How to “Browse” Distribution Db (LSN/seqno explained)

You ever wonder if you can get more information what is actually replicated in your system or how to get exact commands/transactions that been replicated?

I believe you did:). As I mentioned before Transactional Replication operates on LSN (sequence numbers of transactions). Understanding how sequence number works is crucial thing. Lets try to explain.

Once Log Reader read the transaction log of published database it is putting information about transaction and commands into system tables into relevant distribution database. The system tables I mentioned are msrepl_transactions and msrepl_commands. First one contains one row for each transaction and second all commands for given transaction. The most important column is xact_seqno which is the transaction ID and you can join these two tables on that column (and publisher_database_id) to get what commands are included in what transactions.

select top 10 * from MSrepl_transactions (NOLOCK) t
join MSrepl_commands (NOLOCK) c
on t.publisher_database_id=c.publisher_database_id
and t.xact_seqno= c.xact_seqno

Now, How you get current LSN that is being delivered. There are two ways. First one is to check msdistribution_history system table for given distribution agent. Second is to check subscriber and system table msreplication_subscriptions and get current_timestamp. That values is the information about last delivered transaction and as the replication is sequential next seqno in the queue will be the one that is currently delivered. With both there are issues that you need to be aware of. I noticed that misdistribution_history is not live data and it is refreshed with some delay. The problem with second solution is that the next seqno that you get from distribution database could belong to different publication (there is no pubid column in these tables). To over come this you can filter by article id, however then you need to join msrepl_commands table and this can be quite heavy query. Here are the examples:

On Subscriber:

select publisher,publisher_Db, publication,transaction_timestamp from MSreplication_subscriptions

msreplication_subscriptions

On Distribution database:

select top 10 * from MSrepl_transactions (NOLOCK) t
join MSrepl_commands (NOLOCK) c
on t.publisher_database_id=c.publisher_database_id
and t.xact_seqno= c.xact_seqno
join MSarticles a
on a.article_id=c.article_id
join MSpublications pub
on a.publication_id = pub.publication_id
where t.xact_seqno >= <seqno from subscriber>
and pub.publication =< publication name>
order by t.xact_seqno asc

LSN/sequence number is often used in order to troubleshoot replication issues. While u checking replication monitor for failing distribution agent you will get and error that includes seqno and a command id. With these you can find what command exactly failing. In order to do that you need system procedure located on distributor calledĀ  sp_browsereplcmds. This procedure allows you to browse actual commands. More on that next post.

 

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