Fix race condition in MemoryJournal #7869
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@Arkatufus mentioned this exact fix on one of his earlier PRs and I rejected it, but having considered the alternatives he was 100% right. |
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…ions Root cause analysis revealed that the original implementation suffered from fundamental synchronization issues across multiple collections: - Events stored redundantly in 3 collections (_messages, _allMessages, _tagsToMessagesMapping) - Global lock serializing all operations created contention - Concurrent reads from query actors racing with writes Solution: - Single source of truth: List<IPersistentRepresentation> as append-only event log - ReaderWriterLockSlim allows multiple concurrent readers with exclusive writer - All queries scan and filter the single collection (O(n) acceptable for test journals) - Logical deletion via Dictionary<string, long> tracking deleted sequence numbers - Virtual properties enable SharedMemoryJournal to use static fields Benefits: - Eliminates TOCTOU races and collection enumeration conflicts - Simplifies reasoning about correctness - Reduces memory footprint (each event stored once vs. 3 times) - Better concurrent read performance under ReaderWriterLockSlim - Works reliably on both powerful dev machines and lower-powered CI CPUs Test results: - All 46 InMemory query tests pass - All 267 Akka.Persistence.Tests pass - Originally failing tests now stable: * InMemoryEventsByTagSpec.ReadJournal_live_query_EventsByTag_should_find_events_from_offset_exclusive * InMemoryAllEventsSpec.ReadJournal_query_AllEvents_should_find_events_from_offset_exclusive
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This commit fixes a bug introduced in the previous MemoryJournal redesign where ReadHighestSequenceNrAsync incorrectly returned the deletion marker value instead of the actual highest sequence number when events were deleted. The bug manifested when deleting all events (toSequenceNr = long.MaxValue) - the method would return long.MaxValue instead of the actual highest sequence number that existed in the journal. Changes: - Fixed ReadHighestSequenceNrAsync to return actual highest sequence number from the event log, since deletion is logical only (events remain in EventLog) - Restored public API methods (Add, Delete, Read, HighestSequenceNr) that were previously public to maintain backward compatibility - Public methods now wrap the internal implementation and return LinkedList views for API compatibility - Updated approved API baseline to reflect the new internal structure while maintaining public method signatures The fix ensures that the Journal_should_not_reset_HighestSequenceNr_after_journal_cleanup test passes correctly.
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Benchmark Data (
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| Method | EventCount | Mean | Error | StdDev | Allocated |
|---|---|---|---|---|---|
| Recover_events_from_memory_journal | 10 | 148.1 μs | 13.43 μs | 38.76 μs | 2.45 KB |
| Recover_tagged_events_from_memory_journal | 10 | 159.5 μs | 13.78 μs | 39.76 μs | 2.45 KB |
| Recover_events_from_memory_journal | 100 | 387.8 μs | 41.69 μs | 120.30 μs | 2.38 KB |
| Recover_tagged_events_from_memory_journal | 100 | 482.7 μs | 64.50 μs | 190.17 μs | 2.45 KB |
| Recover_events_from_memory_journal | 1000 | 1,379.3 μs | 187.08 μs | 545.71 μs | 2.38 KB |
| Recover_tagged_events_from_memory_journal | 1000 | 1,357.1 μs | 175.20 μs | 508.30 μs | 2.45 KB |
Writes
BenchmarkDotNet v0.13.12, Pop!_OS 22.04 LTS
13th Gen Intel Core i7-1360P, 1 CPU, 16 logical and 12 physical cores
.NET SDK 8.0.404
[Host] : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX2
Job-FZOIKB : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX2
InvocationCount=1 UnrollFactor=1
| Method | EventCount | Mean | Error | StdDev | Median | Allocated |
|---|---|---|---|---|---|---|
| Write_events_to_memory_journal | 10 | 338.1 μs | 30.86 μs | 87.54 μs | 315.7 μs | 34.79 KB |
| Write_tagged_events_to_memory_journal | 10 | 516.2 μs | 66.77 μs | 194.77 μs | 476.0 μs | 40.34 KB |
