Load Balancing Overview
Load balancing is a technique to spread work between two or more computers, network links, CPUs, hard drives, or other resources, in order to get optimal resource utilization, maximize throughput, and minimize response time. Using multiple components with load balancing, instead of a single component, may increase reliability through redundancy. The balancing service is usually provided by a dedicated program or hardware device (such as a multilayer switch).
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, NNTP servers and DNS servers.
Load balancer is usually a software program or a hardware device which 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.
A variety of scheduling algorithms are used by load balancers to determine which backend server to send a request to. Simple algorithms include random choice or round robin. More sophisticated load balancers may take into account additional factors, such as a server’s reported load, recent response times, up/down status (determined by a monitoring poll of some kind), number of active connections, geographic location, capabilities, or how much traffic it has recently been assigned. High-performance systems may use multiple layers of load balancing.
An important issue when operating a load-balanced service is how to handle information that must be kept across the multiple requests in a user’s session. If this information is stored locally on one back end server, then subsequent requests going to different back end servers would not be able to find it. This might be cached information that can be recomputed, in which case load-balancing a request to a different back end server just introduces a performance issue. One solution to the session data issue is to send all requests in a user session consistently to the same back end server. This is known as “persistence” or “stickiness”.
Fortunately there are efficient approaches to solve this problem. In the very common case where the client is a web browser, per-session data can be stored in the browser itself. One technique is to use a browser cookie, suitably time-stamped and encrypted. Another is URL rewriting. Storing session data on the client is generally the preferred solution; then the load balancer is free to pick any backend server to handle a request.
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