asmhatre
66208bcd97
A Spring Boot 4.1.1 module whose test suite is the evidence for the article: 19 tests producing 22 transcripts under docs/output/, plus 12 documentation chapters. Findings the build pins: - @EnableCaching has no exposeProxy attribute; the widely-copied @EnableCaching(exposeProxy = true) does not compile. - Two methods sharing a cache name and an argument type share a key space, and one silently serves the other's answers. - beforeInvocation = true is NOT deferred by TransactionAwareCacheManagerProxy on 7.0.9 - doEvict picks evictIfPresent, which the decorator does not intercept. - Four of five invalid declarations start a clean context and throw at the first call. - Caffeine on the classpath silently displaces the simple provider.
95 lines
3.8 KiB
Markdown
95 lines
3.8 KiB
Markdown
[← versus Hibernate L2](09-versus-hibernate-l2.md) · [next: diagnostics →](11-diagnostics.md)
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# 10. Caching and transactions
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## The ordering
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The caching interceptor runs **inside** the transaction interceptor. A cache write therefore
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happens at method exit — before the commit, and without any knowledge of whether the commit will
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succeed.
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`docs/output/13-transactions.txt` puts an ordinary write-through update inside an outer
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transaction that then fails:
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```
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nameOf(1) -> Alice
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An outer @Transactional method calls the @CachePut update, which succeeds,
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and then fails on the next step. The transaction rolls back.
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what the cache serves : Alice Cooper
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what the database has : Alice
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```
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The cache is now holding a name no transaction ever committed. Nothing will correct it until the
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entry expires or something evicts it — and on the default provider nothing expires.
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The same shape with `@CacheEvict` is self-healing:
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```
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after the rollback, nameOf(2) -> Bob
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database loads: 2 <- the entry was evicted, so this one reloaded
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```
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That asymmetry is the practical takeaway. **An eviction that fires too early costs a lookup; a
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put that fires too early costs correctness.** When in doubt, evict.
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## `TransactionAwareCacheManagerProxy`
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Wrapping the manager defers every put and evict to a post-commit synchronisation:
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```java
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@Bean
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CacheManager cacheManager() {
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return new TransactionAwareCacheManagerProxy(new ConcurrentMapCacheManager());
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}
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```
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`docs/output/19-transaction-aware.txt` runs the identical rollback and the cache still says
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`Alice`. Several `CacheManager` implementations expose the same thing as
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`setTransactionAware(true)` (they extend `AbstractTransactionSupportingCacheManager`);
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`TransactionAwareCacheManagerProxy` is the generic wrapper, and it lives in
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**`spring-context-support`**, not `spring-context`.
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### What it does not cover
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- **Reads are never deferred.** A `@Cacheable` lookup inside the transaction sees whatever the
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shared cache holds.
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- **Outside a transaction it is a pass-through**, so the same method reached from an unmanaged
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path writes immediately.
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- **It does not make the cache transactional.** Two concurrent transactions still race at commit
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time; last writer wins, and it may be the one whose value is older.
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### A correction worth recording
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`spring-framework#23192` reported that `beforeInvocation = true` was swallowed by the
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transaction-aware decorator — the evict got deferred to commit, which is the opposite of what the
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attribute asks for. That is widely repeated and, on 7.0.9, **no longer true**. The measurement in
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`docs/output/19-transaction-aware.txt` came back the other way round, and
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`docs/output/22-decorator-bytecode.txt` shows why:
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```
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doEvict(cache, key, true) -> Cache.evictIfPresent -> straight to the target cache
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doEvict(cache, key, false) -> Cache.evict -> registerSynchronization, runs after commit
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```
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`AbstractCacheInvoker.doEvict(Cache, Object, boolean)` picks `evictIfPresent` for the immediate
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path, and `TransactionAwareCacheDecorator` only registers a synchronisation in `evict`.
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`evictIfPresent` delegates straight to the target cache.
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This was written the wrong way round first and the run corrected it, which is a reasonable
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advertisement for running the thing.
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## The pattern that actually holds up
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For anything that must not serve uncommitted state:
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1. Evict, do not put — `@CacheEvict(beforeInvocation = true)`.
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2. Make the cache transaction-aware if you also need the eviction not to happen on rollback.
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3. Give every mutable cache a TTL, so the worst case is bounded even when all of the above is
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wrong.
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Related reading: [`@Transactional`: propagation, isolation and the silent
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failures](https://ankurm.com/transactional-propagation-isolation-silent-failures/) covers the
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interceptor ordering from the transaction side.
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