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Archive for the ‘Replication’ Category

Transactional Replication Part -1

Posted by Karthick P.K on November 22, 2013

Transactional Replication Part -1 of transactional replication series covers about

1. Architecture and transactional replication data flow.
2. Different entities involved in transactional replication like Publisher Server, Distributor Server and Subscriber Server, publication, publication database, subscription, subscription database, articles are discussed in this video.
3. Replication agents involved in one way transactional replication and their usage of different agents like snapshot agent, log reader agent and distribution agent are also discussed
4. Steps involved in configuring transactional replication like configuring distributor, publisher and subscriber along with configuring publication and subscription are also discussed in this video.
5. Any DBA can look into this video and can learn the Transactional Replication Data flow, working and how to configure Transactional replication.

After watching the below video you can look at the Transactional replication Part 2 demo video which will help you to learn the above concepts practically and will enable you to configure replication on your servers.

Transactional Replication internals and architecture by Gaurav Mathur

Posted in Replication, SQL General | Tagged: , , , , | 2 Comments »

Troubleshooting Transactional replication Latency using Agent Statistics

Posted by Prabhakar Bhaskaran on September 13, 2013

Troubleshooting latency issues in replication is black box for many DBA’s, In this post I will explain how you can leverage the agent statistics to troubleshoot the latency issues.

Before understanding how to decode the agent statistics, lets take a look at the some of the basic things which will help us to troubleshoot the replication performance issue in better way.

The following MSDN diagram depicts the transactional replication architecture in simple manner.

Transactional replication components and data flow

Troubleshooting latency issues is multi step approach, first step is identify which agent is slow,

  • Log reader Agent (Publisher to Distributor)
  • Distribution Agent (Distributor to Subscriber)

So, the problem can be either log reader or distribution agent, we can identify this by just simply inserting the tracer token.

Once we find out the problematic agent the next step is to identify within the agent which particular thread causing the issue.

Let me introduce you to the important threads and its work on these replication agents in nutshell.

Log Reader Agent

Reader Thread – It scans the publisher database transaction log using sp_replcmds

Writer Thread – Add the queued transactions to Distribution database using sp_MSadd_repl_commands

Distribution Agent

Reader thread – It finds the watermark from the table Msreplication_subscriptions(on subscriber) and uses this information to retrieve pending commands from the Distribution database. It basically uses the stored procedure sp_MSget_replcommands to achieve it.

Writer thread – Writer thread uses the Batched RPC calls to write the information to subscriber database.

Now that we understood the threads in the replication agents.  let’s assume we already identified which agent is slow by inserting tracer token. Next is to dig deeper on thread level, this is where our replication agent statistics comes to rescue us.

Agent statistics entries appended to history tables every 5 minutes by default. It provides the historical view of how the agent has been performing and keeps the last 3 days data. You can keep for more days by changing the history retention period.

MSlogreader_history

MSdistribution_history

the above two tables are located in Distribution database. The statistics information is added as XML blob in comments column of these tables.

Now, lets take a look at how to decipher this XML Data for each agents.

Log Reader Agent statistics

<stats state=”1″ work=”948″ idle=”351940″ >
<reader fetch=”859″ wait=”0″/>
<writer write=”822″ wait=”395390″/>
<sincelaststats elapsedtime=”300″ work=”49″ cmds=”176998″ cmdspersec=”3543.000000″><reader fetch=”17″ wait=”0″/><writer write=”29″ wait=”350833″/></sincelaststats></stats>

– State = 1 means stats after batch commit

–Work = cumulative time spent by the agent since restart – idle time

–Idle = Time spent waiting to call sp_replcmds

–Reader fetch = Time to do execute sp_replcmds

Wait = Time spent waiting on writer to release buffer

–Writer write = Time spent writing commands into distribution database

Wait = Time spent waiting on reader to populate buffer

Note: Each thread will have their own buffer with 40k in size.

