CUE Zipline is composed of two parts:
The front end, which provides web services to external clients such as the CUE editor, drop resolvers, CUE Print and so on.
The core, which monitors the Content Store, waiting for changes to content items that have "shadow" content in external systems (CUE Print and DC-X, plus other systems via NewsML export).
When running a CUE Zipline cluster, the front end web services should be load balanced, in order to distribute incoming client requests between the instances in the cluster.
The core activity of monitoring the Content Store for changes cannot, however, be load balanced in this way. Only one CUE Zipline instance (called the "active" instance) monitors the Content Store. The other instances in the cluster are said to be "inactive", even though they are available for processing front end web service requests.