8 Types of Failover

Last Updated on April 28, 2024 by Arnav Sharma

In Information Technology and network management, failover mechanisms stand as the guardians of continuous operational flow, playing a pivotal role in maintaining service availability even in the face of system failures. This comprehensive exploration delves into the various failover configurations and strategies, emphasizing their significance in disaster recovery and high availability planning.

Failover Definition: The Basis of Reliability

Failover mechanisms are designed to offer a seamless transition to a redundant or standby system, component, or network upon the detection of a failure or abnormal termination in the primary system. This automatic switch, aiming to ensure minimal service interruption, is fundamental in maintaining high availability and minimizing downtime.

An Overview of Failover Types: Building a Resilient Infrastructure

Active-Passive Failover (Cold Failover):

• Characteristics: In this model, a secondary system remains in standby, activated only when the primary system encounters a failure.
• Benefits: It’s a cost-effective solution for businesses, as the standby system doesn’t require the same level of resources if only used occasionally, serving efficiently when used as a failover option.
• Considerations: Failover times may be extended since the secondary system needs to initialize and start up services upon activation, underscoring the importance of selecting the appropriate failover mode and at least one reliable failover solution.

Active-Active Failover (Hot Failover):

• Characteristics: Both systems operate concurrently, sharing the workload. If one system fails, the other instantly assumes full responsibility, ensuring uninterrupted service.
• Benefits: Offers a seamless failover experience with minimal disruption, as both systems are always in synchronization.
• Considerations: This setup demands two fully operational systems at all times, which can increase costs. To enhance system resilience, an application failover mechanism is included, ensuring a seamless transition between systems.

Active-Active-Passive Failover:

• Characteristics: This approach merges the active-active and active-passive strategies, incorporating a third, standby system that intervenes if one of the active systems fails.
• Benefits: Introduces an additional layer of redundancy, enhancing system resilience through the use of application failover mechanisms.
• Considerations: While offering increased reliability, this method also adds complexity and potentially higher costs due to the maintenance of an extra standby system.

Manual Failover:

• Characteristics: Requires human intervention to switch from the failed system to the standby system, highlighting the need to learn the definition of failover to improve response times.
• Benefits: Offers controlled transition, suitable for non-critical systems or where automated failover mechanisms are impractical.
• Considerations: Less desirable for critical systems due to the increased risk of downtime.

DNS Failover:

• Characteristics: Utilizes changes in the Domain Name System (DNS) to reroute traffic to a redundant system upon failure.
• Benefits: Provides a DNS-based failover solution that can be implemented with minimal infrastructure changes.
• Considerations: Failover response times may be impacted by DNS caching and propagation delays.

Database Failover:

• Characteristics: Designed for database servers, this type involves replication (synchronous or asynchronous) between primary and secondary database servers, where the cluster must ensure the secondary replica is failover ready.
• Benefits: Ensures data operation continuity with minimal data loss during database server failures.
• Considerations: Requires careful configuration of replication mechanisms to ensure data consistency and availability.

Cloud Failover:

• Characteristics: Leverages cloud computing resources to establish a failover system, using cloud-based services as a backup, which is especially effective in a HA cluster environment.
• Benefits: Exploits cloud flexibility and scalability, potentially reducing the need for dedicated failover hardware and lowering costs.
• Considerations: Depends on cloud service availability and may involve data transfer latency issues.

Geographic Failover:

• Characteristics: Implements failover between systems located in different geographic areas to mitigate regional disruptions.
• Benefits: Enhances disaster recovery by providing high resilience against localized disasters.
• Considerations: Can be complex and costly due to the need for maintaining multiple geographically dispersed systems.

Designing a Failover Strategy: 

Developing an effective failover strategy requires a nuanced understanding of the multiple types of failover mechanisms and their application within specific operational contexts. By carefully selecting and configuring failover solutions—ranging from server failover clusters to cloud and database failover systems—organizations can fortify their disaster recovery plans and ensure high system availability.

