On a daily basis, we use countless websites and applications at the same time as millions of other people on Earth. While we each access these platforms, or local area networks (LAN), from our own personal devices, the connections for each platform come from the same server or set of servers. If a platform has dozens of users at a time, it may use a single server to represent the data to the users; however, if a platform has hundreds or thousands of users at one time, it will require multiple servers. In turn, these servers must be balanced, and this need is met by the installation of a load balancer. Load balancing works as a ‘reverse-proxy’ to represent the application servers to the client through a virtual IP address (VIP). This technology is known as server load balancing (SLB). In order to increase both availability and scalability, a load balancer uses specific algorithms to steer traffic to the available servers, monitor for when additional servers are needed, and identify when servers are not required. This blog will cover the basics of what you need to know about the different types of load balancing for your applications.
Load balancers perform what are known as health checks; they monitor the servers to determine whether they are available for use. If a server fails the health check, it is unavailable for further use and the load balancer takes that instance of the application out of its pool of available servers. When the application comes back online, the health check validates its availability and the server is put back into the availability pool. Additionally, with the location of the load balancer between the server and the platform, it can also perform content switching, provide content-based security like web application firewalls (WAF), and offer authentication enhancements like two factor authentication (2FA). Furthermore, load balancing allows for site maintenance without limiting availability to the user and protects against malicious content. When a user logs onto an application or website with a load balancer, the connection is immediately sent to a specific application instance on a server. The load balancer continues to manage and monitor the connection for the entire duration. Similar to the role of an agent or a representative, SLB is carried out automatically. Depending on the specific requirements, different types of load balancers are available for varying purposes.
When understanding load balancers, it is useful to cover each type chronologically. Network service load balancers entered the market in the 1990s as internet traffic became an increasing problem. They performed basic management of what is known as the 5-tuple: source IP, destination IP, source port, destination port, and IP protocol. Over the years, advancements in technology have led to the innovation of more complex and capable load balancers. Application load balancers introduced the ability to provide content awareness and content switching by looking beyond the packet header into more specific content like the URL, HTTP header, and other things to make load balancing decisions. Application load barriers surpass the Layer 4 load balancing capabilities of the earliest network service load balancers as Layer 7 load balancers. The next revolutionary innovation introduced global server load balancing (GSLB), an entirely separate form of technology from the traditional layer 4-7 load balancer. GSLB is based on domain name server (DNS) and acts as a DNS proxy to provide responses based on GSLB load balancing algorithms in real time. This type of load balancer uses technology that manages and monitors multiple sites through configurations and health checks.
Initially, all load balancers were hardware; however, with the growth of network technologies like software-defined, virtualization, and cloud technologies, there became a need for software-based load balancing solutions. These are more flexible than hardware-based load balancers, but they require additional dependencies such as hypervisors and COTS hardware. Lastly, one might purchase an Elastic Load Balancer (ELB). These are the most sophisticated solutions that offer cloud-computing operators scalable capacity based on traffic requirements at any one time. Furthermore, they scale load balancing instances automatically and on-demand and increase the fault tolerance of your applications.
Regardless of the type of load bearing a server uses, they all rely on load balancing algorithms to function, all of which are designed to connect the user to the best server option. The most common is the algorithm that sends users to the server with the least connection at any time, but many options exist. For example, round robin and weighted round robin algorithms base the connection decision on time slices in equal or unequal rotating orders. Additional algorithms include weighted least connection, agent based adaptive load balancing, chained failover (fixed weighted), weighted response time, source IP hash, and software defined networking (SDN) adaptive options.
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