Replica Set Configuration¶

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Replica Set Configuration Document Example
Replica Set Configuration Fields

You can access the configuration of a replica set using the rs.conf() method or the replSetGetConfig command.

To modify the configuration for a replica set, use the rs.reconfig() method, passing a configuration document to the method. See rs.reconfig() for more information.

Warning

Avoid reconfiguring replica sets that contain members of different MongoDB versions as validation rules may differ across MongoDB versions.

Replica Set Configuration Document Example¶

The following document provides a representation of a replica set configuration document. The configuration of your replica set may include only a subset of these settings:

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{
  _id: <string>,
  version: <int>,
  term: <int>,
  protocolVersion: <number>,
  writeConcernMajorityJournalDefault: <boolean>,
  configsvr: <boolean>,
  members: [
    {
      _id: <int>,
      host: <string>,
      arbiterOnly: <boolean>,
      buildIndexes: <boolean>,
      hidden: <boolean>,
      priority: <number>,
      tags: <document>,
      slaveDelay: <int>,
      votes: <number>
    },
    ...
  ],
  settings: {
    chainingAllowed : <boolean>,
    heartbeatIntervalMillis : <int>,
    heartbeatTimeoutSecs: <int>,
    electionTimeoutMillis : <int>,
    catchUpTimeoutMillis : <int>,
    getLastErrorModes : <document>,
    getLastErrorDefaults : <document>,
    replicaSetId: <ObjectId>
  }
}

Replica Set Configuration Fields¶

_id¶

Type: string

The name of the replica set. Once set, you cannot change the name of a replica set.

_id must be identical to the replication.replSetName or the value of –replSet specified to mongod on the command line.

See

replSetName or --replSet for information on setting the replica set name.

version¶

Type: int

An incrementing number used to distinguish revisions of the replica set configuration document from previous iterations of the configuration.

Changed in version 4.4: Replica set members use term and version to achieve consensus on the “newest” replica configuration. When members compare replica configuration documents, the configuration document with a larger term is considered the “newest”. If term is the same or absent, the configuration document with the larger version is considered “newest”.

term¶

Type: int

New in version 4.4.

Only available with featureCompatibilityVersion (fCV) “4.4” or later.

An incrementing number used to distinguish revisions of the replica set configuration document from previous iterations of the configuration. The term of a configuration document matches the term of the replica set primary which performed the reconfiguration. The primary increments its term each time it steps up after winning an election. The primary ignores the term field if set explicitly in the replSetReconfig operation.

Issuing a force reconfiguration removes the term field. When the primary next issues replSetReconfig without force, it sets the term to its own term.

Replica set members use term and version to achieve consensus on the “newest” replica configuration. When members compare replica configuration documents, the configuration document with a larger term is considered the “newest”. If term is the same or absent, the configuration document with the larger version is considered “newest”.

configsvr¶

New in version 3.2.

Type: boolean

Default: false

Indicates whether the replica set is used for a sharded cluster’s config servers. Set to true if the replica set is for a sharded cluster’s config servers.

See also

Sharded Cluster Enhancements

protocolVersion¶

New in version 3.2.

Type: number

Default: 1

Starting in 4.0, MongoDB only supports protocolVersion: 1 and no longer supports protocolVersion: 0.

See also

Replica Set Protocol Version

writeConcernMajorityJournalDefault¶

New in version 3.2.6.

Type: boolean

Default: true

Determines the behavior of { w: "majority" } write concern if the write concern does not explicitly specify the journal option j.

The following table lists the writeConcernMajorityJournalDefault values and the associated { w: "majority" } behavior:

Value { w: "majority" } Behavior

true

MongoDB acknowledges the write operation after a majority of the voting members have written to the on-disk journal.

Important

All voting members of the replica set must run with journaling when writeConcernMajorityJournalDefault is true.

If any voting member of a replica set uses the in-memory storage engine, you must set writeConcernMajorityJournalDefault to false.

