Impact of DST change on Qlik Replicate
This topic describes the impact of Daylight Saving Time (DST) on Qlik Replicate and provides guidelines for handling changes brought about by DST.
Tasks that move from Full Load to Change Processing when DST comes into effect may encounter data loss. However, such data loss can be prevented by adhering to the guidelines outlined in this appendix.
Additionally, the times displayed in the Replicate Console may not be synced with the server. Should you encounter any time discrepancy issues, either restart the Qlik Replicate Server service or stop and resume the tasks.
There are two types of DST changes:
- DST On - Occurs approximately when Summer starts (actual date is country specific). Its impact on local time is that local time is moved one hour forward (so, for example, 01:00 AM becomes 02:00 AM). This DST change does not impact Qlik Replicate because it does not result in time overlap.
- DST Off - Occurs approximately when Winter starts (actual date is country specific). Its impact on local time is that local time is moved back one hour (so, for example, 02:00 AM becomes 01:00 AM). This DST change results in time overlap where local time travels over the same hour twice in a row.
The comments below assume that the customer has not changed the time but rather the timezone or the DST setting. Changing the actual time (not for minor time adjustments) is a sensitive operation and is best done when Qlik Replicate is stopped.
Running Qlik Replicate tasks do not depend on the timezone or DST for correctly scanning and processing the transaction logs. Internally, Qlik Replicate timers use UTC.
Still, there are several places where DST may have an effect:
- Timestamps in logs and audit messages are in local time. As a result, when Winter time starts, the logs will show the time going back an hour; conversely, when Summer time starts, the logs may appear to be missing one hour.
Scheduled jobs as well as the global and table manipulation variables timestamp and commit_timestamp use local time so these will also be affected. The impact of this depends on the manipulation done and on the intended use of the timestamp based data.Information note
To prevent timestamp and scheduling anomalies resulting from DST starting or ending, the following best practices should be observed:
- DST Off (summer to winter): Do not schedule a task to start from the time the clock changes until the following hour. For example, if DST ends at 02:00 am, do not schedule a task to run between 02:00 and 02:59, as the task will run twice.
- DST On (winter to summer): Do not schedule a task to start from the time the clock changes until the following hour. For example, if DST starts at 02:00 am, do not schedule a task to run between 02:00 and 02:59 as this hour does not exist.
- The initial Full Load of tables or reloading of tables should not be done during the DST change window. It is recommended to perform such operations either an hour before or an hour after DST.
If you have existing jobs scheduled to start at the overlap time and you do not want to modify them, then you need to stop the Qlik Replicate Server. Going in to Winter time, for example, if at 02:00 AM the clock is to be set back to 01:00 AM then when the time is 00:55 AM the Qlik Replicate Server should be stopped and, after an hour and ten minutes (at 01:05 AM), should be started again.
If you forget to do this, all scheduled jobs will run an hour earlier than intended. You can rectify this by setting the desired scheduling time and then restarting the Qlik Replicate Server service.
- Statistics shown on the console are also sensitive to local time and thus may also show confusing/inaccurate data in the overlap period (going in to Winter time) or for the skipped period (going into Summer time).
If the clock on Qlik Replicate Server machine is one hour behind the clock on the Qlik Replicate Console (UI) machine, the following issues are known to occur:
- The Applied Changes circle graph will be updated as the changes are applied, but the information in the Recent Activity tab will not be updated.
- Scheduled jobs will start according to the Qlik Replicate Server time (as expected), but will remain in the Active Jobs list after execution instead of moving to the Expired Jobs tab.
For more information on scheduling jobs, see Scheduling jobs.
In general, it is recommended to avoid non-critical task design changes during the first overlap period (going in to Winter time) so as to prevent confusion about when the changes took place.
In addition to Qlik Replicate, other components are also involved including:
- The source endpoint system
- The target endpoint system
- The local operating system
- The task design (specifically using timestamp based variables)
Given the complexity of the topic and the involvement of many independent components and settings, Qlik generally recommends that customers first verify the impact of DST changes in their test environment.