Load balancer concepts

To use the Rackspace Cloud Load Balancers API effectively, you should understand several key concepts:

Load balancer

A load balancer is a logical device which belongs to a cloud account. It is used to distribute workloads between multiple back-end systems or services, based on the criteria defined as part of its configuration.

Virtual IP

A virtual IP is an Internet Protocol (IP) address configured on the load balancer for use by clients connecting to a service that is load balanced. Incoming connections are distributed to back-end nodes based on the configuration of the load balancer.

Error page

An error page is the html file that is shown to the end user when an error in the service has been thrown. By default every virtual server is provided with the default error file. It is also possible to submit a custom error page via the Load Balancers API.


A node is a back-end device providing a service on a specified IP and port.

Health monitor

A health monitor is a configurable feature of each load balancer. It is used to determine whether or not a back-end node is usable for processing a request. The load balancing service currently supports active health monitoring.

Active health monitoring is a technique that uses synthetic transactions executed at periodic intervals to determine the condition of a node. One of the advantages of active health monitoring is that it does not require active transactions to be processed by the load balancer to determine whether or not a node is suitable for handling traffic. Active health monitoring is not applied by default and must be enabled per load balancer.

The active health monitor can use one of three types of probes:

  • connect

  • HTTP


These probes are executed at configured intervals; in the event of a failure, the node status changes to OFFLINE and the node does not receive traffic. If, after running a subsequent test, the probe detects that the node has recovered, then the node’s status is changed to ONLINE and it is capable of servicing requests.

If active health monitoring is enabled, a customer can then specify a SECONDARY node for routing traffic in the event of a PRIMARY node failure. If all PRIMARY nodes fail, SECONDARY nodes will then start to serve traffic. A common use case is having the SECONDARY nodes return a static page saying “We have a problem; please check back later”.

Session persistence

Session persistence is a feature of the load balancing service. It attempts to force subsequent connections to a service to be redirected to the same node as long as it is online.

Connection logging

The connection logging feature allows logs to be delivered to a Cloud Files account every hour. For HTTP-based protocol traffic, these are Apache-style access logs. For all other traffic, this is connection and transfer logging.