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- Capped Collections
Capped Collections¶
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Overview¶
Capped collections are fixed-size collections that support high-throughput operations that insert and retrieve documents based on insertion order. Capped collections work in a way similar to circular buffers: once a collection fills its allocated space, it makes room for new documents by overwriting the oldest documents in the collection.
See createCollection() or create
for more information on creating capped collections.
Tip
As an alternative to capped collections, consider MongoDB’s TTL (Time To Live) indexes. As described in Expire Data from Collections by Setting TTL, these indexes allow you to expire and remove data from normal collections based on the value of a date-typed field and a TTL value for the index.
TTL indexes are not compatible with capped collections.
Behavior¶
Insertion Order¶
Capped collections guarantee preservation of the insertion order. As a result, queries do not need an index to return documents in insertion order. Without this indexing overhead, capped collections can support higher insertion throughput.
Automatic Removal of Oldest Documents¶
To make room for new documents, capped collections automatically remove the oldest documents in the collection without requiring scripts or explicit remove operations.
Consider the following potential use cases for capped collections:
- Store log information generated by high-volume systems. Inserting documents in a capped collection without an index is close to the speed of writing log information directly to a file system. Furthermore, the built-in first-in-first-out property maintains the order of events, while managing storage use.
- Cache small amounts of data in a capped collections. Since caches are read rather than write heavy, you would either need to ensure that this collection always remains in the working set (i.e. in RAM) or accept some write penalty for the required index or indexes.
For example, the oplog.rs collection that stores a log
of the operations in a replica set uses a capped collection.
Starting in MongoDB 4.0, unlike other capped collections, the oplog can
grow past its configured size limit to avoid deleting the majority
commit point.
_id Index¶
Capped collections have an _id field and an index on the _id
field by default.
Restrictions and Recommendations¶
Updates¶
If you plan to update documents in a capped collection, create an index so that these update operations do not require a collection scan.
Document Size¶
Changed in version 3.2.
If an update or a replacement operation changes the document size, the operation will fail.
Document Deletion¶
You cannot delete documents from a capped collection. To remove all
documents from a collection, use the drop()
method to drop the collection and recreate the capped collection.
Sharding¶
You cannot shard a capped collection.
Query Efficiency¶
Use natural ordering to retrieve the most recently inserted elements from the collection efficiently. This is (somewhat) analogous to tail on a log file.
Transactions¶
Starting in MongoDB 4.2, you cannot write to capped collections in transactions. Reads from capped collections are still supported in transactions.
Procedures¶
Create a Capped Collection¶
You must create capped collections explicitly using the
db.createCollection() method, which is a helper in the
mongo shell for the create command. When
creating a capped collection you must specify the maximum size of the
collection in bytes, which MongoDB will pre-allocate for the collection.
The size of the capped collection includes a small amount of space for
internal overhead.
If the size field is less than or equal to 4096, then the collection will
have a cap of 4096 bytes. Otherwise, MongoDB will raise the provided size to
make it an integer multiple of 256.
Additionally, you may also specify a maximum number of documents for the
collection using the max field as in the following document:
Important
The size argument is always required, even when
you specify max number of documents. MongoDB will remove older
documents if a collection reaches the maximum size limit before it
reaches the maximum document count.
See
Query a Capped Collection¶
If you perform a find() on a capped collection
with no ordering specified, MongoDB guarantees that the ordering of
results is the same as the insertion order.
To retrieve documents in reverse insertion order, issue
find() along with the sort()
method with the $natural parameter set to -1, as shown
in the following example:
Check if a Collection is Capped¶
Use the isCapped() method to determine if a
collection is capped, as follows:
Convert a Collection to Capped¶
You can convert a non-capped collection to a capped collection with
the convertToCapped command:
The size parameter specifies the size of the capped collection in
bytes.
This holds a database exclusive lock for the duration of the operation. Other operations which lock the same database will be blocked until the operation completes. See What locks are taken by some common client operations? for operations that lock the database.
Tailable Cursor¶
You can use a tailable cursor with capped collections. Similar to the
Unix tail -f command, the tailable cursor “tails” the end of a
capped collection. As new documents are inserted into the capped
collection, you can use the tailable cursor to continue retrieving
documents.
See Tailable Cursors for information on creating a tailable cursor.