3.44. MongoDB fixed set (Capped Collections)

3.44.1. Create a fixed collection ¶

We create a fixed collection through createCollection, and the capped option is set to true:

>db.createCollection("cappedLogCollection",{capped:true,size:10000}) 

You can also specify the number of documents, plus the max:1000 attribute:

>db.createCollection("cappedLogCollection",{capped:true,size:10000,max:1000}) 

Determine whether the set is a fixed set:

>db.cappedLogCollection.isCapped() 

If you need to convert an existing collection to a fixed collection, you can use the following command:

>db.runCommand({"convertToCapped":"posts",size:10000}) 

The above code converts our existing posts collection into a fixed collection.

3.44.2. Fixed set query ¶

Fixed collection documents are stored in insertion order, and queries are returned in insertion order by default, or you can use $natural to adjust the return order.

>db.cappedLogCollection.find().sort({$natural:-1}) 

3.44.3. The functional characteristics of fixed sets ¶

You can insert and update, but the update cannot exceed the size of the collection, otherwise the update fails and deletion is not allowed, but you can call drop () to delete all rows in the collection, but you need to explicitly rebuild the collection after drop.

The maximum value of a cappped collection on a 32-bit machine is about 482.5m, which is limited only by the size of the system file on 64-bit.

3.44.4. Fixed set attributes and usage ¶