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setIndexCommitQuorum

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  • Syntax
  • Command Fields
  • Behavior
  • Examples

New in version 4.4.

setIndexCommitQuorum

The setIndexCommitQuorum command sets minimum number of data-bearing members that must be prepared to commit their local index builds before the primary node will commit the index.

Syntax

The command has the following syntax:

db.runCommand(
   {
     setIndexCommitQuorum: <string>,
     indexNames: [ <document> ],
     commitQuorum: <int> | <string>,
     comment: <any>
   }
)

Command Fields

The command takes the following fields:

Field
Type
Description
setIndexCommitQuorum
string

The name of the collection for which the indexes are being built.

array

An array of in-progress index builds to modify. Each element of the array must be the name of the index.

The indexes specified to indexNames must be the entire set of in-progress builds associated to a given index builder, i.e. the indexes built by a single createIndexes or db.collection.createIndexes() operation.

int or string

The minimum number of data-bearing replica set members (i.e. commit quorum), including the primary, that must report a successful index build before the primary marks the indexes as ready.

Starting in MongoDB v5.0, it's possible to resume some interrupted index builds when the commit quorum is set to "votingMembers".

To update the commitQuorum, member replica set nodes must have members[n].buildIndexes set to true. If any voting nodes have members[n].buildIndexes set to false, you can't use the default "votingMembers" commit quorum. Either configure all nodes with members[n].buildIndexes set to true, or select a different commit quorum.

Supports the following values:

  • "votingMembers" - all data-bearing voting replica set members (Default). A "voting" member is any replica set member where members[n].votes is greater than 0.

  • "majority" - a simple majority of data-bearing replica set members.

  • <int> - a specific number of data-bearing replica set members. Specify an integer greater than 0.

  • A replica set tag name.

comment
any

Optional. A user-provided comment to attach to this command. Once set, this comment appears alongside records of this command in the following locations:

A comment can be any valid BSON type (string, integer, object, array, etc).

New in version 4.4.

Behavior

Note

Requires featureCompatibilityVersion 4.4+

Each mongod in the replica set or sharded cluster must have featureCompatibilityVersion set to at least 4.4 to start index builds simultaneously across replica set members.

MongoDB 4.4 running featureCompatibilityVersion: "4.2" builds indexes on the primary before replicating the index build to secondaries.

Index creation is a multistage process. Starting in MongoDB 4.4, the index creation process uses the commit quorum to minimize replication lag on secondary nodes.

When a secondary node receives a commitIndexBuild oplog entry, the node stops further oplog applications until the local index build can be committed. Index builds can take anywhere from moments to days to complete, so the replication lag can be significant if the secondary node builds more slowly than the primary.

To manage the replication lag, the commit quorum delays committing the index build on the primary node until a minimum number of secondaries are also ready to commit the index build.

The commit quorum does not guarantee that indexes on secondaries are ready for use when the command completes. To ensure that a specific number of secondaries are ready for use, set an appropriate write concern.

If a secondary node that is not included in the commit quorum receives a commitIndexBuild oplog entry, the node may block replication until its index build is complete.

Issuing setIndexCommitQuorum has no effect on index builds started with commitQuorum of 0.

Important

Replica set nodes with buildIndexes set to false can't be included in a commit quorum.

Commit Quorum Contrasted with Write Concern

There are important differences between commit quorums and write concerns:

  • Index builds use commit quorums.

  • Write operations use write concerns.

Each data-bearing node in a cluster is a voting member.

The commit quorum specifies how many data-bearing voting members, or which voting members, including the primary, must be prepared to commit a simultaneous index build. before the primary will execute the commit.

The write concern is the level of acknowledgement that the write has propagated to the specified number of instances.

The commit quorum specifies how many nodes must be ready to finish the index build before the primary commits the index build. In contrast, when the primary has committed the index build, the write concern specifies how many nodes must finish the index build before the command returns.

Examples

Starting with MongoDB 4.4, index builds on a replica set or sharded cluster build simultaneously across all data-bearing replica set members. For sharded clusters, the index build occurs only on shards containing data for the collection being indexed. The primary requires a minimum number of data-bearing voting members (i.e commit quorum), including itself, that must complete the build before marking the index as ready for use. See Index Builds in Replicated Environments for more information.

The following operation starts an index build of two indexes:

db.getSiblingDB("examples").invoices.createIndexes(
  [
    { "invoices" : 1 },
    { "fulfillmentStatus" : 1 }
  ]
)

By default, index builds use "votingMembers" commit quorum, or all data-bearing voting replica set members. The following operation modifies the index build commit quorum to "majority", or a simple majority of data-bearing voting members:.

db.getSiblingDB("examples").runCommand(
  {
    "setIndexCommitQuorum" : "invoices",
    "indexNames" : ["invoices_1", "fullfillmentStatus_1"],
    "commitQuorum" : "majority"
  }
)

  • The indexes specified to indexNames must be the entire set of in-progress builds associated to a given index builder, i.e. the createIndexes() operation.

  • The indexNames field specifies the names of the indexes. Since the indexes were created without an explicit name, MongoDB generated an index name by concatenating the names of the indexed fields and the sort order.

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setFeatureCompatibilityVersion