What is server redundancy?

When uptime is all-important, you need redundant servers on hand when trouble strikes

A server corridor for a supercomputer installation

'Redundancy' is a word that most people fear, but for server infrastructure, it's a vital part of keeping everything working. 

For IT professionals, redundancy is a duplication of critical components or functions of a system that aims to increase the reliability of the said system. It usually comes in the form of a backup or fail-safe. Ergo, having so-called redundant servers in your business is actually a key part of your backup, load balancing or maintenance strategies. 

You'll likely find server redundancy in IT environments that are dependent on large-scale server reliability. The redundant server is a mirror image of the production server it reflects. It has the same compute capacity, storage and applications running under the same configurations.

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The difference is the redundant server isn't online and it isn't used as a 'live' one until it is needed. It still receives power and has network connectivity ready to go, should the need arise. This is usually during downtimes or extreme traffic. The redundant server either picks the network back up or it is used to spread the workload. 

The downside is that it can increase costs as running a server - whether online or not - is still an expense. It also takes up space and needs to be kept cool and clean. But, the upside is that your network is backed up to the fullest. 

Why is server redundancy important?

If 2020 taught us anything, it's that an organisation needs to be prepared for the unexpected if it hopes to survive and thrive. The difference between a company that comes through intact and one that fails comes down to how effective and thorough a disaster recovery strategy they have in place - not just in theory but also in implementation.

In a world where most organisations are built around digital infrastructure, redundant servers must form an essential element of a successful disaster recovery plan. Not only is it critical for your organisation to be able to access data in the aftermath of a disaster, but its loss could mean severe, potentially long-lasting disruption.

Hardware failure, application faults, network problems and other such issues can prevent the proper functioning of primary servers, leaving users unable to access services and vital data. At best, this will pose a problem for productivity.

Your business can avoid these contingencies by employing server redundancy. By having critical data duplicated at a second location, it can quickly and easily be retrieved in the event of a fault with a live server. If data integrity and access are vital for applications and the functioning of your organisation - as they are for many - redundant servers are essential.

What are the business benefits of server redundancy?

Redundant servers offer assurance to businesses as they have a cost-effective backup for accessing critical data if disaster strikes and a live server goes offline. If a server goes down, a backup server can take up the slack enabling maximum uptime until the failed server is fixed.

They also feature real-time system monitoring which scans for possible failure, this means your business always knows about the health of their servers.

However, the benefits need to be balanced with the level of risk and the substantial costs associated with it.

Types of redundant server

Redundant servers can take many forms.

Redundant domain, front end, and validation servers: These are used for load-balancing to ensure users can always access a service. For example, a secondary Windows Active Directory server validates user access to the domain if the primary AD server goes down or is busy.

Replicated servers: A replicated backup server can be paired with a production server. Any change to the production server is replicated to a backup server using software-based or hardware-based tools. In the event of a server failure, the replicated server can be brought into service.

Disaster recovery servers: These are semi-hot spares that can have backup files quickly restored and restart processing, should disaster strike.

How to create server redundancy

To create server redundancy in your infrastructure, you need two servers housing identical data - a primary server and a secondary server.

A failover monitoring server checks the primary servers for any problems. Should a problem be detected, it will automatically update DNS records so that network traffic is diverted to a secondary server.

Once the primary server is working properly again, traffic will be rerouted back to the primary server. If the handover and hand back are successful, users should not notice any difference to the service.

What is IP failover?

IP failover is a popular technique for server redundancy. Servers run a so-called heartbeat process and in the event of one server failing to see the heartbeat of the other server, it takes over the IP address of the failing server.

IP takeover is implemented when two servers are connected on the same switch and are running on the same subnet.

What else should be redundant?

In addition to a redundant server, your infrastructure should make sure it has other parts that can be duplicated in case of emergencies and to ensure maximum uptime.

Backups: Backups can be deployed to ensure data held locally is also stored elsewhere (on the cloud, or another data centre in a distant location). This allows you to quickly restore data in the event of a disaster.

Disk drives: Hot spares should be available so that if a disk drive in a primary server fails, another drive can immediately replace it. Using a RAID array should ensure that a server can keep running when there is a single disk failure.

Power supplies: Redundant power supplies should be deployed on critical servers so that if the main power supply fails, it can continue to run.

Internet connectivity: If your server needs to have a connection to the internet at all times, having a line from a different telecoms company is important. If one line fails (e.g. if a workman severs a cable), traffic can shift onto an undamaged line.

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