Versioning and Migration in Visual C#.NET

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A fundamental issue when working with DSLs is what happens to existing models when the DSL de nition changes DSL authors might decide to add, delete, or rename domain properties, domain classes, relationships, shapes, connectors, and so on In such a situation, models corresponding to the original de nition may contain valuable data that should not be discarded In order to help manage this situation, the saved le contains an attribute in the top-level element that records which version of the DSL was used to save the le:
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<issueStateModel dslVersion="1000" >
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This version number can be set in the DSL designer through the properties of the DSL itself Following NET conventions, the version number has four components: {Major, Minor, Build, Revision} Code is generated so that these values are saved in every model le from the generated designer, and the version is checked whenever a model le is loaded If the version number does not match in every respect, then by default the model fails to load, and the user sees an error message that describes the problem
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This behavior can be overridden by custom code It s possible, for example, to call a different serializer in cases where the version number does not match, thus providing for automatic migration for models created using older versions of the domain model The de nition of a DSL can change for reasons other than deploying new versions While the language is being developed, test and example models will be created in the debugging solution that is automatically created by the DSL Tools, and unless it is explicitly changed, all of these models will have the same version number It would be frustrating if these models were invalidated for every change to the DSL de nition Fortunately, the le-reading code generated for a DSL is relatively forgiving and will do a good job of reading in models, even if the DSL de nition has changed, and will issue warnings that indicate the steps it has taken The le reader will take the following approaches when confronted with an incorrect le: Within the scope of a given domain class, elements (properties and relationships) may appear in any order Elements de ned in base classes must appear before elements from derived classes, though Unexpected elements will be ignored, together with everything nested in them Unexpected attributes will be ignored Missing attributes and elements will cause the model to be populated using default values When such a le is encountered, warnings may be given to the user, because the le will fail its schema validation Schemas are discussed in the next section of this chapter If, while reading in a model, monikers are encountered that cannot be resolved, le loading will fail This is likely to happen when the DSL author changes the cross-referencing scheme for one or more relationships for example, changing the key for a domain class from an Id to a name In these cases, the model reader will open the le in the XML editor and will display errors in the error window that show which monikers have failed to resolve and on which lines of the le the corresponding cross-references appear It such cases it is often easiest to edit the XML directly to change the moniker
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6: Serialization
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elements into the form that will successfully load An example is illustrated in Figure 6-7, where the XML le has been edited so that line 12 contains the reference "StateModel/Opened" for which there is no corresponding element Double-clicking on the error message takes the user directly to the offending element In such a situation, the diagram le may also fail to load because its references into the model cannot be resolved
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