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Archive for the ‘Replication’ Category

Quick one today:

In our lab, quite frequently, we need to cross check the replication distribution agent profile settings between each environment to be sure they are set correctly. Replication Monitor provides a good way to check each agent and its profiles, but when you have many, many distribution agents in each environment, manual check is not fun.

This could be done by collecting all agent profile parameters using T-SQL

--
-- Run this query pointed to the Distributor
--
SELECT    D.id
		, D.name
		, D.publisher_database_id
		, D.publisher_id
		, D.publisher_db
		, D.publication
		, D.subscriber_id
		, D.subscriber_db
		, D.subscription_type
		, D.profile_id
		, D.creation_date
		, D.subscriber_security_mode
		, D.subscriptionstreams

		, CASE	WHEN P.agent_type = 1 THEN 'Snapshot Agent'
				WHEN P.agent_type = 2 THEN 'Log Reader Agent'
				WHEN P.agent_type = 3 THEN 'Distribution Agent'
				WHEN P.agent_type = 4 THEN 'Merge Agent'
				WHEN P.agent_type = 9 THEN 'Queue Reader Agent'
				ELSE 'Unknown'
		  END AS [AgentType]
		, P.profile_name
		, CASE	WHEN P.type = 0 THEN 'System Profile'
				WHEN P.type = 1 THEN 'User Defined Profile'
				ELSE 'UNKNOWN'
		  END AS [ProfileType]

		, PML.parameter_name
		, PM.value AS [ParameterValue_InUse]

		, PML.default_value
		, PML.min_value
		, PML.max_value

FROM dbo.MSdistribution_agents AS D
INNER JOIN msdb.dbo.MSagent_profiles AS P
	ON D.profile_id = P.profile_id
	AND P.agent_type = 3 -- Distribution agent

INNER JOIN msdb.dbo.MSagent_parameters AS PM
	ON PM.profile_id = D.profile_id

INNER JOIN msdb.dbo.MSagentParameterList AS PML
	ON PML.agent_type = P.agent_type -- Distributor
	AND PML.parameter_name = SUBSTRING(PM.parameter_name, 2, 50)
GO

Result looks something like this:

Distributor_AgentProfiles.PNG

Hope this helps,
_Sqltimes

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Interesting one today:

This may be common knowledge, but upon running some tests its evident that Snapshot Replication does both the things:

  1. Deliver a complete snapshot (of all articles chosen) to the Subscribers
  2. Also delivers, incremental changes to those articles a.k.a INSERTS/UPDATES / DELETES

Or let’s state it differently:

  1. Snapshot and Transactional replication are two different replication types
  2. When we set up replication, we create:
    1. LogReader Agent : Reads TRN Log at Publisher and writes changes to Distributor
    2. Snapshot Agent: Creates a complete snapshot/backup/data of Publisher database to be created at Subscriber (but does not deliver it to Subscriber)
    3. Distributor Agent: This agent delivers data created in the above two steps to the Subscriber.
      1. Delivers the continuous changes that LogReader brings
      2. Delivers snapshot that Snapshot agent creates
  3. Transactional Replication:
    1. When we create Transactional Replication, the continuous activity at Publisher is replicated to Subscriber. This is carried out by LogReader Agent & Distributor agent.
      1. Distributor has replication procedures for INSERT, DELETE & UPDATE that take each DML activity at Publisher and re-play it as Subscriber
    2. But Transaction replication expects database & tables to already exist at the Subscriber
      1. And that the object structure matches — as the replication procedures (INSERT & UPDATE) depend on the Table & Column names and column order
    3. Creating Snapshot Agent is an optional step while establishing Transactional Replication. Its not needed to create Snapshot Agent, if the database objects already exist at Subscriber;
    4. And all we care about is replicating data from this point onward (and not the old data)
  4. Snapshot Replication:
    1. As part of creating Snapshot replication we create Snapshot Agent & Distributor
    2. We create all replication procedures (for each article) to replicate incremental changes on top of initial snapshot/backup/data

Summary

  • Yes, when we set up Snapshot replication, not only is the initial data from Publisher is replicated to the Subscribers, it also replicates incremental changes happening at Publisher to all Subscribers.
  • Snapshot Agent is different Snapshot Replication

 

Hope this helps,
_Sqltimes

 

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Interesting Problem Today:

Ran into this issue a few times and every time its a variation of the same headache. So, here some ideas will be documented for posterity.

In general terms, the error looks like this:

Connecting to Subscriber ''
Agent message code 20084. The process could not connect to Subscriber ''
Microsoft SQL Server Native Client 11.0
SQL Server Network Interfaces: The target principal name is incorrect.
Cannot generate SSPI context

The error message seems nebulous and confusing — but for trained eyes it makes perfect sense. For me it took a while to make sense out of it.

There could be several things wrong under the hood, but essentially it says that the target SQL server that is is trying to connect to, does not have a valid SPN with Active Directory.

Meer from Microsoft has documented some information on troubleshooting this issue here. For more details, please read his article, as I’ll over simplify things and address a variation of the problem in this articles (which will be slightly different from his).

