Anchor prepare cache entry via PreparedStatement back-reference#893
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nikagra wants to merge 2 commits into
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Anchor prepare cache entry via PreparedStatement back-reference#893nikagra wants to merge 2 commits into
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When the cached CompletableFuture is already done, return it directly instead of creating a defensive copy via thenApply(x -> x). Completed futures are immutable (cancel/complete are no-ops), so the copy only served to release the caller's strong reference to the cached value, causing premature weak-value eviction under GC pressure. This led to repeated PREPARE requests being sent to all nodes on every execution, as the cache entry would be garbage-collected between calls. With prepare-on-all-nodes=true (default), each eviction multiplied the re-prepare cost by the cluster node count. The defensive copy is still used for in-flight futures to protect the shared cache entry from cancellation by concurrent waiters. Ref: CUSTOMER-372
Add a 'cacheRetainer' field to DefaultPreparedStatement that holds a strong reference to the CompletableFuture stored in the weak-value prepare cache. This ensures that as long as the application holds a reference to the PreparedStatement, the cache entry won't be GC'd. Combined with the previous fix (skipping defensive copies for completed futures), this provides a complete solution: the cache entry remains alive for the entire lifetime of the PreparedStatement object, not just for the duration of a single prepare() call. When the PreparedStatement becomes unreachable, both it and the cached future become eligible for GC, preserving the memory-bounded behavior of weak-value caching. Ref: CUSTOMER-372
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Pull request overview
This PR addresses premature eviction of entries in the weak-values prepare cache by anchoring the cached CompletableFuture to the resulting DefaultPreparedStatement, ensuring the cache entry stays alive for as long as the application holds the PreparedStatement.
Changes:
- Add a strong back-reference field (
cacheRetainer) and setter toDefaultPreparedStatementto retain the cached future. - In
CqlPrepareAsyncProcessor, attach the cached future to the prepared statement upon successful prepare completion. - Add unit tests covering weak-value retention/eviction behavior, including the new prepared-statement retainer behavior.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.
| File | Description |
|---|---|
| core/src/main/java/com/datastax/oss/driver/internal/core/cql/CqlPrepareAsyncProcessor.java | Anchors the cached future via the prepared statement, and returns cached future directly for completed entries. |
| core/src/main/java/com/datastax/oss/driver/internal/core/cql/DefaultPreparedStatement.java | Adds cacheRetainer field + setter used to strongly retain the cached future. |
| core/src/test/java/com/datastax/oss/driver/internal/core/cql/CqlPrepareAsyncProcessorTest.java | Adds tests for defensive-copy behavior and weak-value cache retention/eviction, including PS-retainer scenarios. |
Comments suppressed due to low confidence (1)
core/src/test/java/com/datastax/oss/driver/internal/core/cql/CqlPrepareAsyncProcessorTest.java:235
- Same brittleness concern as above: this direct
DefaultPreparedStatementconstruction relies on manynullarguments and could easily break with unrelated production changes. A helper that builds a minimally-valid instance (or a dedicated test fixture) would make the GC/retainer test more stable and easier to maintain.
DefaultPreparedStatement ps =
new DefaultPreparedStatement(
java.nio.ByteBuffer.wrap(new byte[] {1, 2, 3, 4}),
"SELECT 1",
com.datastax.oss.driver.internal.core.cql.EmptyColumnDefinitions.INSTANCE,
java.util.Collections.emptyList(),
null,
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| /** | ||
| * Attaches a strong reference to the prepare cache entry, preventing its weak-value eviction as | ||
| * long as this PreparedStatement is reachable. | ||
| */ | ||
| public void setCacheRetainer(Object retainer) { | ||
| this.cacheRetainer = retainer; | ||
| } |
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| for (int i = 0; i < 10; i++) { | ||
| System.gc(); | ||
| Thread.sleep(50); | ||
| cache.cleanUp(); | ||
| if (cache.getIfPresent(request) == null) { | ||
| break; | ||
| } | ||
| } | ||
|
|
||
| // Cache entry may have been evicted (weak values) | ||
| // This is expected behavior - the fix ensures callers who DO hold a reference keep it alive | ||
| // We just verify the cache doesn't throw | ||
| assertThat(cache.size()).isGreaterThanOrEqualTo(0); |
Comment on lines
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| // Simulate what the processor does: create a real DefaultPreparedStatement and set retainer | ||
| DefaultPreparedStatement ps = | ||
| new DefaultPreparedStatement( | ||
| java.nio.ByteBuffer.wrap(new byte[] {1, 2, 3, 4}), | ||
| "SELECT 1", | ||
| com.datastax.oss.driver.internal.core.cql.EmptyColumnDefinitions.INSTANCE, | ||
| java.util.Collections.emptyList(), | ||
| null, |
Comment on lines
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| if (result.isDone()) { | ||
| // Completed futures are immutable (cancel/complete/completeExceptionally are no-ops), | ||
| // so returning the cached instance directly is safe. This also keeps the cache entry | ||
| // alive via the caller's strong reference, preventing premature weak-value eviction | ||
| // under GC pressure. | ||
| return result; | ||
| } | ||
| // Defensive copy for in-flight preparations only: protects the shared cached future | ||
| // from cancellation by one of multiple concurrent waiters. |
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Summary
Adds a strong back-reference from
DefaultPreparedStatementto the cachedCompletableFuture, preventing weak-value eviction for the entire lifetime of the PreparedStatement.Problem
Even with the defensive-copy fix (PR #892), there is still a window where the cache entry can be evicted: if no caller is currently holding the returned
CompletionStage(e.g., between prepare() calls in a long-running application), the weak-value cache entry has no strong references and can be GC'd.Fix
volatile Object cacheRetainerfield toDefaultPreparedStatementCqlPrepareAsyncProcessor, after the prepare handler completes successfully, callps.setCacheRetainer(mine)before completing the futurecache --weak--> CF --> PS --> CF, but since the weak reference is from cache to CF, the entry stays alive as long as PS is reachable from the applicationLifecycle:
Builds on
Testing
Added 2 new tests to
CqlPrepareAsyncProcessorTest:should_keep_cache_entry_alive_via_prepared_statement_retainer— PS holds retainer → GC cannot evictshould_evict_cache_entry_when_prepared_statement_is_unreachable— PS released → GC evicts (no leak)All existing tests pass.
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