Undocumented Gotchas of Transactional Replication

Undocumented Gotchas of Transactional Replication

Gotcha - Norelco commercial (1977)

This discussion started on Twitter via the #sqlhelp hash tag. The question that was asked is, “What are the undocumented gotchas of Transactional Replication? #SQLHelp”. I responded with several things that came to mind. I thought these would be good to capture in a blog post and hopefully others would contribute some more gotchas to the list.

If you can think of any other gotchas about any type of replication, not just transactional, please add it to a comment. Also, if you have a blog post on it, please include the link to the post. I’m getting the ball rolling with the ones I mentioned on Twitter.

Gotchas of Replication

Transactional replication
• If you run the agent continuously, and it stops (fails), it won’t restart or raise an alert until the subscription expires.
• If the agent is failing, it will retry forever and you may never notice it is failing unless you are monitoring closely.
• If you add/remove an article using the GUI, it will regenerate the entire snapshot. Use TSQL to avoid this.
• Where you place the distribution db is very important for performance and manageability. Plan this point very well.
• Check the VLFs in your db log before configuring repl. as it can have a very high impact on performance of the log reader
• Be wary of non-replicated trans that create lots of log records (eg, index maint.) as it affects log reader performance
• DML ops. to LOB columns in repl. tables will fail if the length of value exceeds the “max text repl size (B)” setting.
• Make sure you are testing deployments in repl. preprod. environ. so you aren’t surprised by related failures in Prod.
• When the publication is mirrored. Transactions will not replicate until hardened on mirror, unless a trace flags used
  • Reported by John Sansom (blog|@JohnSansom)
  • John and Martin Cairns (blog|@MartinCairnsSQL) report that the whitepaper can be downloaded here.
• Log reader fails if it encounters blank (empty) XML in an XML column.
  • Reported and blogged by Kendall Van Dyke (blog|@SQLDBA) here.
• Including an article in multiple publications can lead to performance problems.
  • Reported and blogged by Kendall Van Dyke (blog|@SQLDBA) here.
• Altering articles to not replicate deletes may not be set on all articles as expected.
  • Reported and blogged by Kendall Van Dyke (blog|@SQLDBA) here.

Merge replication
• Changing conflict tracking level from row to column generates conflicts when different columns are updated
  • Reported and blogged by David Williams (blog|@is_brandon) here.