The Anatomy of Interoperability

I’d like to talk a little about about interoperability. Although I’ll concentrate on document formats, the concepts here are of wider applicability. Wherever you have standards and implementations, these are some of the issues that you will want to consider.

Rather than attempting the reckless frontal assault of defining interoperability, I’ll take an oblique approach and discuss the forces that tend to increase or decrease interoperability. By understanding these forces, and how they interact, you can get a good sense of how interoperability works, and have a better idea how the decisions that are being made today will determine the interoperability consumers will experience in the years to come.

I’ll start with the specification itself, the text of the standard. What characteristics might a specification have that hinder interoperability?

1. Ambiguities — The specification may describe a feature in a way that is open to more than one interpretation. This may be caused by imprecise language, or by incomplete description of the feature. For example, if a specification defines a sine and cosine function, but fails to say whether their inputs are in degrees or radians, then this function is ambiguous.
2. Out of scope features — The specification totally lacks description of a feature, making it out of scope for the standard. For example, neither ODF nor OOXML specifes the storage model, the syntax or the semantics of embedded scripts. If a feature is out of scope, then there is no expectation of interoperability with that feature.
3. Undefined behaviors — These may be intentional or accidental. A specification may explicitly call out some behaviors as “undefined”, “implementation-dependent” or “implementation-defined”. This is often done in order to allow an implementation to implement the feature in the best performing way. For example, the size of integers are implementation-defined in the C/C++ programming languages, so they are free to take advantage of the capabilities of different machine architectures. Even a language like Java, which goes much further than many to ensure interoperability, has undefined behaviors in the area of multi-threading, for performance reasons. There is a trade-off here. A specification that specifies everything and leaves nothing to the discretion of the implementation will be unable to take advantage of the features of a particular platform. But a specification that leaves too much to the whim of the implementation will hinder interoperability.
4. Errors — These may range from typographical errors, to incorrect use of control language like “shall” or “shall not”, to missing pages or sections in the specification, to inconsistency in provisions. If one part of the specification says X is required, and another says it is not, then implementations may vary in how feature X is treated.
5. Non-thematic provisions — If a standard is claimed to be platform-neutral or application-neutral, then anything that is tied to a particular platform or application will hinder that type of interoperability.
6. Feature Creep — A standard can collapse under its own weight. There is often a trade-off between expressiveness of a standard (what features it can describe) and the ease of implementation. The ideal is to be very expressive as well as easy to implement. If a standard attempts to do everything that everyone could possibly want, and does so indiscriminately, then the unwieldy complexity of the standard will make it more difficult for implementations to implement, and this will hinder interoperability.

The forces that help counteract these problems are:

1. Public participation — The public gets involved early, reading drafts of the standard before it is approved, and also meeting minutes and discussion list traffic. The public provides comments to the technical committee and reads the comments submitted by others. Note: the mere theoretical ability of the public to participate results in absolutely no benefit to the standard. This is like a beta program that exists in theory only will uncover zero bugs. What counts is the actual participation. Openness is a means to participation, but is insufficient on its own.
2. Expert review — The feature set for a modern word processor or spreadsheet is large and requires interdisciplinary expertise in such diverse areas as vector graphics, text layout, mathematical layout, accessibility, internationalization, schema design, statistical and financial calculations, etc. So the design of a file format will similarly require a broad range of expertise. Tapping into outside experts (for example, from member companies or other technical committees) when designing the standard, or as reviewers, is a way to reduce the types of problems that hinder interoperability.
3. Multi-vendor participation — It is good to have multiple active participants in the standards development process, representing implementors both proprietary and open source, both traditional implementations as well as web-based. You might consider other constituencies as well, such as end-users, accessibility advocates, academic and government representatives, consumer advocates, etc. There is a saying familiar to programmers, “Every class of users finds a new class of bugs”. This is as true with standards as it is will code. More eyeballs, especially with different perspectives, is key. It is better to have 5 active participants with different perspectives than to have 100 participants from a single vendor. Again, I’m talking reality here, not just perception. It is easy to have a technical committee that is large on paper, but still have all of the ideas come from a single company. This does nothing for interoperability. You want to have really multi-vendor participation. This is why having public read-only access to the mailing list of the technical committee is so valuable. That is great way to see what the real participation level is.
4. Multiple implementations — If a standard is intended to be platform-neutral and vendor-neutral, then the participation of multiple implementors working on multiple platforms is essential. They are the ones who truly drive interoperability. Especially if they are implementing while the specification is under development or review, they will find, report and resolve many interoperability issues.
5. Reuse of existing standards — When writing a program, the availability of reusable code or design patterns can improve quality and shorten schedules. In manufacturing, the use of off-the-shelf components can be faster to tool and be less expensive than custom components. Similarly, in standards development, when you reuse another standard you reuse the domain expertise, the standards development effort and the review effort that went into the development of that standard. If you are lucky and you reuse a widely-implemented standard, implementors will likely also be able to reuse knowledge and even code when implementing it.
6. Existence of a reference implementation and test suite — These work, together with the standard itself, as a triad of verifications that reinforce each other. The test suite is written to test the provisions of the specification. The reference implementation is written to implement the provisions of the specification. The test suite is executed against the reference implementation. Any errors indicated are thus either errors in the reference implementation, the specification, or the test suite. Fix the bug and repeat until the reference implementation passes the test suite. Having a reference implementation that passes a test suite that has high or complete coverage of the provisions of the specification is ideal for interoperability.
7. Good engineering — In the end, a technical standard is technical, and it will be influenced by the same kinds of engineering criteria that influence how we architect and design software applications. If you can make a standard that is modular, exhibits internal consistency in its parts, is cohesive, has reasonable dependency chains among its parts, does what it set out to do with minimal complexity, but allows of uses beyond that, then you’ve made a good start. But if you find yourself shaking your head, slapping your forehead or even holding your nose when reading a standard, then you are heading for interoperability hell. The standards that have worked well were as simple as possible, but no simpler. Think XML 1.0 or XSLT 1.0. Engineers know how to build complexity on top of simplicity. But you don’t build anything on top of crap.

