MongoDB : Capped Collections

There are two types of collections normal collection and capped collection

• In comparison to the normal collection, capped collections are created in advance and are fixed in size 
• Normal collections are created dynamically and automatically grow in size to fit extra data
• These capped collections are designed to consume less space and are rotating that means once allocated space is full, it will start writing from older documents again.
• Below operations are not allowed on a capped collection

• Documents can not be removed
• Updates that make documents to grow in size are not allowed 

Capped collections can not be sharded

Creating Capped collection

As discussed above, capped collection needs to be created explicitly, we will discuss commands for the same

> db.createCollection("my_cap_col1",

... {"capped" : true, "size":100000});

{ "ok" : 1 }

> 

above command creates capped collection with below details
capped collection name : my_cap_col1
fixed size : 100000 byes

same way capped collection can be created with limit on number of documents
for example

> db.createCollection("my_cap_col2",

... {"capped":true, "size" : 100000, "max" : 100});

{ "ok" : 1 }

> show collections;

my_cap_col1

my_cap_col2

Once capped collections are created, it can not be modified, it must be dropped and recreated with desired parameters

Converting normal collection to capped collection

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

{ "ok" : 1 }

> 

There is no way to convert capped collection to normal collection