Event Manager
Components in a loosely coupled system like an
Active Space are typically designed to operate by generating and responding to
asynchronous events. The event manager provides a model for decoupled
communication between different entities in an Active Space. It allows
creating channel categories, browsing these categories and their associated
channels and creating and deleting channels associated with particular
categories. All Gaia components use the event manager to learn about changes in
the state of the space and react accordingly. The Presence
Service, for example, listens to different channels to learn about new
entities (e.g., software services and people entering the space), filters the
information, and publishes events to inform the rest of the system about new
entities discovered and entities no longer available.
The event manager distributes load among multiple
machines. It creates event channel factories remotely on pre-configured machines
whenever it decides that the existing event channel factories are already
overloaded. When the event manager is bootstrapped it has to be configured with
a set of available machines on which event channels factories can be
instantiated. The event manager is also responsible for recreating those
channels if any supplier of events complains that the particular event channel
no longer exists. In essence, the event manager keeps state for the channels it
creates and recreates them if they crash, transparent to the event consumers.
]
