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Criteria for evaluating specifications

As Andrew Tannenbaum said, "The nice thing about standards is that there are so many to choose from." Apparently he followed this up by: "And if you really don't like all the standards you just have to wait another year until the one arises you are looking for."

So how does one choose? Why adopt one specification and not another? There are many possible reasons, and this article is an attempt to catalog some of them.

In the course of my work at Infrae, I run into many specifications that we may decide to implement or use, so having a common vocabulary and set of questions about this is important to us.

Note that I use the word 'specification', not 'standard' in this article, as there are far more specifications than true industry standards. Many so-called "standards" are only aiming to become so, but naturally many fail in this process due to a variety of reasons.

What is specified?

Specifications fall into a number of categories. A non-exhaustive list:

  • protocol (TCP/IP, HTTP, FTP, XML-RPC, SOAP, WebDAV)
  • document format ('pure' XHTML, Docbook, TEI, many custom XML document formats)
  • visual design ('pragmatic' XHTML, CSS, SVG, XSL-FO)
  • metadata (RDF, Topic Maps, RSS, Dublin Core)
  • query/programming language (SQL, XPath, XSLT, XQuery, C, C++, Java, C#, etc)
  • API (DOM, SAX, COM specification for MS Office interoperability, UNO specification for OOo API)
  • structural, foundational (XML, RDF)
  • prescriptive/descriptive (DTD, XML Schema, Relax NG, RDF Schema)

Questions to ask when encountering a new specification

Industry adoption

How well is the specification adopted within the industry? I.e., is this specification industry-standard or seems to be on the way to be so? There are a lot of specifications that are really not standards. Industry adoption has two sides to it:

  • Systems that implement the specification.
  • Systems that actually make us of the specification implementation to accomplish specific use cases.

If there are few of the former, there won't be many of the latter. An example is SVG in browsers. Even though it is expected that the uptake of SVG technology will be big once it is established in popular browsers, this uptake is not taking place right now as the browsers don't support it very well. In other areas, like the Linux desktop, SVG is seeing some use now.

Even though systems that conform to the specification may be implementable fairly easily, it may be that few systems exist that actually use this for useful purposes. RDF has a problem like this; even though it is relatively easy to produce some RDF, and tools exist that can then take this RDF and do some analysis, there are actually few systems that use this RDF to fulfill practical use cases. RSS is an exception, but only a partial one, as only few RSS streams are actually fully RDF compliant.

Implementation quality

How well is the specification implemented within the industry? If a specification is well implemented, this helps interoperability between software. CSS is an example of a specification that is implemented commonly by browsers, but not all to the same level of quality. The practical result of this is that the same CSS needs to be tested on all browsers, and pragmatic subsets and tricks are developed in order to work around limitations in the implementation on some platforms.

The DOM specification has been implemented quite a few times in Python, but very little Python software actually works across Python DOM implementations, because the implementations differ a lot in quality, supported feature set, and there is a lot of inconsistency in the translation to Python of the various DOM APIs. In contract, because of more rigidly defined and checked interfaces in Java, this language gains more interoperability between its various DOM implementations. Another example is Javascript-based browser DOMs. While many inconsistencies exist, the browser DOM still allows cross-browser applications to be developed.

XSLT is an example of a specification that seems to be well implemented across many platforms, probably due to the clarity of the specification, the limited scope, and extensive conformance test suites.

Market demand

How much does the market (or a particular market, or a particular client) want to adopt the specification as a standard?

Even in cases where industry adoption is still limited few implementations exist, significant forces in the market may still very well want implementations. They may want this for rational reasons, or due to industry hype, or a combination of both. One good reason for adoption of a specification is its popularity, so industry hype may in fact be a worthwile reason to adopt a specification.

Hype may exist in limited markets. Most people for instance haven't heard of SCORM, but it does have some buzz in the education market.


How much buzz is there in the industry and market? Are vendors promising support? Are there high visibility projects? Glowing magazine articles?


Does the specification fit the actual problem domain? XSLT can be used well for templating and transformations, but it's not very well suited for writing whole applications.

