What is a SQL Server failover clustering

 

SQL Server failover clusters are made of group of servers that run cluster enabled applications in a special way to minimize downtime. A failover is a process that happens if one node crashes, or becomes unavailable and the other one takes over and restarts the application automatically without human intervention

What does SQL Server failover clustering provide?

A SQL Server failover cluster is also known as a High-availability cluster, as it provides redundancy for critical systems. The main concept behind failover clustering it to eliminate a single point of failure by including multiple network connections and shared data storage connected via SAN (Storage area network) or NAS (Network attached storage)

Each node in a cluster environment is monitored all the time via a private network connection called the heartbeat. A system must be able to overcome the situation called “split-brain” which occurs if all heartbeat links go down simultaneously. Then, all other nodes can conclude that one node is down and will try to restart the application on themselves. A failover cluster uses a quorum-based approach to monitor overall cluster health and maximize node-level fault tolerance

Example

A cluster named CLUSTER-01 contains two servers – nodes, named CLUSTER-01-SRV-01, CLUSTER-01-SRV-02. There is one SQL Server instance called SQL-INST-01. Also, there is a shared storage connected to the all three servers

When the server CLUSTER-01-SRV-01 crashes, the failover cluster service in CLUSTER-01 is aware of the situation through the heartbeat and automatically starts the SQL Server instance SQL-INST-01 on the CLUSTER-01-SRV-02 server

 

In a SQL Server failover cluster, data needs to be on a shared storage. The cluster can move the SQL Server instance if one node is having a problem because all the data is shared. This solution can guarantee higher up-time and redundancy. Because there is only one storage space, regular SQL Server maintenance requirements are still needed. Also, if the shared storage isn’t redundant, after a storage failure, the SQL Server database will be unavailable. For the busy SQL Server environments, where the downtime is measured in seconds, the “falling over” time needs to be considered because the change between nodes isn’t instant

SQL Server failover cluster configurations

There are four main node configurations available in SQL Server failover clustering: Active/Active (Multi-Instance Failover Cluster), Active/Passive, N+1, and N+M

An active/active failover cluster or multi-instance failover cluster, shares resources between virtual servers. Each node can host two or more virtual servers at the same time. Traffic can be passed onto the second active node or can be load balanced across the remaining nodes if there is more than one node left active

Active/passive failover clusters have standby nodes that are activated only when the primary node is down. The primary node owns all the resources. In case of a failure, the standby node takes over all the resources and recovers the database from the database files and transaction logs

An N+1 failover cluster is based on active/passive nodes where two or more nodes share the same failover node. In the situation where all N nodes fail, the standby node must be capable to take over all load

An N+M failover cluster has two or more active nodes and two or more standby nodes. It is cheaper for implementation than the N+1 configuration, because the load can be distributed to more than one standby node

When comparing MS-SQL (Microsoft SQL Server) and MySQL for clustering, the choice depends on several factors, including features, costs, and specific use cases. Here’s a breakdown to help you decide which is better and more cost-effective:

1. Feature Set for Clustering

• MS-SQL Server:

o Always On Availability Groups: MS-SQL offers this feature for high availability and disaster recovery. It provides read-scale capabilities and supports failover clusters.

o Failover Cluster Instances (FCI): FCI provides high availability by using shared storage, where the database can failover to a different node in the cluster.

o Replication and Mirroring: MS-SQL also offers database mirroring, transactional replication, and log shipping for clustering and redundancy.

o Enterprise Features: Some advanced features, like advanced clustering and distributed transactions, are only available in the Enterprise edition, which is more expensive.

• MySQL:

o MySQL InnoDB Cluster: This is MySQL’s solution for high availability and clustering. It’s integrated and supports automatic failover.

o MySQL Group Replication: Another solution that provides redundancy, fault tolerance, and scalability. You can use it for active-active clusters.

o Replication: MySQL also supports master-slave and master-master replication for clustering, but may not offer the same high-level management and failover features as MS-SQL without extra tools.

o Cost-effective Options: Open-source tools like Galera Cluster (with MySQL) provide clustering and high availability with a lower cost.

2. Cost Considerations

• MS-SQL Server:

o Licensing Costs: MS-SQL’s licensing model is significantly more expensive. The Enterprise edition, which is required for advanced clustering features, is costly and charges either per core or based on Server + CAL (Client Access License) models.

o Free Version: There is a free Express edition of MS-SQL, but it has very limited capabilities, especially for clustering or high-availability setups.

o Additional Costs: If using Windows Server with MS-SQL, there are additional costs for the OS, making the total cost higher.

• MySQL:

o Free and Open Source: MySQL’s Community Edition is free and open-source, making it significantly cheaper upfront than MS-SQL. You can implement clustering and replication using free tools.

o Paid Versions: MySQL Enterprise Edition offers enhanced support and advanced features, but it’s still less expensive than MS-SQL Enterprise.

o Additional Tools: While MySQL is cost-effective, to get advanced clustering functionality, you might need additional open-source tools like Percona XtraDB Cluster or Galera Cluster, which still keeps the cost low.

3. Performance and Scalability

• MS-SQL: Offers strong performance, built-in integration with Windows, and enterprise-level features for very large databases. However, it requires more resources and cost to scale effectively, especially for very large datasets or complex clustering.

• MySQL: MySQL performs well in smaller to medium-scale environments. MySQL clustering can scale horizontally, but in very large enterprises, MS-SQL might be a better choice due to its robustness for handling high workloads.

4. Ease of Use and Management

• MS-SQL: Comes with SQL Server Management Studio (SSMS), which is user-friendly and provides powerful tools for monitoring, backups, and cluster management. However, advanced features require more technical expertise and come at a higher cost.

• MySQL: MySQL's tools like MySQL Workbench are easier to use and sufficient for most applications. It might require more configuration for clustering, but the community and open-source tools provide extensive support.

5. Platform Compatibility

• MS-SQL: Traditionally tied to Windows Server, though more recent versions are available for Linux. However, many MS-SQL installations still run on Windows, which increases the licensing cost if Windows Server is required.

• MySQL: It is platform-independent and works well on Linux, which is often preferred for clustering setups due to the lower cost and flexibility.

Conclusion:

• Cost-effective and open-source solution: MySQL is the more cost-effective choice, especially when using free or open-source clustering solutions like Galera Cluster or MySQL InnoDB Cluster. It offers flexibility and scales well in many environments, particularly small-to-medium-sized businesses.

• Enterprise-grade features with advanced clustering: MS-SQL is better for large enterprises that require enterprise-level features, complex clustering configurations, or integration with other Microsoft technologies. However, it comes at a significantly higher cost.

If cost is your primary concern and you don't need high-end enterprise features, MySQL is the better option for clustering. If you need advanced features and are willing to invest in a comprehensive, fully-integrated system, MS-SQL may be worth considering.