scripts/three-replicas.sh unlocked
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scripts/three-replicas.sh locked
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```
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or, with Docker:
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```bash
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docker compose up --build # PROFILE=locked docker compose up --build
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```
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next: [The lock](02-the-lock.md)
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