TTL (time-to-live) Options
TTL background thread frequency
The frequency for invoking the TTL background removal thread. The value for this option is specified in milliseconds. The lower this value, the more frequently the TTL background thread will kick in and scan all available TTL indexes for expired documents, and the earlier the expired documents will actually be removed.
TTL maximum total removals
In order to avoid “random” load spikes by the background thread suddenly kicking in and removing a lot of documents at once, the number of to-be-removed documents per thread invocation can be capped.
The TTL background thread will go back to sleep once it has removed the configured number of documents in one iteration. If more candidate documents are left for removal, they will be removed in following runs of the background thread.
TTL maximum per-collection removals
This option controls the maximum number of documents to be removed per collection in each background thread run. This value can be configured separately from the total removal amount so that the per-collection time window for locking and potential write-write conflicts can be reduced.
TTL only for loaded collections
This option will only scan TTL indexes of collections that are already loaded into memory. Setting the option to false will make the background thread ignore collections that are currently not loaded. This saves the background thread from loading all collections into memory.
This option is useful for the MMFiles engine only, where collections need to be loaded into memory first. For the RocksDB engine, collections do not need to be loaded into memory first, so this option does not make any difference.