Feeds:
Posts
Comments

Archive for the ‘Replication’ Category

Quick one today:

Every now and then, when good Sql Server resources are available, they are posted here for more people to benefit from them. Continuing on that tradition, today we have a gold mine. Microsoft has released many, many & many e-books open to public to download for free. In the list there are several Sql Server books along with BI, Windows, Office, SharePoint, etc

Happy learning !!

 

Hope this helps,
_Sqltimes

Read Full Post »

Problem

While gathering replication backlog details, ran into this interesting error. The goal was to run the sp_replmonitorsubscriptionpendingcmds stored procedure and store the output in a table. As we’ve seen in a recent post, redirecting output of a stored procedure execution into a table is possible; But in this case, it throws an error saying that is not allowed.

INSERT INTO #DC1_Repl_Backlog
EXEC  sp_replmonitorsubscriptionpendingcmds
		  @publisher	= 'InstanceName'
		, @publisher_db	= 'DBName'
		, @publication	= 'Publication'
		, @subscriber	= 'Subscriber'
		, @subscriber_db= 'DBName2'
		, @subscription_type = '0'
GO
Msg 8164, Level 16, State 1, Procedure sp_replmonitorsubscriptionpendingcmds, Line 233
An INSERT EXEC statement cannot be nested.

(0 row(s) affected)

With the available information, right now, a clear & coherent explanation fo this behavior is not available from my end. But my guess is this; The code inside this stored procedure must be using a similar INSERT INTO #table EXEC sp_xyz, hence the error “INSERT EXEC statement cannot be nested

Resolution

OPENROWSET helps in getting around this. See the sample code below:

--
--
--
IF OBJECT_ID('tempdb..#DC1_Repl_Backlog') IS NOT NULL
	DROP TABLE #DC1_Repl_Backlog

CREATE TABLE #DC1_Repl_Backlog (
	  pendingcmdcount	BIGINT
	, estimatedprocesstime	BIGINT
)

INSERT #DC1_Repl_Backlog (pendingcmdcount, estimatedprocesstime)
SELECT *
FROM OPENROWSET('SQLOLEDB',
		'Server=InstanceName;Trusted_Connection=yes;',
		'EXEC Ditribution.dbo.sp_replmonitorsubscriptionpendingcmds
					  @publisher = ''PublisherInstance''
					, @publisher_db	= ''DBName''
					, @publication = ''Publication''
					, @subscriber = ''Subscriber''
					, @subscriber_db = ''DBName2''
					, @subscription_type = ''0'''
		) 

SELECT *
FROM #DC1_Repl_Backlog
GO
Hope this helps,
_Sqltimes

Read Full Post »

Quick one today:

In replication, the transactions that come into the system are stored in Distribution database first; From there they are replicated to each Subscriber. The data stays there for several hours. This is determined by a ‘Transaction Retention‘ setting on the Distributor.

Similarly, as the data is replicated, log entries are made in the Distributor database. These entries also have a retention policy that could be set using ‘History Retention‘ setting.

Go to Distribution instance, Replication >> Right click >> Distributor Properties

Distributor Retention Properties

Distributor Retention Properties

Under general tab, we see these settings. Change them as needed to achieve longer/shorter retention of both data & log.

Retention Policy

Retention Policy

Hope this helps,
_Sqltimes

Read Full Post »

Quick one today:

Recently, in one of our lab machines, we ran into this errors during replication reconfiguration (rebuilding) efforts.

Msg 18752, Level 16, State 1, Procedure sp_replcmds, Line 1
Only one Log Reader Agent or log-related procedure (sp_repldone, sp_replcmds, 
and sp_replshowcmds) can connect to a database at a time. If you executed a 
log-related procedure, drop the connection over which the procedure was executed 
or execute sp_replflush over that connection before starting the Log Reader Agent 
or executing another log-related procedure.

The error message, though verbose, it was a bit confusing to understand the underlying cause for this error. Once, we reiterated the steps that resulted in this error, it became clear as to what could have contributed to this error. We had replication set upon a lab machine, and before this error occurred, we were attempting to drop transactional replication using SSMS GUI.