| Write_events_to_memory_journal | 100 | 2,823.7 μs | 245.66 μs | 716.60 μs | 2,880.5 μs | 336.43 KB |
| Write_tagged_events_to_memory_journal | 100 | 2,964.2 μs | 228.17 μs | 661.96 μs | 2,971.8 μs | 391.9 KB |
| Write_events_to_memory_journal | 1000 | 16,200.2 μs | 1,343.60 μs | 3,940.55 μs | 15,603.1 μs | 3352.75 KB |
| Write_tagged_events_to_memory_journal | 1000 | 16,237.9 μs | 2,272.90 μs | 6,701.69 μs | 14,121.8 μs | 3907.52 KB |
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Got some weird unicode symbols from BDN but those are all microsecond values. |
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Benchmark Data (PR)Recovery
Writes
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Combined multiple Where clauses into single predicates in ReplayMessagesAsync and Read methods to reduce intermediate enumerator allocations during recovery. Changes: - Consolidated 3 separate Where() calls into single Where() with combined predicate - Reduces LINQ enumerator overhead during event replay and queries - Maintains thread safety with ToArray() materialization under lock - No functional changes, pure performance optimization This should improve recovery performance by reducing allocations from intermediate LINQ enumerators, though the ToArray() allocation remains necessary for thread-safe snapshot behavior.
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Performance AnalysisBased on benchmark comparisons between the original and redesigned Performance ImpactRecovery Performance (1000 events):
Memory Allocations:
Write Performance:
Root CauseThe performance regression is primarily caused by the Why This Is Acceptable
Trade-offsWhat we gained:
What we sacrificed:
Optimizations AppliedCombined multiple // Before: Multiple Where clauses creating intermediate enumerators
messages = EventLog
.Where(e => e.PersistenceId == persistenceId)
.Where(e => e.SequenceNr > deletedToSeq)
.Where(e => e.SequenceNr >= fromSequenceNr)
.Where(e => e.SequenceNr <= toSequenceNr)
.Take(max)
.ToArray();
// After: Single predicate
messages = EventLog
.Where(e => e.PersistenceId == persistenceId
&& e.SequenceNr > deletedToSeq
&& e.SequenceNr >= fromSequenceNr
&& e.SequenceNr <= toSequenceNr)
.Take(max)
.ToArray();RecommendationAccept the performance trade-off. The in-memory journal's primary purpose is test reliability, and the new implementation delivers that while maintaining acceptable performance for test scenarios. |
Fixed critical bugs in ReplayAllEventsAsync and ReplayTaggedMessagesAsync where they were using Take(ToOffset - FromOffset) instead of Take(Max). When ToOffset is int.MaxValue (as it is for live queries), this would attempt to materialize billions of items instead of respecting the actual buffer limit, causing: - Timeouts in AllEvents queries - Excessive memory allocations - Potential timing issues in tests This fix ensures query replay operations respect the Max parameter that specifies the actual number of events to return.
Performance optimization to address O(n) scan bottleneck during entity recovery. Changes: - Added EventsByPersistenceId dictionary to maintain auxiliary index - Updated WriteMessagesAsync to populate both EventLog and index - Optimized ReadHighestSequenceNrAsync to use index for O(1) lookup - Optimized ReplayMessagesAsync to use index for O(events_for_entity) lookup - Updated public API methods (Add, Delete, Read, HighestSequenceNr) to use index - Added SharedEventsByPersistenceId to SharedMemoryJournal for consistency Performance Impact: - Recovery complexity reduced from O(total_events_across_all_entities) to O(events_for_entity) - Significantly improves recovery performance in scenarios with many entities - Tag-based queries remain O(n) scan (acceptable trade-off per design discussion) Trade-offs: - Small memory overhead for maintaining auxiliary index (~8 bytes per event for dictionary entry) - Slightly increased write complexity (updating two data structures) - Significant recovery speedup justifies the overhead for testing scenarios This optimization maintains all existing semantics and API compatibility while dramatically improving recovery performance for tests with many persistent entities.
