Question about MS_REPLICATION method specification for MS SQL Server source (Behavior during task stoppage)

iti-attunity-sup — Fri, 05 Jun 2026 02:55:07 GMT

Hi community,

I have a question regarding the "Start transactions in the database" mechanism under the MS_REPLICATION method with an MS SQL Server source.

I have reviewed the following official support article:
https://community.qlik.com/t5/Official-Support-Articles/5-Tips-for-Replicate-task-configuration-with-MS-SQL-server-as/ta-p/1793429
- Tip 5: Prevent truncation of unread changes from TLOG
Option 1: Start transactions in the database

From this article, I understand that while the Replicate task is running, truncation of unread TLOGs is prevented. However, I have a question regarding the behavior when the task is stopped.

Based on my verification in a test environment, even when the Qlik Replicate task is stopped, the SQL Server Log Reader Agent continues to run, reading logs and delivering them to the distribution database. My understanding is that the Log Reader Agent executes sp_repldone internally.

Therefore, I initially thought that if the Log Reader Agent completes reading the logs while the task is stopped, the synchronization checkpoint would be advanced (updated). However, even after stopping the task temporarily, I was able to successfully resume it without any issues.

Regarding this behavior, I have hypothesized the following mechanism. Could anyone please let me know if my understanding is correct?

・Why unread log truncation is prevented while the task is stopped:
　By design, SQL Server cannot truncate log space older than the "oldest currently open transaction." Qlik Replicate leaves a pseudo-transaction (backward latch) remaining at the Log Sequence Number (LSN) where it last finished reading.

・Behavior during task stoppage:
　Even if the task is intentionally stopped or fails due to an error, this open transaction remains active ("incomplete/open state") on the database. Therefore, even if the SQL Server Log Reader Agent executes sp_repldone periodically, the logs from the position where Qlik last read onward are protected and exempt 　from truncation.

If my understanding is incorrect or if there are other underlying mechanisms, please let me know.

Thank you in advance for your help!

Re: Question about MS_REPLICATION method specification for MS SQL Server source (Behavior during task stoppage)

DesmondWOO — Fri, 05 Jun 2026 08:24:58 GMT

Hi @iti-attunity-sup ,

Regarding "Start transactions in the database", once the task is stopped, the active open transactions will be released and gone. The reason you observed that the task could still be resumed properly without data loss after being stopped temporarily is due to the following two scenarios:

The TLOG hasn't been overwritten yet: Even though the open transaction was released, the corresponding transaction logs in the online have not yet been truncated or overwritten by new incoming database activities. Qlik Replicate can still pick up where it left off directly from the active TLOG.
Qlik Replicate fetched data from Backup Logs: If a scheduled log backup occurred while the task was stopped, the active TLOG might have been truncated. However, because Qlik Replicate has the capability to read from backup files, it automatically goes to the SQL Server backup history, locates the relevant .trn files, and extracts the missing changes to resume the CDC chain seamlessly.

Regards,
Desmond

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Question about MS_REPLICATION method specification for MS SQL Server source (Behavior during task stoppage)

Re: Question about MS_REPLICATION method specification for MS SQL Server source (Behavior during task stoppage)