Rest – Publish / Subscribe via HTTP Callbacks

Solution B only works if you request the data from System A every time it is required by System B. Otherwise you cannot guarantee the information is up to date. This is not as bad as it sounds and is in fact the basis of all SOA architectures.

The decision of whether to push (Solution A) or pull (Solution B) depends on the ratio of updates in system A to the number of queries in System B.

Where you have lots of updates in system A versus a few queries in System B then "pull" is the obvious choice.

Where you have relatively few updates in system A and lots of queries in System B then "push" is the way to go.

Where you have lots of updates and lots of queries either choice is equally good (or bad!).

There are two possibilities:

1. Pass the "message tray" to whatever component needs it
2. Make each component that can have controls implement the "tray" interface. That way, the holder of the internal panels is also a tray and it can dispatch the messages to it's parent tray (if any).

I like more #2 because it allows you to be more flexible in how the controls hierarchy looks like (aka, a control that is tray might decide that only some messages are passed to parents). Also, in this design, each child has only a reference to the parent and the parent decides what to do with the incoming messages.