[Editorial Draft] Extending and Versioning XML Languages Part 1

1.1 Terminology

The terminology for describing languages, namespaces, constraints, evolvability etc. follows. Let us consider an example. Two or more systems need to exchange name information. Names may not be the perfect choice of example because of internationalization reasons, but it resonates strongly with a very large audience. The Name language is created to be exchanged. [Definition: A producer is an agent that creates text. ] Continuing our example, Fred is a producer of Name language text. [Definition: A Production is the creation of text.]. A producer produces text for the intent of conveying information. When Fred does the actual creation of the text, that is a production. [Definition: A consumer is an agent that consumes text.] We will use Barney and Wilma as consumers of text. [Definition: A Consumption is the processing of text of a language.] Wilma and Barney consume the text separately from each other, each of these being a consumption event. A consumer is impacted by the instance that it consumes. That is, it interprets that instance and bases future processing, in part, on the information that it believes was present in that instance. Text can be consumed many times, by many consumers, and have many different impacts.

Generally, a language has one or more vocabularies that each may have multiple terms. Formally, [Definition: a language is an identifiable set of vocabulary terms with defined syntactic and semantic constraints. ] By language, we mean the set of text that are members of that language and used by a particular application. [Definition: A vocabulary is a set of terms]. The Name language consists of 3 terms: name, given, family. In order to identify the terms in the Name language in XML, a namespace is assigned to the terms. Other examples include the elements and attributes of XHTML 1.0 or the names of built-in functions in XPath 2.0. The Name language could consist of terms from other vocabularies, such as Dublin Core or UBL. These terms each have their own namespaces, illustrating that a language can comprise vocabularies from multiple namespaces.

The name language takes the 3 terms and specifies the constraints: that a name consists of a given and a family. [Definition: A language has a set of constraints that apply to the vocabulary terms in the language. ] These constraints can be defined in machine processable syntactic constraint languages such as XML Schema, human readable textual descriptions such as HTML descriptions, or are embodied in software. Languages may or may not be defined by a schema in any particular schema language. The constraints on a language determine the strings that qualify for membership in the language. There is a close relationship between terms and strings that we have not chosen to call out in an effort to keep the terminology as minimal as possible. From a text or string perspective, a language is the set of strings that are part of the language. Vocabulary terms contribute to the set of strings, but they are not the only source of characters to the set of strings in a given language. The language strings may include characters outside of terms, such as punctuation. One reason for additional characters are to distinguish or separate terms, such as whitespace and markup. Languages can also relate to the interpretation of a given string, that is 2 languages may have the exact same string but different meanings. We have chosen to elide that notion from our terminology

<name>
  <given>Dave</given>
  <family>Orchard</family>
</name>

name="Dave Orchard" 

<span class="fn">Dave Orchard</span>

urn:nameschem:given:Dave:family:Orchard

In general, the intended meaning of a vocabulary term is scoped by the language in which the term is found. However, there is some expectation that terms drawn from a given vocabulary will have a consistent meaning across all languages in which they are used. Confusion often arises when terms have inconsistent meaning across language. The Name terms might be used in other languages, but it is generally expected that they will still be "the same" in some meaningful sense.

[Definition: Text is a specific, discrete sequence of characters]. Given that there are constraints on a language, any particular text may or may not have membership in a language. Indeed, a particular string of characters may be a member of many languages, and there may be many different strings of characters that are members of a given language. The text of the language are the units of exchange. Documents are texts of a language.

These terms and their relationships are shown below

There are many different systems for exchanging texts in languages, such as SQL, Java, XML, ECMAScript, C#. We will briefly describe some key refinements to our lexicon for XML. An XML language has a vocabulary that may use terms from one or more XML Namespaces (or none), each of which has a namespace name. [Definition: An XML language is an identifiable set of vocabulary terms with defined XML syntactic and semantic constraints. ] By XML language, we mean the set of elements and attributes, or instances, used by a particular application. The Name language - consisting of name, given, family - has a namespace is assigned to the terms. We use the prefix "namens" to refer to that namespace. The Name language could consist of terms from other vocabularies, such as Dublin Core or UBL. These terms each have their own namespaces, illustrating that a language can comprise vocabularies from multiple namespaces. An XML Namespace is a convenient container for collecting terms that are intended to be used together within a language or across languages. It provides a mechanism for creating globally unique names.

