Capped collections are fixed-size collections and they act like queues. The documents added to it are added towards the end of the collection, removing the oldest entry in the collection, if the space allocated to the collection becomes full. They provide fast access to the limited-sized collections even without the use of the index. They are naturally sorted by the order of the insertion, and any retrieval needed on them ordered by time can be retrieved using the $natural sort order. The following diagram gives a pictorial representation of a capped collection whose size is enough to hold up to three documents of equal size (which is too small for any practical use, but good for illustration purposes). As we see in the diagram, the collection is similar to a circular queue, where the oldest document is replaced by the newly added document, should the collection become full:

Tailable cursors are a special type of cursor that tails...