Chapter 6. Special Index and Collection Types
This chapter covers the special collections and index types MongoDB has available, including:
Capped collections for queue-like data
TTL indexes for caches
Full-text indexes for simple string searching
Geospatial indexes for 2D and spherical geometries
GridFS for storing large files
“Normal” collections in MongoDB are created dynamically and automatically grow in size to fit additional data. MongoDB also supports a different type of collection, called a capped collection, which is created in advance and is fixed in size (see Figure 6-1). Having fixed-size collections brings up an interesting question: what happens when we try to insert into a capped collection that is already full? The answer is that capped collections behave like circular queues: if we’re out of space, the oldest document will be deleted, and the new one will take its place (see Figure 6-2). This means that capped collections automatically age-out the oldest documents as new documents are inserted.
Certain operations are not allowed on capped collections. Documents cannot be removed or deleted (aside from the automatic age-out described earlier), and updates that would cause documents to grow in size are disallowed. By preventing these two operations, we guarantee that documents in a capped collection are stored in insertion order and that there is no need to maintain a free list for space from removed documents.