We shall use the term instance when speaking of sequences of characters (aka text) in XML. [Definition: An instance is a specific, discrete sequence of terms]. Documents are instances of a language. In XML, they must have a root element. A name document might be a name element as the root element. Alternatively, the name vocabulary may be used by a language such as purchase orders. The purchase order documents may contain name elements. Thus instances of a language are always part of a document and may be the entire document. XML instances (and all other instances of markup languages) consist of markup and content. In the name example, the given and family elements including the end markers are the markup. The values between the start and end markers are the content. An instance has an information model. There are a variety of data models within and without the W3C, and the one standardized by the W3C is the XML infoset.

The XML related terms and their relationships are shown below

A stylesheet processor is a consumer of the XML document that it is processing (the producer isn't mentioned); in the Web services context the roles of producer and consumer alternate as messages are passed back and forth.Note that most Web service specifications provide definitions of inputs and outputs. By our definitions, a Web service that updates its output schema is considered a new producer. A service that updates its input schema is a new consumer.

Versioning is an issue that effects almost all XML applications eventually. Whether it's a processor styling documents in batch to produce PDF files or Web services engaged in financial transactions, the language and instances will likely change over time. The versioning policies for a namespace, particularly whether the namespace is mutable or immutable, should be specified by the namespace owner. Versioning is closely related to extensibility as extensible languages may allow different versions of instances than known by the language designer. Applications may receive versions of a language that they aren't expecting.

We now return to our discussion of languages in general. Extensibility is a property that enables evolvability of software. It is perhaps the biggest contributor to loose coupling in systems as it enables the independent and potentially compatible evolution of languages. Languages are defined to be [Definition: Extensible if instances of the language can include terms from other vocabularies.]. The name language is extensible if it can include terms from other vocabularies, like a new middle term.

As languages evolve, it becomes possible to speak of both backwards and forwards compatability. We base our definitions of backwards and forwards compatibility on the [FOLDOC] definitions. There are two aspects of compatibility that are called out: software compatibility and schema compatibility. While it is often the case that they are directly related, sometimes they are not.

[Definition: A language change is backwards compatible if newer processors can process all instances of the old language. ] Backwards compatibility means that a newer version of a consumer can be rolled out in a way that does not break existing producers. A producer can send an older version of a message to a consumer that understands the new version and still have the message successfully processed. A software example is a word processor at version 5 being able to read and process version 4 documents. A schema example is a schema at version 5 being able to validate version 4 documents. In the case of Web services, this means that new Web services consumers, ones designed for the new version, will be able to process all instances of the old language. This means that a producer can send an old version of a message to a consumer that understands the new version and still have the message successfully processed.

[Definition: A language change is forwards compatible if older processors can process all instances of the newer language.] Forwards compatibility means that a newer version of a producer can be rolled out in a way that does not break existing consumers. Of course the older consumer will not implement any new behavior, but a producer can send a newer version of an instance and still have the instance successfully processed. An example is a word processing software at version 4 being able to read and process version 5 documents. A schema example is a schema at version 4 being able to validate version 5 documents. In the case of Web services, this means that existing Web service consumers, designed for a previous version of the language, will be able to process all instances of the new language. This means that a producer can send a newer version of a message to an existing consumer and still have the message successfully processed.

In general, backwards compatibility means that existing documents can be used by updated consumers, and forwards compatibility means that newer documents can be used by existing consumers. Another way of thinking of this is in terms of message exchanges. Backwards compatibility is where the consumer is updated and forwards compatibility is where the producer is updated, as shown below:

Example 2: Evolution of Producers and/or Consumers

In broad terms, backwards compatibility means that newer producers can continue to use existing services, and forwards compatibility means that existing producers can use newer services

One form of compatibility is with respect to the strings, that is whether all the strings in one language are also strings in another language. Another form of compatibility is with respect to the information conveyed, that is whether the information conveyed by the strings in one language is conveyed by the same strings in another language. The strings could be compatible but the information conveyed is not compatible.

