Server: Ongoing Tasks: External Replication

What IS being replicated

All database documents and related data:

• Attachments
• Revisions
• Counters
• Time Series

What is NOT being replicated

Server and cluster level features:

• Indexes
• Conflict resolver definitions
• Compare-Exchange
• Subscriptions
• Identities
• Ongoing tasks
  • ETL
  • Backup
  • Hub/Sink Replication

Why are Cluster-Level Features Not Replicated?

To provide for architecture that prevents conflicts between clusters, especially when ACID transactions are important, RavenDB is designed so that data ownership is at the cluster level.
To learn more, see Data Ownership in a Distributed System.

Conflicts

• Two databases that have an External Replication task defined between them will detect and resolve document conflicts according to each database's conflict resolution policy.
• It is recommended to have the same policy configuration on both the source and the target databases.

Sharding Support

External replication is supported by both sharded and non-sharded databases.
Learn more about the way ETL works on a sharded database here.

The required elements of an External Replication task are:

• The UpdateExternalReplicationOperation() method.
• The destination server needs the certificate from the source server so that it will trust the source.
• The connection string with the destination server URL and any other details needed to access the destination server.
• The following properties in the ExternalReplication object:
  • ConnectionStringName
    The connection string name.
  • Name
    The External Replication task name.

Properties
        public class ExternalReplication
        {
            public TimeSpan DelayReplicationFor { get; set; }
            public string Name { get; set; }
            public string ConnectionStringName { get; set; }
            public string MentorNode { get; set; }
        }

Optional elements include the following properties in the ExternalReplication object:

• MentorNode
  The preferred responsible node in the source server.
• DelayReplicationFor
  The amount of time to delay replication.
  The following sample shows a 30-minute delay. Delay can also be set by days, hours, and seconds.

//define external replication with mentor node and delay timespan
await sourceStore.Maintenance.SendAsync(
    new UpdateExternalReplicationOperation(new ExternalReplication
{
    ConnectionStringName = connectionStrName,
    Name = "task-name",
    MentorNode = "B",
    DelayReplicationFor = TimeSpan.FromMinutes(30)
}));
           

ExternalReplication properties:

public class ExternalReplication
{
    public TimeSpan DelayReplicationFor { get; set; }
    public string Name { get; set; }
    public string ConnectionStringName { get; set; }
    public string MentorNode { get; set; }
}

To create an external replication task via the RavenDB Studio, see the Step-by-Step Guide

To learn how to define an external replication task via code, see code sample.

You can also configure external eplication tasks via RavenDB Studio.

• When the source cluster is down (and there is no leader):

  • Creating a new Ongoing Task is a Cluster-Wide operation,
    thus, a new Ongoing External Replication Task cannot be scheduled.

  • If an External Replication Task was already defined and active when the cluster went down,
    then the task will not be active and no replication will take place.

• When the node responsible for the external replication task is down

  • If the responsible node for the External Replication Task is down,
    then another node from the Database Group will take ownership of the task so that the external replica is up to date.

• When the destination node is down:

  • The external replication will wait until the destination is reachable again and proceed from where it left off.

  • If there is a cluster on the other side, and the URL addresses of the destination database group nodes are listed in the connection string, then when the destination node is down, the replication task will simply start transferring data to one of the other nodes specified.