Example:

From SQLServerA, using UserA, if I’m trying to connect to SqlServerB, sometimes I get this error. Essentially, means SqlServerB does not have a valid SPN.

Resolution:

First, log into the machine that has SqlServerB. Open command prompt with Administrative privileges. Run the command below to see if there is a valid SPN.

 

SETSPN -L <SQL Server Instance Service Account>

 

If the output looks like the first image below, then the Sql Server instance does not have a valid SPN. Now its time to generate one.

Output without valid SPN:

Invalid SPN

Invalid SPN

Step 2: Download Kerberos Configuration Manager for SQL Server from here, and start generating one.

Step 3: Open Kerberos Configuration Manager for SQL Server from the same machine that has SqlServerB instance. When you run it, it shows something like this:

 

Kerberos Tool Output

Kerberos Tool Output

Notice that for Sql Server service, there is not valid SPN or misplaced SPN. So its time to generate one.

Step 4: Hit the “Fix it” button right next to it and generate one. Make sure the user account that is logged into the machine has domain controller permissions.

Step 5: Now run the same command as in Step 1, and the output looks different.

Output with valid SPN:

Valid SPN

Valid SPN

 

Voila !! Now you are able to connect to SqlServerB from SqlServerA using UserA

 

 

Hope this helps,
_Sqltimes

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Interesting one today:

Earlier in our lab environment, ran into this interesting error:

2019-06-20 00:18:46.67 Connecting to Distributor ''
2019-06-20 00:18:46.78 The replication agent had encountered an exception.
2019-06-20 00:18:46.78 Source: Replication
2019-06-20 00:18:46.78 Exception Type: 
Microsoft.SqlServer.Replication.ReplicationAgentException
2019-06-20 00:18:46.78 Exception Message: The snapshot could not be 
generated because the publisher is inactive.
2019-06-20 00:18:46.78 Message Code: 54057
2019-06-20 00:18:46.78

Replication from publisher to subscriber was set up correctly, but when the agent runs, the SQL Agent job stops with this error.

Resolution:

Go to Distributor and run this query to check the status of the publisher instance.


EXEC sp_helpdistpublisher

PublisherInvalid

As we can see, one of the publisher instance is set to Inactive. So now we need to reset it.

Run the script below to change the publisher status at distributor:


EXEC sp_changedistpublisher  @publisher = 'Publisher',  @property = 'active', @value = 'true'
GO

The result looks something like this:

Publisher_Active

Now  run the previous ‘sp_helpdistpublisher’ again and now the status is active:

PublisherValid

 

Hope this helps,
_Sqltimes

 

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Interesting one today:

Last time we walked through the T-SQL steps to insert a tracer token to measure latency in replication topology. Today, we’ll look at an alternate method to query tracer token details; a.k.a. MStracer_tokens &  MStracer_history meta tables.

Along with the T-SQL procedures (sys.sp_helptracertokenhistory), Sql Server also provides a way to query the tracer tokens using metadata tables i.e. MStracer_tokens &  MStracer_history.  They keep track of details for each token. Querying them will provide us necessary information.

--
-- Query tracer token tables
--
SELECT	  publication_id
	, agent_id
	, t.publisher_commit
	, t.distributor_commit
	, h.subscriber_commit

FROM MStracer_tokens t
JOIN MStracer_history h
	ON t.tracer_id = h.parent_tracer_id

ORDER BY t.publisher_commit DESC
GO
Query Tracer Token Details

Query Tracer Token Details

Hope this helps,
_Sqltimes

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Interesting one today:

This is Part 2 on blogs related to Replication Setup. Full list is here.

In a recent post, we walked through the steps for setting up replication using T-SQL commands. Today, we’ll look at the commands to remove/drop replication
using T-SQL commands.

Essentially there are 3 major steps to dropping replication; And all steps are executed at Publisher instance. As and when needed, these steps will communicate with Subscriber & Distributor to remove relevant artifacts at each step.

Main Steps:

  1. Remove definitions for Publication, Subscription & all relevant articles
  2. Change database settings
  3. Change Distributor settings

As expected, each of these major steps could have multiple sub-steps, which we’ll get into in future posts.

NOTE: All steps are carried out at Publisher instance

Replication Step T-SQL Step
1. Remove Publication, Subscriptions, etc

a. Remove subscription to each subscriber

b. Remove subscription with articles

c. Remove articles associated with the publication

d. Finally, remove the Publication

1. Run Publisher Instance

a. sp_dropsubscription

b. sp_dropsubscription

c. sp_droparticle

d. sp_droppublication

2. Change database settings

a. Disable database from Publishing

b. Remove associations with all Subscribers

2. Run at Publisher

a. sp_replicationdboption

b. sp_dropsubscriber

3. Distributor Settings

a. Remove association with Distributor

3. Run at Publisher

a. sp_dropdistributor

In a future post, we’ll get into the next set of details.

Hope this helps,
_Sqltimes

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