Essential Considerations:

• Failover and Failback: A comprehensive failover plan not only addresses the switch to a standby system but also the subsequent return to the primary system once stability is restored.
• Failover Testing: Regular testing of failover mechanisms is crucial to validate the effectiveness of the failover strategy and to ensure that systems can handle actual failover scenarios.
• Disaster Recovery and High Availability: Failover is a key component of broader disaster recovery and high availability strategies, requiring integration with data backup, system redundancy, and business continuity planning.

FAQ: 

Q: What is failover and how is it implemented in server environments?

A: Failover is the ability of a system to automatically transfer control to a redundant or standby computer server, application, or network upon the failure or termination of the previously active server, application, or network. In server environments, failover can be implemented through various methods such as automatic failover, where the failover process occurs without human intervention, and forced failover, initiated by a database administrator. Failover ensures high availability, reduces downtime, and minimizes data loss by switching to a secondary server or database on the failover target that is synchronized and failover ready. Failover configurations can range from active-passive clusters, where the secondary server merely rests until needed, to active-active clusters that allow both servers to run simultaneously, handling different sets of workloads to achieve failover in a different way. Failover cluster solutions, especially Windows Server Failover Clustering (WSFC), are commonly used to provide failover capability in servers, including SQL Server failover clusters, which ensure that a secondary replica of the SQL Server database is always on standby in case the primary fails.

Q: How do automatic and forced failovers differ, and what are the scenarios for each?

A: Automatic failover and forced failover are two different types of failover mechanisms used to maintain high availability and continuity of service, critical in environments where a HA cluster is deployed. Automatic failover is supported by systems configured with failover mode set to automatic, where the failover occurs without human intervention, typically when a system detects a failure in the primary server. This type of failover is essential for critical applications where even minimal downtime cannot be afforded, as it ensures an immediate switch to a standby computer server or secondary database on the failover target. On the other hand, forced failover is initiated by a database administrator, often in scenarios where manual intervention is necessary, such as during failover and recovery testing or when the secondary server is not configured for manual failover mode. Forced failover allows more control over the failover process but requires that the failover target is synchronized and ready to take over the responsibilities of the primary server.

Q: What are the key components and practices in designing a failover strategy for SQL Server?

A: Designing a failover strategy for SQL Server involves several key components and practices to ensure high availability and disaster recovery. Firstly, implementing SQL Server Failover Cluster Instances (FCI) with Windows Server Failover Clustering (WSFC) offers a solid failover solution by allowing another node in the cluster to take over in case of a server failure. It’s important to have a primary failover server and a secondary replica that is failover ready to minimize downtime and data loss. The failover process should be tested regularly through failover and recovery testing to ensure the entire failover set, including application server failover and database failover, operates as expected. Utilizing SQL Server Always On Availability Groups can also offer a more granular level of failover capability, providing an active-standby cluster configuration where a secondary database can automatically become the primary failover server in case of a failure. Additionally, configuring SQL Server with failover mode set to automatic ensures failover automation and high availability without manual intervention. Best practices also include using failover cluster manager tools to manage and monitor the health of the failover cluster and implementing application server load balancing and active-active high availability cluster configurations to distribute the load and reduce the risk of a single point of failure.

Q: How does cloud computing enhance failover strategies?

A: Cloud computing significantly enhances failover strategies by offering flexible, scalable, and cost-effective solutions for high availability and disaster recovery. Cloud providers offer a cloud failover strategy that involves deploying applications and data across multiple geographically dispersed data centers, ensuring that an application can continue to operate from another data center in the event of a disaster or failure. This approach not only minimizes downtime but also reduces the risk of data loss. Cloud environments support various forms of failover, such as application server failover and database failover, with configurations ranging from active-passive to active-active clusters. The cloud’s ability to dynamically allocate resources allows for more efficient failover implementations, where resources can be scaled up or down based on demand, and failover can be achieved more seamlessly. Furthermore, cloud services often come with built-in failover capabilities, where failover automation is integrated into the service, thereby simplifying the failover process and reducing the need for extensive failover and recovery testing. Offering a cloud failover strategy also means that organizations can leverage the cloud provider’s expertise and infrastructure to achieve high availability and disaster recovery without the need for significant capital investment in their own data center resources.