What is Load Balancing?

The load balancing feature distributes user requests for Web site pages and other protected applications across multiple servers that all host (or mirror) the same content. You use load balancing primarily to manage user requests to heavily used applications, preventing poor performance and outages and ensuring that users can access your protected applications. Load balancing also provides fault tolerance; when one server that hosts a protected application becomes unavailable, the feature distributes user requests to the other servers that host the same application.

You can configure the load balancing feature to:

• Distribute all requests for a specific protected Web site, application, or resource between two or more identically-configured servers.
• Use any of several different algorithms to determine which server should receive each incoming user request, basing the decision on different factors, such as which server has the fewest current user connections or which server has the lightest load.

The load balancing feature is a core feature of the NetScaler appliance. Most users first set up a working basic configuration and then customize various settings, including persistence for connections. In addition, you can configure features for protecting the configuration against failure, managing client traffic, managing and monitoring servers, and managing a large scale deployment.

How Load Balancing Works

In a basic load balancing setup, clients send their requests to the IP address of a virtual server configured on the NetScaler appliance. The virtual server distributes them to the load-balanced application servers according to a preset pattern, called the load balancing algorithm. In some cases, you might want to assign the load balancing virtual server a wildcard address instead of a specific IP address. For instructions about specifying a global HTTP port on the appliance, see Global HTTP Ports.

Load Balancing Algorithms

The load balancing algorithm defines the criteria that the NetScaler appliance uses to select the service to which to redirect each client request. Different load balancing algorithms use different criteria. For example, the least connection algorithm selects the service with the fewest active connections, while the round robin algorithm maintains a running queue of active services, distributes each connection to the next service in the queue, and then sends that service to the end of the queue.

Some load balancing algorithms are best suited to handling traffic on websites, others to managing traffic to DNS servers, and others to handling complex web applications used in e-commerce or on company LANs or WANs. The following table lists each load balancing algorithm that the NetScaler appliance supports, with a brief description of how each operates.

Depending on the protocol of the service that it is load balancing, the NetScaler appliance sets up each connection between client and server to last for a different time interval. This is called load balancing granularity, of which are three types: request-based, connection-based, and time-based granularity. The following table describes each type of granularity and when each is used.

During startup of a virtual server, or whenever the state of a virtual server changes, the virtual server can initially use the round robin method to distribute the client requests among the physical servers. This type of distribution, referred to as startup round robin, helps prevent unnecessary load on a single server as the initial requests are served. After using the round robin method at the startup, the virtual server switches to the load balancing method specified on the virtual server.

Before you use Load Blancer in netscaler, you must enable it first. For this, open settings, and click configure basic features. Then enable LoadBalancing and content switching.

LB service state:

• up: within the timeout netscaler monitor probes were able to reach the server and successful.

• Down: within the timeout netscaler monitor probes were NOT able to reach the server within the timeout.

• out of service: when admin disables the service or server

• Going out of service: when admin disables it, or set a time limit on when it can be disabled, it will be in going out of service state.

• Down going out of service: monitor for a service failed while it is going out of service.

Similarly, LB vservers also has up, down or out of service states. LB vserver will be in up state if atleast one of the services bound to the lb vserver are up. down, if all services bound to vserver are down or lb feature is not enabled. If Admin has disabled a server, it will be in out of service.

• servers and monitors are bound to services.

• services are bound to virtual servers.

• When you have multiple services, you can manage them as a group by using service groups.

For load balancing, both IPV4 and IPV6 are supported as well as wildcard values for IP or port on the virtual server.

LB Methods:

Least connection: routes the request to the server which has least number of connections.

Round Robin: routes connection to the first server, then server2 will get the second request and so on.. servers are sent back to end of the queue once they receive a request.

Slow Start: Initially when state of lb vserver is changed or when it is starting up, if slow start is configured, that lb vserver will use round robin to send traffic. after it is fully up, it uses the lb method configured on the lb vserver. This is to prevent any requests going to one server only at the initial phase when the vserver is starting up.