Release 10.1A: OpenEdge Application Server:
Administration

> ## Fault-tolerant AppServers and load balancing

> When an AppServer registers with a NameServer, it specifies a list of application services. In the simplest configuration, only one AppServer instance can register a specific application service with the same controlling NameServer. Thus, the NameServer satisfies each request for the registered application service by directing the client connection to the same AppServer. However, if you install the NameServer Load Balancer, you can configure fault-tolerant AppServers by having multiple AppServer instances register the same application service with a single controlling NameServer.

> Note: If you have not installed the NameServer Load Balancer and an AppServer tries to register the same application service on the same controlling NameServer of an already-registered AppServer, the AppServer attempting to register receives an error.

> When a client requests an AppServer connection provided by fault-tolerant AppServers, the specified NameServer resolves the connection request using one of the registered AppServer instances that support the specified application service. By default, the NameServer chooses the AppServer instance at random from among the available instances that can satisfy the connection request, thus distributing connections equally.

> The NameServer Load Balancer also allows you to have the NameServer distribute client connections across a set of fault-tolerant AppServers proportionately, according to a weight factor that you specify when you configure each AppServer instance. When each AppServer registers with the controlling NameServer, in addition to its location and supported application service list, the AppServer registers any weight factor you specify. The NameServer uses the weight factors configured for all instances in a set of fault-tolerant AppServers to determine how to distribute client connections among them.

> The values of these weight factors are arbitrary. It is the relative differences between these values that determine how the NameServer balances load. To achieve effective load balancing, you must coordinate weight factor assignments according to individual AppServer performance. Thus, the most correct weight factor for each AppServer instance depends on the relative performance (for your application) of the particular AppServer platform and its configuration compared to other AppServers that register support for the same application service.

> Note that the NameServer is flexible enough that you can dynamically start an AppServer instance to register at any time among a set of running fault-tolerant AppServers. As an AppServer instance starts up and registers, the NameServer adds it to the list of available AppServers that support the same application services. The NameServer then apportions client connection requests according to the latest set of weight factors.

> For more information on setting weight factors and the effects of these settings, see OpenEdge Getting Started: Installation and Configuration .

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