The synchronization process takes place whenever a Main server connects or re-connects to another server. The steps that are taken during the synchronization process differ depending on whether the Duty Mode is enabled or Disabled on the servers. In this section, we describe the synchronization process for servers that have Duty Mode disabled (which is the default Duty Mode setting).
The synchronization process starts when:
- A Main server establishes or re-establishes a connection with another server
- ClearSCADA is started on a Standby server that is connected to a Main server that is already running.
For example, if a Main server loses its connection with a Standby server due to a disconnected network cable, when that cable is reconnected, the synchronization process begins.
The synchronization process consists of these steps (which are performed in order):
- The Main server transfers some of its Boolean (Enabled/Disabled) server configuration settings to the Standby server: Exclusive Control, Event Journal, Historic Data, Historic Validation, Historic Files, Alarms & Events, Quality Classification, Alarm Response, and Configuration Changes Auditing. Only the Enabled/Disabled settings are transferred, the other settings for each of the features need to be configured manually.
- The Main server transfers any metadata (custom fields) to the Standby server.
- The Main server transfers the database structure to the Standby server. The database structure is the hierarchy of groups, database items etc.
- Configuration CRC (Cyclic Redundancy Checks) values are calculated from the specific configuration details of the database items. The Main server retrieves the configuration CRCs from the Standby server and compares them to the configuration CRCs on the Main server.
- Any configuration data on the Main server that has different CRCs to the configuration on the Standby server is transferred to the Standby server. This means that the configuration data is the same on both the Main server and Standby server.
- Non-configuration data such as point states, time stamps, quality values etc., for database items is transferred from the Main server to the Standby server.
- Alarm data and event data is transferred from the Main server to the Standby server.
NOTE: Steps 1-7 inclusive are referred to as the ‘initial’ synchronization. When steps 1-7 have taken place, clients can connect to the Standby server and the remaining steps 8-11 continue in the background.
- Alarm Redirection data is transferred from the Main server to the Standby server.
- The Main server queries the historic files on the Standby server (it uses file size and time stamps). On small to medium sized systems, this should take a matter of minutes. On systems with large amounts of historic data (the design limit is 6 billion records on 32 bit systems and 30 billion records on 64 bit systems), the historic data query could take many minutes or even hours.
- Any historic files on the Main server that have different file sizes and time stamps to the historic files on the Standby server are transferred to the Standby server. This will include files that have been modified for events logged since the last synchronization as well as any new historic files that have been created.
- The Main server queries the file sizes and modified time stamps of the Event Journal data files on the Standby server.
- ClearSCADA detects if there are event journal files on the Main server that are different to the event journal files on the Standby server. If there are, the files that are different are transferred from the Main server to the Standby server. This will include files that have been modified for events logged since the last synchronization as well as any new files that have been created.
- The Main server transfers the historic files (generated reports) to the Standby server.
Each time a Main server connects to another server, the synchronization process has to take place. This means that the other server is regularly updated with the data on the Main server. The first time that this synchronization takes place is called a ‘first pass’. Following the completion of the ‘first pass’, the Main server and the other server will continue to synchronize on a regular basis. Each of these continuous synchronisations is called a ‘short pass’. A ‘short pass’ consists of the same steps as a ‘first pass’ but is completed more quickly as there is less data to be transferred and so the synchronization takes less time to complete.
During the ‘first pass’, steps 1-7 of the synchronization has to be completed before the Standby server can connect to the Main server. After step 7 has been completed, the Standby server can be used to access the database on the Main server. The remaining steps of the ‘first pass’ will continue to take place.
When the ‘first pass’ has completed, the Main server and the Standby server will continue to synchronize on a regular basis. The frequency of the continued synchronizations is set via the Transfer Interval and Interleave settings on the Partners section of the Server Configuration Tool (see Define the Transfer Interval and Transfer Timeouts).
The ongoing synchronizations that take place after the ‘first pass’ are called ‘short passes’ (see Short Pass’ Synchronization Process).
Further Information
‘First Pass’ Synchronization Process (Duty Mode Enabled)