Upgrading a Replication Topology to SQL Server 2016


Cross posting from Tiger blog.

SQL Server Replication provides multi-faceted data movement capabilities across SQL Server releases which has been used by customers across the globe for a large number of years. When moving from one major release of SQL Server to another, replication topology upgrade has been a constant topic of lengthy discussions. In this post, we shall outline some of the challenges of upgrading SQL Server replication environments to SQL Server 2016. The requirements of upgrading a replication topology need to abide by the following guidelines:

  • A Distributor can be any version as long as it is greater than or equal to the Publisher version (in many cases the Distributor is the same instance as the Publisher).
  • A Publisher can be any version as long as it less than or equal to the Distributor version.
  • Subscriber version depends on the type of publication:
    • A Subscriber to a transactional publication can be any version within two versions (n-2) of the Publisher version. For example: a SQL Server 2012 Publisher can have SQL Server 2014 and SQL Server 2016 Subscribers; and a SQL Server 2016 Publisher can have SQL Server 2014 and SQL Server 2012 Subscribers.
    • A Subscriber to a merge publication can be any version less than or equal to the Publisher version.

If you had to draw a support matrix for the major release versions for transactional and merge replication, the output would be the two tables shown below.

For the remainder of this post, transactional replication refers to Transactional Replication excluding heterogeneous replication, P2P replication and updateable subscribers.

Transactional Replication Matrix

Publisher Distributor Subscriber
SQL Server 2016 SQL Server 2016 SQL Server 2016SQL Server 2014

SQL Server 2012

SQL Server 2014 SQL Server 2016SQL Server 2014 SQL Server 2016SQL Server 2014

SQL Server 2012

SQL Server 2008 R2

SQL Server 2008

SQL Server 2012 SQL Server 2016SQL Server 2014

SQL Server 2012

SQL Server 2016SQL Server 2014

SQL Server 2012

SQL Server 2008 R2

SQL Server 2008

SQL Server 2008 R2SQL Server 2008 SQL Server 2016SQL Server 2014

SQL Server 2012

SQL Server 2008 R2

SQL Server 2008

SQL Server 2014SQL Server 2012

SQL Server 2008 R2

SQL Server 2008

SQL Server 2005

SQL Server 2000

Merge Replication Support Matrix

Publisher Distributor Subscriber
SQL Server 2016 SQL Server 2016 SQL Server 2016SQL Server 2014

SQL Server 2012

SQL Server 2014 SQL Server 2016SQL Server 2014 SQL Server 2014SQL Server 2012

SQL Server 2008 R2

SQL Server 2008

SQL Server 2012 SQL Server 2012 SQL Server 2012SQL Server 2008 R2

SQL Server 2008

SQL Server 2008SQL Server 2008 R2 SQL Server 2008 R2SQL Server 2008 SQL Server 2008 R2SQL Server 2008

SQL Server 2005

SQL Server 2000

If you notice the line items for SQL Server 2016, you will see that a topology is unable to support SQL Server 2016 in a number of scenarios when you are running SQL Server 2016 as a publisher. Replication topologies have three common deployment patterns as shown in the visio diagram below. The distributor could be on the publisher or subscriber or a remote distributor. We do come across different deployments of the publisher and subscriber which are a mix of standalone instances, SQL Server failover cluster instances or Always On Availability Group replica instances.

image

Depending on the deployment pattern, the upgrade path to SQL Server 2016 would be different. Let us explore the different possibilities. SQL Server offers two upgrade paths in general:

  • Side-by-side: This approach involves setting up a new parallel environment and moving the databases along with the associated instance level objects like logins, jobs etc. to the new environment.
  • In-place upgrade: With this approach, the SQL Server setup program upgrades the existing SQL Server installation by replacing the existing SQL Server bits with the SQL Server 2016 bits and then upgrades each of the system and user databases. For environments running SQL Server failover cluster instances or Always On Availability Groups, an in-place upgrade is combined with a rolling upgrade to minimize downtime.

The scenarios below apply to Transactional Replication (without P2P Replication, Queued Updating Subscription and Immediate Updating Subscription) and Merge Replication. The options below outline how a phased approach can be adopted for your replication topology upgrade so that you don’t have to upgrade all the SQL Server instances in one big upgrade operation.

