Capped Collections
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. You
can create an index on the collection, but mongod doesn't delete
expired documents.
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 Replica Set Oplog that stores a log
of the operations in a replica set uses a capped collection.
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
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 similar to using the tail
command on a log file.
Aggregation $out
The aggregation pipeline stage $out
cannot write results to a capped collection.
Transactions
You cannot write to capped collections in transactions.
Procedures
Create a Capped Collection
You must create capped collections explicitly using the
db.createCollection() method, which is a
mongo shell helper for the create command.
When creating a capped collection you must specify the maximum size of
the collection in bytes, which MongoDB pre-allocates for the
collection. The size of the capped collection includes a small amount of
space for internal overhead.
db.createCollection( "log", { capped: true, size: 100000 } )
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:
db.createCollection("log", { capped : true, size : 5242880, max : 5000 } )
Important
The size argument is always required, even when
you specify the max number of documents. MongoDB removes older
documents if a collection reaches the maximum size limit before it
reaches the maximum document count.
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:
db.cappedCollection.find().sort( { $natural: -1 } )
Check if a Collection is Capped
Use the isCapped() method to determine if a
collection is capped, as follows:
db.collection.isCapped()
Convert a Collection to Capped
You can convert a non-capped collection to a capped collection with
the convertToCapped command:
db.runCommand({"convertToCapped": "mycoll", size: 100000});
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.