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MessageCentral and message-oriented programming
Component-structure fragility
Message-oriented programming
aims to eliminate the denotation of one of the parameters
from the clients' view of method call as the special object to which
the "message" is sent, and focus, instead, on the message as
a request for service by any server.
This permits client code to be reused even when the implementation of a
method is moved from one object to another. Consider, for example, a
fragment of code written to record the fact that an item was sold to a
customer at a store. Assuming that "item, customer, and store" are variables
that refer to objects of those types then the sale could be
registered by calling the register_sale method of the item
being sold. A Java programmer might write:
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| "item.register_sale(store, customer)", or perhaps |
| "store.register_sale(item, customer)", or perhaps |
| "customer.register_sale(item, store)". |
This example shows the essence of component-fragility. Source code
calling for a service is dependent on which of the data involved happen
to carry the implementation. This severely limits the use and reuse of
software developed by different organizations that made different
choices in deciding which objects should carry which function. The use
of a message broker makes it possible to have the client code simply
read
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| "register_sale(item, store, customer)" |
and to move the knowledge of which object carries the function into
the broker. This makes both the serving objects and their clients much
more reusable.
Issues in moving to message-oriented software
Successfully supporting a programming style that eliminated
component-structure fragility requires addressing several issues:
- What to do instead of interfaces?
- What happens to object references?
- What about multiple servers?
- How do we avoid "message not understood"?
Good support for message-oriented programming will rely upon a broker
and a composition system capable of great flexibility, like that
provided by MessageCentral.
For further information, please contact
Bill Harrison.
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