We can show the terminology diagram with instances. Fred produces, aka creates, an instance of the name language. The Name instance is in the set of strings that have membership in the Name Language version 1. Barney is a consumer of the Name instance. The particular consumption of the instance has an impact on Barney of giving him Name information. The consumption of the instance is with respect to the Name language V1 that Barney knows. Wilma is also a consumer of the Name instance. The particular consumption of the instance has an impact on Wilma of giving her Name information. The consumption of the instance is with respect to the Name language V2 that Wilma knows. Name Language V2 relates to V1 by relationship r1, which is forwards compatible comparing language V1 to V2 instances, and backwards compatible comparing language V2 to V1 instances. Similarly, the information 2 - as conveyed by consumption 2 - relates to information 1 - as conveyed by consumption 1 - by relationship r2.

Compatibility is defined for the producer and consumer of an individual document instance. Most messaging specifications, such as Web Services, provide inputs and outputs. Using these definitions of compatibility, a Web service that updates its output message is considered a newer producer. If a Web service updates the output message, then it is sending a newer version of the message, hence it is considered a newer producer. Conversely, updating the input message makes the service a newer consumer.

The cost of changes that are not backward or forward compatible is often very high. All the software that uses the language must be updated to the newer version. The magnitude of that cost is directly related to whether the system in question is open or closed.

[Definition: A closed system is one in which all of the producers and consumers are more-or-less tightly connected and under the control of a single organization.] Closed systems can often provide integrity constraints across the entire system. A traditional database is a good example of a closed system: all of the database schemas are known at once, all of the tables are known to conform to the appropriate schema, and all of the elements in the each row are known to be valid for the schema to which the table conforms.

From a versioning perspective, it might be practical in a closed system to say that a new version of a particular language is being introduced into the system at such and such a time and all of the data that conforms to the previous version of the schema will be migrated to the new schema.

[Definition: An open system is one in which some producers and consumers are loosely connected or are not controlled by the same organization. The internet is a good example of an open system.]

In an open system, it's simply not practical to handle language evolution with universal, simultaneous, atomic upgrades to all of the software components. Existing producers and recievers outside the immediate control of the organization that's publishing a changed language will continue to use the previous version for some (possibly long) period of time.

Finally, it's important to remember that systems evolve over time and have different requirements at different stages in their life cycle. During development, when the first version of a language is under active development, it may be valuable to persue a much more aggressive, draconian versioning strategy. After a system is in production and there is an expectation of stability in the language, it may be necessary to proceed with more caution. Being prepared to move forward in a backwards and forwards compatible manner is the strongest argument for worrying about versioning at the very beginning of a project.

1.2 Why Worry About Extensibility and Versioning?

As documents, or messages, are exchanged between applications, they are processed. Most applications are designed to discriminate between valid and invalid inputs. In order to have any sort of interoperability, a language must be defined or described in some normative way so that the terms "invalid" and "valid" have meaning.

There are a variety of tools that might be employed for this purpose (DTDs, W3C XML Schema, RELAX NG, Schematron, etc.). These tools might be augmented with normative prose documentation or even some application-specific validation logic. In many cases, the schema language is the only validation logic that is available.

It is almost unheard of for a single version of a language to be deployed without requiring some kind of augmentation. Invariably, the original language designer did not include certain terms and constraints. In fact, good designers should not try to define all the possible terms and constraints. This is sometimes called "boiling the ocean". Knowing that a language will not be all things to all people, a language designer can allow parties to extend instances of the language or the language itself. Typically the tools will allow the language designer to specify where extensions in the instance and extensions in the language are allowed. Of note, we do not call extending an instances of a language a new version. This scopes our discussion of versioning to changes in a language, not changes to instaces.

Whether you've deployed ten services, or a hundred, or a million, if you change a language in such a way that all those services will consider instances of the new language invalid, you've introduced a versioning problem with real costs.

Once a language is used outside of its development environment, there will be some cost associated with changing it: software, user expectations, and documentation may have to be updated to accomodate the change. Once a language is used in environments outside of a single realm of control, any changes made will introduce multiple versions of the language.

