Introduction
Teams that care about uptime, recovery, and growth often ask which SQL Server high availability option they should build around. The short answer is availability groups. They keep databases online during failures, and they also add capabilities that change how you run the platform. You can scale out reads, offload backups, fail over related databases together, and move across data centers and clouds with fewer surprises. Failover cluster instances and mirroring solved older problems well. Availability groups solve those and open the door to modern patterns.
What each option really provides
A failover cluster instance protects at the instance level and relies on shared storage. If a node fails, the instance starts on another node and uses the same data files. It is reliable, familiar, and simple to operate. It does not provide read scale because there is only one copy of the data.
Database mirroring works per database. It was the right answer for many years. It is now a legacy technology. Microsoft has been clear that new builds should use availability groups.
An availability group protects a set of user databases that fail over as a unit. Each replica holds its own copy of the data. Secondaries can be readable and can take backups. That design changes your day to day operations.
Why availability groups are the better default
Read scale that removes pressure from the primary
You can direct reports, analytics, extracts, and ad hoc queries to readable secondary replicas. Connection strings can include read intent, and the listener routes those sessions to the right place. The primary stays focused on OLTP and critical writes.
Backup offload that smooths peak hours
Backups can run on secondaries. You set a preference, and your jobs follow it. The result is less I/O and CPU on the primary during business hours.
More replicas and flexible commit modes
One primary and multiple secondaries give you design choices. Keep one or more replicas in synchronous commit for local high availability. Use asynchronous commit for remote disaster recovery. This mix matches how most enterprises span sites.
Database group failover for multi database apps
Many applications depend on several databases that must move together. Availability groups treat them as a unit with a single client endpoint. Mirroring fails over one database at a time. FCIs move the whole instance, including databases unrelated to the application.
Modern connection behavior
Listeners simplify client connectivity. SQL Server 2019 and later can also redirect read and write connections from a secondary to the primary when needed. Client drivers support the MultiSubnetFailover option to speed failovers across subnets.
Hybrid and cross platform mobility
Distributed availability groups span two independent availability groups, which is useful for cross region and cross cloud design and for low downtime migrations. Managed Instance link builds on this to replicate from SQL Server to Azure SQL Managed Instance and to support controlled failover and testing.
Operational consistency on SQL Server 2022 and later
Contained availability groups keep key environment objects like logins and Agent jobs consistent at the availability group scope. This reduces the housekeeping that used to favor FCIs.
Trade offs to plan for
More moving parts than an FCI
High availability groups use a cluster manager on Windows or Linux. On Windows this is Windows Server Failover Clustering. On Linux this is Pacemaker. You design quorum, configure a listener, and monitor multiple replicas. Read scale groups with cluster type set to NONE remove the cluster dependency but they do not provide automatic failover or high availability.
Commit latency in synchronous mode
Synchronous commit waits for a secondary to harden the log. Keep synchronous replicas close to the primary. Use asynchronous commit for distance where small data loss is acceptable during failover.
Extra storage and network
Every replica has its own copy of the data and ships logs continuously. That is more capacity than an FCI with shared storage. In return, you get read scale, backup offload, and better disaster recovery shapes.
Edition and feature limits
The full availability group feature set, including multiple readable secondaries and backups on secondaries, requires Enterprise Edition. Standard Edition supports Basic availability groups, which are intentionally limited to a single database and two replicas, with no readable secondary and no backups on the secondary.
When an FCI can still be the right tool
If your priority is instance level coverage that includes system databases, SQL Agent, and linked servers, and you already have reliable shared storage, an FCI remains straightforward and effective. Many enterprises use a hybrid pattern. An FCI provides local instance high availability. An availability group provides disaster recovery and read scale. This is a practical middle path.
For DBAs who prefer FCIs or mirroring
Trust in FCIs and mirroring comes from years of stable operations and muscle memory. That experience matters. It is also why learning availability groups is worth the investment. AGs do not replace your instincts. They extend them with tools that match modern workloads and architectures.
The first payoff is immediate. Readable secondaries and backup offload lower contention on the primary without new hardware. The second payoff is cleaner failovers for multi database applications. An AG moves the databases your app depends on as a unit. The third payoff is flexibility. You can keep a synchronous replica close for high availability and place an asynchronous replica in another region for disaster recovery. Contained availability groups in SQL Server 2022 reduce the login and job drift that used to favor FCIs.
The learning curve is real but manageable. Start with a read scale AG in a non critical environment. Point reporting and backups at a secondary. Add a synchronous secondary and practice planned failovers until they are routine. If you rely on FCIs for instance level protection, keep them and layer an AG on top for read scale and disaster recovery. This phased approach lets you deliver value while you build confidence.
In almost every enterprise scenario, AGs deliver better outcomes over time. They align with vendor guidance, support hybrid and cloud moves, and reduce pressure on the primary during business hours. The skills you build around routing, commit modes, and monitoring carry forward across on premises and cloud platforms. That is a good trade for a team that wants stability today and options tomorrow.
A note on mirroring
Mirroring taught the platform valuable lessons, but it is a legacy feature. Microsoft guidance is to avoid it for new development and to plan migration paths to availability groups. If you are running mirrored pairs today, begin planning that move.
Conclusion
If you are building for the next decade, choose availability groups as your default. You get high availability and disaster recovery, and you also gain read scale, backup offload, coordinated failover across related databases, and flexible topologies that reach across sites and clouds. Accept the extra design work, be intentional about synchronous and asynchronous commit, and use contained availability groups where you can. Keep FCIs in your toolbox for simple instance level protection with shared storage. Put mirroring into maintenance mode and plan its retirement. That strategy gives you a SQL Server platform that runs steadier today and adapts faster tomorrow.