High Availability for SQL Server

Ensuring your SQL Server databases are continuously available is critical for business operations. Microsoft SQL Server offers a robust set of features designed to minimize downtime and protect against data loss.

Key High Availability Technologies

1. Always On Availability Groups

Always On Availability Groups is the premier high availability and disaster recovery solution for SQL Server. It provides an environment for high availability of a user-defined set of user databases, called availability databases, that fail over together as a unit.

Primary Replica: The read/write copy of the availability databases.
Secondary Replicas: Read-only copies of the availability databases. Can be synchronous (for high availability) or asynchronous (for disaster recovery).
Availability Databases: A collection of user databases that fail over together.
Availability Mode:
- Synchronous commit: Transactions are committed on both primary and secondary replicas before acknowledging the commit to the client. Guarantees no data loss but can impact performance.
- Asynchronous commit: Transactions are committed on the primary replica and then sent to the secondary. Offers better performance but carries a risk of data loss.
Failover Mode:
- Automatic: Failover can occur without human intervention if the primary replica becomes unavailable (requires synchronous mode and healthy secondary replicas).
- Manual: Failover requires administrator intervention.

2. Failover Cluster Instances (FCI)

Failover Cluster Instances provide instance-level protection. An FCI uses Windows Server Failover Clustering (WSFC) to provide automatic failover for a SQL Server instance. If a server node in the cluster fails, SQL Server automatically restarts on another node.

Requires shared storage (e.g., SAN, SMB file share).
Provides instance-level redundancy for all databases within the instance.
Simpler to manage for applications that connect to the SQL Server instance rather than specific databases.

3. Log Shipping

Log shipping is a solution that allows you to send transaction log backups from one or more primary databases on a SQL Server instance to one or more secondary SQL Server instances. This enables disaster recovery and can also be used for read-only reporting.

Requires manual configuration of backup, copy, and restore jobs.
Can have a delay between primary and secondary due to the restore process.
Cost-effective for disaster recovery scenarios.

4. Mirroring (Deprecated)

Database mirroring was a solution that provided redundancy at the database level. While still supported in older versions, it is largely superseded by Always On Availability Groups.

Database-level redundancy.
Supported principal/mirror server pairs.
Automatic or manual failover.

Choosing the Right Solution

The best choice depends on your specific requirements:

For maximum availability and disaster recovery with minimal downtime: Always On Availability Groups.
For instance-level protection and ease of application connection: Failover Cluster Instances.
For cost-effective disaster recovery with acceptable downtime: Log Shipping.

Configuration and Best Practices

Implementing high availability requires careful planning and configuration. Consider the following:

Example: Configuring Availability Groups

Setting up an Availability Group involves several steps:

Ensure WSFC is configured and healthy.
Enable the Availability Groups feature on each SQL Server instance.
Create a new Availability Group, specifying the primary replica and desired secondary replicas.
Add databases to the Availability Group.
Configure listener(s) for client connections.

Refer to the detailed configuration guides in the Always On Setup guide.

Important Considerations:

Network Latency: High latency can impact synchronous commit performance in Availability Groups.
Storage Performance: Ensure your storage can handle the workload for both primary and secondary replicas.
Monitoring: Implement robust monitoring for WSFC, SQL Server health, and Availability Group status.
Testing: Regularly test failover procedures to ensure they work as expected.

Tip:

Use multiple availability modes and failover modes to balance availability, performance, and disaster recovery needs across different databases.

For in-depth technical details, performance tuning, and advanced scenarios, please consult the full documentation on specific technologies.