Replica set mode

By default every node presents itself to drivers as a mongos. If an application or tool expects a replica set (it connects with replicaSet=, reads the oplog or uses the replSet* commands), change the presentation live, without restarting any node.

Requirements

• A running cluster and access to teseladb admin from one of its nodes.
• A name for the replica set, the one your applications will put in replicaSet=.

Change the presentation

teseladb admin topology set replicaset --name rs0

You can also change it from mongosh with the topologyPresentation cluster parameter, with the same effect as the administration command.

What drivers see

The designated node answers hello as primary and the others as secondaries, with setName, hosts, primary and electionId. If the designated node stops being alive, the cluster designates another on its own and drivers see it as a MongoDB election.

mongodb://node-01:27017,node-02:27017,node-03:27017/?replicaSet=rs0&tls=true

Oplog

local.oplog.rs is a read-only collection generated from the versions of the documents. Tools that read the oplog and mongodump --oplog work in both modes. applyOps applies entries as in MongoDB.

Check

In mongosh, db.hello() returns setName: "rs0" and the node acting as primary in primary.

Common problems

• If the driver finds no primary, the replicaSet= name in the string does not match the --name you gave when changing the presentation.

See also

• Sharding and zones, the default presentation.
• Differences with MongoDB, with reading the oplog under the mongos presentation.