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What is Load Balancing ?

In networking, load balancing is a technique to distribute workload evenly across two or more computers, network links, CPUs, hard drives, or other resources, in order to get optimal resource utilization, maximize throughput, minimize response time, and avoid overload. Using multiple components with load balancing, instead of a single component, may increase reliability through redundancy. The load balancing service is usually provided by a dedicated program or hardware device (such as a multilayer switch or a DNS server).

One of the most common applications of load balancing is to provide a single Internet service from multiple servers, sometimes known as a server farm. Commonly, load-balanced systems include popular web sites, large Internet Relay Chat networks, high-bandwidth File Transfer Protocol sites, Network News Transfer Protocol (NNTP) servers and Domain Name System (DNS) servers. Lately, some load balancers evolved to support databases, these are called database load balancers. For Internet services, the load balancer is usually a software program that is listening on the port where external clients connect to access services.

The load balancer forwards requests to one of the "backend" servers, which usually replies to the load balancer. This allows the load balancer to reply to the client without the client ever knowing about the internal separation of functions. It also prevents clients from contacting backend servers directly, which may have security benefits by hiding the structure of the internal network and preventing attacks on the kernel's network stack or unrelated services running on other ports. Some load balancers provide a mechanism for doing something special in the event that all backend servers are unavailable.

In this technique, multiple IP addresses are associated with a single domain name (e.g. www.example.org); clients themselves are expected to choose which server to connect to. Unlike the use of a dedicated load balancer, this technique exposes to clients the existence of multiple backend servers. The technique has other advantages and disadvantages, depending on the degree of control over the DNS server and the granularity of load balancing desired. This might include forwarding to a backup load balancer, or displaying a message regarding the outage. An alternate method of load balancing, which does not necessarily require a dedicated software or hardware node, is called round robin DNS.

A short TTL on the A-record will also help to ensure traffic is quickly diverted if a server goes down. Consideration must be given the possibility that this technique may cause individual clients to switch between individual servers mid-session. This way, if a server is down, its DNS will not respond and so the web service will not receive any traffic. Also if the line to one server becomes congested the unreliability of DNS will ensure less HTTP traffic will reach that server, further the DNS response that gets back to the resolver the quickest will nearly always be from the network closest server, ensuring geo-sensitive load-balancing.

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