What is Always On SQL Server?

Raj Verma • Last Modified: August 21st, 2023

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SQL Server is a leading Relational Database Management System (RDBMS) trusted by Data Professionals all over the world. Business enterprises primarily use SQL Server as a Database Server for storing and retrieving data. However, enterprises are constantly exploring the number of ways in which they can secure their data in SQL Server and avail high availability. This is where the Always On SQL Server comes in.

Always On SQL Server refers to the High Availability and Disaster Recovery solution introduced in the 2012 version. Always On availability groups increase the availability of a set of user Databases for a business enterprise. Proving to be an alternative to Database Mirroring, Always On SQL Server basically extends a failover environment for Availability Databases that failover together. Before getting started with Always On SQL Server, let’s discuss this robust RDBMS in brief.

Table of Contents

What is SQL Server?

Always On SQL Server: SQL Server | Hevo Data
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SQL Server is a popular Relational Database Management System (RDBMS) offered by Microsoft. Launched in 1989, it is written in programming languages like C and C++. The architecture of SQL Server is based on the relational model created by E.F.Codd, and it is capable of storing petabytes of data in structured formats of rows and columns.

Microsoft SQL Server has established itself as an easy-to-use, reliable, and efficient Database System. It works on ANSI SQL, the standard SQL (Structured Query Language). The SQL Database is vertically scalable and its comprehensive querying layer aids in Business Analysis and readily integrates with Reporting & Visualization tools like Power BI.

Key Features of SQL Server

Take a look at the prominent features responsible for the immense popularity of SQL Server.

  • The Database Engine: The Database Engine supports fast exchange processing and streamlines data storage and data security.
  • The Server Agent: It basically acts as a task scheduler and can be triggered by any occasion or according to request.
  • The SQL Server Browser: The Server Browser accepts approaching queries and connects them with the ideal SQL Server instance. 
  • The SQL Server Full-Text Search: As the name suggests, users can run a full-text search against Character information in SQL Tables.
  • Cloud Database Support: SQL Server can be easily integrated with Azure SQL or Microsoft Cloud with built-in security. Cloud Database support makes SQL Server highly available and allows for fast failover with minimum uptime.
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What is an Always On SQL Server Availability Group?

An Availability Group provides a replicated environment for a specific set of user Databases called Availability Databases. This replicated environment comprises a set of read-write Primary Databases and corresponding sets of Secondary Databases. An Availability Group can be configured for either High Availability (HA) or read-scale. An HA Availability Group comprises Databases that failover at the same time.

The Always On Availability Group is a High Availability and Disaster Recovery feature. It doesn’t require shared disk storage for hosting the SQL Server. However, the Always On Availability Group is supported only by the Enterprise edition. The diagram below depicts an implementation of Always On SQL Server Availability Group for High Availability and Disaster Recovery.

Always On SQL Server: AlwaysOn Availability Groups | Hevo Data
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Asymmetric storage refers to a cluster disc shared only by a subset of the nodes. Asymmetric disc capability enables a disc witness to be set up and accessible only to nodes in a single site, the primary site generally.

Let’s take a look at the different components of SQL Server Always On Availability Groups:

Availability Databases

For a Database to be eligible to be added to an Availability Group, it must be an online, read-write Database existing on the server instance that hosts the primary replica. After adding a Database into an Availability Group, it becomes a Primary Database while being accessible to clients. Corresponding Secondary Databases cease to exist until backups of the new Primary Databases are restored to the server hosting the secondary replica. The new Secondary Database remains to be in the RESTORING state until it is added to the Availability Group.

After the Secondary Database is joined to the Availability Group, it goes into the ONLINE state and initiates the process of Data Synchronization with the corresponding Primary Database. Data Synchronization refers to the process of reproducing the changes from a Primary Database to a Secondary Database. The Primary Database sends transaction log records to the Secondary Database during Data Synchronisation.

Availability Replicas

Each Availability Group has 2 or more failover partners known as availability replicas. The Availability Databases in the Availability Group are replicated in the Availability Replicas. The Availability Replicas of a specific Availability Group must be hosted by separate instances of SQL Server running on different WSFC cluster nodes. Always On must be enabled for each of these server instances.

Each server instance can host only one Availability Replica per Availability Group. However, a given server instance can be used for many Availability Groups. A SQL Server instance can be either a stand-alone or a Failover Cluster Instance (FCI).