A common approach that has been adopted for side-by-side upgrades of replication topologies is to move publisher-subscriber pairs in parts to the new side-by-side environment as opposed to a movement of the entire topology. This phased approach helps to control downtime and minimize the impact to a certain extent for the businesses dependent on replication.

Upgrading a Replication Topology with a Remote Distributor

Transactional Replication

Upgrading from

Distributor

Publisher/Subscriber

SQL Server 2014

SQL Server 2012

Step 1:

In-place upgrade (Can be upgraded due to n-2 support)

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs in the replication topology*

Step 2:

In-place upgrade (Can be upgraded due to n-2 support)

OR

Side-by-side upgrade of subscriber requires reinitialization of subscriber

Side-by-side upgrade of publisher requires reconfiguring all the publisher-subscriber pairs

The publisher and subscriber can be upgraded in any order

SQL Server 2008 R2

SQL Server 2008

Step 1:

In-place upgrade

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs in the replication topology*

Step 2:

In-place upgrade would need to occur for both publisher and subscriber at the same time as publisher and subscriber need to be within two major releases. A SQL Server 2008/R2 publisher/subscriber cannot have a SQL Server 2016 publisher/subscriber.

OR

Intermediate In-place upgrade to SQL Server 2012/2014 of publisher or subscriber

The other server in the publisher/subscriber pair can then be upgraded to SQL Server 2016

OR

Side-by-side upgrade will require the upgrade of publisher and subscriber to happen together and requires a re-setup of the publisher/subscriber pairs

*See “Side-by-side Upgrade of the Distributor without re-initialization” below

Merge Replication

Upgrading from

Distributor

Publisher

Subscriber

SQL Server 2014

SQL Server 2016

Step 1:

In-place upgrade

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs in the replication topology*

Step 2:

In-place upgrade

OR

Side-by-side upgrade of publisher requires reconfiguring all the publisher-subscriber pairs

Step 3:

In-place upgrade

OR

Side-by-side upgrade of subscriber requires reinitialization of subscriber

SQL Server 2008 R2

SQL Server 2008

Step 1:

In-place upgrade

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs in the replication topology*

Step 2:

In-place upgrade

OR

Side-by-side upgrade of publisher requires reconfiguring all the publisher-subscriber pairs. Needs to happen with the upgrade of the subscriber.

Step 3:

In-place upgrade

OR

Side-by-side upgrade of subscriber requires reinitialization of subscriber. Needs to happen with the upgrade of the publisher.

*See “Side-by-side Upgrade of the Distributor without re-initialization” below

Upgrading a Replication Topology with Publisher acting as the Distributor

Transactional Replication

Upgrading from

Publisher/Distributor

Subscriber

SQL Server 2014

SQL Server 2012

Step 1:

In-place upgrade (Can be upgraded due to n-2 support)

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs in the replication topology*

Step 2:

In-place upgrade (Can be upgraded due to n-2 support)

OR

Side-by-side upgrade of subscriber requires reinitialization of subscriber

SQL Server 2008 R2

SQL Server 2008

Step 1:

In-place upgrade (Requires subscriber to be upgraded because publisher and subscriber need to be within two major releases. A SQL Server 2016 publisher cannot have a SQL Server 2008/R2 subscriber.)

OR

Intermediate in-place upgrade to SQL Server 2012/2014 for the publisher which is acting as the distributor also

The subscriber can be upgrade to SQL Server 2016 post the intermediate publisher upgrade

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs served by this distributor in the replication topology. Needs to happen with the upgrade of the subscriber.

Step 2:

In-place upgrade would need to occur for both publisher and subscriber at the same time as publisher and subscriber need to be within two major releases. A SQL Server 2008/R2 publisher/subscriber cannot have a SQL Server 2016 publisher/subscriber.

OR

Intermediate In-place upgrade to SQL Server 2012/2014 of the publisher

The subscriber can then be upgraded to SQL Server 2016

OR

Side-by-side upgrade will require the upgrade of subscriber to happen together with the publisher and requires a re-initialization of the subscribers. Needs to happen with the upgrade of the publisher.