If any voting member of a replica set uses the in-memory storage engine and writeConcernMajorityJournalDefault is true, "majority" write operations may fail. These include operations that inherently use "majority" write concern, such as the replSetStepDown command, or various mongo shell methods that by default use "majority" write concern, such as user management methods and role management methods.

Starting in version 4.2 (and 4.0.13 and 3.6.14 ), if a replica set member uses the in-memory storage engine (voting or non-voting) but the replica set has writeConcernMajorityJournalDefault set to true, the replica set member logs a startup warning.

false

MongoDB acknowledges the write operation after a majority of the voting members have applied the operation in memory.

Warning

If any voting member of a replica set uses the in-memory storage engine, you must set writeConcernMajorityJournalDefault to false.

Starting in version 4.2 (and 4.0.13 and 3.6.14 ), if a replica set member uses the in-memory storage engine (voting or non-voting) but the replica set has writeConcernMajorityJournalDefault set to true, the replica set member logs a startup warning.

You cannot run transactions on a sharded cluster that has a shard with writeConcernMajorityJournalDefault set to false (such as a shard with a voting member that uses the in-memory storage engine).

`members`¶

members¶

Type: array

An array of member configuration documents, one for each member of the replica set. The members array is a zero-indexed array.

Each member-specific configuration document can contain the following fields:

members[n]._id¶

Type: integer

An integer identifier of every member in the replica set. Values must be between 0 and 255 inclusive. Each replica set member must have a unique _id. Avoid re-using _id values even if no members[n] entry is using that _id in the current configuration.

Once set, you cannot change the _id of a member.

Note

When updating the replica configuration object, access the replica set members in the members array with the array index. The array index begins with 0. Do not confuse this index value with the value of the members[n]._id field in each document in the members array.

members[n].host¶

Type: string

The hostname and, if specified, the port number, of the set member.

The hostname name must be resolvable for every host in the replica set.

Warning

members[n].host cannot hold a value that resolves to localhost or the local interface unless all members of the set are on hosts that resolve to localhost.

members[n].arbiterOnly¶

Optional.

Type: boolean

Default: false

A boolean that identifies an arbiter. A value of true indicates that the member is an arbiter.

When using the rs.addArb() method to add an arbiter, the method automatically sets members[n].arbiterOnly to true for the added member.

For the following MongoDB versions, pv1 increases the likelihood of w:1 rollbacks compared to pv0 (no longer supported in MongoDB 4.0+) for replica sets with arbiters:

MongoDB 3.4.1
MongoDB 3.4.0
MongoDB 3.2.11 or earlier

See Replica Set Protocol Version.

members[n].buildIndexes¶

Optional.

Type: boolean

Default: true

A boolean that indicates whether the mongod builds indexes on this member. You can only set this value when adding a member to a replica set. You cannot change members[n].buildIndexes field after the member has been added to the set. To add a member, see rs.add() and rs.reconfig().

Do not set to false for mongod instances that receive queries from clients.

Setting buildIndexes to false may be useful if all the following conditions are true:

you are only using this instance to perform backups using mongodump, and
this member will receive no queries, and
index creation and maintenance overburdens the host system.

Even if set to false, secondaries will build indexes on the _id field in order to facilitate operations required for replication.

Warning

If you set members[n].buildIndexes to false, you must also set members[n].priority to 0. If members[n].priority is not 0, MongoDB will return an error when attempting to add a member with members[n].buildIndexes equal to false.

To ensure the member receives no queries, you should make all instances that do not build indexes hidden.

Other secondaries cannot replicate from a member where members[n].buildIndexes is false.

members[n].hidden¶

Optional.

Type: boolean

Default: false

When this value is true, the replica set hides this instance and does not include the member in the output of db.isMaster() or isMaster. This prevents read operations (i.e. queries) from ever reaching this host by way of secondary read preference.

Hidden members can acknowledge write operations issued with Write Concern. For write operations issued with "majority" write concern, the member must also be a voting member (i.e. votes is greater than 0).

