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In a world where data is increasingly valuable, ensuring its security and the continuity of its access is of utmost importance. SQL Server Distributed Availability Groups (DAG) is a feature that caters to these needs, providing solutions for disaster recovery and alternative site availability for readable replicas. Let’s dive into understanding what they are and how to use them.
What are SQL Server Distributed Availability Groups?
An advanced feature of SQL Server, DAG offers a simple yet effective way to manage and ensure data availability. A Distributed Availability Group is an element of AlwaysOn Availability Groups that spans separate SQL Server instances. This provides a layer of abstraction over server instances, allowing a secondary server to take over when a primary server is unavailable, therefore ensuring the availability of your database during a disaster.
Creating a Distributed Availability Group
The process of creating a Distributed Availability Group involves the establishment of two different AlwaysOn Availability Groups and a sort of ‘linkage’ group that binds them in a distributed model. Let’s dive into the actual SQL code, this is a very basic model for educational purposes:
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-- Create the Availability Groups CREATE AVAILABILITY GROUP [AG1] WITH (DB_FAILOVER = ON, DTC_SUPPORT = PER_DB) FOR REPLICA ON 'Instance1' WITH (ENDPOINT_URL = 'TCP://Instance1:5022', AVAILABILITY_MODE = SYNCHRONOUS_COMMIT, FAILOVER_MODE = MANUAL, SEEDING_MODE = AUTOMATIC) -- Repeat the above steps for AG2 on 'Instance2' |
This sets the stage by creating two availability groups, AG1 and AG2, on separate instances. Here, we are using synchronous commit mode, which means the changes must be written to the log on both replicas before the transaction is considered committed.
Linking the Availability Groups
After successfully setting up the Availability Groups on their respective instances, the next step involves linking them. This is done by creating a Distributed Availability Group that spans the previously created Availability Groups, which is done with the following command:
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-- Create the Distributed Availability Group CREATE AVAILABILITY GROUP [DAG1] WITH (DISTRIBUTED) FOR REPLICA ON 'Instance1' WITH (ENDPOINT_URL = 'TCP://Instance1:5022', AVAILABILITY_MODE = SYNCHRONOUS_COMMIT, FAILOVER_MODE = MANUAL, SEEDING_MODE = AUTOMATIC) -- Repeat the above steps for AG2 on 'Instance2' |
Conclusion
Once created, SQL Server Distributed Availability Groups provide a robust solution for disaster recovery, ensuring your data’s safety and continuation of access. Remember, setting up Distributed Availability Groups may take some time and requires careful planning. However, the confidence and data recovery assurance it provides are invaluable to businesses of any size.
Mastering the use of distributed availability groups is a stepping stone towards becoming a proficient SQL Server administrator. Don’t be afraid to explore more about their other features and the myriad of possibilities they open up!
References:
– Microsoft SQL Server Documentation