Document Versioning

Each document in Xenon has documentVersion field that gets incremented on every update to that document. Using this field and along with documentOwner and documentEpoch Xenon resolves conflicts during synchronization between multiple nodes.

Changing version retention limit

By default Xenon keeps 1000 old document versions. You can change that setting by implementing getDcoumentTemplate() method in your service to tune the disk requirements of your service. Following is the example showing how to set the version retention limit in your service.

@Override
public ServiceDocument getDocumentTemplate() {
    ServiceDocument template = super.getDocumentTemplate();

    // instruct the index to only keep the most recent N versions
    template.documentDescription.versionRetentionLimit = ThisService.VERSION_RETENTION_LIMIT;
    return template;
}

Querying a particular version

Whenever we do GET on a service, latest version of that service document is returned by Xenon. We can create a query to get a particular version. Following is an example code that creates a QueryTask for retrieving a document with particular version. It includes any deleted versions as well by setting QueryOption.INCLUDE_DELETED.

public static QueryTask buildVersionQueryTask(long targetVersion, String documentSelfLink) {
    QueryTask.Query selfLinkClause = new QueryTask.Query()
            .setTermPropertyName(ServiceDocument.FIELD_NAME_SELF_LINK)
            .setTermMatchValue(documentSelfLink)
            .setTermMatchType(QueryTask.QueryTerm.MatchType.TERM);

    QueryTask.QuerySpecification querySpecification = new QueryTask.QuerySpecification();
    querySpecification.resultLimit = 10;
    querySpecification.options = EnumSet.of(
            QueryTask.QuerySpecification.QueryOption.INCLUDE_ALL_VERSIONS,
            QueryTask.QuerySpecification.QueryOption.INCLUDE_DELETED,
            QueryTask.QuerySpecification.QueryOption.TOP_RESULTS,
            QueryTask.QuerySpecification.QueryOption.EXPAND_CONTENT);

    QueryTask.NumericRange<?> versionRange =
            QueryTask.NumericRange.createEqualRange((Long)targetVersion);

    versionRange.precisionStep = Integer.MAX_VALUE;
    QueryTask.Query versionClause = new QueryTask.Query()
            .setTermPropertyName(ServiceDocument.FIELD_NAME_VERSION)
            .setNumericRange(versionRange);

    querySpecification.query.addBooleanClause(selfLinkClause);
    querySpecification.query.addBooleanClause(versionClause);

    return QueryTask.create(querySpecification).setDirect(true);
}