Real-Time Search Indexing: LSM-Trees, Write-Ahead Logs & Segment Merging

Balancing continuous real-time document ingestion with microsecond query performance requires log-structured storage architectures.

1. The LSM-Tree Ingestion Lifecycle

When new documents are indexed, they are immediately recorded to an append-only Write-Ahead Log (WAL) on disk for crash recovery while simultaneously updating an in-memory inverted buffer (MemTable). Queries search across both committed segments and live memory buffers.

2. Immutable Segment Flushes

When the memory buffer reaches threshold capacity, it is written to disk as an immutable index segment. Because segments are never modified in-place, OS file system read caches operate with maximum efficiency without lock contention.

3. Tiered Background Segment Merging

To prevent segment proliferation from degrading query performance, background threads continuously execute tiered merges, consolidating smaller segments into larger optimized structures while permanently purging deleted DocIDs.