Turns out Log Reader agent was not properly stopped before attempting this; To resolve this, just stop the Log Reader Agent. To be sure, go to SSMS > Replication > Right click on Publication and go to ‘View Log Reader Agent Status‘.

Once Log Reader Agent is stopped, we were able proceed with dropping Transactional replication configuration.

Hope this helps,
_Sqltimes

Read Full Post »

Quick one today:

Edit: Update on April 10th – Added some comments to repeat some steps, as needed

Earlier today, we ran into this error in our lab environment with replication. Two different errors, on the same environment, due to similar sequence of events.

Msg 20584, Level 16, State 1, Procedure sp_MSrepl_check_server, Line 67
Cannot drop server because it is used as a Subscriber to remote Publisher in replication.

Cannot drop server because it is used as a distributor in replication

This type of errors are rare, but one of these three options below might provide some relief.

  1. Reset Dist flag
  2. Force drop distributor
  3. Manually remove Publishers (from Distributor properties)
    1. Invalid object name ‘dbo.MSmerge_agents’

Also, see the notes in the bottom section (points to keep in mind)

Reset Dist Flag for Distributor Instance

Sometimes, the dist flag for Distributor stays set, when not supposed to. So manually resetting it helps.

--
--  Reset Distributor flag
--
EXEC master.dbo.sp_ServerOption   @server = 'REPLDist'
				, @OptName = 'dist'
				, @OptValue = 'false'
GO

Force Drop Distributor

One of the last options is to ‘force’ drop distributor. This allows removing distributor database without running through the proper checks. This step is a high risk, high reward option; Since all checks were previously performed and all components of replication are removed, except distributor, this step might provide relief.

EXEC sp_dropdistributor @no_checks = 1, @ignore_distributor = 1
GO

Manually Remove Publishers

Usually, the above two steps will resolve the issue. But in rare occasions, the issue persists, like in our lab. For those stubborn situation, this technique has given some relief.

Go to Replication >> Distributor Properties >> Go to Publishers (tab)

Now, un-check each Publisher and save. Important to note that, this needs to be done for one publisher at a time.

Distributor_Properties_RemovePublishers.PNG

3.1 Invalid object name MSmerge_agents

In Step 3, sometimes we might run into an error like this:

Invalid object name 'dbo.MSmerge_agents'.

Not sure what the root cause is, but this seems to help:

  1. Create two tables in Distributor database
  2. Rerun Step 3

1. Create two tables in Distributor database

NOTE: This step is not necessary in all situation. So, proceed with caution.

Looks like these two tables are missing in Distribution database. Create them in the Distribution database. And then go back to Step 3 again. Or Step 3 & 2, as needed.

Note: There is no harm in adding these two tables because, after you create these tables and then successfully drop replication (including the distributor database) the new objects will be removed along with the Distributor database.

So you have a clear system at the end.

create table msmerge_agents(
	  id						INT
	, name						NVARCHAR(100)
	, publisher_id				SMALLINT
	, publisher_db				SYSNAME
	, publication				SYSNAME
	, subscriber_id				SMALLINT
	, subscriber_db				SYSNAME
	, local_job					BIT
	, job_id					BINARY(16)
	, profile_id				INT
	, anonymous_subid			UNIQUEIDENTIFIER
	, subscriber_name			SYSNAME
	, creation_date				DATETIME
	, offload_enabled			BIT
	, offload_server			SYSNAME
	, sid						VARBINARY(85)
	, subscriber_security_mode	SMALLINT
	, subscriber_password		NVARCHAR(524)
	, publisher_login			SYSNAME
	, publisher_password		NVARCHAR(524)
	, job_step_uid				UNIQUEIDENTIFIER
)
GO

CREATE table MSmerge_subscriptions(
	  publisher_id		SMALLINT
	, publisher_db		SYSNAME
	, publication_id	INT
	, subscriber_id		SMALLINT
	, subscriber_db		SYSNAME
	, subscription_type INT
	, sync_type			TINYINT
	, status			TINYINT
	, subscription_time DATETIME
)
GO

