High availability (HA) is essential for any modern SQL Server deployment. From traditional options like Log Shipping to advanced Always On Availability Groups, SQL Server provides a wide array of features to ensure business continuity and minimize downtime.
Log Shipping
Overview:
Log Shipping involves backing up transaction logs from a primary server and restoring them on a secondary server at regular intervals. It’s a time-tested and simple disaster recovery solution.
Key Features:
- Scheduled log backup, copy, and restore
- Supports multiple secondary servers
- Easy to set up and maintain
Limitations:
- No automatic failover
- Potential data loss depending on backup frequency
- Manual monitoring and maintenance required
Best For:
Disaster recovery with low cost and tolerance for manual intervention.
Database Mirroring
Overview:
Database Mirroring provides database-level redundancy by maintaining a hot standby mirror of the principal database. It supports synchronous or asynchronous operation.
Key Features:
- Automatic failover (with witness)
- Synchronous replication
- Fast failover time
Limitations:
- One database per session
- Deprecated as of SQL Server 2016
- No read access on the mirror
Best For:
Legacy systems requiring simple automatic failover.
Always On Failover Cluster Instances (FCIs)
Overview:
FCIs provide high availability at the instance level by using Windows Server Failover Clustering (WSFC) with shared storage (e.g., SAN).
Key Features:
- Protects entire SQL Server instance
- Automatic failover with WSFC
- No changes required at database level
Limitations:
- Requires shared storage infrastructure
- No readable secondaries
- Windows-only
Best For:
On-premises environments with access to SAN storage and need for full-instance protection.
Since there is no data synchronization between nodes (all nodes access the same shared storage), there is no latency due to data replication or synchronization processes, making it ideal for performance-sensitive workloads.
Always On Availability Groups (AGs)
Overview:
AGs are SQL Server’s flagship HA/DR feature that allows multiple databases to be grouped and replicated across multiple servers for both high availability and disaster recovery.
Key Features:
- Supports up to 9 secondary replicas
- Readable secondaries and backup offloading
- Synchronous and asynchronous replication
- No shared storage required
Limitations:
- Requires Windows Server Failover Clustering (WSFC)
- More complex to configure and monitor compared to simpler HA options
Best For:
Mission-critical workloads needing high performance, low RTO/RPO, and read scale-out.
Basic Availability Groups
Overview:
Basic AGs offer a limited version of Always On Availability Groups, designed for SQL Server Standard Edition with reduced features.
Key Features:
- Automatic failover
- Included in Standard Edition
Limitations:
- Supports only one database per group
- No read access on secondary
- No backup offloading
Best For:
Small-scale production systems on Standard Edition that need basic HA.
Distributed Availability Groups
Overview:
Distributed AGs connect two separate AGs across different clusters or regions. Useful for geo-distributed deployments or migration scenarios.
Key Features:
- Cross-data center or region replication
- Supports complex DR topologies
- Each AG retains local control
Limitations:
- Complex setup and management
- Enterprise Edition only
- Higher latency depending on distance
Best For:
Multi-region HA and cloud migration use cases requiring high resiliency.
Cluster-less Availability Groups
Overview:
Introduced in SQL Server 2022, this feature removes the requirement for Windows Server Failover Clustering, enabling more lightweight and container-friendly HA setups.
Key Features:
- No WSFC dependency
- Supports read-scale and backups
Limitations:
- No automatic failover (manual failover only)
- Enterprise Edition only
Best For:
Modern DevOps workflows, lightweight environments, and containerized deployments.
Feature Comparison Table
| Feature | Auto Failover | Readable Secondary | Shared Storage | Max DBs | Edition |
|---|---|---|---|---|---|
| Log Shipping | No | Yes (delayed) | No | All | All |
| Database Mirroring | Yes (with witness) | No | No | 1 | All |
| Failover Cluster Instance | Yes | No | Yes | All | All |
| Availability Groups | Yes | Yes | No | Many | Enterprise |
| Basic Availability Groups | Yes | No | No | 1 | Standard |
| Distributed Availability Groups | No | Yes | No | Many | Enterprise |
| Cluster-less AGs | No | Yes | No | Many | Enterprise |