Information Technology Reference
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placed in a situation where another correctly functioning component sends it
a message that it cannot interpret. One example of the application of this rule
can be found in the computational language, where a series of constraints on
computational interface types is laid down in the reference model to ensure
that evolutionary changes in interface type can be carried out in a smooth
and orderly way. These rules describe the relationships between client and
server operational interfaces that must be satisfied in order for a binding to
be created between them; they state that:
1. The operations defined in the client interface must be a subset of those
defined in the server interface and must have the same number of pa-
rameters;
2. The parameters generated by the client must be subtypes of those ex-
pected by the server; and
3. The terminations issued by the server must be a subset of those under-
stood by the client, and their parameters must be subtypes of those the
client expects.
What these rules (summarized in figure 13.1) amount to is that neither
party will receive anything they are not expecting, although they may not re-
ceive things they could well handle because the sender still lacks the capability
to generate them.
client
operation
set
server
operation
set
<
parameters sent
are subtypes
Client
Server
parameters sent
are subtypes
client
termination
set
server
termination
set
>
client
interface
server
interface
FIGURE 13.1: Computational interface type compatibility rules.
