MongoDB Documentation, Release 3.0.4 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. Warning: This command obtains a global write lock and will block other operations until it has completed. Changed in version 2.2: Before 2.2, capped collections did not have an index on _id unless you specified autoIndexId to the create , after 2.2 this became the default. Automatically Remove Data After a Specified Period of Time For additional flexibility when expiring data, con- sider MongoDB’s TTL indexes, as described in Expire Data from Collections by Setting TTL (page 211). 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 (page 211) 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. See Create Tailable Cursor (page 123) for information on creating a tailable cursor. New in version 2.2. This document provides an introduction to MongoDB’s “ time to live ” or TTL collection feature. TTL collections make it possible to store data in MongoDB and have the mongod automatically remove data after a specified number of seconds or at a specific clock time. Data expiration is useful for some classes of information, including machine generated event data, logs, and session information that only need to persist for a limited period of time. A special TTL index property (page 503) supports the implementation of TTL collections. The TTL feature relies on a background thread in mongod that reads the date-typed values in the index and removes expired documents from the collection. Expire Data from Collections by Setting TTL 211 5.1. Administration Concepts