Points to keep in mind:

Sometimes, just the one of these steps might be enough to resolve the issue; Sometimes, it might take two of the steps. In our lab, we ended up needing all the 3 steps. the first step is easy to implement.
The second and third steps might need repeated attempts. Something like:
  1. Try Step 2, if it fails,
    1. then try step 3;
    2. If it fails, try Step 3.1
    3. If that succeeds, then go back to Step 2.
  2. Once Step 2 is successful, the distributor is fully cleared and cleaned.
Hope this helps,
_Sqltimes

Read Full Post »

Interesting one today:

MSDTC is one of the popular software components that is on all Windows systems. It is one of the Windows  Operating System components that Sql Server relies on it to perform some crucial tasks (when needed).

What does it do?

MSDTC, Microsoft Distributed Transaction Coordinator, is essentially, as name suggests, a coordinator/manager to handle transactions that are distributed over multiple machines. Let’s say we start a transaction, where one of the steps includes querying data from a different Sql Server instance on a different physical machine; MSDTC comes into action with these specific tasks that need transaction coordination across different physical machines. It executes the section of code that is supposed to run on remote machines and brings back the results to local Sql instance. In this process, if any issue were to occur, on the remote machine that results in rollback, MSDTC makes sure the original transaction on this machine also rolls-back safely.

How does it do?

MSDTC comes with necessary Operating System controls and memory structures to carry out these operations independent of the Sql Instances, while keeping integrity of the transaction across the multiple physical Sql machines a.k.a. the complete two-phase distributed commit protocol and the recovery of distributed transactions.

Where does Sql Server use it?

The key point here is that these need to be Sql Instances on different physical machines. Queries that request data across different instances on the same physical box do not go through MSDTC.

MSDTC is used by query activities like

  • Linked Servers
  • OPENROWSET
  • OPENQUERY
  • OPENDATASOURCE
  • RPC (Remote Procedure Calls)
  • Ones with
    • BEGIN DISTRIBUTED TRANSACTION
  • etc…

So, every time we run SQL queries that utilize above techniques, they rely on MSDTC to carry out operation while maintaining transaction integrity.

Who else uses it?

MSDTC is an Operating System resource that is used by applications other than Sql Server, to perform any distributed transaction activities; Like eXtended Architecture applications.

Is MSDTC required?

MSDTC is not required for Sql Server installation or operation. If you are only going to use Database Engine, then it is not required or used. If your Sql uses any of the above mentioned query techniques (Linked Server, OPENQUERY, etc), or SSIS or Workstation Components then MSDTC is required.

If you are installing only the Database Engine, the MSDTC cluster resource is not required. If you are installing the Database Engine and SSIS, Workstation Components, or if you will use distributed transactions, you must install MSDTC. Note that MSDTC is not required for Analysis Services-only instances.

What about Sql Cluster?

Same rules as above apply to Sql Clusters as well with one additional rule. If you have two instances on the same machine (that are clustered across different physical machines), then you’ll need MSDTC. Since the Cluster could failover to remote machine at anytime.

Let’s take an example:

Let’s say Instance1 is on physical machines A & B, with B as active node. Instance2 is on machines B & C, with B as active node. A query going from Instance1 to Instance2 will need MSDTC (even if both the instances are active on the same physical machine B at that given point in time.).

This is because, there is no guarantee that they will remain on the same physical machine at any given time; They might failover to other machines, resulting in instances being on physically different machines. So MSDTC is required (when distributed operations are performed).

Also the recent Sql Server versions do not required MSDTC during Sql Server installations.

Other points in a Clustered Environment

We could have multiple instances of MSDTC as different clustered resource (along with default MSDTC resource).

In scenario with multiple MSDTC, we could configure each Sql Cluster resource to have a dedicated MSDTC instance. If such mapping does not exist, it automatically falls back to use the default MSDTC resource.

Hope this helps,
_Sqltimes

Read Full Post »

Older Posts »