Even if a specification is not optimally suited to a problem domain, other factors may still cause it to be adopted -- increased interoperability for instance, or reduced learning time.


How easy is the specification to implement? There are a whole set of reasons why a specification would be easier or harder to implement.

  • How clear is the specification? Is the specification very readable by a software developer?
  • How complete is the specification? Are there areas essential for interoperability which are glossed over?
  • How big is the specification? If all other factors stay the same, the larger specification is harder to implement.
  • How conceptually pure is the specification? Does the specification specify a hodge-podge of features or is everything reducible to basic principles? The more pure a specification is, the easier it tends to be to implement.
  • Do test suites exist to verify conformance? How extensive are these test suites? If these are lacking, successful implementation becomes a lot harder.
  • Do proof of concept implementations to server as developer examples? Often an implementation in software actually clarifies many of the details that may otherwise be left undescribed.
  • Does the specification have dependencies on other ones? A specification may have a requirement on one or more other specifications that are not commonly implemented.
  • Is the specification tied to a particular operating environment or programming language? If there are many requirements before a specification can be implemented, this may limit the implementability.
  • On the other hand, a specification that tries to be very general may turns out to be harder to implement as well, as it may be more complicated as a result.


Will implementing a specification in fact increase interoperability?

There are a number of categories of interoperability:

  • Interoperability between client and server, swap out server. Here the server implements the specification (and the client makes use of it). As an example, HTTP makes it easy to swap out one webserver for other one, while the same client (a browser) is used for both.
  • Interoperability between client and server, swap out client. Here the client implements the specification, and the server produces something that fits the specification. HTML and CSS make it easier to swap out the client (browsers such as Mozilla, IE, Konqueror, Opera) as each implement HTML and CSS, even though the server may always be the same.
  • Interoperability between "peers". XML makes it easier for one system to read basic structured information coming from another, as they can make use of well established XML processing tools.

The more difficult it is to implement a specification (correctly or completely), the fewer the interoperability benefits.


Will the implementation specification likely make it easier for the developer to learn the system (or learn how to interoperate with the system)?

  • If the specification is popular, it's more likely there's plenty of reading material about it that the developers heard about. There's also a bigger chance the developer already heard about it, or used it in the context of another system. XML is the traditional example of how this works. XSLT is another good example, though only in the last few years, now that it has been commonly implemented and much documentation exists.
  • Implementability and learnability are related. If the specification is for some reason difficult to understand (difficcult to read, hugely complicated, badly written), it will be more difficult for a developer to understand it in other to use it.

If only everybody

How much of a network effect is needed to make implementing or using a specification useful? If first everybody needs to implement the specification to actually make its use worthwhile, a chicken and egg problem exists.

An example of this is the semantic web. If only everybody had their web systems produce large amounts of semantic information in the form of RDF, it'd be very useful in many ways and useful tools would be created which process this information. Unfortunately nobody is doing this as there are no tools to process the information.

Reasons for usefulness

Specifications can be useful for a number of reasons:

  • Interoperability. This is the obvious one. Your system can use another system that implements the specification, or vice versa. Implementability is the main factor that determines whether this can be accomplished, and popularity determines whether this is worthwhile (see also tool leverage).
  • Attracting developers. We expect many developers to be already be familiar with the specification, or alternatively, there are copious resources for developers to familiarize themselves with a specification. Implementing the specification makes our system easier to learn and develop for. Learnability and popularity are important factors here.
  • Reuse of design. Can we take useful ideas from a specification? Even if an interoperable specification is not developed, we may still be helped as we may not have to reinvent the wheel. Learnability is the most important factor here.
  • Tool leverage. By implementing the specification, other existing toolsets and libraries can be used to extend the feature set of the software. Important factors here are popularity; lots of other systems that build on top of this specification will exist that now become useful.
  • Marketing. Implementing the specification will provide us with buzzword compliant "checkbox features" for marketing. This will be helped by popularity (at least perceived popularity). The specification itself might be pure nonsense, or our implementation may be bad, it can still work, at least for a while until everybody finds out.


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