Each Availability Replica is given either a primary role or a secondary role, which is inherited by that replica’s Availability Databases. Depending on the role, it is determined whether a replica hosts read-write Databases or read-only Databases. The primary role is assigned to the Primary Replica which hosts read-write Databases (also known as Primary Databases), whereas the secondary role is assigned to at least one other replica, known as a Secondary Replica. The Secondary Replica hosts read-only Databases (also known as Secondary Databases).

Availability Modes

The Availability Mode is a property of every Availability Replica. Based on the Availability Mode it is determined whether the Primary Replica waits to commit Database transactions until the transaction log records have been written to the disc by a specific Secondary Replica. Always On SQL Server Availability Groups support 2 Availability Modes: Asynchronous-commit Mode and Synchronous-commit Mode.

  • Asynchronous-commit Mode: Asynchronous-commit Replica uses the Asynchronous-commit Mode. The Primary Replica performs transactions without waiting for acknowledgment from Asynchronous-commit Secondary Replicas to harden their transaction logs in Asynchronous-commit Mode.

    The Asynchronous-commit Mode reduces transaction latency on Secondary Databases while allowing them to lag behind Primary Databases, potentially leading to data loss.
Always On SQL Server: Synchronous Data Commit | Hevo Data
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  • Synchronous-commit Mode: Synchronous-commit Replica uses the Synchronous-commit Mode. A Synchronous-commit Primary Replica waits for acknowledgment from Synchronous-commit Secondary Replicas to harden their transaction logs before committing transactions in Synchronous-commit Mode.

    Synchronous-commit Mode makes sure that the committed transactions are fully secure once a given Secondary Database is synchronized with the Primary Database. However, this protection comes at the cost of increased transaction latency.

Benefits of an Always On SQL Server

There are numerous benefits of deploying Always On Availability Groups in the SQL Server, let’s take a look at some of them.

  • Always On SQL Server Availability Groups support up to 9 Availability Replicas. Breaking this down, every Availability Group supports 1 Primary Replica and up to 8 Secondary Replicas.
  • Always On Availability Group supports different forms of Availability Group Failover, namely Automatic Failover, Planned Manual Failover, and Forced Manual Failover.
  • It supports Automatic Page Repair to prevent page corruption.
  • It provides secure, high-performing transport of data as it readily supports encryption and compression.
  • One can easily monitor the Always On SQL Server Availability Groups on the Always On Dashboard. It further monitors the Availability Replicas and Availability Databases and evaluates results for Always On policies.

Understanding Always On Failover Clustering Instances

Always On FCI requires shared storage, such as an iSCSI or Fibre Channel SAN, that can be accessed by all nodes in the cluster. However, you can also do this over Cloud or for virtual machines by using third-party Data Replication technologies. It allows multisite clustering across subnets, allowing SQL Server instances to fail over across Data Centres. However, this requires Data Replication across shared storage in every Data Centre.

Available on both SQL Server Standard and Enterprise Edition, Always On FCI imposes restrictions on SQL Server Standard Edition such as a 2-node limit. While installing SQL Server, you should select the “New SQL failover cluster installation” option. The diagram below depicts the implementation of a single-site two-node AlwaysOn FCI.

Always On SQL Server: AlwaysOn Failover Clustering Instances (FCI) | Hevo Data
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Characteristics of Always On FCI and Always On AG

Both the technologies serve different purposes. However, you can combine Always On FCI and Always On AG as per your requirement of High Availability and Disaster Recovery. Some of your tasks may require local High Availability within a Data Center via Always On FCI and cross Data Center Disaster Recovery via Always On AG. This means the solution would hold a combination of shared storage and non-shared storage.

The below table summarizes the characteristics of Always On SQL Server AG and FCI to help you understand both the solutions.

Always On FCIAlways On AG
Shared Storage solutionNon-Shared Storage solution
Supports Instance-level HASupports Database-level HA  
Provides Instance-level protection without data redundancy.Secondary AG Database(s) are redundant copies of Primary Databases.
Have ActivePassive nodes. No concept of a secondary database.DR replica can be Active Secondary for the backup, read-only workload.
The application connects via a virtual server name.The application connects via the AG listener name.
A redundant copy of the data is not stored, hence it does not protect against an I/O subsystem failure.Automatic Page Repair provides protection against an I/O subsystem failure.
Always On FCI vs Always On AG


Always On SQL Server is a High Availability and Disaster Recovery solution introduced in SQL Server 2012. It increases the availability by providing a failover environment for Availability Databases that failover together. This article introduced you to MS SQL Server and later took you through various aspects of Always On SQL Server.

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Share your experience of understanding Always On SQL Server in the comments section below.

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