See also

Hidden Replica Set Members

members[n].priority¶

Changed in version 3.6: Starting in MongoDB 3.6, arbiters have the priority 0. If an arbiter has a priority of 1, MongoDB 3.6 reconfigures the arbiter to have a priority of 0.

Optional.

Type: Number between 0 and 1000 for primary/secondary; 0 or 1 for arbiters.

Default: 1.0 for primary/secondary; 0 for arbiters.

A number that indicates the relative eligibility of a member to become a primary.

Specify higher values to make a member more eligible to become primary, and lower values to make the member less eligible. A member with a members[n].priority of 0 is ineligible to become primary.

Members with priority greater than 0 cannot have 0 votes.

Changing the balance of priority in a replica set will trigger one or more elections. If a lower priority secondary is elected over a higher priority secondary, replica set members will continue to call elections until the highest priority available member becomes primary.

Members with priority of 0 can acknowledge write operations issued with Write Concern. For write operations issued with "majority" write concern, the member must also be a voting member (i.e. votes is greater than 0).

See also

Replica Set Elections.

members[n].tags¶

Optional.

Type: document

Default: none

A tags document contains user-defined tag field and value pairs for the replica set member.

{ "<tag1>": "<string1>", "<tag2>": "<string2>",... }

For read operations, you can specify a tag set in the read preference to direct the operations to replica set member(s) with the specified tag(s).
For write operations, you can create a customize write concern using settings.getLastErrorModes and settings.getLastErrorDefaults.

For more information, see Configure Replica Set Tag Sets.

members[n].slaveDelay¶

Optional.

Type: integer

Default: 0

The number of seconds “behind” the primary that this replica set member should “lag”.

Use this option to create delayed members. Delayed members maintain a copy of the data that reflects the state of the data at some time in the past.

Delayed members can contribute to acknowledging write operations issued with Write Concern. However, they return write acknowledgment no earlier than the configured delay value. For write operations issued with "majority" write concern, the member must also be a voting member (i.e. votes is greater than 0).

See also

Delayed Replica Set Members

members[n].votes¶

Optional.

Type: integer

Default: 1

The number of votes a server will cast in a replica set election. The number of votes each member has is either 1 or 0, and arbiters always have exactly 1 vote.

Members with priority greater than 0 cannot have 0 votes.

A replica set can have up to 50 members but only 7 voting members. If you need more than 7 members in one replica set, set members[n].votes to 0 for the additional non-voting members.

Non-voting (i.e. votes is 0) members must have priority of 0.

Non-voting members cannot acknowledge write operations issued with a "majority" write concern.

See also

`settings`¶

settings¶

Optional.

Type: document

A document that contains configuration options that apply to the whole replica set.

The settings document contain the following fields:

settings.chainingAllowed¶

Optional.

Type: boolean

Default: true

When settings.chainingAllowed is true, the replica set allows secondary members to replicate from other secondary members. When settings.chainingAllowed is false, secondaries can replicate only from the primary.

See also

Manage Chained Replication

settings.getLastErrorDefaults¶

Optional.

Type: document

A document that specifies the write concern for the replica set. The replica set will use this write concern only when write operations or getLastError specify no other write concern.

If settings.getLastErrorDefaults is not set, the default write concern for the replica set only requires confirmation from the primary.

Important

Starting in version 4.4, MongoDB deprecates specifying a settings.getLastErrorDefaults value other than the default of { w: 1, wtimeout: 0 }. MongoDB 4.4 honors any write concern value that you specify, however future MongoDB versions might not honor values other than the default. Instead, use the setDefaultRWConcern command to set the default read or write concern configuration for a replica set or sharded cluster.

settings.getLastErrorModes¶

Optional.

Type: document

A document used to define a custom write concern through the use of members[n].tags. The custom write concern can provide data-center awareness.