1.3 Why Extend languages?

The primary motivation to allow instances of a language to be extended is to decentralize the task of designing, maintaining, and implementing extensions. It allows producers to change the instances without going through a centralized authority. It means that changes can occur at the producer or consumer without the language owner approving of them. Consider the effort that the HTML Working Group put into modularity of HTML. Without some decentralized process for extension, every single variant of HTML would have to be called something else or the HTML Working Group would have to agree to include it in the next revision of HTML.

1.4 Why Do Languages Change?

There are many reasons why a different version of a language may be needed. A few of them include:

1. Bugs may need to be fixed. Production use may reveal defects or oversights that need to be fixed. This may involve changes to components of the language or changes to the semantics of existing components.

2. Changing requirements may motivate changes in the schema design. For example, a callback may be added to a service that performs some processing so that it is able to notify the caller when processing has completed.

3. Different flavors of a schema may be desirable. For example, the XHTML 1.0 Recommendation defines strict, transitional, and frameset schemas. All three of those schemas purport to define the same namespace, but they describe very different languages.

   And additional schemas may be defined by other specifications, such as the XHTML Basic Recommendation.

Whatever the cause, over time, different versions of the language exist and designing applications to deal with this change in a predictable, useful way requires a versioning strategy.

1.5 How Do Languages Change?

REWRITE STARTS HERE. There will be discussion of non-xml specific language changes.

At the most basic level, languages can change in only a few ways:

• Content: The allowable content can evolve through addition or deletion. In XML, this becomes

  • ElementsNew elements can be added, existing elements can be removed, or the acceptable number of occurences of an element can change. In addition, the content of an element could change from element only content to mixed content, or vice versa.

    For elements with simple content, the type or range of values that are acceptable can change.

  • Attributes: New attributes can be added, existing attributes can be removed, or the type or range of values that are acceptable can change.

• Semantics: The meaning of a an existing element or attribute can change.

Of course, the difference between two versions of a language can be an arbitrary number of these changes.

One of the most important aspects of a change is whether or not it is backwards or forwards compatible.

Some typical backwards- and forwards-compatible changes:

• adding optional components (elements and/or attributes)

• adding optional content, for example extending an enumeration

Some typical forwards-compatible changes:

• Decreasing the maximum allowed number of occurrences of an element but not to less than the minimum

• Decreasing the allowed range of an element or attribute

Some typical backwards-compatible changes:

• Increasing the maximum allowed number of occurrences of an element

• Increasing the allowed range of an element or attribute

Some typical incompatible changes:

• changing the meaning or semantics of existing components

• adding required components

• removing required components

• restricting a components content model, such as changing a choice to a sequence

1.6 Kinds of Languages

Ultimately, there are different kinds of languages. The versioning approaches and strategies that are appropriate for one kind of language may not be appropriate for another. Among the various kinds of vocabulares, we find:

• Just Names: some languages don't actually identify elements or attributes, they're just lists of names. Using QNames to identify words in the WordNet database, for example, or the names of functions and operators in XPath2 are examples of "just name" languages.

• Standalone: languages designed to be used more-or-less by themselves, for example XHTML, DocBook, or The TEI.

• Containers: languages designed to be used as a wrapper or framework for some other language or payload, for example SOAP or WSDL.

• Container Extensions: languages designed to extend or augment a particular class of container. Specifications that extend SOAP by defining SOAP header blocks, for example, to provide security, asynchrony or reliable messaging are examples of container extension languages.

  There are a couple types of extension languages, element extension and attribute extension.

  • Element Extension. Languages that are elements. SOAP, etc. are element extensions.

  • Attribute or type Extensions. Langages that are types or attributes. These languages must exist in the context of an element. Sometimes called "parasite" languages as they require a "host" element. XLink is an example.

• Mixtures: languages designed for, or often used for, encapsulating some semantics inside another language. For example, MathML might be mixed inside of another language.

This is by no means an exhaustive list. Nor are these categories completely clear cut. MathML can certainly be used standalone, for example, and languages like SVG are a combination of standalone, containers, and mixtures.

2.1 Can 3rd parties extend the language?

It is rarely desirable to prevent 3rd parties from extending languages, but it does happen. An example may be a tightly constrained security environment where distributed authoring is considered a _bug_ rather than a feature.