*See “Side-by-side Upgrade of the Distributor without re-initialization” below

Merge Replication

Upgrading from

Publisher/Distributor

Subscriber

SQL Server 2014

SQL Server 2016

Step 1:

In-place upgrade

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs served by this distributor in the replication topology*

Step 2:

In-place upgrade

OR

Side-by-side upgrade of subscriber requires reinitialization of subscriber

SQL Server 2008 R2

SQL Server 2008

Step 1:

In-place upgrade

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs served by this distributor in the replication topology. Needs to happen with the upgrade of the subscriber.

Step 2:

In-place upgrade

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs in the replication topology. Needs to happen with the upgrade of the publisher.

*See “Side-by-side Upgrade of the Distributor without re-initialization” below

Upgrading a Replication Topology with Subscriber acting as the Distributor

Transactional Replication

Upgrading from

Distributor/Subscriber

Publisher

SQL Server 2014

SQL Server 2012

Step 1:

In-place upgrade (Can be upgraded due to n-2 support)

OR

Side-by-side upgrade requires re-setup of publisher/subscriber pairs in the replication topology served by this distributor*

Step 2:

In-place upgrade (Can be upgraded due to n-2 support)

OR

Side-by-side upgrade of subscriber requires re-setup of all the publisher/subscriber pairs

SQL Server 2008 R2

SQL Server 2008

Step 1:

In-place upgrade (Requires publisher to be upgraded also because subscriber and publisher need to be within two major releases. A SQL Server 2008/R2 publisher cannot have a SQL Server 2016 subscriber.)

OR

Intermediate in-place upgrade to SQL Server 2012/2014 for the subscriber which is acting as the distributor also

The publisher can then be upgraded to SQL Server 2016 post this intermediate distributor upgrade

OR

Side-by-side upgrade of distributor/subscriber requires re-setup of all the publisher/subscriber pairs served by this distributor. Needs to happen with the upgrade of the publisher.

Step 2:

In-place upgrade would need to occur for both publisher and subscriber at the same time as publisher and subscriber need to be within two major releases. A SQL Server 2008/R2 publisher/subscriber cannot have a SQL Server 2016 publisher/subscriber.

OR

The publisher can then be upgraded to SQL Server 2016 (Requires

intermediate In-place upgrade to SQL Server 2012/2014 of the subscriber)

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs in the replication topology. Needs to happen with the upgrade of the distributor/subscriber.

*See “Side-by-side Upgrade of the Distributor without re-initialization” below

Merge Replication

Upgrading from

Distributor/Subscriber

Publisher

SQL Server 2014

SQL Server 2016

Step 1:

In-place upgrade

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs in the replication topology served by this distributor*. This also requires upgrade of the publisher as the publisher version has to be higher than the subscriber.

Step 2:

In-place upgrade

OR

Side-by-side upgrade of subscriber requires reinitialization of all publisher/subscriber pairs. Requires simultaneous upgrade of the subscriber because it is acting as the distributor.

SQL Server 2008 R2

SQL Server 2008

Step 1:

In-place upgrade (Requires simultaneous upgrade of the publisher)

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs in the replication topology. Requires simultaneous upgrade of the publisher.

Step 2:

In-place upgrade (Requires simultaneous upgrade of the subscriber acting as the distributor)

OR

Side-by-side upgrade requires re-setup of all the publisher/subscriber pairs in the replication topology. Requires simultaneous upgrade of the subscriber because it is acting as the distributor.

*See “Side-by-side Upgrade of the Distributor without re-initialization” below

Side-by-side Upgrade of the Distributor without re-initialization

If you are running your SQL Server instance to be upgraded on Windows Server 2008 or Windows Server 2008 R2, then you will need to perform a side-by-side upgrade of the distributor first to Windows Server 2012 R2 or Windows Server 2016 before upgrading to SQL Server 2016. The reason for this intermediate OS upgrade is that SQL Server 2016 cannot be installed on a Windows Server 2008/2008 R2 server. The side-by-side approach can also help reduce downtime if you are upgrading the hardware of the Windows Server hosting the distributor instance. Downtime of the publisher and subscriber can be reduced using SQL Server Failover Cluster instances or Always On Availability Groups.

