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Database · Picking the Right DB · 2 articles
TopicsAllDatabaseReal ProblemsAWSSystem Design
DatabaseAll in sectionSchema DesignQuery OptimizationPicking the Right DBClickHouse
DatabasePicking the Right DB·15 min read

How Redis Actually Deletes Expired Keys — The Lazy + Active Hybrid You Didn't Know About

A senior-engineer walkthrough of how Redis really handles TTLs. Why TTL=0 doesn't free memory. The two strategies — lazy (on-access) and active (the random-sample-and-25%-threshold sampler). The hz parameter, active-expire-effort, lazyfree-lazy-expire. DEL vs UNLINK and why big-key expiry used to pause the event loop. How replicas handle expiration via the replication stream. maxmemory + eviction policies as the actual safety net. The metrics from INFO that tell you when the sampler is falling behind.

DatabaseRedisCachingTTLRead →

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Database·Picking the Right DB

LSM Trees, the Deep Cut — MemTable, WAL, SSTables, Compaction and Why Your Writes Are So Fast

A senior-engineer walkthrough of Log-Structured Merge Trees: MemTable + WAL durability, SSTable layout with sparse index and bloom filters, the L0→Ln level hierarchy, leveled vs tiered vs FIFO compaction, the three amplifications (write/read/space), bloom filter math, and the RocksDB and Cassandra tuning knobs that actually matter. Why RocksDB, LevelDB, Cassandra, HBase, ScyllaDB, ClickHouse and TiKV all picked it — and when you should not.

DatabaseStorage EngineLSM Tree22 min read·
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