Replication: Operational considerations
Summary
The following sections describe some specific operational issues that may be encountered when using Replication. These are not exhaustive descriptions of all possibilities. They are listed to provide you with some considerations for your environment(s):
- "Behavior of Replication DS-System when the primary DS-System is not reachable"
- "Multi-Directional Replication will replicate DS-Client database dump files"
- "Considerations about Multi-Directional Replication speed"
- "Synchronization process for DS-Clients connecting to different DS-Systems in the same Replication Group"
- "Time between backup windows to allow replication to complete"
- "Which DS-System is the 'source' and which is the 'destination'"
- "Automatic fail-over (of DS-Clients to alternate DS-System) - critical notes"
Behavior of Replication DS-System when the primary DS-System is not reachable
A DS-Client can be configured with one or more DS-System connections (For instructions see the DS-Client User Guide (dsuser.pdf) > 2 - Setup and Configuration > Change the DS-Client Configuration > Edit the Settings for DS-System Access).
Each primary and secondary DS-System connection can have one or multiple IP addresses. When DS-Client tries to connect to a DS-System, it will try to connect to each IP address specified in its "Primary" Connection. DS-Client only considers secondary addresses after trying all the IP addresses listed in the "Primary" Connection.
If DS-Client connects to a secondary DS-System, it can always perform restores, however its ability to back up new data depends on the type of license running on the secondary DS-System:
- Production DS-System: full backup / restore (uninterrupted)
- Replication DS-System: restores only - no new backups allowed
Multi-Directional Replication will replicate DS-Client database dump files
This feature is managed from the DS-Operator > Setup Menu > Advanced Configurations: "ClientDBDumpDays" parameter.
Any backed up dsclient database dump files are replicated from one DS-System to another in the Replication Group based on whichever DS-System has the latest dump. If a DS-Client must be recovered (e.g. from a total hardware failure) and it connects to a Replication DS-System (where it has never run a Daily / Weekly Admin activity), that DS-Client will still be able to recover the databases (provided the dumps have been replicated).
On each DS-System, the "ClientDBDumpDays" parameter is configured separately, to determine how many days' worth of dumps to keep. Note the size implications of increasing this parameter, since it applies to all the DS-Clients on the DS-System.
Considerations about Multi-Directional Replication speed
In a basic Replication Group configuration, both the primary DS-System and secondary DS-System may reside on the same subnet / LAN. As a result, the Replication process will always be up-to-date on the secondary DS-System, since the Replication process speed between the DS-Systems is faster than the backup speed to the production DS-System (i.e. LAN speed vs. WAN speed).
In a configuration where the DS-Systems are replicating over the WAN, the replication speed may not be as fast as the backup to the primary DS-System. In these situations, the Replication DS-System may not always be up-to-date.
Synchronization process for DS-Clients connecting to different DS-Systems in the same Replication Group
- Replication Groups with at least two Production DS-System licenses (i.e. the DS-Systems will allow immediate backup from shared DS-Clients).
- A DS-Client that is configured (in its "DS-Client Configuration Connections Tab" screen) with the IP address of both these Production DS-Systems.
When DS-Client backup sets are marked as "out-of-sync", DS-Client will try to connect to all DS-Systems by using the IP addresses in its "DS-Client Configuration Connections Tab" screen. DS-Client will determine the most up-to-date DS-System, and will synchronize its data with that one.
Time between backup windows to allow replication to complete
The DS-System replication is asynchronous and a replication will only be done after backups have completed. Backup and replication will not run concurrently.
- As a result, it is important to make sure that there is enough time between the backup windows to allow for the replication to complete.
Which DS-System is the 'source' and which is the 'destination'
When replicating data, the DS-System Group will pick one of the DS-Systems as a "source" and another as the "destination". This is done based on the last connection time of the DS-Client:
- The last DS-System that the DS-Client connected to for each backup set will be considered as the data source. The other DS-System (with the older DS-Client connection time) will be considered as the destination.
- After the replication process finishes successfully, the data on the destination DS-System will be the same as on the source DS-System (any extra data on the destination will be discarded).
Automatic fail-over (of DS-Clients to alternate DS-System) - critical notes
Multi-directional replication has the following impact on automatic fail-over:
- If there is not enough time for the replication to start and/or finish and the DS-Client fails-over to a secondary DS-System (e.g. because of network problems on its primary DS-System), the DS-Client will send new data to the secondary DS-System.
- When a new replication process is started, that secondary DS-System will be considered authoritative: any new data that was sent to the primary DS-System and was not replicated will be discarded.
- If you want to be completely sure that no data will ever be discarded by this scenario, you must configure the DS-Client to use only one DS-System for backup (configure only one "Primary DS-System Connection" and do not create any secondary connection entries). In this scenario, in case of a disaster, you must manually update the DS-Client to connect to the secondary DS-System (because it was prevented from automatic fail-over).
See Also
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