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

When it comes to jobs, redundancy is not a word you want to hear. But for server infrastructure, it is absolutely vital if you need to guarantee uptime.

In IT circles, redundancy is the "duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the form of a backup or fail-safe, or to improve actual system performance, such as in the case of GNSS receivers, or multi-threaded computer processing."

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Having redundant servers in your enterprise is key to ensuring key functions such as backup, load balancing, or maintenance can be carried out.

Server redundancy is initiated in an IT environment where server availability is utterly essential. A redundant server is a mirror image of the production server it shadows with same compute capacity and storage as well as having the same applications running and the same configuration.

A redundant server remains offline and is not used as a live server until it is required. That said, it is powered up and has network connectivity so it can spin up when needed.  When failure, downtime or extreme traffic happens to a live production server, the redundant server can be used to either take the place of the production server or share its traffic load to reduce problems. However, it can bump up the cost of having a server and running costs. Also, you will also need space for two servers.

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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