Helpers for Adding Entries to RDF/Files Output Data Unit

This section contains a short guide on how to use helpers for adding RDF triples and files to output data units.

For basic information about data units, please refer to: Basic Concepts for DPU Developers.

To see the core data unit interfaces and how the particular types of data units (RDF, Files) extend such interfaces, please look at .

Helpers described on this page are advanced helpers which support adding of entries (files, triples) to output data units (Files, RDF data units). There are also data unit helpers, such as FilesHelper, which should be also considered. Such helpers have certain disadvantage, but are a bit simpler to be used.

The DPU extensions described on this page do not support reading of entries. For reading entries from input data units, please see this tutorial.

Refer to this tutorial how to use these simpler helpers. In general, you should use DPU extensions described on this page (rather than the simpler helpers) for adding entries to an output data unit if you to write your DPUs fault tolerant. For a more detailed comparison of these helpers, see here.

This helper is an extension.

Before this extension can be used, you have to insert the following code to the Main DPU class, where param in Line 1 contains the name of the data unit the helper wraps (in this case 'output').

@ExtensionInitializer.Init(param = "output")
public WritableSimpleFiles outputFilesHelper;

There are two methods DPU developers may use to add files to the output files data unit (using the helper):

Without using this helper, the task of adding an existing file may be executed as follows:

Symbolic symbolicName = output.addExistingFile(fileName, file.toURI().toString());
MetadataUtils.set(output, symbolicName, FilesVocabulary.UV_VIRTUAL_PATH, fileName);

The advantage of the helper is that the code is cleaner: compare the code needed to add existing file to the output file data unit, which is one line (with helper) vs. two lines (when the helper is not used).

Additionally, when the helper is not used, you as a DPU developer must be aware of virtualPath metadata, must know that the recommended practice is to set virtualPath = symbolicName.

This helper is an extension.

Before this extension can be used, the following code has to be inserted to the Main DPU class, where param in Line 1 contains name of the data unit the helper wraps (in this case 'output').

@ExtensionInitializer.Init(param = "output")
public WritableSimpleRdf outputRdfHelper;

There are two methods DPU developers may use to add RDF triples to the output RDF data unit (using the helper):

Sample usage of the first method add(Resource s, URI p, Value o) is depicted below:

org.openrdf.model.ValueFactory vf = outputRdfHelper.getValueFactory();
org.openrdf.model.URI s = valueFactory.createURI(http://data.example.com/resource/subjectS);
org.openrdf.model.URI p = valueFactory.createURI(http://data.example.com/resource/predicateP);
org.openrdf.model.Value = valueFactory.createLiteral(rowNumber)
add(s,p,o);

By default, calling the add methods above, the triples are added to the default entry (RDF graph), automatically generated for you in the output data unit, the wrapper wraps. The symbolicName of the default entry is set to default-output.In typical cases, preparing one entry (RDF graph), where all the data (triples) is loaded, is sufficient.

In certain cases, if you already prepared an entry (e.g. by using RDFDataUnitUtils), you may specify that you want to add triples to this prepared entry (and not to the default entry generated) by calling:

E-SparqlEndpoint

T-Tabular

T-Metadata