# Atlas Stream Processing cached lookup TTL and eviction > Source-anchored research for Atlas Stream Processing cached lookup TTL and eviction. Run status: COMPLETED. Parent: [MongoDB Atlas Stream Processing](https://llms-explorer.com/tree/mongodb-atlas-stream-processing/) · 4 facets · 8 facts · page: https://llms-explorer.com/tree/atlas-stream-processing-cached-lookup-ttl-and-eviction/ ## Structure and components - The required ttl document contains integer size and unit: ms, second, minute, hour, or day. Entries are evicted when TTL elapses. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-cachedlookup.md) ## How it works - MongoDB recommends caching static or slowly changing reference data; its tier guide recommends higher tiers when large reference sets need more cache RAM. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-cachedlookup.md) - Inference: TTL is not a guaranteed residence time, because memory-pressure eviction can remove cached results earlier. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-cachedlookup.md) ## Problems, failure modes and limitations - At the memory limit, eviction frees space for new results. The syntax table describes older/expired results first, then random eviction if insufficient. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-cachedlookup.md) - maxMemUsageBytes defaults to 10% of workspace RAM and cannot exceed 12.5%, according to the syntax table. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-cachedlookup.md) ## Facts and statements - $cachedLookup joins stream messages to an Atlas collection registered in the Connection Registry and caches query results. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-cachedlookup.md) - $cachedLookup does not support let or pipeline fields. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-cachedlookup.md) - The 19 August 2025 release added $cachedLookup; the 27 August release added lifetime cache hit/miss statistics under stats.operatorStats. (low confidence) — [source](https://www.mongodb.com/docs/atlas/atlas-stream-processing/changelog.md)