Synchronous vs. Asynchronous: Demystifying Always On Availability Group Modes

Always On Availability Groups in SQL Server offer high availability and disaster recovery solutions. But under the hood, two distinct data synchronization methods dictate how changes flow between the primary and secondary replicas: synchronous and asynchronous. Choosing the right one hinges on your specific needs and performance priorities.

Synchronous Replicas: Dancing To the Same Beat

Imagine a synchronized dance troupe. Synchronous replicas mirror the primary replica’s actions step-by-step. Every transaction committed on the primary waits for confirmation from a quorum of synchronous replicas before finalizing. This ensures data consistency across all replicas. However, this wait introduces latency, potentially impacting user experience, especially on geographically dispersed replicas.


  • Guaranteed data consistency: No risk of data divergence between replicas.
  • Fast failover: Minimal data loss on failover due to close synchronization.
  • Read-scale availability: Secondary replicas can serve read-only workloads, reducing pressure on the primary.


  • Higher latency: Waiting for synchronous commits can impact performance.
  • Limited scalability: Large numbers of synchronous replicas can burden the primary.
  • Single point of failure: If the primary or quorum replica fails, transactions stall.

Asynchronous Replicas: Flying Solo with Lag

Asynchronous replicas, like solo dancers, apply changes independently. Transactions committed on the primary are queued and sent asynchronously to the secondary. This eliminates waiting, improving performance and scalability. However, a data lag can exist between replicas, creating potential inconsistencies.


  • Lower latency: Transactions commit faster without synchronous waits.
  • Higher scalability: Can handle more secondary replicas without impacting performance.
  • Resilience to failures: Asynchronous replicas continue receiving data even if the primary fails.


  • Potential data inconsistencies: Data lag can occur, leading to temporary data differences.
  • Slower failover: More data might need to be replayed on failover, increasing downtime.
  • Limited read-scale availability: Asynchronous replicas might not be fully synchronized for read workloads.

Choosing the Right Rhythm:

The choice between synchronous and asynchronous boils down to your tolerance for latency vs. data consistency:

  • Synchronous: Choose for mission-critical applications requiring constant data consistency and fast failover, even at the cost of performance.
  • Asynchronous: Choose for performance-sensitive applications where read-scale availability and scalability are crucial, despite a potential data lag.

A Final Note:

Always On Availability Groups offer flexibility with hybrid configurations. You can mix synchronous and asynchronous replicas within the same group based on specific needs. Remember to carefully evaluate your workload characteristics and desired recovery times before settling on a rhythm for your availability group.

Stay tuned for further articles delving deeper into specific aspects of Always On Availability Groups!