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SERVER-49602 Architecture guide: Clustered Indexes

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Louis Williams 2021-04-02 13:45:12 -04:00 committed by Evergreen Agent
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src/mongo/db/catalog/README.md
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@ -1718,6 +1718,27 @@ schema, per returned document:
}
```
# Time-series Collections
## Clustered Collections
Collections clustered by _id store documents in _id-order on the RecordStore. Unlike regular
collections, they do not require a separate index from _id values to RecordIds. RecordIDs are
encoded using each document's _id value, rather than a 64-bit integer. Currently, this value must be
an ObjectId.
Clustered collections may be created with the `clusteredIndex` collection creation option, but this
feature is limited to internal time-series buckets collections only.
## TTL Deletions
Like a secondary TTL index, clustered collections can delete old data when created with the
`expireAfterSeconds` collection creation option.
The TTL monitor will only delete data from a time-series bucket collection when a bucket's minimum
time, _id, is past the expiration plus the bucket maximum time span (default 1 hour). This
procedure avoids deleting buckets with data that is not older than the expiration time.
# Glossary
**binary comparable**: Two values are binary comparable if the lexicographical order over their byte
representation, from lower memory addresses to higher addresses, is the same as the defined ordering