Question: How do you back up a MongoDB replica set?

Answer

Backing up a MongoDB replica set is crucial for ensuring data durability and availability. In MongoDB, a replica set is a group of mongod instances that maintain the same data set. Backups can be performed using several methods, including mongodump, filesystem snapshots, and MongoDB Cloud Manager or Ops Manager for a more managed approach.

Using mongodump

mongodump is a utility for creating a binary export of the contents of a database. To backup a replica set, you can connect to a secondary member to perform the backup to minimize impact on the performance of the primary. Use the --oplog option to capture oplog entries during the backup process for point-in-time restore capabilities.

mongodump --host replicaSet/secondaryHost:port --oplog --gzip --archive=/path/to/backup/archive.gz

Replace replicaSet/secondaryHost:port with your replica set name and the host and port of one of your secondary members. The --gzip compresses the output, and --archive specifies the file to store the backup.

Filesystem Snapshots

Filesystem snapshots are another method to backup a replica set. This method requires running the MongoDB instance on a filesystem that supports snapshot functionality (e.g., LVM, ZFS, AWS EBS). Snapshotting is instantaneous and has a minimal impact on the database. However, it's essential to freeze the database or ensure that the snapshot is taken from a secondary member to avoid inconsistencies.

1. Flush all writes to disk and lock the database (if taking a snapshot from a secondary, step into the SECONDARY state first).

use admin;
db.fsyncLock();

2. Take the filesystem snapshot according to your filesystem's or storage provider's instructions.

3. Unlock the database:

db.fsyncUnlock();

MongoDB Cloud Manager or Ops Manager

MongoDB offers Cloud Manager and Ops Manager for automated backup solutions. These tools provide continuous, online backups with point-in-time recovery of replica sets. They are part of MongoDB's suite of cloud services and require a subscription.

To use Cloud Manager or Ops Manager for backups:

1. Register your deployment with Cloud Manager or install Ops Manager.
2. Configure your replica set in the service.
3. Enable backup for your deployment. The service handles the backup process automatically.

Choosing the right backup method depends on your specific requirements, including the acceptable backup window, restore times, and infrastructure capabilities.