2.2 Can 3rd parties extend the language in a compatible way?

If so, a substitution mechanism is required for forwards compatibility. If an older consumer has no mechanism for dealing with new content, then forwards-compatible evolution isn_t possible. One simple substitution mechanism is simply ignoring the unrecognized components.

2.3 Can 3rd parties extend the language in an incompatible way?

If so, and if compatible extensions are also possible, then it must be possible to identify incompatible changes so that they can override the substitution mechanism used for extensible changes.

In environments where unrecognized components are ignored, a _must understand_ component can be added to identify incompatible changes.

If compatible changes are not possible, then incompatible changes simply become the default. For example, WS-Security mandates that 3rd parties can only provide incompatible extensions. Unlike most languages, a security language has unique requirements where the consequences of ignored data can be severe. WS-Security accomplished this by specifying that all extensions are required to be understood and there is no substitution mechanism.

2.4 Can the designer extend the language in a compatible way?

As with 3rd parties compatible extensions, a substitution mechanism for the designer_s extensions is required for forwards compatibility.

A question that does not need to be asked is: _Can the designer extend the language in an incompatible way?_ They can always do this by using new namespace names, element names or version numbers.

2.5 Is the vocabulary a stand-alone language or an extension of another vocabulary?

A part of this question is whether the language depends on another language. That determines which, if any, facilities are provided for the containing language and what must be provided by a contained language.

SOAP is an example of a container language. The SOAP processing model applies uniformly to all headers, which may employ soap:mustUnderstand to identify incompatible changes, even though the contents of the SOAP headers are languages independent from SOAP.

2.6 What Schema language(s)?

Choosing a schema language or languages guides the language design in many ways. Some features, particularly extensibility, must be anticipated in the first version of a language in order to take advantage of the features of some schema languages.

In addition, various features may be incompatible across different languages. For example, writing a V2 compatible schema in W3C XML Schema requires special design, which is not required in a schema language such as RELAX NG. Some of the language design choices mandated by W3C XML Schema are discussed in other sections of this Finding.

2.7 Should extensions or versions be expressible in the Schema language?

The ability to write a schema for extensions or versions is directly affected by the schema design and the compatibility desires.

3.1 Schema language design choices or constraints.

If the language can be extended in a compatible way, then a few specific schema design choices must be followed.

Wildcards are used to provide extensibility in XML Schema. If revisions to the Schema are to support substitution, specific schema designs must be used in conjunction with the wildcard. The main choices are: provide wildcards, provide Extension elements, or provide delimiter elements. Extension and delimiter elements are described in the new components in existing or new namespaces section.   If Extension/delimiter elements are not provided, then a compatible V2 Schema cannot be written.

3.2 Substitution Mechanism.

Forwards compatibility can only be achieved by providing a substitution mechanism for Version 2 instances or Version 1 extensions to V1 without knowledge of V2.  A V1 consumer must be able to transform any instances, such as V1 + extensions, to a V1 instance in order to process the instance. The _Must Ignore unknown_ rule is a simple substitution mechanism. This rule says that any extensions are _ignored_. Using it, a V1 + extensions document is transform into a V1 document by ignoring the extensions. Others substitution mechanisms exist, such as the fallback model in XSLT.

3.3 Component identification

The identification of components into language versions or extensions has a variety of general mechanisms related to namespaces. These are detailed in the Versioning section.

3.4 Identification of incompatible extensions

The identification of versions is covered by language identification, but 3rd parties cannot arbitrarily change versions or change namespaces. They may need a mechanism to indicate that an extension is an incompatible change. A couple of mechanisms are a _Must Understand_ identifier (such as a flag or list of required namespaces) or requiring that extensions are in substitution groups.

7.1 Versioning Strategies

Versioning is a broad and complex issue. Different communities have different notions about what constitutes a version, what constitutes a reasonable policy, and what the appropriate behavior is in the face of deviations from that policy. Historically, it has always proved more complicated in practice than in theory.

In broad terms, the approaches to versioning fall into a number of classes ranging from "none" to a "big bang":

• None. No distinction is made between versions of the language. Applications are either expected not to care, or they are expected to cope with any version they encounter.

• Compatible. Designers are expected to limit changes to those that are either backwards or forwards compatible, or both.