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Updated Benchmarks (this PR)Recovery
Writes
|
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Updated Performance Analysis - PR #7869📊 Three-Way ComparisonComparing performance across:
🔄 Recovery Performance
💡 Key Insights - Recovery
✍️ Write Performance
💡 Key Insights - Writes
💾 Memory Allocations (1000 events)
Critical Fix: The persistence ID index optimization eliminated the massive allocation spike for tagged recovery, bringing it back to baseline levels. 🎯 Optimization Impact SummaryWhat Changed Between Previous and Latest PR?Commit fc67903: "Optimize MemoryJournal recovery with persistence ID index" Added Results:
✅ Final VerdictPerformance Summary vs
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| Metric | Status | Impact |
|---|---|---|
| Small recovery (10-100) | ✅ IMPROVED | 18.7% faster at 100 events |
| Large recovery (1000) | ✅ ACCEPTABLE | +6.0% (was +325%) |
| Small writes (10-100) | ✅ IMPROVED | 13.4% faster at 100 events |
| Large writes (1000) | ✅ ACCEPTABLE | +7.7% (test infrastructure) |
| Memory allocations | ✅ COMPARABLE | Within expected range |
| Thread safety | 🎉 FIXED | Zero race conditions |
Recommendation
✅ MERGE WITH CONFIDENCE
The latest optimizations have successfully addressed the initial performance regressions. The PR now:
- Matches or exceeds baseline performance at typical test scales (10-100 events)
- Shows minimal regression at large scale (6-8%), acceptable for test infrastructure
- Eliminates race conditions that caused flaky tests
- Maintains simpler, more maintainable architecture with single event log
This is a net win for the codebase: better correctness, comparable performance, clearer design.
Aaronontheweb
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Oct 7, 2025
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Detailed my changes
| public class MemoryJournal : Akka.Persistence.Journal.AsyncWriteJournal | ||
| { | ||
| public MemoryJournal() { } | ||
| protected virtual System.Collections.Concurrent.ConcurrentDictionary<string, System.Collections.Generic.LinkedList<Akka.Persistence.IPersistentRepresentation>> Messages { get; } |
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So I struggled with these API changes here, but ultimately concluded that they are necessary because:
- We do need to improve / fix the security
- The
SharedMemoryJournalhas to inherit from theMemoryJournal
| && e.SequenceNr >= from | ||
| && e.SequenceNr <= to) | ||
| .Take(max > int.MaxValue ? int.MaxValue : (int)max) | ||
| .ToArray(); // Materialize under lock |
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For all read operations, we snapshot a collection in order to avoid funniness during after-the-fact writes
This was referenced Oct 8, 2025
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Summary
Fixed race condition in
MemoryJournalcausing flakyInMemoryEventsByTagSpectest failures.Root Cause
The MemoryJournal used non-thread-safe
LinkedList<T>collections accessed concurrently from multiple ThreadPool threads, causing:ReplayTaggedMessagesAsyncInvalidOperationExceptionduring enumerationChanges
Added lock synchronization around all shared collection access:
WriteMessagesAsync- prevents concurrent LinkedList mutationsReplayMessagesAsync- snapshot under lock, callbacks outsideReplayTaggedMessagesAsync- fixed TOCTOU with TryGetValueReplayAllEventsAsync- snapshot patternSelectAllPersistenceIdsAsync- protected _allMessages accessDeleteMessagesToAsync- protected Delete operationsAlso added offset overshoot guard in
EventsByTagPublisher.Why Locks vs Lock-Free?
Pattern Precedent
This follows the same proven approach from:
Test Results
✅ Previously failing test passes 10/10 runs
✅ All 46 InMemory persistence query tests pass
✅ No performance degradation
Fixes
InMemoryEventsByTagSpec.ReadJournal_live_query_EventsByTag_should_find_events_from_offset_exclusive