A load balancer, or server load balancer (SLB), is a hardware or software-based device that efficiently distributes network or application traffic across a number of servers. With a load balancer, if a server's performance suffers from excessive traffic or if it stops responding to requests, the load-balancing capabilities will automatically switch the requests to a different server.
In this way, load balancers improve the performance of networks and applications by automatically monitoring and managing application and network sessions.
In addition to providing simple distributed service to multiple servers, load balancers can help prevent denial-of-service attacks, allow legitimate users uninterrupted access to services, protect against single point of failure outages and prevent traffic bottlenecks to systems.
Types of Load Balancers and Application Delivery Controllers
Load balancers are frequently categorized as being Layer 4 or Layer 7 capable, in reference to the 7 Layers of the OSI Model. Layer 4 load balancers respond to data found in network and transport layer protocols such as TCP, IP, FTP and UDP, while Layer 7 load balancers distribute requests based upon data found in application layer protocols such as HTTP, as well as data within the application messages.
Load-balancing capabilities are found in hardware routers from vendors like F5, Cisco, Citrix and Kemp Technologies. Hardware load balancer devices have mostly evolved into what are now referred to as application delivery controllers (ADCs), but load balancing remains at the heart of ADCs.
Software-based load balancers from vendors like Zen, Resonate and NGINX provide load-balancing capabilities that are typically less expensive than hardware load balancer solutions, and they offer the flexibility of being easily deployed on-premises on existing hardware or in the cloud.
Additional types of load balancers marketed by vendors include virtual load balancers, cloud load balancers and load balancers for bare metal servers. Many load balancer vendors offer a variety of load balancer solutions that includes hardware and software-based load servers as well as virtual and cloud-based load servers.
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