  • Backwards compatibility. Applications are expected to behave properly if they receive an instance document of the "older" version of a language. Backwards compatible changes allow applications to behave properly if they receive an "older" version of the language.

  • Forwards compatibility. Applications are expected to behave properly if they receive an instance document of the "newer" version of a language. Forwards compatible changes allow existing applications to behave properly if they receive a "newer" version of the language.

• Flavors. Applications are expected to behave properly if they receive one of a set of flavors of the document type.

• Big bang. Applications are expected to abort if they see an unexpected version.

There's no single approach that's always correct. Different application domains will choose different approaches. But by the same token, the approaches that are available depend on other choices, especially with respect to namespaces. This dependency makes it imperative to plan for versioning from the start. If you don't plan for versioning from the start, when you do decide to adopt a plan for versioning, you may be constrained in the available approaches by decisions that you've already made.

A language goes through a common lifecycle of iterative development followed by deployment. These place in the lifecycle will affect the selection of the versioning strategy

Just as there are a number of approaches, there are a number of strategies for implementing an approach. The internet - including MIME, markup languages, and XML languages have succesfully used various strategies, either singly or in combination. Summaries of strategies and requirements have been produced for earlier technologies and guided XML Namespaces and Schema, such as [Web Architecture: Extensible Languages].

For any given approach, some strategies may be more appropriate than others. Among the strategies we find:

• Must Understand. consumers must understand all of the elements and attributes received and are expected to abort processing if they do not. SOAP processors must understand headers that are explicitly identified to be mandatory.

• Must Ignore. consumers must ignore elements or attributes that they do not understand. Sometimes the must understand and must ignore approaches can be combined for more selective use. SOAP processors must ignore headers they do not recognize unless the header explicitly identifies itself as one that must be understood.

  There are 2 variations of the Must Ignore strategy:

  • Must Ignore All This variation on must ignore requires the consumer to ignore an element or attribute it does not understand and, in the case of elements, all of the descendents of that element. Most data applications, such as Web services that use SOAP header blocks or WSDL extensions, adopt this approach to dealing with unexpected markup. For XML, the Must Ignore all rule was first standardized in the WebDAV specification RFC 2518 [WebDAV] section 14 and later separately published as the [FlexXMLP].

  • Must Ignore Container. This variation on must ignore requires the consumer to ignore an element or attribute that it does not understand, but in the case of elements, to process the children of that element. The Must Ignore Container practice was described in [HTML 2.0]

• Explicit Fallback. A language can provide mechanisms for explicit fallback if the extension is not supported. [MIME] provides multipart/alternative for equivalent, and hence fallback, representations of content. [HTML 4.0] uses this approach in the NOFRAMES element. In XML, the XML Inclusions specification [XInclude] provides a fallback element to handle the case where the putatively included resource cannot be retreived.

• Explicit Testing. A language can provide a mechanism for explicit testing. The XSLT Specification provides a conditional logic element and a function to test for the existence of extension functions. This allows designers of stylesheets to deal with different consumer capabilities in an explicit fashion.

Languages can choose a mixture of approaches. For example, XSLT provides both an explicit fallback mechanism for some conditions and explicit testing for others. The SOAP specification, another example, specifies Must Ignore as the default strategy and the ability to dynamically mark components as being in the Must Understand strategy.

7.2 Why Have a Strategy?

Different kinds of languages and different versioning strategies expose different problems. If you don't have a strategy at all, you are effectively choosing the "no versioning" strategy.

It's probably obvious that attempting to deploy a system that provides no versioning mechanism is frought with peril. Putting the burden of version "discovery" on consumers is probably impractical in anything except a closed system.

At the other end of the spectrum is the "big bang" approach which is also problematic.

"Big bang" is a very coarse-grained approach to versioning. It establishes a single version identifier, either a version number or namespace name, for an entire document.

The semantics of the "big bang" are that applications decide on the basis of the document version whether or not they know how to process that document. If the version isn't recognized, the entire document is rejected.. Typically, when introducing a new version using the big bang approach, all of the software that produces or consumes the instance documents is updated in a sweeping overhaul in which the entire system is brought down, the new software deployed and the system is restarted. This big bang approach to versioning is practical only in circumstances where there is a single controlling authority, and even in that case, it carries with it all manner of problems. The process can take a considerable amount of time, leaving the system out of commission for hours if not days. This can result in significant losses if the system is a key component of a revenue generating business process and the cost of coordinating the system overhaul can also be quite costly as well.

