An application delivery controller (ADC) or load balancer does exactly what its name suggests – efficiently balances server load, enable the efficient delivery of applications. It sits in between the user, or ‘client’, and the server cluster and distributes all the requests from users across all servers capable of fulfilling those requests, which may be local within the same data center or geographically dispersed across the internet or private networks.

By utilizing a ‘least connection’ algorithm, load balancers can evenly distribute the incoming application and network traffic across a group of backend servers, making sure no one server is overloaded – which could degrade performance, or result in a server crashing. They manage and intelligently spread out server load, increasing the capacity and reliability of applications, leading to improved overall application performance. If one server goes down, a load balancer immediately redirects traffic to the remaining healthy servers, thus resulting in a super-fast failover. When any new servers are added to a server pool, a load balancer automatically starts sending requests to it.

In simple words, here’s what happens: Traffic comes to your application or site, an application delivery controller distributes the traffic across appropriate server nodes, the node receives the request and delivers a response to the user.

Customer expectations are rising for digital channels for metrics such as site speed and stability. And to meet these customer expectations, businesses must ensure that their digital experiences are truly ‘zero friction’. Therefore, organizations that have high levels of website activity or mission-critical web and mobile applications that are used by large numbers of customers or employees – should use load balancers.

Such organizations need more than one server to manage the high volume of concurrent requests, and using a load balancer helps them share the traffic load across different physical or virtual servers. While layer 4 ADCs act upon data found in network and transport layer protocols (IP, TCP, FTP, UDP), layer 7 distributes requests based upon data found in application layer protocols such as HTTP.

• High availability: ensuring applications are always accessible
• Scalability: making it easy to change server infrastructure without disrupting the user experience
• Security: adding additional layers of security to your IT applications
• Zero downtime: instantly and automatically rerouting traffic to an alternate server in the cluster, eliminating interruption in application availability
• Redundancy: if something goes wrong with one server, the backup servers take over and restart applications that were running on the failed server
• IT flexibility: allowing technicians to disconnect, patch, upgrade and reconnect servers without any disruption for users
• Efficiency: providing consistent and fast application performance for thousands of concurrent users

At the most fundamental level, there are two types of application delevery controller (ADC)

• Hardware devices that physically live in organizations’ IT departments or data centers
• Software solutions that can be installed on physical or virtual machines in data centers, on-premise, or in the cloud.

Organizations looking for simple load balancing capabilities should go for standard hardware or software-based load balancers as these are cost-effective and easy to install and manage. Some vendors combine load balancing functionality with additional capabilities, such as security features. For example, Loadbalancer.org appliances come complete with a web application firewall (WAF) as a standard feature.

Load balancers are most commonly deployed in active-passive pairs to ensure zero downtime in business-critical applications. All traffic can then be redirected to the redundant device, both to provide business continuity in the event of a failure and to remove the need for scheduled downtime for maintenance.