Here,we need to look at the wait time to understand where the bottleneck exist.For example, if you notice wait time for Reader thread is high then it essentially means your writer thread is slow since reader thread is waiting for writer to release the buffer. Similarly, if you notice high wait time for writer thread then your reader thread is performing slow.

The simple way to decode this is,

HIGH wait time on Reader thread = Writer thread is slow ( thread which writes the commands to distribution database)

HIGH Wait time on Writer thread =  Reader thread is slow ( thread which scans the transaction log)

Distribution Agent Statistics

<stats state=”1″ work=”154″ idle=”351464″>
<reader fetch=”144″ wait=”11″/>
<writer write=”12″ wait=”338″/>
<sincelaststats elapsedtime=”305″ work=”10″ cmds=”81262″ cmdspersec=”8041.000000″><reader fetch=”0″ wait=”9″/><writer write=”10″ wait=”0″/></sincelaststats></stats>
– State =1 means stats after a batch commit

– Work = cumulative time spend by the agent since restart – idle time (seconds)

– Idle = Time spend waiting to call sp_msget_repl_commands

– Reader fetch = Time to do execute sp_msget_repl_commands

Wait = Time spent waiting on writer to release buffer.

– Writer write = Time spend writing commands into distribution database

Wait = Time spent waiting on reader to populate buffer.

Similar to log reader agent, the decoding of wait time is same way we did for log reader agent.

HIGH wait time on Reader thread = Writer thread is slow ( thread which writes the subscriber database using batched RPC Calls)

HIGH wait time on Writer thread = Reader thread is slow ( thread which takes the pending commands from Distribution database)

Distributor Writer thread Slow Scenario

We would be able to understand this concepts better by looking at the example statistics, In this below case, I explicitly started the transaction on subscriber table to simulate blocking at the subscriber side making the writer thread of distribution agent to wait and build up latency.

This is how stats looked,

  • <stats state=”1″ work=”755″ idle=”354505″>
  • <reader fetch=”153″ wait=”604″/>
  • <writer write=”613″ wait=”346″/>
  • <sincelaststats elapsedtime=”636″ work=”515″ cmds=”45033″ cmdspersec=”87.000000″><reader fetch=”0″ wait=”515″/><writer write=”515″ wait=”0″/></sincelaststats></stats>

We can clearly see Reader thread wait time is high(515) which means writer thread is slow since we simulated the blocking on subscriber side.

Similarly,we can simulate the blocking on replication tables msrepl_commands and msrepl_transactions which will cause Log reader writer thread to be slow and stats will show Reader thread wait time as high.

Ok, now we isolated the source of bottleneck in thread level, After this we can just follow the standard performance troubleshooting approach described in this Whitepaper to troubleshoot the slowness of the replication session.

For instance, check out the video where Joe Sack talks about using Extended events to troubleshoot the Distributor writer thread slowness.

In Summary

1. Find which agent is causing slowness using tracer token.

2. Leverage the Agent statistics to narrow down problem to thread level .

3. Follow standard performance troubleshooting approach to resolve the issue.

Thanks for reading! I hope this will help you to troubleshoot the replication performance better next time.

-Prabhakar Bhaskaran

Posted in Performance, Replication, SQL General | Tagged: , , , , , | 2 Comments »

Copy database wizard or replication setup might fail due to broken dependency

Posted by Karthick P.K on May 4, 2012

Copy database wizard would fail with below error while creating views or user defined functions if the dependency lists of objects is  broken.

Error:

failed with the following error: “Invalid object name ‘dbo. .”. Possible failure reasons: Problems with the query, “ResultSet” property not set correctly, parameters not set correctly, or connection not established correctly.

helpFile= helpContext=0 idofInterfaceWithError={8BDFE893-E9D8-4D23-9739-DA807BCDC2AC}

StackTrace: at Microsoft.SqlServer.Management.Dts.DtsTransferProvider.ExecuteTransfer()

at Microsoft.SqlServer.Management.Smo.Transfer.TransferData()

at Microsoft.SqlServer.Dts.Tasks.TransferObjectsTask.TransferObjectsTask.TransferDatabasesUsingSMOTransfer()

Replication setup would also fail while applying the scripts if the dependency lists of objects is broken.

