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C++ implementation of the HDT compression format

Header Dictionary Triples (HDT) is a compression format for RDF data that can also be queried for Triple Patterns.

Getting Started

Prerequisites

In order to compile this library, you need to have the following dependencies installed:

• GNU Autoconf

  • sudo apt install autoconf on Debian-based distros (e.g., Ubuntu)
  • sudo dnf install autoconf on Red Hat-based distros (e.g., Fedora)

• GNU Libtool

  • sudo apt install libtool on Debian-based distros (e.g., Ubuntu)
  • sudo dnf install libtool on Red Hat-based distros (e.g., Fedora)

• GNU zip (gzip) Allows GNU zipped RDF input files to be ingested, and allows GNU zipped HDT files to be loaded.

  • sudo apt install zlib1g zlib1g-dev on Debian-based distros (e.g., Ubuntu)
  • sudo dnf install gzip on Red Hat-based distros (e.g., Fedora)

• pkg-config A helper tool for compiling applications and libraries.

  • sudo apt install pkg-config on Debian-based distros (e.g., Ubuntu)
  • sudo dnf install pkgconf-pkg-config on Red Hat-based distros (e.g., Fedora)

• Serd v0.28+ The default parser that is used to process RDF input files. It supports the N-Quads, N-Triples, TriG, and Turtle serialization formats.

  • sudo apt install libserd-0-0 libserd-dev on Debian-based distros (e.g., Ubuntu)
  • sudo dnf install serd serd-devel on Red Hat-based distros (e.g., Fedora)

  Sometimes the version of Serd that is distributed by package managers is too old. In that case, Serd can also be built manually: see https://github.com/drobilla/serd for the installation instructions.

Installation

To compile and install, run the following commands under the directory hdt-cpp. This will also compile and install some handy tools.

./autogen.sh
./configure
make -j2
sudo make install

Installation issues

Sometimes, the above instructions do not result in a working HDT installation. This section enumerates common issues and their workaround.

Package requirements (serd-0 >= 0.28.0) were not met

When getting

Package requirements (serd-0 >= 0.28.0) were not met: Requested 'serd-0 >= 0.28.0' but version of Serd is 0.X

Serd is not 0.28+, probably because of the package manager. Built it manually at https://github.com/drobilla/serd.

./configure cannot find Serd

While running ./configure you get a message similar to the following:

Package 'serd-0', required by 'virtual:world', not found

This means that ./configure cannot find the location of the serd-0.pc file on your computer. You have to find this location yourself, e.g., in the following way:

find /usr/ -name serd-0.pc

Once you have found the directory containing the serd-0.pc file, you have to inform the ./configure script about this location by setting the following environment variable (where directory /usr/local/lib/pkgconfig/ is adapted to your situation):

export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig/

Using HDT

After compiling and installing, you can use the handy tools that are located in hdt-cpp/libhdt/tools. We show some common tasks that can be performed with these tools.

RDF-2-HDT: Creating an HDT

HDT files can only be created for standards-compliant RDF input files. If your input file is not standards-compliant RDF, it is not possible to create an HDT files out of it.

$ ./rdf2hdt data.nt data.hdt

HDT-2-RDF: Exporting an HDT

You can export an HDT file to an RDF file in one of the supported serialization formats (currently: N-Quads, N-Triples, TriG, and Turtle). The default serialization format for exporting is N-Triples.

$ ./hdt2rdf data.hdt data.nt

Querying for Triple Patterns

You can issue Triple Pattern (TP) queries in the terminal by specifying a subject, predicate, and/or object term. The questions mark (?) denotes an uninstantiated term. For example, you can retrieve all the triples by querying for the TP ? ? ?:

$ ./hdtSearch data.hdt
>> ? ? ?
http://example.org/uri3 http://example.org/predicate3 http://example.org/uri4
http://example.org/uri3 http://example.org/predicate3 http://example.org/uri5
http://example.org/uri4 http://example.org/predicate4 http://example.org/uri5
http://example.org/uri1 http://example.org/predicate1 "literal1"
http://example.org/uri1 http://example.org/predicate1 "literalA"
http://example.org/uri1 http://example.org/predicate1 "literalB"
http://example.org/uri1 http://example.org/predicate1 "literalC"
http://example.org/uri1 http://example.org/predicate2 http://example.org/uri3
http://example.org/uri1 http://example.org/predicate2 http://example.org/uriA3
http://example.org/uri2 http://example.org/predicate1 "literal1"
9 results shown.

>> http://example.org/uri3 ? ?
http://example.org/uri3 http://example.org/predicate3 http://example.org/uri4
http://example.org/uri3 http://example.org/predicate3 http://example.org/uri5
2 results shown.

>> exit

Exporting the header

The header component of an HDT contains metadata describing the data contained in the HDT, as well as the creation metadata about the HDT itself. The contents of the header can be exported to an N-Triples file:

$ ./hdtInfo data.hdt > header.nt

Replacing the Header

It can be useful to update the header information of an HDT. This can be done by generating a new HDT file (new.hdt) out of an existing HDT file (old.hdt) and an N-Triples file (new-header.nt) that contains the new header information:

$ ./replaceHeader old.hdt new.hdt new-header.nt

Contributing

Contributions are welcome! Please base your contributions and pull requests (PRs) on the develop branch, and not on the master branch.

License

hdt-cpp is free software licensed as GNU Lesser General Public License (GPL). See libhdt/COPYRIGHT.