What is load balancing?

A load balancer is a device that distributes network or application traffic across a cluster of servers. Load balancing improves responsiveness and increases availability of applications.

A load balancer sits between the client and the server farm accepting incoming network and application traffic and distributing the traffic across multiple backend servers using various methods. By balancing application requests across multiple servers, a load balancer reduces individual server load and prevents any one application server from becoming a single point of failure, thus improving overall application availability and responsiveness.

Load balancing is the most straightforward method of scaling out an application server infrastructure. As application demand increases, new servers can be easily added to the resource pool, and the load balancer will immediately begin sending traffic to the new server. Core load balancing capabilities include:

  • Layer 4 (L4) load balancing - the ability to direct traffic based on data from network and transport layer protocols, such as IP address and TCP port
  • Layer 7 (L7) load balancing and content switching – the ability to make routing decisions based on application layer data and attributes, such as HTTP header, uniform resource identifier, SSL session ID and HTML form data
  • Global server load balancing (GSLB) - extends the core L4 and L7 capabilities so that they are applicable across geographically distributed server farms

How do load balancers work?

When one application server becomes unavailable, the load balancer directs all new application requests to other available servers in the pool.

To handle more advanced application delivery requirements, an application delivery controller (ADC) is used to improve the performance, security and resiliency of applications delivered to the web. An ADC is not only a load balancer, but a platform for delivering networks, applications and mobile services in the fastest, safest and most consistent manner, regardless of where, when and how they are accessed.

Load balancing algorithms and methods

Load balancing uses various algorithms, called load balancing methods, to define the criteria that the ADC appliance uses to select the service to which to redirect each client request. Different load balancing algorithms use different criteria.

  • The Least Connection Method
    The default method, when a virtual server is configured to use the least connection, it selects the service with the fewest active connections.
  • The Round Robin Method
    This method continuously rotates a list of services that are attached to it. When the virtual server receives a request, it assigns the connection to the first service in the list, and then moves that service to the bottom of the list.
  • The Least Response Time Method
    This method selects the service with the fewest active connections and the lowest average response time.
  • The Least Bandwidth Method
    This method selects the service that is currently serving the least amount of traffic, measured in megabits per second (Mbps).
  • The Least Packets Method
    This method selects the service that has received the fewest packets over a specified period of time.
  • The Custom Load Method
    When using this method, the load balancing appliance chooses a service that is not handling any active transactions. If all of the services in the load balancing setup are handling active transactions, the appliance selects the service with the smallest load.

Why are load balancers needed?

Traffic volumes are increasing and applications are becoming more complex. Load balancers provide the bedrock for building flexible networks that meet evolving demands by improving performance and security for many types of traffic and services, including applications.

What is Citrix NetScaler?

NetScaler ADC is an industry leading application delivery controller that delivers business applications to any device and any location with unmatched security, superior L4-7 load balancing, reliable GSLB, and 100 percent uptime.

Related terms: application acceleration, business continuity plan, cloud network, disaster recovery, GSLB, network functions virtualization

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