Each time a Main server connects or reconnects to another server, a synchronization process has to take place. Synchronization is used to allow the Standby servers in multi-server architectures to accurately reflect the data on the Main server. During the synchronization process, the Main server checks the data on the Standby servers and updates them as required.
There are three types of synchronization that can occur that depend on the state of the servers when they are connected or reconnected.
The ' First Pass' Synchronization process occurs when the following conditions are met:
- The Main server has been restarted.
- The Standby server has been restarted.
- The Standby server has attempted to synchronize with another server.
- The Standby server has become a Main.
- ClearSCADA is started on a Standby server that is connected to a Main server that is already running.
Following the completion of the ‘First pass’, the Main server and the other server will continue to synchronize on a regular basis.
A 'Short pass' Synchronization process can only occur while servers remain connected and repeats continuously after the ‘First pass’ has completed. It ensure that the Main server updates the Standby servers regularly.
A ‘Short pass’ consists of the same steps as a ‘First pass’ but, if the initial synchronization settings do not change it is completed more quickly. Only changes in the data need to be transferred and so the synchronization takes less time to complete.
'Warm' Synchronization refers to a process that can occur when the network connection between a Main server and a Standby is interrupted for a short period of time and providing the following conditions are met:
- The Main server has not been restarted.
- The Standby server has not been restarted.
- The Standby server has not attempted to synchronize with another server.
- The Standby server has not become a Main, for example it remains an Isolated Standby or is a Permanent Standby.
When the network connection is re-established between the Main and the Standby, a verification of each server is performed and providing all the conditions are met the Main server performs a 'Warm' synchronization. This restarts the synchronization of the data from the time just before the network connection was interrupted.
Once 'a Warm' synchronization has completed the servers will continue to update with 'Short pass' synchronization.
The way that the synchronization process works differs slightly depending on whether the servers have Duty Mode enabled or disabled. If you are unsure as to whether your server has Duty Mode enabled or disabled, you can view the setting on the Partners section of the Server Configuration Tool (see Enable or Disable Duty Mode).
The synchronization process is performed as quickly as possible via each re-established Main-Standby connection. If connections with more than one Standby server are re-established at the same time, the Main server will attempt to perform the synchronization process on both Standby servers as quickly as possible. The amount of time it takes to fully synchronize the Standby servers will vary according to the speed of the connection and the data on the Standby servers. For example, if Standby server A requires more data from the Main server than Standby server B, it will take a longer amount of time to fully synchronize Standby server A than Standby server B (as less data has to be transferred).
For more information on the ‘first pass’ or ‘short pass’ part of the synchronization process, see the topics that are listed in the gray footer section at the bottom of this topic. Select the relevant entry to display the topic that you require.