When a Main server detects a new connection or re-establishes a connection with a server, it runs the ‘first pass’ synchronization process. During this process, the Main server updates the other server. Once the ‘first pass’ is complete, the Main server continues to update the Standby server on a regular basis. These regular updates are called ‘short passes’ as they usually take less time to complete than a ‘first pass’ (as the Main server only has to update the Standby server with new data, not all data).
‘Short passes’ take place continuously after a ‘first pass’. The frequency of the ‘short passes’ is defined by the Transfer Interval and Interleave settings on the Server Configuration Tool (though it is possible for ‘short passes’ to be delayed, which we explain later in this section).
During a ‘short pass’:
- The Main server checks to see if its database structure has changed since the previous 'pass'. Any changes to the database structure are transferred to the Standby server. The database structure is the hierarchy of groups, items etc.
- The Main server checks to see if there have been any configuration changes since the previous 'pass'. Any configuration changes are transferred to the Standby server.
- The Main server checks to see if there have been any changes to the saved data or dynamic data since the previous 'pass'. Any saved data or dynamic data changes are transferred to the Standby server.
- The Main server checks to see if there have been any updates to the alarm or event data since the previous 'pass'. Any new alarm or event data is transferred to the Standby server.
- The Main server checks to see if any new historic records have been created on the Main server since the previous 'pass'. Any new historic records or modified historic records are transferred to the Standby server. Any historic record that is on the Main server but has not yet been transferred to the Standby server is regarded as ‘new’.
- If there are new historic records, at least 1 of them has to be transferred during a ‘short pass’ (the next ‘short pass’ cannot occur until at least 1 historic record has been transferred).
- The Main server checks to see if any new event records have been created on the Main server since the previous 'pass'. Any new event records are transferred to the Standby server.
- The Main server transfers the historic files (generated reports) to the Standby server.
If the Standby server is not fully synchronized with the Main server by the time the next ‘short pass’ is due, the current ‘short pass’ is ended and the next ‘short pass’ takes place. The only exception to this rule is if there are new historic records to be transferred, but no historic records were transferred during the ‘short pass’. In this situation, the next ‘short pass’ is delayed until at least 1 historic record has been transferred.
For example, let’s say that the Main server has new historic records that need to be transferred to the Standby server during a ‘short pass’. The Transfer Interval has been set to 2 seconds and the Interleave is 1. The ‘short pass’ begins, but after 2 seconds, it has yet to transfer any historic records. The next ‘short pass’ is due to start, but as no new historic records have been transferred, it has to be delayed. A further second passes and 1 historic record is transferred. With 1 historic record transferred, the ‘short pass’ can end and the next ‘short pass’ can begin (1 second later than expected).
NOTE: 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).
>Further Information.
‘First Pass’ Synchronization Process (Duty Mode Disabled)