Entities

Entities are the fundamental units of observation/monitoring in the system.

Essentially, an entity is a process (pid). As processes, all entities are uniquely identifiable by their pids. Associated with each entity is a symbolic name that can be used to refer to that specific entity. Again, the names associated with entities are unique across the system. As we'll see later, this uniqueness requirement is very similar to the naming scheme used in a hierarchical filesystem.

There are three fundamental entity types:

• Self-attached entities (HA-aware components)—processes that choose to send heartbeats to the HAM, which will then monitor them for failure. Self-attached entities can, on their own, decide at exactly what point in their lifespan they want to be monitored, what conditions they want acted upon, and when they want to stop the monitoring. In other words, this is a situation where a process says, Do the following if I die.
• Externally attached entities (HA-unaware components)—generic processes (including legacy components) in the system that are being monitored. These could be arbitrary daemons/service providers whose health is deemed important. This method is useful for the case where Process A says, Tell me when Process B dies but Process B needn't know about this at all.
• Global entity—a placeholder for matching any entity. The global entity can be used to associate actions that will be triggered when an interesting event is detected with respect to any entity on the system. The term global refers to the set of entities being monitored in the system, and allows a process to say things like, When any process dies or misses a heartbeat, do the following. The global entity is never added or removed, but only referred to. Conditions can be added to or removed from the global entity, of course, and actions can be added to or removed from any of the conditions.