Capped Collections

Capped Collections

Capped collections are fixed-size collections that support high-throughput operations that insert, retrieve, and delete 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. Capped collections have the following behaviors:

• 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, they can support higher insertion throughput.
• Capped collections guarantee that insertion order is identical to the order on disk (natural order) and do so by prohibiting updates that increase document size. Capped collections only allow updates that fit the original document size, which ensures a document does not change its location on disk.
• Capped collections automatically remove the oldest documents in the collection without requiring scripts or explicit remove operations.

For example, the oplog.rs collection that stores a log of the operations in a replica set uses a capped collection. 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.

Recommendations and Restrictions

• You can update documents in a collection after inserting them. However, these updates cannot cause the documents to grow. If the update operation causes the document to grow beyond their original size, the update operation will fail.
• If you plan to update documents in a capped collection, create an index so that these update operations do not require a table scan.
• You cannot delete documents from a capped collection. To remove all records from a capped collection, use the ‘emptycapped’ command. To remove the collection entirely, use the drop() method.
• You cannot shard a capped collection.
• Capped collections created after 2.2 have an _id field and an index on the _id field by default. Capped collections created before 2.2 do not have an index on the _id field by default. If you are using capped collections with replication prior to 2.2, you should explicitly create an index on the _id field.
• Use natural ordering to retrieve the most recently inserted elements from the collection efficiently. This is (somewhat) analogous to tail on a log file.
• The aggregation pipeline operator $out cannot write results to a capped collection.

Create a Capped Collection – You must create capped collections explicitly using the 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.

db.createCollection( “log”, { capped: true, size: 100000 } )

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 } )

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.

Automatically Remove Data After a Specified Period of Time – For additional flexibility when expiring data, consider MongoDB’s TTL indexes, as described in Expire Data from Collections by Setting TTL. These indexes allow you to expire and remove data from normal collections using a special type, based on the value of a date-typed field and a TTL value for the index. TTL Collections are not compatible with capped collections.

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.

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