Error:

The schema script ‘XXX_4.sch’ could not be propagated to the subscriber. (Source: MSSQL_REPL, Error number: MSSQL_REPL-2147201001)

Get help: http://help/MSSQL_REPL-2147201001

Unable to replicate a view or function because the referenced objects or columns are not present on the Subscriber. (Source: MSSQL_REPL, Error number: MSSQL_REPL20164)

Get help: http://help/MSSQL_REPL20164

Invalid object name ‘. (Source: MSSQLServer, Error number: 208)

Get help: http://help/208

--Below script will fix the broken dependencies on all the objects
----------------------------------------------------------------------------
--List of objects for which referenced objects are missing.
--ex: View created on table XYZ and table XYZ is dropped
----------------------------------------------------------------------------
SELECT OBJECT_NAME (referencing_id),referenced_database_name, referenced_schema_name, referenced_entity_name
FROM sys.sql_expression_dependencies
WHERE referenced_entity_name not in (select name from sysobjects)

create table #t_excluded_modules (module_name sysname)
go

create table #t_modules_refreshed_in_end (module_name sysname)

go

------------------------------------------------------------------------------
--
-- get the list of modules whose dependencies have to be refreshed
--
-- Comment:
-- in the list we're not considering procedures or triggers because
-- because they can be created in any order, which means they can be refreshed
-- in any order
--
------------------------------------------------------------------------------
create table #t_user_views_or_tables (module_id int)
insert into #t_user_views_or_tables(module_id)
		select object_id from sys.objects where
			type in ('V', 'FN', 'IF', 'TF')
			and name not like 'MSMerge%'
			and is_ms_shipped <> 1
			and name not in (select * from #t_modules_refreshed_in_end)
			and name not in (select * from #t_excluded_modules)

insert into #t_user_views_or_tables(module_id)
		select object_id from sys.objects where
			name in (select * from #t_modules_refreshed_in_end)

----------------------------------------
--
-- get the dependency table
-- |---------------------------------|
-- |  referencing_id | referenced_id |
-- | ----------------|---------------|
-- |      XXX        |     XXX       |
-- | ----------------|---------------|
--
----------------------------------------
Declare @module int, @message varchar(1000), @str nvarchar(1000)
create table #t_dependency_table (referencing_id int, referenced_id int)
DECLARE modules_cursor CURSOR FOR SELECT module_id FROM #t_user_views_or_tables
open modules_cursor
fetch next from modules_cursor into @module

IF @@FETCH_STATUS <> 0
	PRINT '            <<None>>	No module to refresh'

while @@FETCH_STATUS = 0
	begin
		select @str = quotename(schema_name(objectproperty(@module, 'schemaid'))) + '.' + quotename(object_name(@module))
		select @message = '            trying to refresh ' + @str
		print @message
		exec sys.sp_refreshsqlmodule @str
		select @message = '            ' + @str + ' was refreshed'
		print @message

		insert into #t_dependency_table (referencing_id, referenced_id)
			select distinct object_id as referencing_id, referenced_major_id as referenced_id
				from sys.sql_dependencies
				where object_id <> referenced_major_id      -- to avoid self recursion for functions
					  and object_id = @module

		fetch next from modules_cursor into @module
	end

close modules_cursor
deallocate modules_cursor

-------------------------------------------------------------------------------------------
--
-- get the the bottom of the dependency list i.e. independent modules
-- i.e.
-- get the list of referenced_ids in the dependency table which
-- don't occur in the referencing_ids column
--
-- Comment:
-- if there are circular dependencies then the few modules which form a circular dependency
-- would be ignored in the independent modules list
--
-------------------------------------------------------------------------------------------
create table #t_independent_modules (modules int)

insert into #t_independent_modules (modules)
	select #t_dependency_table.referenced_id from
			#t_dependency_table left outer join #t_dependency_table t2
			on #t_dependency_table.referenced_id = t2.referencing_id
	where
			t2.referencing_id is NULL

