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Back Up a Sharded Cluster with Database Dumps

To take a consistent backup of a sharded cluster with mongodump, you must first stop the balancer, stop writes, and stop any schema transformation operations on the cluster. This ensures that the cluster remains in a consistent state for the duration of the backup.

MongoDB provides backup and restore operations that can run with the balancer and running transactions through the following services:

  • MongoDB Atlas

  • MongoDB Cloud Manager

  • MongoDB Ops Manager

This task uses mongodump to back up a sharded cluster. Ensure that you have a cluster running that contains data in sharded collections.

This procedure requires a version of MongoDB that supports fsync locking from mongos.

Starting in MongoDB 7.0.2 (also available starting in 6.0.11, 5.0.22, and 4.4.25) the fsync and fsyncUnlock commands can run on mongos to lock and unlock a sharded cluster.

To perform these tasks, your user must have the fsync authorization, which allows the user to run the fsync and fsyncUnlock commands.

To take a self-managed backup of a sharded cluster, complete the following steps:

1

To find a good time to perform a backup, monitor your application and database usage to find a time when chunk migrations, resharding, and schema transformation operations are unlikely to occur, as these can cause an inconsistent backup.

For more information, see Schedule Backup Window for Sharded Clusters.

2

To prevent chunk migrations from distruping the backup, connect to mongos and use the sh.stopBalancer() method to stop the balancer:

sh.stopBalancer()

If a balancing round is in progress, the operation waits for balancing to complete.

To confirm that the balancer is stopped, use the sh.getBalancerState() method:

sh.getBalancerState()
false

The command returns false when the balancer is stopped.

3

The sharded cluster must remain locked during the backup process to protect the database from writes, which may cause inconsistencies in the backup.

To lock a sharded cluster, connect to mongos and use the db.fsyncLock() method:

db.getSiblingDB("admin").fsyncLock()

To confirm the lock, on mongos and the primary mongod of the config servers, run the following aggregation pipeline and ensure that all the shards are locked:

db.getSiblingDB("admin").aggregate( [
{ $currentOp: { } },
{ $facet: {
"locked": [
{ $match: { $and: [
{ fsyncLock: { $exists: true } },
{ fsyncLock: true }
] } }],
"unlocked": [
{ $match: { fsyncLock: { $exists: false } } }
]
} },
{ $project: {
"fsyncLocked": { $gt: [ { $size: "$locked" }, 0 ] },
"fsyncUnlocked": { $gt: [ { $size: "$unlocked" }, 0 ] }
} }
] )
[ { fsyncLocked: true }, { fsyncUnlocked: false } ]
4

To back up the sharded cluster, use mongodump to connect to mongos and perform the backup:

mongodump \
--host mongos.example.net \
--port 27017 \
--username user \
--password "passwd" \
--out /opt/backups/example-cluster-1
5

After the backup completes, you can unlock the cluster to allow writes to resume.

To unlock the cluster, use the db.fsyncUnlock() method:

db.getSibling("admin").fsyncUnlock()

To confirm the unlock, on mongos and the primary mongod of the config servers, run the following aggregation pipeline and ensure that all shards are unlocked:

db.getSiblingDB("admin").aggregate( [
{ $currentOp: { } },
{ $facet: {
"locked": [
{ $match: { $and: [
{ fsyncLock: { $exists: true } },
{ fsyncLock: true }
] } }],
"unlocked": [
{ $match: { fsyncLock: { $exists: false } } }
]
} },
{ $project: {
"fsyncLocked": { $gt: [ { $size: "$locked" }, 0 ] },
"fsyncUnlocked": { $gt: [ { $size: "$unlocked" }, 0 ] }
} }
] )
[ { fsyncLocked: false }, { fsyncUnlocked: true } ]
6

To restart the balancer, use the sh.startBalancer() method:

sh.startBalancer()

To confirm that the balancer is running, use the sh.getBalancerState() method:

sh.getBalancerState()
true

The command returns true when the balancer is running.

You can restore a database from a mongodump backup using mongorestore.

  • To restore a sharded cluster, execute mongorestore against mongos.

  • To migrate to a replica set or standalone server, execute mongorestore against mongod.

For more information, see Back Up and Restore with MongoDB Tools.

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