SQL Server Always On: Can Your Secondary Be a Future-Proof, Writable Wonder? (2019 to 2022 Guide)

Curious about leveraging a SQL Server 2022 secondary with a SQL Server 2019 primary in an Always On Availability Group? You're not alone! Many DBAs are exploring the possibilities of this configuration. This article dives deep into the compatibility, limitations, and best practices, offering actionable insights to guide your decision. Learn if your secondary can be writable and how transactional replication plays a role.

SQL Server 2019 Primary, 2022 Secondary: Is It Supported?

The short answer: yes, but with major caveats. Microsoft allows a newer SQL Server version (like 2022) as a secondary replica for an older primary (like 2019). This is primarily intended for migration purposes, acting as a stepping stone to upgrade.

Here's what you need to know:

• One-Way Ticket: Failover to the SQL Server 2022 secondary is possible. However, failback to the SQL Server 2019 primary is not supported once the failover occurs. The database on the secondary will be upgraded, rendering it incompatible with the older primary.
• Migration Strategy: This setup facilitates migrating to a newer SQL Server version with minimal downtime. However, it's crucial to meticulously plan the entire process including application testing to ensure compatibility.

Can Your SQL Server 2022 Secondary Be a Writable Copy?

Unfortunately, Always On Availability Groups do not inherently offer a writable secondary replica. You cannot directly modify data or create indexes on the SQL Server 2022 secondary while it's in its secondary role. Any modifications are applied to the primary replica first and then propagated to the secondary.

Transactional Replication: Your Key to a Readable Secondary?

While Always On doesn't provide writability, transactional replication can be your solution. This feature is completely independent of Always On.

Here's how it works:

• Publisher & Subscriber: Configure your SQL Server 2019 primary database as a publisher and your SQL Server 2022 instance as a subscriber.
• Data Synchronization: Transactional replication ensures that changes made on the primary database are propagated to the secondary (subscriber) database.
• Near Real-Time Data: You can use the replicated data on the SQL Server 2022 instance for reporting, analytics, or other read-intensive operations.

Key benefits of using transactional replication with a SQL Server 2022 secondary:

• Offload read workloads from the primary server.
• Maintain a synchronized, readable copy on a newer SQL Server version.
• Facilitate reporting/analytics without impacting the primary database performance.

Best Practices and Considerations for SQL Server Always On with Mixed Versions

• Thorough Testing: Before implementing this configuration, rigorously test your applications and SQL queries against the SQL Server 2022 secondary to ensure compatibility.
• Planned Migration: Clearly define your migration strategy. This setup is best suited for migrations, not as a long-term standard configuration.
• Backup Strategy: Ensure that your backup and recovery strategy accounts for the failover and potential upgrade process.
• Monitor Replication: If using transactional replication, closely monitor its performance and ensure data consistency between the primary and secondary databases.

By understanding the limitations and implementing the right strategies, you can successfully leverage a SQL Server 2022 secondary with a SQL Server 2019 primary for migration and read-scale purposes. Remember to carefully plan your deployment and rigorously test your applications.