Always connect & query against the Primary in ReplicaSet

[Richard Seldon]

Hello, 

Have a requirement to always connect & query (find / delete operations) against the PRIMARY MongoDB in a replica set.

Was looking at this StackOverflow answer for how to do this from the Command Line :  http://stackoverflow.com/a/28120454/1882064

```

db.runCommand("ismaster")

{

     "ismaster" : false,

     "secondary" : true,

     "hosts" : [

             "ny1.acme.com",

             "ny2.acme.com",

             "sf1.acme.com"

     ],

     "passives" : [

          "ny3.acme.com",

          "sf3.acme.com"

     ],

     "arbiters" : [

         "sf2.acme.com",

     ]

     "primary" : "ny2.acme.com",

     "ok" : true

}

```

Above, actually to do the opposite, and always connect to a Secondary MongoDB in a replica set.

How can I achieve the same using the Node Mongo DB Driver - is it just a case of adding the above as options to the Connection ?

```

const options = {  // complete here... };

MongoClient.connect(config.MONGO_URL, options, (err, db) => {

```

Any details / docs / code snippets most welcome. Thank you.

[Wan Bachtiar]

Have a requirement to always connect & query (find / delete operations) against the PRIMARY MongoDB in a replica set.

Hi Richard,

The isMaster() command is normally used to find out whether or not an instance is the primary of the replica set. Not to set a client to read from the primary of a replica set.

The connection preference to perform queries (i.e. find() ) can be configured by setting Read Preference. However, write/update operations (i.e. deleteOne()), can only be performed on the replica set primary member. Also see Replication for more information.

How can I achieve the same using the Node Mongo DB Driver - is it just a case of adding the above as options to the Connection ?

See MongoDB Node.JS Driver: Connect to a Replica Set for code example and information. Note that the default read preference value of the driver is to read from primary.

You may find the following resources useful:

• MongoDB Node.JS driver v2.2: Tutorials
• MongoDB Replica Set Tutorials

If you have further questions, could you provide the following:

• MongoDB version.
• MongoDB Node.JS driver version.
• Snippet code of how you’re making the connection, along with the options.
• The MongoDB connection string URI that you’re using.
• What led you to believe that you’re not connecting to the primary from your application?

Regards,

Wan.

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