Each Zato cluster has exactly one high-availability load-balancer in front of it to fairly distribute incoming HTTP traffic across all the servers forming a cluster. A load-balancer is a cluster-specific entity, each cluster needs to have its own load-balancer.

Zato’s load-balancer is implemented using HAProxy running in a subprocess and can handle large number of incoming connections.

The web admin can be used to graphically update most commonly used aspects of the load-balancer and it’s always possible to edit the HAProxy’s configuration directly through web admin, meaning that Zato’s load-balancer can offer every feature supported by HAProxy.

A companion of each load-balancer, its agent, written using Spring Python, is accessed by administrators through the web admin console. The console communicates with agents through XML-RPC over SSL, optionally using client certificates. The agent exchanges messages with the load-balancer through the latter’s UNIX admin socket and can be used to update and reload a load-balancer’s configuration as well as to query the load-balancer for its status and run-time data, such as the number of connected HTTP clients. It is always the agent that initiates the connection, never the load-balancer.

Each web console offers a link to load-balancer statistics provided by HAProxy itself. The statistics offer a view into how many requests the load-balancer has processed, the transfer rate, how long the servers messages are relayed to have been up and similar information.

Administrators use the command-line interface to create, start and stop load-balancers along with their agents. Note that it’s not possible to create a load-balancer without an agent nor vice-versa.