Why should you use a hidden replica set member

MongoDB brought a lot of innovation to the database ecosystem.
One of this innovation is the concept of a hidden replica set member.
A hidden replica set is a member that keeps a copy of primary's data set, but it is invisible for the client.
This means that this member won't receive any read when the client is connect using the replica set connection string.

Use Cases for the Hidden Replica set member

You can do the backup in production from the hidden member without causing any performance issues to your production workflow.

You can connect your BI (Business Intelligence) to the hidden member, doing a lot of heavy queries without compromising your production workflow .

You can create a new index from a big collection in the hidden member, so you can create the index faster.

Configuring a Hidden Replica Set member

Use mongosh to connect to a MongoDB node

 mongosh mongodb://mongo1:27017,mongo2:27017/db?replicaSet=rs0

Run rs.status() to see what node is a SECONDARY

rs.status()

Example output:

members: [
    {
      _id: 0,
      name: 'mongodb1:27017',
      stateStr: 'PRIMARY',
    },
    {
      _id: 1,
      name: 'mongodb2.mongo-prd:27017',
      stateStr: 'SECONDARY',
    },
]

We need to modify the replica set config like this:

const config = rs.config();

config.members[1].hidden = true;
config.members[1].priority = 0;

rs.reconfig(config)

This will set the members in the position 1 of the array (mongodb2) to the be a hidden replica set member.

In Conclusion

Production environment are much more sensitivity than localhost and staging.
You need to design your process to avoid affecting production workflow.
A hidden member is perfect for backups, running heavy queries in your BI and doing database maintenance like building a new index for a big collection.

References

https://www.mongodb.com/docs/manual/core/replica-set-hidden-member/

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