MongoDB Glossary - Hidden Replica Set Member

In MongoDB, a replica set is a group of MongoDB instances that store the same data. Replica sets provide high availability and data redundancy, ensuring that your data remains accessible even in the event of hardware failures or planned maintenance.

Each replica set consists of multiple members, with one member acting as the primary and the others as secondary members. The primary member handles all write operations and replicates the data to the secondary members, which serve read operations and act as backups.

What is a Hidden Replica Set Member?

A hidden replica set member is a secondary member that is not eligible to become the primary member. It does not participate in the election process for primary member selection. Hidden members are useful for specific use cases where you want to offload read operations from the primary member without impacting its performance.

Hidden members are configured with the hidden:true option, which prevents them from becoming primary. They still receive replicated data from the primary member and can serve read operations, but they do not appear in the replica set configuration or participate in elections.

Use Cases for Hidden Replica Set Members

Hidden replica set members offer several benefits and can be used in various scenarios:

Reporting and Analytics

By configuring a hidden member, you can direct read operations from reporting and analytics tools to the hidden member, reducing the load on the primary member. This allows you to perform complex queries and generate reports without impacting the performance of your primary member.

Backup and Disaster Recovery

Hidden members can act as hot backups for disaster recovery purposes. By replicating data from the primary member, hidden members ensure that you have an up-to-date copy of your data that can be quickly promoted to a primary member in case of a failure.

Geographically Distributed Deployments

In geographically distributed deployments, hidden members can be placed in different regions to serve read operations for local users. This reduces network latency and improves the overall user experience. The hidden members can replicate data from the primary member located in a different region.

Configuring Hidden Replica Set Members

To configure a hidden replica set member, you need to add the member to the replica set configuration with the hidden:true option. Here's an example of adding a hidden member using the MongoDB shell:

cfg = rs.conf()
cfg.members.push({ _id: 2, host: "hidden_member:27017", hidden: true })
rs.reconfig(cfg)

In this example, the hidden member is added with the hostname "hidden_member" and the port "27017". Once added, the hidden member will start receiving replicated data and can serve read operations.

Conclusion

Hidden replica set members in MongoDB provide a powerful way to offload read operations from the primary member and improve performance. They are particularly useful for reporting and analytics, backup and disaster recovery, and geographically distributed deployments. By configuring hidden members, you can ensure high availability, data redundancy, and optimal performance for your MongoDB replica set.

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