parallelCollectionScan¶

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parallelCollectionScan¶

New in version 2.6.

Allows applications to use multiple parallel cursors when reading all the documents from a collection, thereby increasing throughput. The parallelCollectionScan command returns a document that contains an array of cursor information.

Each cursor provides access to the return of a partial set of documents from a collection. Iterating each cursor returns every document in the collection. Cursors do not contain the results of the database command. The result of the database command identifies the cursors, but does not contain or constitute the cursors.

Important

The server may return fewer cursors than requested.
This command will not return more than one cursor for the WiredTiger storage engine.

The command has the following syntax:

{
  parallelCollectionScan: "<collection>",
  numCursors: <integer>,
  readConcern: <document>,
  maxTimeMS: <integer>
}

The parallelCollectionScan command takes the following fields:

Field	Type	Description
`parallelCollectionScan`	string	The name of the collection.
`numCursors`	integer	The maximum number of cursors to return. Must be between 1 and 10000, inclusive.
`readConcern`	document	Optional. Specifies the read concern. The option has the following syntax: readConcern: { level: <value> } Possible read concern values are: `"local"`. This is the default read concern level. `"majority"`. Available for replica sets that use WiredTiger storage engine. `"linearizable"`. Available for read operations on the `primary` only. For more formation on the read concern levels, see Read Concern Levels.
`maxTimeMS`	non-negative integer	Optional. Specifies a time limit in milliseconds for processing operations on a cursor. If you do not specify a value for maxTimeMS, operations will not time out. A value of `0` explicitly specifies the default unbounded behavior. MongoDB terminates operations that exceed their allotted time limit using the same mechanism as `db.killOp()`. MongoDB only terminates an operation at one of its designated interrupt points.

parallelCollectionScan is only available for mongod, and it cannot operate on a sharded cluster.

Example¶

Override Default Read Concern¶

To override the default read concern level of "local", use the readConcern option.

The following operation on a replica set specifies a Read Concern of "majority" to read the most recent copy of the data confirmed as having been written to a majority of the nodes.

Note

To use read concern level of "majority",
- you must start the mongod instances with the --enableMajorityReadConcern command line option (or the replication.enableMajorityReadConcern set to true if using a configuration file).
- replica sets must use WiredTiger storage engine and election protocol version 1.
Regardless of the read concern level, the most recent data on a node may not reflect the most recent version of the data in the system.

db.runCommand(
   {
      parallelCollectionScan: "restaurants",
      numCursors: 5,
      readConcern: { level: "majority" }
   }
)

To ensure that a single thread can read its own writes, use "majority" read concern and "majority" write concern against the primary of the replica set.

Output¶

The parallelCollectionScan command returns a document containing the array of cursor information:

{
    "cursors" : [
        {
            "cursor" : {
                "firstBatch" : [ ],
                "ns" : "<database name>.<collection name>",
                "id" : NumberLong("155949257847")
            },
            "ok" : true
        }
    ],
    "ok" : 1
}

parallelCollectionScan.cursors¶: An array with one or more cursors returned with the command.

parallelCollectionScan.cursors.cursor¶: For each cursor returned, a document with details about the cursor.

parallelCollectionScan.cursors.cursor.firstBatch¶: An empty first batch is useful for quickly returning a cursor or failure message without doing significant server-side work. See cursor batches.

parallelCollectionScan.cursors.cursor.ns¶: The namespace for each cursor.

parallelCollectionScan.cursors.cursor.id¶: The unique id for each cursor.

parallelCollectionScan.cursors.ok¶: The status of each cursor returned with the command.

parallelCollectionScan.ok¶: A value of 1 indicates the parallelCollectionScan command succeeded. A value of 0 indicates an error.

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