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{ getLastErrorModes: {
   <name of write concern> : { <tag1>: <number>, .... },
   ...
} }

The <number> refers to the number of different tag values required to satisfy the write concern. For example, the following settings.getLastErrorModes defines a write concern named datacenter that requires the write to propagate to two members whose dc tag values differ.

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{ getLastErrorModes: { datacenter: { "dc": 2 } } }

To use the custom write concern, pass in the write concern name to the w Option, e.g.

copy

{ w: "datacenter" }

See Configure Replica Set Tag Sets for more information and example.

settings.heartbeatTimeoutSecs¶

Optional.

Type: int

Default: 10

Number of seconds that the replica set members wait for a successful heartbeat from each other. If a member does not respond in time, other members mark the delinquent member as inaccessible.

Note

For pv1, settings.electionTimeoutMillis has a greater influence on whether the secondary members call for an election than the settings.heartbeatTimeoutSecs.

settings.electionTimeoutMillis¶

New in version 3.2.

Optional.

Type: int

Default: 10000 (10 seconds)

The time limit in milliseconds for detecting when a replica set’s primary is unreachable:

Higher values result in slower failovers but decreased sensitivity to primary node or network slowness or spottiness.
Lower values result in faster failover, but increased sensitivity to primary node or network slowness or spottiness.

The setting only applies when using protocolVersion: 1.

Note

Changed in version 4.0.2: If the parameter enableElectionHandoff is true (default), when a primary steps down from rs.stepDown() (or the replSetStepDown command without the force: true), the stepped-down primary nominates an eligible secondary to call an election immediately. Otherwise, secondaries can wait up to settings.electionTimeoutMillis before calling an election. The stepped down primary does not wait for the effects of the handoff. For more information, see enableElectionHandoff.

settings.catchUpTimeoutMillis¶

New in version 3.4.

Optional.

Type: int

Changed in version 3.6:

Default: -1, infinite catchup time.

Time limit in milliseconds for a newly elected primary to sync (catch up) with the other replica set members that may have more recent writes. Infinite or high time limits may reduce the amount of data that the other members would need to roll back after an election but may increase the failover time.

The newly elected primary ends the catchup period early once it is fully caught up with other members of the set. During the catchup period, the newly elected primary is unavailable for writes from clients. Use replSetAbortPrimaryCatchUp to abort the catchup then complete the transition to primary.

The setting only applies when using protocolVersion: 1.

Note

To downgrade a replica set initiated in version 3.6 to 3.4, change catchUpTimeoutMillis from -1 to a positive number. Failure to change this value to a positive number causes nodes running version 3.4 to neither restart nor join the replica set.

settings.catchUpTakeoverDelayMillis¶

New in version 3.6.

Optional.

Type: int

Default: 30000 (30 seconds)

Time in milliseconds a node waits to initiate a catchup takeover after determining it is ahead of the current primary. During a catchup takeover, the node ahead of the current primary initiates an election to become the new primary of the replica set.

After the node initiating the takeover determines that it is ahead of the current primary, it waits the specified number of milliseconds and then verifies the following:

It is still ahead of the current primary,
It is the most up-to-date node among all available nodes,
The current primary is currently catching up to it.

Once determining that all of these conditions are met, the node initiating the takeover immediately runs for election.

For more information on Replica Set Elections, see Replica Set Elections.

Note

Setting catchUpTakeoverDelayMillis to -1 disables catchup takeover. Setting catchUpTimeoutMillis to 0 disables primary catchup and consequently also catchup takeover.

settings.heartbeatIntervalMillis¶

New in version 3.2.

Internal use only.

The frequency in milliseconds of the heartbeats.

settings.replicaSetId¶

New in version 3.2.

Type: ObjectId

The ObjectId associated with the replica set and automatically created during rs.initiate() or replSetInitate. You cannot change the replicaSetId.

← Replication Reference Replica Set Protocol Version →

Replica Set Configuration¶

Replica Set Configuration Document Example¶

Replica Set Configuration Fields¶

members¶

settings¶

`members`¶

`settings`¶