Advanced SQL Server Topics
High Availability and Disaster Recovery
This section delves into strategies and technologies that ensure your SQL Server environment remains available and resilient to failures.
Understanding High Availability (HA)
High Availability aims to minimize downtime by keeping applications and services running even in the event of hardware or software failures. Key technologies include:
- Failover Clustering: Provides instance-level availability by clustering multiple servers. If one node fails, another takes over automatically.
- Log Shipping: A disaster recovery solution that sends transaction log backups from a primary server to one or more secondary servers.
- Database Mirroring: A simpler high-availability solution than clustering, offering automatic or manual failover for a single database.
Disaster Recovery (DR) Strategies
Disaster Recovery focuses on recovering your data and services after a catastrophic event, such as a natural disaster or a major data center outage. While HA is about minimizing downtime, DR is about business continuity.
- Always On Availability Groups: The modern, comprehensive HA/DR solution for SQL Server, providing database-level availability with advanced features like readable secondaries and automatic failover.
- Geo-Replication: Used to replicate data across geographically distant data centers, crucial for DR.
- Backup and Restore: The fundamental DR strategy, involving regular backups and a well-defined restore plan.
Key Concepts and Configurations
Always On Availability Groups
Always On Availability Groups (AGs) offer a robust solution for HA and DR. They provide a set of primary databases and one to four sets of secondary databases that are kept in sync. Key components include:
- Availability Replicas: Instances of SQL Server hosting availability databases.
- Availability Databases: User databases that are part of an availability group.
- Listener: A virtual network name and IP address that client applications connect to, directing traffic to the current primary replica.
Configuration Options:
| Feature | Description |
|---|---|
| Failover Mode | Automatic, Manual, or Forced |
| Availability Mode | Synchronous Commit (HA), Asynchronous Commit (DR) |
| Readable Secondaries | Configurable to allow read-only workloads on secondary replicas |
Failover Clustering
SQL Server Failover Cluster Instances (FCIs) provide instance-level protection. A clustered instance shares storage and runs on one of multiple nodes in a Windows Server Failover Cluster (WSFC). If a node fails, the instance can be brought online on another node.
Prerequisites:
- Windows Server Failover Clustering installed and configured.
- Shared storage accessible by all nodes.
- Network configuration for virtual network name and IP addresses.
Best Practices for HA/DR
- Regularly test failover procedures.
- Monitor performance and health of all replicas.
- Maintain consistent network configurations.
- Implement a robust backup and restore strategy.
- Document your HA/DR architecture and procedures.
For detailed configuration steps and advanced scenarios, please refer to the Always On Availability Groups Reference and the SQL Server Failover Clustering Tutorial.