Another approach to mitigate against a complete overhaul is to run parallel versions of the system, often with proxies or gateways between them. However, this too has its costs as multiple versions of the software must be supported and maintained over time and there is the added cost of developing the proxy or gateway between the two environments.

The "big bang" approach is appropriate when the new version is radically different from its predecessor. But in many cases, the changes are incremental and often a consumer could, in practice, cope with the new version. For example, it might be that there are many messages that don't use any features of the new version or perhaps it is appropriate to simply ignore elements that are not recognized.

For example, consider two services exchanging messages. Imagine that some future version of the language that they are using defines a new "priority" element. Because producers and consumers are distributed, it may happen that an old consumer, one unprepared for a priority element, encounters a message sent by a newer producer.

If big bang versioning is used, old systems will reject the new message. However, if the versioning strategy instead allowed the old consumer to simply ignore unrecognized content, it's quite possible that other components of the system could simply adapt to the previous behavior. In effect, the old system would ignore the priority element and its descendents so it would "see" a message that looks just like the old format it is expecting.

For the producer, the result would be that the request is fulfilled, though perhaps in a more or less timely fashion than expected. In many cases this may be better behavior than receiving an error. In particular, producers using the new format can be written to cope with the possibility that they will be speaking to old consumers.

If the new system needs to make sure that priority is respected, then it can change the purchase order's name or namespace to indicate that the new behavior is not considered backwards compatible.

What is needed is some sort of middle ground solution. An evolving system should be designed with backwards and forwards compatibility in mind.

One approach is to use version numbers, with a goal of using "major version" changes for incompatible developments and "minor version" changes for compatible ones.

Unfortunately, version numbers often wind up looking very similar to the big bang approach. Each language is given a version identifier, almost always a number, that's incremented each time the language changes. Although it's possible to design a system with version numbers that enables both backward and forward compatibility, for example XSLT, typically, a version change is treated as if that the new language is not backwards compatible with the old language.

Some efforts, such as HTTP, try to have the best of both worlds by allowing for extensibility (in HTTP's case, via headers) as well as version numbers that explicitly identify when a new version is backwards compatible with an old version.

One argument in favor of version numbers is that they allow one to determine what is a 'new version' and what is an 'old version'. But in practice this is not necessarily true. For example, RSS has 0.9x, 1.x, and 2.x versions, all being actively developed in parallel. In effect the version numbers, even though they appear to be ordered, are simply opaque identifiers. Using version numbers does not gaurantee that version 1+x has any particular relationship to version 1.

The self-describing and extensible nature of XML markup, and the addition of XML Namespaces, provide a much better framework for developing languages that can evolve.

8.1 Version Strategy: all components in new namespace(s) for each version (#1)

The following names would be valid:

<name xmlns="http://www.openuri.org/name/1">
  <given>Dave</given>
  <family>Orchard</family>
</name>

<name xmlns="http://www.openuri.org/name/2">
  <given>Dave</given>
  <family>Orchard</family>
  <middle>Bryce</middle>
</name>

<name xmlns="http://www.openuri.org/name/3">
  <given>Dave</given>
  <family>Orchard</family>
  <pref1:middle xmlns:pref1="http://www.openuri.org/name/mid/1">Mr.</pref1:middle>
</name>

<name xmlns="http://www.openuri.org/name/3">
  <given>Dave</given>
  <family>Orchard</family>
  <mid:middle xmlns:pref2="http://www.example.org/name/mid/1">Mr.</mid:middle>
</name>

The 2^nd example shows all the components in the same new namespace. The 3rd and 4^th example show an additional middle element in 2 different namespace names. The first middle, the 3rd example, comes from a namespace name that is in the same domain as the name element_s new namespace name. One reason for 2 namespaces is to modularize the language. The 4^th example shows a complete different namespace name for the middle. It is probable that the pref1:middle was created by the name author, and the mid:middle was created by a 3rd party.