The assumption here is that the edition of the SQL Server instance will not change and a failover cluster instance of SQL Server will be upgraded to a failover cluster instance where as a standalone instance will be upgraded to a standalone instance using the steps mentioned below.

Steps for side-by-side migration of the distributor to Windows Server 2012 R2

  • Setup a new failover cluster or standalone instance running the same major release, edition and version as your distributor on Windows Server 2012 R2/2016 with a different windows cluster and SQL Server FCI name or standalone host name. You will need to keep the directory structure same as the old distributor to ensure that the replication agents executables, replication folders and database file paths are found at the same path on the new environment. This will reduce any post migration/upgrade steps required.
  • Make sure that the current synchronization is complete and post that shut down all the replication agents
  • Shut down the current SQL Server failover cluster instance or standalone instance running as the distributor. If this is a standalone instance of SQL Server, you will need to shutdown the Windows Server hosting  the SQL Server instance to ensure that there is no conflict while renaming the server.
  • Remove the DNS entries for the old (current distributor instance) environment and the AD entries for the computer object for the SQL Server FCI
  • If this is a SQL Server Failover Cluster instance, rename the new SQL Server Failover Cluster instance name with the old virtual server name. If this is a standalone SQL Server instance, then rename the new standalone host with the old hostname.
  • Copy the database files from the previous instance using SAN redirection or storage copy or file copy
  • Bring the new SQL Server instance online
  • Restart all the replication agents and verify if the agents are running successfully
  • Validate if replication is working as expected

In-place upgrade to SQL Server 2016

  • Run in-place upgrade for SQL Server 2016 on the new cluster
  • If required, rebuild old nodes and add to the cluster to re-use existing hardware
  • Validate if replication is working fine

If you want to reduce the downtime, we recommend that you perform the side-by-side migration of the distributor as one activity and the in-place upgrade to SQL Server 2016 as another activity. This will allow you to take a phased approach, reduce risk and minimize downtime.

Summary

Upgrading a replication topology is a multi-step process. We recommend attempting an upgrade of a replica of your replication topology in a test environment before running the upgrade on the actual production environment. This will help iron out any operational documentation that is required for handling the upgrade smoothly without incurring expensive and long downtimes during the actual upgrade process. We have seen customers reduce downtime significantly with the use of Always On Availability Groups and/or SQL Server Failover Cluster Instances for their production environments while upgrading their replication topology. Additionally, we recommend taking backups of all the databases including MSDB, Master, Distribution database(s) and the user databases participating in replication before attempting the upgrade.

Resources

Recovering a Deleted Cluster Name Object (CNO) in a Windows Server 2008 Failover Cluster

Upgrade Replicated Databases

Supported Version and Edition Upgrades for SQL Server 2016

Hardware and Software Requirements for SQL Server 2016

SQL Server Upgrade

Rename a Computer that hosts a Standalone instance of SQL Server

Rename a SQL Server Virtual Server Name

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MaxBCPThreads not causing parallel export


I had recently run into an issue where the MaxBCPThreads value was set to a non-zero value and the Snapshot Agent was still not creating multiple snapshot files for the table in question which was over 70GB in size.

On further inspection, I did find that the issue was with the statistics distribution of the table in question was affecting the parallel file export by the snapshot agent. It has already been documented that the concurrent snapshot generation option prevents the use of parallel BCP threads by the Distribution Agent. This is documented on the ReplTalk blog on MSDN.

imageNow to find out why the parallel threads were not being generated. So I created a repro of the situation on my side with a transaction replication publication having three tables with the same schema options. The snapshot folder after the snapshot agent’s execution is as shown in the screenshot.

The tblNewArticle6 has 60398 rows, tblNewArticle3 has 100,000 rows and the tblNewArticle1 has 2 rows.

The MaxBCPThreads configured value for the snapshot agent is 6. Surprisingly, tblNewArticle6 has 12 BCP files and the tblNewArticle3 has only one BCP file.

Looking into a profiler trace and the verbose log for the Snapshot Agent, I found out the following information:

1. There is a thread which performs an exploration on what ranges the parallel BCP threads will be fetching. The application name in the profiler trace will show up as “Auxiliary bcp load ordering hint and partitioning resolution thread # for publication <publication name>”. See Screenshot 2 for more details.

