load balancing servers, routers and hardware, server clusters, farms and failover

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During my one day pre-conference session at Exchange Connections, I heard a desire for something that doesn't exist today. Later in the week, I sat in the back during a Harold Wong session, and the same topic/request/demand/whatever came up. The basic…(read more)
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I wrote up some information on deploying CCR on Windows Server 2008 after doing a full day workshop. After trying to explain how to perform some of the steps, I found it was easier to demo some of the steps, so I used Camtasia to capture them and save…(read more)
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One of the major changes moving forward into Windows Server 2008 R2 is that some of the tools are being deprecated (I love that word) in favor of PowerShell. Some people just screamed, “Wooo hooo, fantastic!” while others screamed, “Great…(read more)
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What is Load Balancing?, well basically A network load balancer is an appliance device that is used to split network load across multiple servers.

Unsurprisingly Wikipedia have a great definition.-

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.

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. 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. In this technique, multiple IP addresses are associated with a single domain name (i.e. www.example.org); clients themselves are expected to choose which server to connect to. Unlike the use of a dedicated load balancer, this technique is not “transparent” to clients, because it exposes 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.

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.

In addition to using dedicated hardware load balancers, software-only solutions are available, including open source options. Examples of the latter include the Apache web server’s mod_proxy_balancer extension and the Pound reverse proxy and load balancer.

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