Of course, the standard is only one half of the equation. The implementations are the other half, and they have their own list of ways they can hinder or help interoperability.

The factors that hinder interoperability include:

1. Implementation bugs — Conformance to a standard, like any other product feature, gets weighed against a long list of priorities for any given product release. There is always more work to do than time to do it. Whether a high-quality implementation of a standard becomes a priority will depend on factors such as user-demand, competition, and for open source projects, the level of interest of developers contributing to the community.
2. Functional subsets — Even in heavily funded commercial ventures standards support can be partial. Look at Microsoft’s Internet Explorer, for example. How many years did it take to get reasonable CSS2 support? When an application supports only a subset of a standard, interoperability with applications that allow the full feature set of the standard, or a different subset of the standard, will suffer.
3. Functional supersets — Similarly, an application can extend the standard, often using mechanisms allowed and defined by the standard, to create functional supersets that, if poorly designed, can cause interoperability issues.
4. Varying conceptual models — For example, a traditional WYSIWYG word processor has a page layout that is determined by the metrics of the printer the document will eventually print to. But a web-based editor is free from those constraints. In fact, if the eventual target of the document is a web page, these constraints are irrelevant. So we have here a conceptual difference, where one implementation sees the printed page as a constraint on layout, and another application is in an environment where page width is more flexible. Document exchange between two editors with different conceptual models of page size will require extra effort to ensure interoperability.
5. Different code bases — The more distinct implementations, the more paths of exchange. If you have only a single implementation of your standard, then there will be no interoperability issues, at least not with others using the same version of your application on the same platform. But if there are two implementations, then you have 2 paths of exchange to worry about. If there are 3 implementations then you have 6 paths, and so on with N*(N-1) possible paths for exchanging files with N applications. Consider the ODF format for word processing documents, where we will soon have implementations in Office (at least two different Plugins), OpenOffice, Lotus Notes, KOffice, Google Docs and Abi Word. This is 42 exchange paths (or more if you consider platform variations) though obviously not all of these paths will be common.

And the forces that counteract these and lead to improved interoperability are:

1. Test suites and reference implementations — As described earlier, these are key. During the development of the standard they improve the standard, and after the standard is approved they continue paying dividends by helping improve the quality additional implementations.
2. Conformance certification — This can take several forms, from self-testing and self-certifying, to more formal tests by 3rd party labs. The result is a compliance report, listing which features of the standard were correctly implemented, and which were not. Having this information available to consumers can help interoperability to the extent it allows them to narrow their consideration to the applications that most fully and correctly implement the standard.
3. Profiles — This is a formal mechanism for defining an interoperable subset of a standard. For example, XHTML Basic is a W3C-defined subset of XHTML for use on mobile devices. By defining such a subset, interoperability is enhanced in several ways. First, a single web service can produce XHTML Basic that is understood by mobile devices from several vendors. Second, XHTML Basic is a proper subset of XHTML, so it is also interoperable with all the tools that accept XHTML.
4. Adaptive extension mechanisms — It is good to have a formal mechanism for extending the specification while at the same time allowing a graceful recovery in the case where the document is loaded in an application that doesn’t understand the extension.
5. Inter-vendor cooperation — Make no mistake. Once an interoperability problem is found, where application A and application B are not rendering the same document in an acceptably similar fashion, then we have a bug. Such a bug may be reported by the customers of application A or the customers of application B. Ideally, A and B will cooperate and share information on interoperability bugs found. The customer that reports A’s bug to company B, may tomorrow be matched by another that reports B’s bug to customer A. So some sort of interoperability bug clearinghouse or other protocols can help here.

I may have missed some important factors that help or hinder interoperability. Please share your ideas.

3/29/07 — Corrected error sent in by an anonymous reader. The number of interoperability concerns scale as N(N-1), not as N!.