A Theory of Compatible Versions
by David Orchard
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Pages: 1, 2
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The Name Language Version 2 contains a new term, an optional middle, which must contain alpha characters only. In the texts of the language, middle goes after last: name = first, last, ?middle, *.
The V2 Defined Text Set, V2 Accept Text Set, and Substitution between them looks like:
Figure 4.
There are four sets defined: V1 Defined Text Set, V2 Defined Text Set, V1 Accept Text Set, and V2 Accept Text Set. We added an item to the Defined Text Set so the change in the Defined Text Set from V1 to V2 is an expansion. As a result, the V2 Defined Text Set is greater than (a superset of) the V1 Defined Text Set. The V2 Accept Text Set is a subset of the V1 Accept Text Set. This is because some of the middle names that are in V1 Accept Text are not in V2 Accept Text Set. We can show the expansion of the Defined Text Sets and the contraction of the Accept Text Sets by combining the V1 and V2 diagrams together:
Figure 5.
Compatibility
Any document that is in the V1 Defined Text Set is in the V2 Defined Set, so V2 is backward compatible with V1.
We had to pull a slight trick to achieve backward compatibility: we specified that it was one of the items in the V1 Defined Text Set. If there was an extension to a language--say, a "suffix" was added--then that language is no longer V1. It is V.Not1. Our compatibility guarantees are only for V1 and V2, not for V.Not1.
More formally:
V2 is backward compatible with V1 if V2 Defined Text Set > (superset) V1 Defined Text Set.
Any Text in the V2 Defined Text Set is in the V1 Accept Text Set, so V1 is forward compatible with V2. Formally:
V1 is forward compatible with V2 if V1 Accept Text Set > (superset) V2 Defined Text Set.
Combined together:
V2 is compatible with V1 if V1 Accept Text Set > V2 Defined Text Set > V1 Defined Text Set.
We have shown that forward and backward compatibility is only achievable through extensibility, and the process of compatible versioning is gradually narrowing the extensibility gap between the Defined Text Set and the Accept Text Set.
Incompatibility
We can show an incompatible extension. Imagine that the V2 Defined Text Set and Accept Text Set specify that the middle name must occur between the first and last, rather than at the end. That means some of the members of V2 Defined Text Set are not in V1 Defined Text Set or V1 Accept Text Set. We can show this as V2 not being completely inside V1:
Figure 6.
Partial Understanding Extends Compatibility
The previous examples showed a V1 Name language processor that used the entire name text set and required the correct ordering per the Accept and Defined Text Sets. There are many scenarios where a processor will only consume part of the Defined Text Set. We can imagine an application that only looks at first names and ignores everything else. My favorite example of this is a "Baby Name" Wizard. It might use a simple XPath expression to extract the first name from inside the name. Its Accept Text Set is *, first,*.
Partial understanding affects both the Defined Text Set and Accept Text Set. Partial understanding increases, or supersets, the Accept Text Set because the process of extracting the subset needed means that extra content, legal and illegal under V1 Accept Text Set, is ignored. Partial understanding decreases, or subsets, the V1 Defined Text Set because only part of the Defined Text Set is understood.
We can call the V1 Accept Text Set superset that is used the "V1' Accept Text Set" and the V1 Defined Text Set subset that is used the "V1' Defined Text Set." In this scenario, the sets look like:
Figure 7.
Partial Understanding and Compatibility
The compatibility guarantee that accounts for partial understanding and the subset of V1(V1') that is used is:
V2 is compatible with V1' if V1' Accept Text Set > V2 Defined Text Set > V1' Defined Text Set.
V1 is compatible with V1' if V1' Accept Text Set > V1 Defined Text Set > V1' Defined Text Set.
In this particular case, we see that V1 and V2 are compatible with V1' because the V1' Accept Set is a superset of V1 and V2. V1 is incompatible with V2 because V2 Defined Text Set is not compatible with V1 Accept Text Set. Because the V1' Accept Text Set is more relaxed than the V2 Accept Text Set, the addition of the middle name between the first and last is actually a compatible change using partial understanding!
The use of partial understanding has changed an incompatible change into a compatible change. Extrapolating from this, the more that partial understanding is used, the more changes will be compatible rather than incompatible.
A language designer will usually not know which processors consume which subsets of the Defined Text Set. Effectively, it will not know whether consumers are V1-, V1'-, or V2-compliant. Generally, a processor should produce a text that is valid to the V1 Defined Text, rather than the V1' Defined Text Set.
The previous three examples show a mixture of compatible and incompatible evolution. In one instance, the addition of a new term is an incompatible change. In two others, it is a compatible change because the addition is in the expected extension area, or because of partial understanding.
Conclusion
Languages can be compatibly versioned successfully if the first version of a language defines an Accept Text Set that is a superset of the Defined Text Set, as well as a substitution rule for transforming texts in the Accept Text Set into the Defined Text Set. After that, a compatible change can be made if a second version of the language must have a Defined Text Set larger than the first version and an Accept Text Set smaller than the first. Partial understanding increases compatibility by the creation of an Accept Text Set that is a superset of the first version that accepts a subsequent Text.
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- Missing single quotes
2007-01-02 13:53:33 David Orchard [Reply]
Nick, you are absolutely right. All of the trailing single quotes got dropped during publication. I've asked for them to be added in and that should result in an article that conveys the right intent.
- Track
2007-01-01 04:34:33 rjelliffe [Reply]
Some comments on this at
http://www.oreillynet.com/xml/blog/2007/01/david_orchards_theory_of_compa.html
Rick
- Compatibility == Interoperability
2006-12-27 13:49:33 nickgall [Reply]
One more thing. Just to ride my hobby-horse one more time, wouldn't you agree that everything you say in this discussion regarding compatibility also applies to interoperability between two vendor's "languages". V1 could be Vendor One's implementation of the Name Language and V2 could be Vendor Two's extended implementation. The two vendors implementation of contact functionality would be interoperable if they adopted the specifications illustrated in Figure 5. Another way to make the point about compatibility being equivalent to interoperability is just to think of versions across time (first and second) as simply verisons by different vendors (Vendor "First" and Vendor "Second").
-- Nick
- Compatibility == Interoperability
2007-01-03 14:08:19 David Orchard [Reply]
Nick, you are absolutely right on the point about compatibility == interoperability. I have gone to considerable lengths to describe languages in terms of sets that are independent of production and consumption, ie V1 and V2 Accept, rather than in terms of V1 Producer and V1 Consumer.
It is very likely that many of the Consumers and Producers will have different definitions of the Language. Taking this to it's fullest suggests using a non-linear versioning, that is foo and bar and baz Accept (note that even X and Y Accept seems to imply linear versioning). Then the discussion would be around the set relationships between foo, baz, and bar and these could be Consumer 1, Producer 1 and Producer 2 or any other combination.
- Compatibility == Interoperability
- Typos?
2006-12-27 13:35:51 nickgall [Reply]
Dave,
Like the approach in the article, but I think it is marred by some serious typos (or else I am just not getting it at all <grin>). For example, "In this particular case, we see that V1 and V2 are compatible with V1 because the V1 Accept Set is a superset of V1 and V2." Shouldn't it be "In this particular case, we see that V1 and V2 are compatible with V1' because the V1' Accept Set is a superset of V1 and V2"?
Also, when you first introduce V2, in Figure 4 you define "V2 Accept[sic] first, last, ?middle, *" (missing a colon after Accept) and "V2 Defined: first, middle, last". $64K Question: If V2 Accept requires "middle" AFTER last, how can V2 Defined require "middle" BEFORE "last"?!
These are just a couple of things that have me VERY confused.
-- Nick