2. You will also find dynamic SQL commands executing DBCC SHOW_STATISTICS against the tables in question. image

In my case, tblNewArticle6 has a data distribution graph as shown in screenshot 3. It is clear from the screenshot that the tblNewArticle6 has multiple ranges available for the index id = 1. These were utilized by the Snapshot agent to generate 12 BCP files.

 

image

On looking into the Snapshot agent verbose log, I find find the following

Partitioning where clauses for article ‘tblnewArticle6’:

([SalesOrderNumber] is null) or ([SalesOrderNumber] <= convert(nvarchar(7), 0x53004F0035003000380034003400) collate SQL_Latin1_General_CP1_CI_AS)
([SalesOrderNumber] > convert(nvarchar(7), 0x53004F0035003000380034003400) collate SQL_Latin1_General_CP1_CI_AS) and ([SalesOrderNumber] <= convert(nvarchar(7), 0x53004F0035003300310039003500) collate SQL_Latin1_General_CP1_CI_AS)


([SalesOrderNumber] > convert(nvarchar(7), 0x53004F0037003300300037003600) collate SQL_Latin1_General_CP1_CI_AS)

The above ranges translated to 12 ranges which is the same number of files that I see in my snapshot folder.

Now looking into tblNewArticle3 which has 100,000 rows, I have a DBCC SHOW_STATISTICS output which only showed me two distinct ranges. Another point to note was that the primary key of the table has a non-clustered index defined on it and the clustered index key is a non primary key column. On increasing the number of rows in the table, I still find that the number of rows in the table, tblNewArticle3, I still found that the number of BCP files that were being generated was only 1. Then on additional investigation, I found that there is a small requirement of the clustered index to contain the primary key columns for the published article as well. This is evaluated in one of the system stored procedures for replication.

Some deep diving into the profiler traces, snapshot agent logs and debugging the stored procedures will help you arrive at the following conclusion:

Multiple BCP files will be created by the Snapshot Agent only if the following are true:

1. There are multiple distinct ranges available in the clustered index’s statistics histogram to create partitions which can be used by the BCP program to extract the data in parallel.

2. The clustered index must be defined on the primary key columns.

3. MaxBCPThreads value is set to a value other than 1.

If the above is true, then you will see multiple BCP files created by the Snapshot agent for the table of interest. Note that the above information is simplified to ensure that I comply with my NDA. After making the above changes, I see the following output in the repldata folder shown below in the screenshot:

image

Additional Reference:
Improving snapshot performance using MaxBCPThreads

 

Enabling Transactional Replication: A bit of help


Over the past few months, I have discussed the feasibility of enabling transaction replication for customer databases on various occasions. Every time I end up writing queries to answer certain questions about the database… the most common one being if the tables that need to be replicated have primary keys.

So I finally decided to write a T-SQL script which will help me answer the most common questions asked about a database while deciding on the feasibility of enabling transaction replication.

The script doesn’t capture information like workload, performance metrics etc. to decide if the replication workload (snapshot and distribution agent) can be supported on the existing hardware and resources available in the environment.

My take on the matter is that this information is required only once we have figured out if transactional replication can be enabled on the database or not. Eg. If the main tables that need to be replicated do not have primary keys, then the question of resource availability and hardware capability is moot point!

The script below checks the following:

1. Existing of primary keys on the tables in the database. Objects (articles) without primary keys cannot be replicated as part of a transactional replication publication.
2. If the database has transparent database encryption enabled. The subscriber database is not automatically enabled for TDE in such a scenario.
3. Constraints, primary keys, triggers and identify columns which have  NOT FOR REPLICATION bit set and which objects do not. You might choose to replicate or not replicate some of these objects. However, you need to be aware of what you are replicating.
4. Tables having ntext, text and image columns as there are special considerations for handling DMLs on such columns.
5. XML schema collections present in the database. Modifications to the XML Schema collection are not replicated.
6. Tables with sparse column sets as they cannot be replicated.
7. Objects created using WITH ENCRYPTION option. Such objects cannot be replicated either.

As always, in case you think that there are additional checks that could be included in the script, then please leave a comment on my blog and I will add the same into the script.

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