What is Capped Collections in MongoDB?

Capped collections are fixed-size collections that support high-throughput operations that insert and retrieve documents based on insertion order. Capped collections work in a way similar to circular buffers: once a collection fills its allocated space, it makes room for new documents by overwriting the oldest documents in the collection.

Circular buffers are filled and emptied, so capped collection has similar behaviours.

Some notes:
- Automatic Removal of Oldest Documents
- Capped collections have an _id field and an index on the _id field by default.
- You cannot delete documents from a capped collection. To remove all documents from a collection, use the drop() method to drop the collection and recreate the capped collection.
- You cannot shard a capped collection.
- You must create capped collections explicitly

To create a capped collection:

db.createCollection( "cpu_metrics", { capped: true, size: 100000 } )

The name of the collection in our example is cpu_metrics with the flag of capped=true and size of the collection.

You can also specify maximum number of documents for the collection using max as following example:

db.createCollection("cpu_metrics", { capped : true, size : 5242880, max : 5000 } )

NOTE: the size argument is always required, even when you specify max number of documents.

Check whether collection is capped or not:

db.collection.isCapped()

Convert a non-capped collection to capped:

db.runCommand({"convertToCapped": "mycoll", size: 100000});

#mongodb #mongo #capped #collection #capped_collection

Earlier we explained about capped collection in MongoDB. Today we just want to add something more to it.


Query a Capped Collection:
If you perform a find() on a capped collection with no ordering specified, MongoDB guarantees that the ordering of results is the same as the insertion order.

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 } )

#mongodb #mongo #capped_collection #natural #natural_order

If you go to your mongodb data directory where all database data will be stored you will see:

$ ls -lh YOUR_DATABASE.*
 -rw------- 1 mongodb mongodb  64M Feb  6 07:26 YOUR_DATABASE.0
 -rw------- 1 mongodb mongodb 512M Feb  6 07:26 YOUR_DATABASE.1
 -rw------- 1 mongodb mongodb  16M Feb  6 07:26 YOUR_DATABASE.ns

If you have given the size of 524288000 in collection creation, then you would see 512MB for your DB size. You can also see the whole size inside of mongo shell.

rs0:PRIMARY> show dbs
 local         0.203GB
 YOUR_DATABASE 0.578GB

#mongodb #mongo #capped_collection

You cannot #shard a #MongoDB #capped #collection.

#capped_collection