Observed Signal · Apr 25, 2026 · Technical Release · Source: DEV Community · Impact: 2/5 · Sentiment: Positive
Rebuilt Three‑Layer Redis–L1–MongoDB Cache
A developer rebuilt the caching layer of the Nexus backend to fix hierarchy, correctness and concurrency bugs across a three-layer system: Redis (master), an in-memory L1 mirror, and MongoDB (persistent backup). The post documents eight classes of failures in the original implementation — including an inverted master hierarchy, silent data loss on flush failures, TOCTOU remove races, deadlock risk from nested task submission, unbounded MongoDB request storms, ignored Redis evictions, incomplete add paths, and O(n) id lookups — and shows code-level fixes and tests. Key changes: treat Redis as source of truth, only clear dirty flags after confirmed Mongo writes, use atomic removes for concurrency, batch reconciliation with a configurable RECONCILE_BATCH_SIZE (50), restore evicted Redis keys from L1, unify write paths, and maintain an id→key reverse index for O(1) lookups. Source code is available in the project's v1.1.0 release on GitHub. Publication date: 2026-04-25.
Practical, runnable engineering fixes that improve backend cache correctness and reliability; useful to backend teams but not industry-shifting.
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Key Takeaways & Evidence Grounding
- The cache architecture uses three layers: Redis (master), L1 in-memory mirror, and MongoDB as persistent backup.
- Three scheduled tasks keep layers in sync: L1 Sync (10s), Auto Flush (15s), and Reconciliation (3 minutes).
- Author identified and fixed eight issues including inverted hierarchy, silent flush data loss, TOCTOU race, deadlock risk, MongoDB request storm, ignored Redis evictions, incomplete add path, and O(n) ID lookup.
- Fixed behaviors include making Redis the source of truth, only removing dirty flags after Mongo confirms writes, batching reconciliation (RECONCILE_BATCH_SIZE = 50) with CompletableFuture.allOf(), atomic remove() for concurrency safety, and restoring evicted Redis keys from L1.
- Full source for the fixes is published in the project's v1.1.0 release on GitHub.
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