Online replication modes

When Update Settings specify that a replica server uses an online connection to the master server, the servers can exchange data in direct mode or non-blocking mode.

The replication mode is selected in the master server, and all its replica servers use the same mode.

The choice of mode depends largely on the stability of the network connection.

Direct replication mode

Online COS replication consists of two steps:

  1. The replica sends its changes to the master.

  2. The master sends its changes to the replica.

In direct replication mode, both servers operate simultaneously.

Under normal network conditions, this is efficient. However, during replication, local users have limited access to the databases:

  • They cannot write to the databases while replication is reading or writing data.

  • They cannot even read from the databases while replication is writing data.

If there are many changes to replicate or if the network connection is slow or unstable, local users might be unable to access the COS server for a considerable time.

Non-blocking replication mode

In non-blocking replication mode, replication is sequential and based on file transfer:

  • Local changes are first written to a file.

  • The file is sent over the network to the other server.

In this mode, users are only blocked when data is being written to or read from a replication file at the local site. They can continue working while files are transferred between sites. Because file operations are typically faster than network operations, downtime for users is shorter than in direct mode. However, overall replication time may be longer since the servers operate sequentially, not simultaneously.