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Category: Availability Groups

Thinking About Availability Group Outages

Published 2017-01-25 by Kevin Feasel

Brent Ozar reminds us to think about graceful degradation of applications:

There’s a gray bar across the top that says, “This site is currently in read-only mode; we’ll return with full functionality soon.”

That’s not a hidden feature of Always On Availability Groups. Rather, it’s a hidden feature of really dedicated developers whose application:

This is where a bit of foresight and hard work can really pay off.  Read the whole thing.

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Network Latency And Availability Groups

Published 2017-01-11 by Kevin Feasel

Anthony Nocentino discusses network latency and how that can affect Availability Groups:

Now let’s talk about Availability Group replication and network latency. Availability Groups replicate data over your network using Database Mirroring Endpoints which are TCP sockets used to move data between the primary and it’s replicas. When designing Availability Groups, we often think about things in terms of bandwidth…how much data do I need to move between my replicas. But there’s another design factor your need to consider, network latency. Why? Hint, it’s not going to have anything to do with synchronous availability mode and HADR_SYNC_COMMIT waits. Let’s talk about some fundamentals of TCP for a second.

Click through for some discussion of TCP fundamentals.

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Testing Transactional Replication

Published 2017-01-03 by Kevin Feasel

Jes Borland wraps up her series on transactional replication from an on-prem Availability Group to Azure SQL Database:

Congratulations, you’ve configured a remote distributor, configured all of your AG replicas as publishers, and configured your SQL Database as a subscriber! Now you want to ensure that transactions are replicating to the database, and that they continue to do so if there is a failover in the AG.

Read on for the two testing scenarios.

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Replication Subscriber To Azure SQL DB

Published 2016-12-30 by Kevin Feasel

Jes Borland continues her series on setting up transactional replication between an on-prem SQL Server Availability Group and Azure SQL Database:

This subscription is going to use an Azure SQL Database.

Go to the AG primary replica. (In this demo, this is SQL2014AG2.)

Expand Replication. Expand Local Publications. Right-click the publication and select New Subscription.

It turns out that this is a basic push subscription.  Jes’s post is full of screenshots, making it even easier to follow.

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Replication Publisher To Azure SQL DB

Published 2016-12-29 by Kevin Feasel

Jes Borland continues her series on transactional replication from on-prem SQL Server + Availability Groups into Azure SQL Database:

After initializing, check the Snapshot Agent and Log Reader Agent for success. (To do so, go to Replication, right-click the publication name, and select Snapshot Agent Status and Log Reader Agent Status.) I ran into problems with the Snapshot account not having high enough permissions in the databases (it needs db_owner), and then not having enough permissions on the snapshot folder (it needs Full). (This forum post, answered by Hilary Cotter, helped: https://social.msdn.microsoft.com/Forums/sqlserver/en-US/899857db-e38e-4026-a34c-2a8c2628c6fc/access-denied-to-sql-replication-snapshot-folder?forum=sqlreplication.)

Except for the final section, it’s pretty much the same as dealing with on-prem SQL Server sans Availability Groups.

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Transactional Replication To Azure SQL Database

Published 2016-12-28 by Kevin Feasel

Jes Borland has a five-part series on replicating a series of databases in an Availability Group to Azure SQL Database.  Part 1 involves planning:

There are tasks you’ll need to take care of in SQL Server, the AG, and the SQL DB before you can begin.

This blog series assumes you already have an AG set up – it won’t go through the setup of that. It also assumes you have an Azure SQL server and a SQL Database created – it won’t go through that setup either.

Ideally, the publishers, distributor, and subscribers will all be the same version and edition of SQL Server. If not, you have to configure from the highest-version server, or you will get errors.

Part 2 prepares the replication distributor:

The first step in this process is to set up the remote distributor. As I mentioned in the first blog, you do not want your distribution database on one of the AG replicas. You need to set this up on a server that is not part of the AG.

Start by logging on to the distributor server – in this demo, SQL2014demo.

Stay tuned for the remainder of the series.

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Syncing Logins Between AG Replicas

Published 2016-10-13 by Kevin Feasel

Daniel Hutmacher has a new series on Availability Group synchronization, starting with logins:

You’ll need a linked server from your secondary replica to the primary replica. This linked server should feature the absolute bare-minimum of permissions, preferably with only the “be made using the login’s current security context” selected.

The account running the procedure (or the mapped login in the linked server) will need SELECT access to the following DMVs on the remote (primary) server:

  • master.sys.server_principals

  • master.sys.sql_logins

  • master.sys.server_role_members

  • master.sys.server_permissions

The code itself is a download from Daniel’s website; go check it out.

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Errors With Linked Server AG Connections Over ODBC

Published 2016-09-30 by Kevin Feasel

Andrea Allred has set up a linked server connection (using ODBC) to talk to an Availability Group, but certain columns fail:

Some of the problems that we have noticed are querying tables that have big datatypes like time(3-7), timestamp, and a few others.  Casting or converting the datatypes doesn’t help. If we pull the table into a view without the big datatype columns, we are able to query the view from another server, but never the base table. It has been a bit frustrating, but we are still hopeful that we can find a solution or that Microsoft with fix ODBC connections. If there is a better way to do this, please reach out to me.  We have things we need to solve and could use some help.

This is a nice walkthrough of how she set it up, but that sounds like a rather frustrating error.

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Asynchronous Mirroring And Asynchronous AGs

Published 2016-09-21 by Kevin Feasel

Tara Kizer contrasts asynchronous database mirroring with asynchronous Availability Groups:

ASYNCHRONOUS DATABASE MIRRORING

For asynchronous Database Mirroring, all we need is two servers: the principal at the primary site and the secondary at the DR site. Setup async mirroring between the two, and you’re done. If the secondary server goes down, production still continues. The transaction log isn’t clearing when the log backups occur because the principal still needs to send those log records to the secondary. As long as you have enough disk space where the transaction log resides to support this until the secondary server comes back online, production still continues. Of course if you run out of disk space, users will start receiving errors. But that can take some time and often times is enough time to get the secondary back online.

Availability Groups are more difficult to implement than database mirroring, but the flipside is that they’re much more powerful.

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