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Replication Commands

replSetSyncFrom

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  • Description
  • Syntax
  • Command Fields
  • Behavior

Description

replSetSyncFrom

Temporarily overrides the default sync target for the current mongod. This operation is useful for testing different patterns and in situations where a set member is not replicating from the desired host.

Tip

In mongosh, this command can also be run through the rs.syncFrom() helper method.

Helper methods are convenient for mongosh users, but they may not return the same level of information as database commands. In cases where the convenience is not needed or the additional return fields are required, use the database command.

Run replSetSyncFrom in the admin database.

Syntax

The command has the following syntax:

db.adminCommand(
   {
     replSetSyncFrom: "hostname<:port>"
   }
 )

Command Fields

The command takes the following field:

Field
Type
Description
replSetSyncFrom
string

The name and port number of the replica set member that this member should replicate from. Use the [hostname]:[port] form.

Replica set members with 1 vote cannot sync from members with 0 votes.

Behavior

Sync Logic

If an initial sync operation is in progress when you run replSetSyncFrom, replSetSyncFrom stops the in-progress initial sync and restarts the sync process with the new target.

Only modify the default sync logic as needed, and always exercise caution.

Target

The member to sync from must be a valid source for data in the set. To sync from a member, the member must:

  • Have data. It cannot be an arbiter, in startup or recovering mode, and must be able to answer data queries.

  • Be accessible.

  • Be a member of the same set in the replica set configuration.

  • Build indexes with the members[n].buildIndexes setting.

  • A different member of the set, to prevent syncing from itself.

If you attempt to replicate from a member that is more than 10 seconds behind the current member, mongod will log a warning but will still replicate from the lagging member. See also Replication Lag and Flow Control.

Persistence

replSetSyncFrom provide a temporary override of default behavior. mongod will revert to the default sync behavior in the following situations:

  • The mongod instance restarts.

  • The connection between the mongod and the sync target closes.

  • If the sync target falls more than 30 seconds behind another member of the replica set.

For more information the use of replSetSyncFrom, see Configure a Secondary's Sync Target.

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