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Description

This error indicates that an operation requiring a replica set context was attempted on a MongoDB instance that is not configured or recognized as part of a replica set. It commonly occurs when a client expects replica set behavior from a standalone `mongod` server. The instance is operating in a non-replica set mode.

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Error Message

Not A Replica Set

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Known Causes

3 known causes

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Standalone MongoDB Instance

The `mongod` process was started without the `--replSet` option, meaning it operates as a standalone server, not part of a replica set.

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Replica Set Not Initialized

The `mongod` instance was configured with `--replSet` but the replica set itself has not been initiated using `rs.initiate()` or `rs.add()` commands.

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Mismatched Client Configuration

The client application is configured to connect to a replica set (e.g., using `?replicaSet=myReplicaSet` in the connection URI) but the target MongoDB instance is standalone.

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Solutions

3 solutions available

1. Start a New Replica Set easy

Initialize a new replica set if the instance was not intended to be part of one.

1

Ensure the MongoDB instance is not running. Stop it if it is.

2

Modify the MongoDB configuration file (e.g., mongod.conf) to include replica set settings. Add or uncomment the `replication` section and specify a `replSetName`.

replication:
  replSetName: "myReplicaSetName"

3

Start the MongoDB instance with the updated configuration.

mongod --config /path/to/your/mongod.conf

4

Connect to the MongoDB instance using the mongo shell.

mongo

5

Initiate the replica set configuration.

rs.initiate()

6

Verify the replica set status. You should see the primary and secondary members.

rs.status()

2. Connect to a Replica Set Member easy

Ensure your application or client is configured to connect to a replica set, not a standalone instance.

1

Identify the hostname and port of at least one member of the intended replica set.

2

When connecting using the `mongo` shell or a MongoDB driver, provide the replica set name and a comma-separated list of replica set members.

mongo --host "mongo1.example.com:27017,mongo2.example.com:27017" --replicaSet "myReplicaSetName"

3

Ensure your application's MongoDB connection string includes the `replicaSet` parameter.

mongodb://mongo1.example.com:27017,mongo2.example.com:27017/?replicaSet=myReplicaSetName

3. Remove Replica Set Configuration (for Standalone Use) medium

If the instance was mistakenly configured as a replica set and you want it to be standalone, remove the replica set configuration.

1

Connect to the MongoDB instance using the `mongo` shell.

mongo

2

If the instance is currently a primary, attempt to step down to allow other members to become primary (if applicable). This step might not be necessary if you are sure it's the only instance.

rs.stepDown()

3

Stop the MongoDB instance.

4

Edit the MongoDB configuration file (e.g., mongod.conf). Remove or comment out the `replication` section, including the `replSetName`.

#replication:
#  replSetName: "myReplicaSetName"

5

Start the MongoDB instance with the updated configuration.

mongod --config /path/to/your/mongod.conf

6

Verify that the instance is now running as a standalone server by checking its status or running `rs.status()` which should now throw an error.

rs.status()

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Related Errors

5 related errors

MongoDB Error 123: Not A Replica Set