What is the role of the Write-Ahead Log (WAL) in Qdrant?

The WAL is an append-only log that records every write operation before it's considered durable, serving as the recovery mechanism that lets Qdrant reconstruct any recent changes that might not have been fully persisted to segment storage if the process crashes or restarts unexpectedly.

Because appending to a sequential log is a comparatively cheap operation, writing to the WAL first — before the more involved work of updating in-memory structures, the HNSW graph, and payload indexes — lets Qdrant acknowledge a write quickly while still guaranteeing that the change won't be silently lost even if something goes wrong immediately afterward.

On restart after an unclean shutdown, Qdrant replays the WAL to bring segment storage back to a consistent state reflecting every acknowledged write, which is the mechanism that gives Qdrant durability guarantees without needing every single write to be fully reflected in the final, optimized index structures before it can be acknowledged as complete.