8.2 Version Strategy: all new components in new namespace(s) for each compatible version (#2)

In this strategy, the following names would be valid:

<name xmlns="http://www.openuri.org/name/1">
  <given>Dave</given>
  <family>Orchard</family>
</name>

<name xmlns="http://www.openuri.org/name/1">
  <given>Dave</given>
  <family>Orchard</family>
  <pref1:middle xmlns:pref1="http://www.openuri.org/name/mid/1">Mr.</pref1:middle>
</name>

<name xmlns="http://www.openuri.org/name/1">
  <given>Dave</given>
  <family>Orchard</family>
  <mid:middle xmlns:pref2="http://www.example.org/name/mid/1">Mr.</mid:middle>
</name>

The 2^nd and 3^rd example show an additional middle element in 2 different namespace names. The first middle, the 2^nd example, comes from a namespace name that is in the same domain as the name element_s namespace name. The 3^rd example shows a complete different namespace name for the middle. It is probable that the pref1:middle was created by the name author, and the mid:middle was created by a 3rd party.

8.3 Version Strategy: all new components in new or existing namespace(s) for each compatible version (#3)

In this strategy, the following names would be valid:

<name xmlns="http://www.openuri.org/name/1">
  <given>Dave</given>
  <family>Orchard</family>
</name>

<name xmlns="http://www.openuri.org/name/1">
  <given>Dave</given>
  <family>Orchard</family>
  <middle>Bryce</middle>
</name>

<name xmlns="http://www.openuri.org/name/1">
  <given>Dave</given>
  <family>Orchard</family>
  <pref1:middle xmlns:pref1="http://www.openuri.org/name/mid/1">Mr.</pref1:middle>
</name>

<name xmlns="http://www.openuri.org/name/1">
  <given>Dave</given>
  <family>Orchard</family>
  <mid:middle xmlns:pref2="http://www.example.org/name/mid/1">Mr.</mid:middle>
</name>

The 2^nd example shows the use of the optional middle name in the name namespace. The 3rd and 4^th example show an additional middle element in 2 different namespace names. The first middle, the 3rd example, comes from a namespace name that is in the same domain as the name element_s namespace name. The 4^th example shows a complete different namespace name for the middle. It is probable that the pref1:middle was created by the name author, and the mid:middle was created by a 3rd party.

8.4 Version Strategy: all new components in existing or new namespace(s) for each version and a version identifier(#4)

Using a version identifier, the name instances would change to show the version of the name they use, such as:

<name xmlns="http://www.openuri.org/name/1" version="1.0">
  <given>Dave</given>
  <family>Orchard</family>
</name>

<name xmlns="http://www.openuri.org/name/1" version="1.1">
  <given>Dave</given>
  <family>Orchard</family>
  <middle>Bryce</middle>
</name>

<name xmlns="http://www.openuri.org/name/1" version="1.1">
  <given>Dave</given>
  <family>Orchard</family>
  <pref1:middle xmlns:pref1="http://www.openuri.org/name/mid/1">Mr.</pref1:middle>
</name>

<name xmlns="http://www.openuri.org/name/1" version="1.0">
  <given>Dave</given>
  <family>Orchard</family>
  <mid:middle xmlns:pref2="http://www.example.org/name/mid/1">Mr.</mid:middle>
</name>

<name xmlns="http://www.openuri.org/name/1" version="2.0">
  <given>Dave</given>
  <family>Orchard</family>
  <pref1:middle xmlns:pref1="http://www.openuri.org/name/mid/1">Mr.</pref1:middle>
</name>

The last example shows that the middle is now a mandatory part of the name.

A significant downside with using version identifiers is that software that supports both versions of the name must perform special processing on top of XML and namespaces. For example, many components _bind_ XML types into particular programming language types. Custom software must process the version attribute before using any of the _binding_ software. In Web services, toolkits often take SOAP body content, parse it into types and invoke methods on the types. There are rarely _hooks_ for the custom code to intercept processing between the _SOAP_ processing and the _name_ processing. Further, if version attributes are used by any 3rd party extensions_say mid:middle has a version_then the schema cannot refer to the correct middle.

9.1 Type changes

The various schema languages have a variety of defined mechanisms to indicate extensions that are incompatible. For example, XML Schema provides Type Extension and Substitution Groups. These mechanisms are described in Part 2.