Developed by Microsoft in the 1980s, SQL Server is a relational database management system designed to help store, retrieve, and manage data. SQL Server’s strong data processing capabilities, robust security, and high scalability make it an excellent option for enterprise environments that need to process high volumes of advanced analytics, transactions, and more.
Data availability is vital for businesses of all sizes, so organizations strive for high availability (HA). In this article, we’ll go over what SQL Server HA is, how it works, why it’s important, what the difference between SQL AlwaysOn and HA is, and what SQL availability groups (AGs) are.
- What is high availability in SQL Server?
- How does high availability work?
- Importance of high availability
- SQL AlwaysOn vs. high availability
- SQL availability groups
What is high availability in SQL Server?
HA refers to SQL Server’s ability to operate continuously, ensuring that databases remain accessible when faced with failures, maintenance, or unexpected disruptions. SQL Server HA is achieved when the system has enough redundancy and failover mechanisms to minimize data loss, maximize SQL Server uptime, and meet a previously agreed-upon operational performance level.
While the mean time between failures (the expected time between two SQL Server failures), the mean downtime (the average amount of time the system is down), and the recovery time objective (estimated time of repair from unplanned outages) all provide organizations with insights into system availability, SQL Server availability adds another layer of insight.
SQL Server HA is measured in percentages. More specifically, it represents the percentage of uptime over a given period and can be calculated using this formula: (minutes in the time period − minutes of downtime) × 100 / minutes in the time period.
The period can be an hour, day, month, or year depending on the organization’s requirements. However, SQL Server availability is often calculated by the month or year, particularly when it comes to service level agreements (SLAs), as these timeframes provide a better idea of overall system reliability.
The goal for most organizations is to reach five-nines availability, or 99.999% SQL Server availability, which is close to zero outages. This level of HA in SQL Server results in 5.26 minutes of downtime each year. However, 99.999% uptime is difficult to achieve, and many are willing to accept 99.99% availability (52.6 minutes of downtime per year) or 99.9% availability (roughly 8.76 hours of downtime each year).
How does SQL Server high availability work?
Having a system be available 100% of the time is impossible, even for the most resilient IT infrastructures and solutions. However, users can achieve HA SQL Server and reach three-nines (99.9%), four-nines (99.99%), or five-nines (99.999%) availability by:
- Eliminating single points of failure: Having a single point of failure¾one component that would take the whole system down if it were to fail¾is a recipe for disaster. For example, if a business only relies on one server to run its application and that server fails, the application would be unavailable until the server is back up and running. By adding redundant components as backups, organizations can ensure that if one component fails, another can take over seamlessly, allowing them to minimize downtime and maintain business continuity.
- Introducing failover mechanisms: By building reliable failover mechanisms for SQL Server and ensuring that when a system component fails, the backup system can take over without impacting performance or losing data, organizations can significantly reduce downtime and improve overall system resilience.
- Having real-time data replication: If an SQL Server fails and there aren’t any data protections in place, the data may be lost. So, it’s important to implement real-time data replication to ensure there is a copy of the most recent data available on another server. With real-time data replication, the backup server will have an up-to-date copy of the data, allowing for seamless failover, reducing data loss risk, and enabling applications to quickly resume operations without major disruptions.
- Using database mirroring and log shipping: Database mirroring involves separately maintaining two copies of one database that reside in different instances of SQL Server to increase availability and support failover, while log shipping transfers transaction logs to backup servers for additional redundancy and recovery options.
- Optimizing load balancing: A load balancer that can route workload requests to the system resource with the most available capacity helps prevent any single server from becoming overwhelmed and can lead to better performance and increased system stability. Load balancing distributes traffic across SQL Server instances to reduce bottlenecks, improve response times, and eliminate single points of failure.
- Having a solution with automatic failure detection: SQL Server failures or faults can wreak havoc on operations, so detecting and resolving issues quickly is essential. That’s where a tool with automatic failure detection can help. Some solutions can automatically take action to resolve issues and failures on their own quickly.
- Proactively monitoring and maintaining systems: Finally, it’s a good idea to proactively monitor SQL Server and its performance, schedule maintenance, and regularly test failover processes. This keeps SQL Server performance in good shape, allowing for higher availability and more consistent performance.
Importance of high availability in SQL Server
Having SQL Server up, running, and operating as it should be is vital, particularly in situations where system downtime could have a major impact on people’s health, safety, and economic well-being.
For example, autonomous vehicles rely on real-time data processing and decision-making. As a result, a brief SQL Server failure could lead to significant navigation errors or put passengers in danger. HA ensures that the databases behind the vehicle’s telemetry, mapping, and sensor data remain operational at all times, which can reduce the risk of accidents caused by system failures.
Similarly, hospitals need to have access to electronic health records 24/7. Otherwise, doctors couldn’t retrieve patient information, including medication history, allergies, and treatment plans. The slightest downtime of SQL Server could impact patient care and put someone’s life at risk.
In high-pressure industries and situations like these, companies must strive for 99.999% uptime to maintain critical operations. This can involve engaging in proactive monitoring, formulating and executing disaster recovery strategies, using failover clustering, and implementing real-time data replication.
Beyond autonomous vehicles and healthcare, financial institutions, emergency response systems, and large-scale e-commerce platforms depend on HA SQL Servers to function. A short system failure in online banking could disrupt transactions for thousands of customers, while downtime in emergency dispatch systems could prevent responders from providing life-saving assistance.
A few minutes of SQL Server downtime can have a major impact. By prioritizing HA in SQL Server and taking action to increase SQL Server HA options, organizations can better protect sensitive data, increase operational efficiency, and ensure their systems remain reliable when faced with unexpected failures. This can lead to fewer disruptions, stronger compliance with industry and government regulations, improved business continuity, closer adherence to SLAs, and increased trust with customers and other users.
SQL AlwaysOn vs. high availability
It’s important to note that SQL AlwaysOn and SQL Server HA aren’t the same. SQL Server HA options are broader and can include strategies such as log shipping, database mirroring, failover clustering, and optimized load balancing.
AlwaysOn refers to one of SQL Server’s specific HA and disaster recovery features and frameworks. More specifically, SQL AlwaysOn encompasses AlwaysOn failover cluster instances, which use Windows Server Failover Clustering to provide HA at the instance level, and Always On AGs. AGs replicate data across servers with automatic failover to increase availability at the database level.
SQL availability groups
SQL Server AGs are key elements of SQL AlwaysOn. They allow organizations to replicate databases across multiple servers, ensuring continuous data, decreased downtime, protection against failures, and improved system resilience.
Each SQL AG consists of a few parts:
- The primary replica: The primary replica, or the main operational database, is responsible for handling read and write operations and sending any data changes to secondary replicas.
- Secondary replicas: Secondary replicas are standby copies of the primary replica that can easily take over if the primary server goes down. They can be used for read-only queries, failover, and backup operations. Some companies have one secondary replica, while others have multiple copies of their main operational database.
SQL Server has two types of AGs: AlwaysOn AGs and Read-Scale AGs. While AlwaysOn AGs focus on providing read-scale balancing, disaster recovery, and HA, Read-Scale AGs provide replicas for read-only workloads. Since there are no cluster managers in Read-Scale AGs, failover is not automatic.
SQL AGs work on the database level, provide real-time data synchronization, and support automatic failover. As a result, they are ideal for businesses that require HA, excellent disaster recovery, and strong data integrity.
To ensure these systems operate efficiently and remain optimized, many organizations turn to third-party solutions such as SolarWinds® SQL Sentryâ or Database Performance Analyzer to help monitor and analyze database performance, gain deep insights, and keep their SQL Servers running smoothly.
