#cluster controlling

Network Load Balancing Services.

Network Load balancing is a computer networking methodology to distribute workload across multiple computers or a computer cluster, network links, central processing units, disk drives, or other resources, to achieve optimal resource utilization, maximize throughput, minimize response time, and avoid overload.

Network Load Balancing Services (NLBS) is a Microsoft implementation of clustering and load balancing that is intended to provide high availability and high reliability, as well as high scalability. Every client request to a stateless application is a separate transaction, so it is possible to distribute the requests among multiple servers to balance the load. For example, if a web server is returning an error page instead of correct content, it is still perceived as "alive" by NLBS. Configuration Tips:

The network load balancing service requires for all the machines to have the correct local time. Unsyncronized times will cause a network login screen to pop up which doesn't accept valid login credentials.

The server console can't have any network card dialogue boxes open when you are configuring the "Network Load Balancing Manager" from your client machine.

You have to manually add each network load balancing server individually to the network load balancing cluster after you've created a cluster host.

To allow communication between servers in the same NLB cluster, each server requires the following registry entry: a DWORD key named "UnicastInterHostCommSupport" and set to 1, for each network interface card's GUID (HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\WLBS\Parameters\Interface\{GUID}).

Network load balancing.

Each and every version of Windows 2003 Server has NLB. NLB is by default installed, but by default is not enabled.

Network Load Balancing is characteristically used to supply high accessibility for Internet application. A Network Load Balancing cluster is able to host up to thirty two servers.

The NLB servers make use of a statistical mapping which determines host precedence for an inward packet, to recognize the NLB server what should knob the call. Just the once the NLB server is recognized, the rest of the servers in the Network Load Balancing cluster castoffs the packet. One can configure an IP address for network traffic which should be trafficked to an identical cluster server. Every server in the Network Load Balancing cluster uses and transmits heartbeat signals to recognize the state. Signals for the protocol identified in the protocol parameters which are hold by the port rule. Most clustering applications as well use different hardware for cluster controlling.

Network Load balancing (computing).

Using multiple components with network load balancing, instead of a single component, may increase reliability through redundancy. The load balancing service is usually provided by dedicated software or hardware, such as a multilayer switch or a Domain Name System server.

One of the most common applications of network 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. Some load balancers provide a mechanism for doing something special in the event that all backend servers are unavailable. Unlike the use of a dedicated load balancer, this technique exposes to clients the existence of multiple backend servers. A variety of scheduling algorithms are used by load balancers to determine which backend server to send a request to. High-performance systems may use multiple layers of network load balancing.

Examples of the latter include the Apache web server's mod_proxy_balancerextension, Varnish, or the Pound reverse proxy and load balancer. In a multitier architecture, terminology for designs behind a load balancer or network dispatcher may include Bowties and Stovepipes. One solution to the session data issue is to send all requests in a user session consistently to the same backend server. The same problem is usually relevant to central database servers; even if web servers are "stateless" and not "sticky", the central database is.

Random assignments must be remembered by the load balancer, which creates a burden on storage. Sophisticated load balancers use multiple persistence techniques to avoid some of the shortcomings of any one method.

Microsoft's ASP.net State Server technology is an example of a session database. All servers in a web farm store their session data on State Server and any server in the farm can retrieve the data.

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.

Load balancer features

Hardware and software load balancers may have a variety of special features.

Priority activation: When the number of available servers drops below a certain number, or load gets too high, standby servers can be brought online

To remove this demand from the Web Server a Load Balancer may be used to terminate the SSL at the Load Balancer. Some Load Balancer appliances include specialized hardware to process SSL. When a Load Balancer terminates the SSL connections the requests are converted from HTTPS to HTTP in the Load Balancer before being passed to the Web Server. Adding a few web servers may be significantly cheaper than upgrading a Load Balancer. One clear benefit to SSL offloading in the Load Balancer is that it enables the ability for the Load Balancer to do load balancing or content switching based on data in the HTTPS request.

The tradeoff is that this feature puts additional CPU demand on the Load Balancer and it is a feature which could be done by web servers instead.

This feature utilizes HTTP/1.1 to consolidate multiple HTTP requests from multiple clients into a single TCP socket to the back-end servers.

TCP buffering: the load balancer can buffer responses from the server and spoon-feed the data out to slow clients, allowing the server to move on to other tasks.

Direct Server Return: an option for asymmetrical load distribution, where request and reply have different network paths.

Health checking: the balancer will poll servers for application layer health and remove failed servers from the pool.

HTTP caching: the load balancer can store static content so that some requests can be handled without contacting the web servers.

Content filtering: some load balancers can arbitrarily modify traffic on the way through.

HTTP security: some load balancers can hide HTTP error pages, remove server identification headers from HTTP responses, and encrypt cookies so end users can't manipulate them.

Content-aware switching: most load balancers can send requests to different servers based on the URL being requested.

Programmatic traffic manipulation: at least one load balancer allows the use of a scripting language to allow custom network load balancing methods, arbitrary traffic manipulations, and more.

Firewall: direct connections to backend servers are prevented, for network security reasons

Network Load balancing can be useful in applications with redundant communications links. Using load balancing, both links can be in use all the time. Many telecommunications companies have multiple routes through their networks or to external networks.

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