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Capped Collections in MongoDB
  • Last Updated : 17 Feb, 2021
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Capped collections are fixed-size collections means when we create the collection, we must fix the maximum size of the collection(in bytes) and the maximum number of documents that it can store. After creation, if we try to add more than documents to their capacity, it overwrites the existing documents. It supports high-throughput operations, that are useful when we insert and retrieve documents based on insertion order. The working of the capped collection is similar to circular buffers, which means once the fixed space is allocated for the capped collection, it creates/makes spaces for the new documents by overwriting the oldest documents in the given collection. Capped collection can also have _id field and by default, each _id field has an index.

How to create a Capped collection?

We create the capped collections using the createCollection() method in MongoDB. When we create a new capped collection, we must specify the maximum size of the collection in bytes, which will MongoDB pre-allocate for the collection. The size of the capped collection contains a small amount of space for the internal overhead and if the value of the size field is less than or equal to 4096, then the capacity of the collection is 4096 bytes. Otherwise, MongoDB will automatically increase the size to make it an integer multiple of 256 and specify a maximum document count using the max field. If the given collection reaches the maximum size limit before it reaches the maximum document count, then MongoDB removes older documents to create space for the new document.

Syntax:

db.createCollection(<name>, {capped: <boolean>, autoIndexId: <boolean>, size: <number>, max: <number>, storageEngine: <document>, validator: <document>, validationLevel: <string>, validationAction: <string>, indexOptionDefaults: <document>, viewOn: <string>, pipeline: <pipeline>, collation: <document>, writeConcern: <document>})

Parameters:



  • name: It represents the name of the collection and it is of string type.
  • options: It is an optional parameter.

Optional parameters:

  • capped: It is used to create a capped collection. If the value of this field is true then you must specify the maximum size in the size field. It is of boolean type.
  • autoIndexId: If the value of this field is false then it disables the automatic creation of an index on the _id field. It is of boolean type.
  • size: It specifies the maximum size of the collection in bytes. It is of number type.
  • max: It specifies the maximum number of documents allowed in the capped collection. It is of number type.
  • storageEngine: It is available only for the WiredTiger storage engine only. It is of the document type.
  • validator: It specifies the validation rules for the collection. It is of the document type.
  • validationLevel: It illustrates how strictly MongoDB applies the validation rule on the existing document during updates. It is of sting type.
  • validationAction: It illustrates whether to error on invalid documents or just warn about the violations but allows invalid documents to be inserted. It is of string type.
  • indexOptionDefaults: It allows the user to specify the default configuration for indexes when creating a collection. It is of document type.
  • viewOn: Name of the source collection or view from which to create the view. It is of string type.
  • pipeline: It is an array that contains the aggregation pipeline stages. Or using this parameter we can apply pipeline to the view or viewOne collection. It is of an array type.
  • collation: It specifies the default collation from the given collection. It is of the document type.
  • writeConcern: It expresses the write concern for the operation. If you want to use default write concern then avoid this parameter. It is of document type.

Examples:

In the following example, we are working with the “gfg” database in which we are creating a new capped collection of name “student” with maximum document capacity “4” using createCollection() method.

db.createCollection("student", {capped:true, size:10000, max:4})

Now we insert the documents in the student collection:

After insert documents, we will display all the documents present in the student collection using find() method.



Now, if we try to insert one more document, it will override the existing document. In the example, we are inserting a new document with the name Akash that overrides the existing document with the name Mihir:

After inserting one or more document:

How to check if the collection is capped or not?

We can check whether the collection is capped or not with the isCapped() method in MongoDB. This method returns true is the specified collections capped collection. Otherwise, return, false.

Syntax:

db.Collection_name.isCapped()

Example:

db.student.isCapped()

After convert student collection to a capped:

How to convert a collection to a capped?

If there is an existing collection that we want to change it to capped, we can do it using the convertToCapped command. It changes the current collection to capped collection:

Syntax:

    convertToCapped: <collection>,

    size: <capped size>,

    writeConcern: <document>,

    comment: <any>

}

Example:

db.runCommand({"convertToCapped":"student",size:5000})

Before convert to capped:

After convert to capped:

Advantages of Capped Collections

  • To return documents in insertion order, queries do not need an index, since it provides greater insertion throughput.
  • Capped collections allow changes that match the size of the original document to ensure that the document does not change its position on the disk.
  • Holding log files is useful.

Disadvantages of Capped Collections

  • The update operation fails if the update of a document exceeds the original size of the collection.
  • Deleting documents from a capped collection is not possible. If you want to delete all records from a capped collection, then use the given command:
{ emptycapped: Collection_name }
  • Capped collection cant be shard.

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