-------------------------------------------------------------------------------------------
--
-- build the ordered list of dependencies starting with the independent modules
-- in the beginning first few rows, ones dependent on it in the following rows and so on...
--
-- there can be tricky cases of dependencies such as
-- V1 -> V2 -> V3
--  |           ^
--  +-----------+
--
-- in the above example the refresh order would be V3, V1, V2, V1.
-- note that V1 is being refreshed twice, the last refresh of V1 after V2 is important.
--
-------------------------------------------------------------------------------------------
create table #t_final_dependency_list (id_num int IDENTITY(1,1), modules int)

while exists (select * from #t_independent_modules)
	begin
		-- append the set of independent modules into a list
		insert into #t_final_dependency_list select * from #t_independent_modules

		-- get the set of dependent modules
		select distinct #t_dependency_table.referencing_id into #temp_table
			from #t_dependency_table
			where #t_dependency_table.referenced_id in (select * from #t_independent_modules)

		-- clear up the list of independent modules
		truncate table #t_independent_modules

		-- the dependent modules now become the independent modules
		insert into #t_independent_modules select * from #temp_table

		-- delete the dependent modules list
		drop table #temp_table
	end

-----------------------------------------------------------
--
-- refresh the modules once more but in the right order now
--
-----------------------------------------------------------
declare modules_cursor_final cursor for
	select modules from #t_final_dependency_list order by #t_final_dependency_list.id_num

open modules_cursor_final
fetch next from modules_cursor_final into @module

IF @@FETCH_STATUS <> 0
	PRINT '            <<None>>	No module to refresh'

while @@FETCH_STATUS = 0
	begin
		if (select type from sys.objects where object_id = @module) in ('V', 'FN', 'IF', 'TF')
			and (select is_schema_bound from sys.sql_modules where object_id = @module) = 0
			begin
				select @str = quotename(schema_name(objectproperty(@module, 'schemaid'))) + '.' + quotename(object_name(@module))
				select @message = '            trying to finally, once more, refresh ' + @str
				print @message
				exec sys.sp_refreshsqlmodule @str
				select @message = '            ' + @str + ' was finally refreshed once again'
				print @message
			end

		fetch next from modules_cursor_final into @module
	end

close modules_cursor_final
DEALLOCATE modules_cursor_final

-----------
--
-- cleanup
--
-----------
drop table #t_excluded_modules
drop table #t_modules_refreshed_in_end
drop table #t_user_views_or_tables
drop table #t_dependency_table
drop table #t_independent_modules
drop table #t_final_dependency_list

Posted in Copy database wizard, Replication, SQL General | Tagged: , , , , | 5 Comments »

I get error while I update a row in immediate updating subscription what is the cause?

Posted by Karthick P.K on September 15, 2010

Error:

{

Another user has modified the contents of this table or view; the database row you
are modifying no longer exists in the database.
Database error: ‘[Microsoft][ODBC SQL Server Driver][SQL Server]Updatable
Subscriptions: Rows do not match between Publisher and Subscriber. Run the
Distribution Agent to refresh rows at the Subscriber.
[Microsoft][ODBC SQL Server Driver][SQL Server]Updatable Subscriptions: Rolling
back transaction.’
}

 

In Transactional Replication with immediate updating subscription

Update to a row in subscriber which is already updated in publisher is not
possible.
When we update the row in subscriber it compares the value of msrepl_tran_version
in Publisher and subscriber. If it matches then update is allowed.
If msrepl_tran_version in Publisher and subscriber doesn’t match Update is not
possible for that row in subscriber.

Posted in Replication | Tagged: , , , | 3 Comments »

 
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