- Forward compatibility
Forward compatibility (sometimes confused with
extensibility) is the ability of a system to gracefully accept inputintended for later versions of itself.
A forward-compatible system is expected to gracefully handle input which is intended for a newer version, by ignoring the unknowns and selecting the known subset of the data that the system is capable of handling. Forward compatibility is harder to achieve than
backward compatibilitybecause a system needs to cope gracefully with an unknown future data formator requests for unknown future features. Backward compatibility does not have this issue because it accepts a known data format.
An example of forward compatibility is a
web browserignoring the HTMLtags that it does not (yet) recognize. It is typical for forward compatible systems to ignore unknown dataor application instructions.
application software systemsare designed with a robust and self-sufficient systems architectureso that they can operate adequately even when inputfor a more advanced version is entered.
In all cases, when the application system accepts the input data or operating system is not as expected, it will produce an
outputthat will identify the problem accurately for the user.
A code example
Assume that version "v0" of a program expects inputs of the form <t>X</t>, where t is an envelope containing some data X. Version "v1" requires some extra functionality and thus it is also able to accept inputs of the form <s>X</s>, where s is a specialization of t. Now "v0" cannot read the input <s>X</s>, since "v0" has no knowledge that s can be treated as t. It would appear that since s is a specialization of t, the new input could have been coded as <t type="s">X</t> where the ‘type’ attribute denotes the specialized nature. This is indeed a better form of the input, but if "v0" wasn’t designed to gracefully ignore the ‘type’ attribute, this won’t present forward-compatibility either. In fact, the design of "v0" determines its forward compatibility. The choice now is simple: forward compatibility or new functionality?
If the choice is new functionality, then we break forward compatibility. But it is important now to build into "v1" the ability to gracefully treat future inputs we could foresee (e.g., ignore attributes, such as "type", that the program does not understand).
* [http://www.catb.org/~esr/writings/taoup/html/ch01s06.html#id2879112 Rule of Extensibility: Design for the future, because it will be here sooner than you think] , in
The Art of Unix Programming
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