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The Schema Difference tool can use schema reference XML files to perform validation in addition to that performed by simple differencing.

Validation allows the application of more complex rules than whether there is a difference between two property values. Validation is performed by DbValidator objects. A chain of DbValidator objects is associated with each database object in the reference schema. Each of these is executed in turn and given the chance to create validation errors based on the corresponding object in the target schema.

If an index has not been given a specific name then the RDBMS will auto-generate one at creation time. This means that the reference schema cannot specify the exact name that the index in the target database will have. This would lead to schema differences being reported if it were not for the use of validators. A NameValidator can be specified for such an index:

<index name="SQL120116153558430" unique="true">
    <validator class="org.alfresco.util.schemacomp.validator.NameValidator">
        <property name="pattern">SQL[0-9]+</property>


This example is from a schema reference file (Schema-Reference-ALF.xml) and indicates that although in the original reference schema the index was named SQL120116153558430 any index having the appropriate parent table, column names (and column order) is valid as long as the name matches the regular expression SQL[0-9]+.

When the validator is invoked, it checks that the name property of the index matches the supplied regular expression. In addition to this, the validator reports, when configured to, that it takes responsibility for the name property of the index. This stops the Schema Difference Tool from applying the differencing logic to the property. A DbValidator can choose to apply its validation in addition to the differencing logic by not taking sole responsibility for any properties. Conversely a validator can also take sole responsibility for an entire database object in which case no differencing logic is applied to any part of the object.

Perhaps a specific unsupported upgrade path has introduced an unexpected schema change - it might not be a problem, but it is important that differences are highlighted so that a decision can be made on whether the difference represents a problem and whether a fix will need to be made. On running the Schema Difference Tool, the following might be observed in the log files:

2012-01-31 14:28:50,697  WARN  [domain.schema.SchemaBootstrap] [main] Schema validation found 1 potential problems, results written to:
2012-01-31 14:28:54,682  INFO  [domain.schema.SchemaBootstrap] [main] Compared database schema with reference schema (all OK):
class path resource [alfresco/dbscripts/create/org.hibernate.dialect.Oracle9Dialect/Schema-Reference-ACT.xml]

The ACT database object is as expected, but there is a difference between the target schema and the ALF (alf_ prefixed database objects) schema reference. Looking at that file it can be seen that an index that is expected to have been auto-generated has been created with an explicit name:

Validation: index"idx_alf_ace_auth" fails to match rule: name must match pattern 'SQL[0-9]+'

Specifically, the error report is stating that the index defined in the schema reference having the name SQL120131142718040 belonging to the table ALF_ACCESS_CONTROL_ENTRY is expected to be named in the same way: prefixed with SQL then a string of one or more digits.

A similar problem to the auto-generated name problem is when a database object is created automatically. Some databases create indexes on the fly, rather than being an explicit part of the schema declaration. It is not known whether they will exist at the time the Schema Difference Tool is run. To suppress such errors an IgnoreObjectValidator can be used - it takes responsibility for validation of the associated database object, but performs no actual validation

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