Database Manual / Reference / mongosh Methods / Replication

rs.reconfigForPSASet() (mongosh method)

Definition

rs.reconfigForPSASet( memberIndex, config, { options } )

New in version 5.0.

Safely perform some reconfiguration changes on a primary-secondary-arbiter (PSA) replica set or on a replica set that is changing to a PSA architecture. You should only use this method in one of the following cases:

You want to reconfigure a secondary in an existing three-member replica set with a PSA architecture to become a voting, data-bearing node with a non-zero priority.
You want to add a new voting, data-bearing node with a non-zero priority to an existing two-member replica set that contains one voting, data-bearing node and one arbiter.

Warning

If the secondary you are adding is lagged and the resulting replica set is a PSA configuration, the first configuration change will change the number of nodes that need to commit a change with "majority". In this case, your commit point will lag until the secondary has caught up.

For details about the behavior of this method, see Behavior.

Compatibility

This method is available in deployments hosted in the following environments:

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

The rs.reconfigForPSASet() method has the following syntax:

rs.reconfigForPSASet(
  memberIndex: <num>,
  config: <configuration>,
  {
    "force" : <boolean>,
    "maxTimeMS" : <int>
  }
)

Parameter	Type	Description
`memberIndex`	integer	The index of the secondary that is being added or modified.
`config`	document	A document that specifies the new configuration of a replica set.
`force`	boolean	Optional WARNING: Running the `rs.reconfigForPSASet()` method with `force: true` is not recommended and can lead to committed writes being rolled back. Specify `true` to force the available replica set members to accept the new configuration. Defaults to `false`. Force reconfiguration can result in unexpected or undesired behavior, including rollback of `"majority"` committed writes.
`maxTimeMS`	integer	Optional Specifies a cumulative time limit in milliseconds for processing each reconfiguration during the `rs.reconfigForPSASet()` operation. By default, `rs.reconfigForPSASet()` waits indefinitely for the replica configurations to propagate to a majority of replica set members.

Behavior

The rs.reconfigForPSASet() method reconfigures your replica set in two steps:

The method reconfigures your replica set to add or modify a secondary with { votes: 1, priority: 0 }.
Once the added or modified secondary has caught up with all committed writes, the method reconfigures the secondary to have the priority specified in the rs.reconfigForPSASet() command ({ votes: 1, priority: <num> }).

The two-step approach avoids the possibility of rolling back committed writes in the case of a failover to the new secondary before the new secondary has all committed writes from the previous configuration.

Example

A replica set named rs0 has the following configuration:

{
   "_id" : "rs0",
   "version" : 1,
   "term": 1,
   "members" : [
      {
         "_id" : 0,
         "host" : "mongodb0.example.net:27017",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {},
         "secondaryDelaySecs" : Long("0"),
         "votes" : 1
      },
      {
         "_id" : 2,
         "host" : "mongodb1.example.net:27017",
         "arbiterOnly" : true,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 0,
         "tags" : {},
         "secondaryDelaySecs" : Long("0"),
         "votes" : 1
      }
   ],
   "protocolVersion" : Long("1"),
   "writeConcernMajorityJournalDefault": true,
   "settings" : {
      "chainingAllowed" : true,
      "heartbeatIntervalMillis" : 2000,
      "heartbeatTimeoutSecs" : 10,
      "electionTimeoutMillis" : 10000,
      "catchUpTimeoutMillis" : 2000,
      "getLastErrorModes" : {},
      "getLastErrorDefaults" : {
         "w" : 1,
         "wtimeout" : 0
      },
      "replicaSetId" : ObjectId("60e6f83923193faa336889d2")
   }
}

The following sequence of operations add a new secondary to the replica set. The operations are issued in the mongosh shell while connected to the primary.

cfg = rs.conf();
cfg["members"] = [
  {
     "_id" : 0,
     "host" : "mongodb0.example.net:27017",
     "arbiterOnly" : false,
     "buildIndexes" : true,
     "hidden" : false,
     "priority" : 1,
     "tags" : {},
     "secondaryDelaySecs" : Long("0"),
     "votes" : 1
  },
  {
     "_id" : 1,
     "host" : "mongodb1.example.net:27017",
     "arbiterOnly" : false,
     "buildIndexes" : true,
     "hidden" : false,
     "priority" : 2,
     "tags" : {},
     "secondaryDelaySecs" : Long("0"),
     "votes" : 1
  },
  {
     "_id" : 2,
     "host" : "mongodb2.example.net:27017",
     "arbiterOnly" : true,
     "buildIndexes" : true,
     "hidden" : false,
     "priority" : 0,
     "tags" : {},
     "secondaryDelaySecs" : Long("0"),
     "votes" : 1
  }
]
rs.reconfigForPSASet(1, cfg);

The first statement uses the rs.conf() method to retrieve a document containing the current configuration for the replica set and stores the document in the local variable cfg.
The second statement adds the new secondary to the members array. In this configuration the new secondary is added at memberIndex 1. The memberIndex is the same as the array index. For additional settings, see replica set configuration settings.

The last statement calls the rs.reconfigForPSASet() method with the memberIndex 1 and the modified cfg. The memberIndex is the array position of the new member in the members array. Upon successful reconfiguration, the replica set configuration resembles the following:

{
   "_id" : "rs0",
   "version" : 1,
   "term": 1,
   "members" : [
      {
         "_id" : 0,
         "host" : "mongodb0.example.net:27017",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {},
         "secondaryDelaySecs" : Long("0"),
         "votes" : 1
      },
      {
         "_id" : 1,
         "host" : "mongodb1.example.net:27017",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 2,
         "tags" : {},
         "secondaryDelaySecs" : Long("0"),
         "votes" : 1
      },
      {
         "_id" : 2,
         "host" : "mongodb2.example.net:27017",
         "arbiterOnly" : true,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 0,
         "tags" : {},
         "secondaryDelaySecs" : Long("0"),
         "votes" : 1
      }
   ],
   "protocolVersion" : Long("1"),
   "writeConcernMajorityJournalDefault": true,
   "settings" : {
      "chainingAllowed" : true,
      "heartbeatIntervalMillis" : 2000,
      "heartbeatTimeoutSecs" : 10,
      "electionTimeoutMillis" : 10000,
      "catchUpTimeoutMillis" : 2000,
      "getLastErrorModes" : {},
      "getLastErrorDefaults" : {
         "w" : 1,
         "wtimeout" : 0
      },
      "replicaSetId" : ObjectId("60e6f83923193faa336889d2")
   }
}

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