3.3.6 Loading and saving to file

The RDF library can read and write triples in RDF/XML and a proprietary binary format. There is a plugin interface defined to support additional formats. The library(semweb/turtle) uses this plugin API to support loading Turtle files using rdf_load/2.

[det]rdf_load(+FileOrList)

Same as rdf_load(FileOrList, []). See rdf_load/2.

[det]rdf_load(+FileOrList, :Options)

Load RDF data. If this predicate is called a second time for the same file, it is by default treated as a no-op. See option =if(changed)=.

Options provides additional processing options. Defined options are:

blank_nodes(+ShareMode)

How to handle equivalent blank nodes. If share (default), equivalent blank nodes are shared in the same resource.

base_uri(+URI)

URI that is used for rdf:about="" and other RDF constructs that are relative to the base uri. Default is the source URL.

concurrent(+Jobs)

If FileOrList is a list of files, process the input files using Jobs threads concurrently. Default is the mininum of the number of cores and the number of inputs. Higher values can be useful when loading inputs from (slow) network connections. Using 1 (one) does not use separate worker threads.

format(+Format)

Specify the source format explicitly. Normally this is deduced from the filename extension or the mime-type. The core library understands the formats xml (RDF/XML) and triples (internal quick load and cache format). Plugins, such as library(semweb/turtle) extend the set of recognised extensions.

graph(?Graph)

Named graph in which to load the data. It is not allowed to load two sources into the same named graph. If Graph is unbound, it is unified to the graph into which the data is loaded. The default graph is a file:// URL when loading a file or, if the specification is a URL, its normalized version without the optional #fragment.

if(Condition)

When to load the file. One of true, changed (default) or not_loaded.

modified(-Modified)

Unify Modified with one of not_modified, cached(File), last_modified(Stamp) or unknown.

cache(Bool)

If false, do not use or create a cache file.

register_namespaces(Bool)

If true (default false), register xmlns namespace declarations or Turtle @prefix prefixes using rdf_register_prefix/3 if there is no conflict.

silent(+Bool)

If true, the message reporting completion is printed using level silent. Otherwise the level is informational. See also print_message/2.

prefixes(-Prefixes)

Returns the prefixes defined in the source data file as a list of pairs.

multifile Boolean+

Indicate that the addressed graph may be populated with triples from multiple sources. This disables caching and avoids that an rdf_load/2 call affecting the specified graph cleans the graph.

Other options are forwarded to process_rdf/3. By default, rdf_load/2 only loads RDF/XML from files. It can be extended to load data from other formats and locations using plugins. The full set of plugins relevant to support different formats and locations is below:

:- use_module(library(semweb/turtle)).        % Turtle and TriG
:- use_module(library(semweb/rdf_ntriples)).
:- use_module(library(semweb/rdf_zlib_plugin)).
:- use_module(library(semweb/rdf_http_plugin)).
:- use_module(library(http/http_ssl_plugin)).

See also

rdf_db:rdf_open_hook/3, library(semweb/rdf_persistency) and library(semweb/rdf_cache)

[det]rdf_unload(+Source)

Identify the graph loaded from Source and use rdf_unload_graph/1 to erase this graph.

deprecated

For compatibility, this predicate also accepts a graph name instead of a source specification. Please update your code to use rdf_unload_graph/1.

[det]rdf_save(+Out)

Same as rdf_save(Out, []). See rdf_save/2 for details.

[det]rdf_save(+Out, :Options)

Write RDF data as RDF/XML. Options is a list of one or more of the following options:

graph(+Graph)

Save only triples associated to the given named Graph.

anon(Bool)

If false (default true) do not save blank nodes that do not appear (indirectly) as object of a named resource.

base_uri(URI)

BaseURI used. If present, all URIs that can be represented relative to this base are written using their shorthand. See also write_xml_base option.

convert_typed_literal(:Convertor)

Call Convertor(-Type, -Content, +RDFObject), providing the opposite for the convert_typed_literal option of the RDF parser.

document_language(+Lang)

Initial xml:lang saved with rdf:RDF element.

encoding(Encoding)

Encoding for the output. Either utf8 or iso_latin_1.

inline(+Bool)

If true (default false), inline resources when encountered for the first time. Normally, only bnodes are handled this way.

namespaces(+List)

Explicitly specify saved namespace declarations. See rdf_save_header/2 option namespaces for details.

sorted(+Boolean)

If true (default false), emit subjects sorted on the full URI. Useful to make file comparison easier.

write_xml_base(Bool)

If false, do not include the xml:base declaration that is written normally when using the base_uri option.

xml_attributes(+Bool)

If false (default true), never use xml attributes to save plain literal attributes, i.e., always used an XML element as in <name>Joe</name>.

Out	Location to save the data. This can also be a file-url (file://path) or a stream wrapped in a term stream(Out).

See also

rdf_save_db/1

rdf_make

Reload all loaded files that have been modified since the last time they were loaded.

Partial save

Sometimes it is necessary to make more arbitrary selections of material to be saved or exchange RDF descriptions over an open network link. The predicates in this section provide for this. Character encoding issues are derived from the encoding of the Stream, providing support for utf8, iso_latin_1 and ascii.

rdf_save_header(+Fd, +Options)

Save XML document header, doctype and open the RDF environment. This predicate also sets up the namespace notation.

Save an RDF header, with the XML header, DOCTYPE, ENTITY and opening the rdf:RDF element with appropriate namespace declarations. It uses the primitives from section 3.5 to generate the required namespaces and desired short-name. Options is one of:

graph(+URI)

Only search for namespaces used in triples that belong to the given named graph.

namespaces(+List)

Where List is a list of namespace abbreviations. With this option, the expensive search for all namespaces that may be used by your data is omitted. The namespaces rdf and rdfs are added to the provided List. If a namespace is not declared, the resource is emitted in non-abreviated form.

[det]rdf_save_footer(Out:stream)

Finish XML generation and write the document footer.

See also

rdf_save_header/2, rdf_save_subject/3.

[det]rdf_save_subject(+Out, +Subject:resource, +Options)

Save the triples associated to Subject to Out. Options:

graph(+Graph)

Only save properties from Graph.

base_uri(+URI)

convert_typed_literal(:Goal)

document_language(+XMLLang)

See also

rdf_save/2 for a description of these options.

Fast loading and saving

Loading and saving RDF format is relatively slow. For this reason we designed a binary format that is more compact, avoids the complications of the RDF parser and avoids repetitive lookup of (URL) identifiers. Especially the speed improvement of about 25 times is worth-while when loading large databases. These predicates are used for caching by rdf_load/2 under certain conditions as well as for maintaining persistent snapshots of the database using library(semweb/rdf_persistency).

[det]rdf_save_db(+File)

[det]rdf_save_db(+File, +Graph)

Save triples into File in a quick-to-load binary format. If Graph is supplied only triples flagged to originate from that database are added. Files created this way can be loaded using rdf_load_db/1.

[det]rdf_load_db(+File)

Load triples from a file created using rdf_save_db/2.