The HDM Event Model

Now that we have an undestanding of the type of structures we are trying to observe, what type of events are services waiting for? The following are the events generated by the HDM and their meaning:

• A HaystackEvent is generated for changes in the structure of the graph. Specifically, there are two extensions to HaystackEvent:
  • Whenever a change in graph structure happens (as indicated by the setting of both the ``forward'' and ``back'' pointers of a Tie) a HaystackCreateEvent is generated.
  • When a Needle that is connected to at least one other Straw has its data changed (by means of a setData(...) invocation) a HaystackChangeEvent is generated.
• An ObjectEvent is generated for changes and creations in any independent Straw (regardless of what it is connected to). This is useful for services that are interested in keeping track of all object changes in the HDM (for example the HsLore service will want to update each Straw entered in the database as it happens). There are three types of ObjectEvents:
  • An ObjectCreateEvent is generated whenever a new HDM object is created.
  • An ObjectDeleteEvent is generated whenever an HDM object is deleted.
  • Finally, Haystack defines an abstract (i.e. non-instantiatable) ObjectChangeEvent class. This class is extended by two specific implementations:
    • An ObjectChangeLinkEvent is generated when any pointer (i.e. the pointers in Ties as well as the StrawTies in Straw) are changed.
    • An ObjectChangeDataEvent is generated when a Needle's data is changed.

What we have indirectly achieved is the ability to differentiate between different granularity of events, from structure to individial ``atomic'' components.