What are Failover and Redundancy in SaaS?

Failover and redundancy are methods used to maintain the functionality of your systems, even during unexpected failures. Redundancy involves having backup parts or systems as substitutes, while failover is the switch, usually automatic, to those backups when the original system fails. Amazon RDS, for example, switches a Multi-AZ DB instance to its standby replica; failover times are typically 60–120 seconds.

On its own, failover solves smaller or localized issues, which are usually limited in scope, while disaster recovery (DR) is a solution for extensive issues such as natural disasters, cyber threats, or system crashes.

The presence of redundancy and failover configurations often relates to the level of disruption experienced during maintenance and upgrades, impacting service continuity. This is crucial in business, where even minor disruptions can be costly.

Redundancy is a technique used specifically to achieve high availability. Consider redundancy as one instrument in the toolbox. It involves having a backup of one or more pieces of equipment to ensure contingency in the event that the initial one fails.

High availability (HA), on the other hand, is the overall strategy of ensuring that your systems are reliably operational with a minimum of interruption. It includes a broader range of strategies, not just redundancy, but also:

• Load Balancing: Splitting loads among various servers to avoid a task overloading some of the servers.
• Regular Maintenance: Anticipating and rectifying issues before failures occur and compromise the workflow in a business organization.
• Robust Monitoring: Quickly identifying failures and executing the necessary failover or other recovery processes.

Furthermore, to ensure high availability in dynamic environments, systems need to incorporate scalability and elasticity to adapt to changing workloads.

Therefore, redundancy is not the sole factor of high availability but an important component.

Three common types of redundancy are:

• Active-Active: Both redundant systems simultaneously share the workload for maximum resource utilization.
• Active-Passive: One system is fully operational (active), and the other system is not (passive) and only comes into operation when the active system fails.
• N+1 Redundancy: You have the number of systems required for normal operations (N), plus one more system for backup purposes.

These models can apply at several layers: power (UPS units and generators), network links from different providers, storage such as RAID, and servers. A redundant network keeps a second path ready if one link fails.

Deciding whether you need failover and redundancy depends on several factors. Consider the following questions:

• Can your business handle any downtime?
• How essential are your services to your clients?
• What are the financial impacts of service disruptions?

If downtime translates into loss of sales or erosion of customer confidence, or breaks the uptime promised in your service level agreement, then redundancy and failover may prove to be cost-effective solutions.

Managed services can run failover for parts of your infrastructure: on Amazon RDS, failover times for a Multi-AZ DB instance are typically 60–120 seconds (AWS). Test it before you need it; RDS lets you force a failover by rebooting the instance.

Redundancy and disaster recovery (DR) both contribute to keeping your systems operational, but they tackle different scenarios:

Redundancy:

• Focus: Minimizing the chances of experiencing downtime in the first place.
• How it works: Uses spare parts or an extra similar system to take over when the main one stops working.

• When it’s used: For minor problems such as hardware failure or single node/facility level power failure.

• Think of it as: A car spare tire kept as a backup in case you get a flat.

Disaster Recovery:

• Focus: Restoring systems and data after a major disruption event.
• How it works: Entails a comprehensive plan for backing up data, moving operations to a secondary site, and restoring systems online.
• When it’s used: In cases of major events like natural disasters, cyber threats, or even situations where the entire system crashes.
• Think of it as: Similar to business insurance policies, it is used to facilitate recovery in the event of a disaster.

In short, redundancy is used to prevent unexpected downtime, and disaster recovery is used to restore systems and data after a major disruption. Redundancy differs from a backup, as issues like data deletion or encryption may be replicated to standby systems, indicating that distinct backup strategies are relevant for disaster recovery.