MongoDB: The Definitive Guide, 2nd Edition

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

Capped Collections

“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.

Figure 6-1. New documents are inserted at the ...