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# Versioning

One of Strolch’s features that sets it apart from other frameworks, is that versioning is baked into Strolch’s fabric. The feature is opt-in, as it is not required in all projects, but it only needs enabling, for all modifications to objects to be versioned, so that rollbacks can be done when needed.

The feature is enabled for each realm. In the `StrolchConfiguration.xml` file enable it by adding the `enableVersioning` propery per realm:

    <StrolchConfiguration>
      <env id="dev">
        ...
        <Component>
          <name>RealmHandler</name>
          <api>li.strolch.agent.api.RealmHandler</api>
          <impl>li.strolch.agent.impl.DefaultRealmHandler</impl>
          <depends>PrivilegeHandler</depends>
          <Properties>
            <realms>defaultRealm, otherRealm</realms>
            <enableVersioning>true</enableVersioning>
            <dataStoreMode>TRANSIENT</dataStoreMode>
            <dataStoreFile>StrolchModel.xml</dataStoreFile>
            <enableVersioning.otherRealm>true</enableVersioning.otherRealm>
            <dataStoreMode.otherRealm>TRANSIENT</dataStoreMode.otherRealm>
            <dataStoreFile.otherRealm>StrolchModel.xml</dataStoreFile.otherRealm>
          </Properties>
        </Component>
      </env>
      ...
    </StrolchConfiguration>

Once versioning is enabled, versioning is handled automatically. The API for versioning is implemented on the ElementMaps.

Example: Revert to previous version of a Resource:

    Resource res = tx.getResourceBy("TestType", "MyTestResource");
    ResourceMap resourceMap = tx.getResourceMap();
    Resource previousVersion = resourceMap.revertToVersion(tx, res);
    // or
    Resource previousVersion = resourceMap.revertToVersion(tx, "TestType", "MyTestResource", 1);

Example: Retrieve all versions of a Resource:

    List<Resource> versions = resourceMap.getVersionsFor(tx, "TestType", "MyTestResource");

Note: When reverting to a previous version, it is important to remember, that any references on an element to other elements will also be restored. As long as the relationship is to the same element, then this is not an issue, but should the relationship have changed, then it this must be handled and the user performing a revert be allowed to decided which element to reference in the reverted version.