A capped collection in MongoDB is a fixed-size collection that maintains insertion order. Once a capped collection reaches its maximum size, it overwrites the oldest documents with the newest ones. Creating a capped collection is fairly simple, just issue the ‘createCollection()‘ command with the ‘capped‘ option set to ‘true‘, and specify the ‘size‘ of the collection in bytes (or ‘max‘ number of documents) as an optional parameter:
db.createCollection("my_capped_collection", { capped: true, size: 10000000 })This creates a capped collection named ‘my_capped_collection‘ that has a maximum size of 10 megabytes (in this case). The maximum number of documents is specified using the ‘max‘ option.
The benefits of using a capped collection are:
- **Insertion order preservation**: Since documents are always written to the end of the collection and older documents are purged as new documents are added, a capped collection preserves the insertion order of its documents.
- **Automatic maintenance**: With the automatic purging of older documents, capped collections do not require any manual maintenance.
- **Fast writes**: Capped collections are designed to be highly efficient for write-heavy workloads. Inserting a new document is an O(1) operation, and updates to existing documents that do not increase the document size or require an index rebuild are similarly efficient.
The limitations of capped collections are:
- **Size limited**: The size of a capped collection is fixed and cannot be changed. When a capped collection reaches its maximum size, older documents are automatically purged to make room for new ones.
- **No updates to documents that would increase their size**: Once a document is inserted into a capped collection, you cannot update it in a way that would increase its size or require an index rebuild. If you try to do so, an error will be thrown.
- **No deletions or updates of individual documents by _id**: Because of the way capped collections are implemented, it is not possible to delete or update individual documents based solely on their ‘_id‘. If you need to remove or modify a document, the only way to do so is to perform a query that returns it and then remove or update it as part of a bulk operation.
In general, capped collections are best used in situations where you need to store a fixed number of documents in a collection, and where you are primarily concerned with tracking the most recent documents that have been added. Some common use cases for capped collections include logging, real-time data processing, and message queues.