ARC2

ARC2 is a PHP 5.6+ library for working with RDF. It also provides a MySQL-based triplestore with SPARQL support.

Installation

Package available on Composer.

If you're using Composer to manage dependencies, you can use

composer require semsol/arc2:^2

Branches

2.3.1 was latest stable version for a long time. But recent developments lead to version 2.4, which is the next minor version of the 2.x-branch, containing stabilizations, wider test coverage and a couple of new features (e.g. PDO adapter for RDF store). Primary focus was to keep backward compatibility, so that an upgrade doesn't mess up your application. Its not clear how long the 2.x-branch will be maintained, so please consider upgrading to 3.x.

Version 3.x introduces new features and develops the backend further. Unfortunately, overall backward compatibility can not be maintained. One of the major changes is the transition from MyISAM to InnoDB table engine (RDF store).

Requirements

PHP

5.6	7.0	7.1	7.2
✔️(1)	✔️	✔️	✔️

(1) It is compatible with PHP 5.3+ but old versions are no longer tested.

Database systems

	5.5	5.6	5.7	8.0
MySQL	✔️	✔️	✔️	💥 (1)

	10.0	10.1	10.2	10.3
MariaDB	✔️	✔️	✔️	✔️

(1) As long as ARC2 uses mysqli, a connection to MySQL Server 8.0 is not possible. For more information, please look here. But its planned to switch to PDO (issue)

RDF triple store

SPARQL support

Please have a look into SPARQL-support.md to see which SPARQL 1.0/1.1 features are currently supported.

Use cache

The RDF store implementation provides a hash-based query cache. It works on two levels: SQL and SPARQL, which means, that it checks given SPARQL queries as well as internally generated SQL queries.

To use it, just add the following to the database configuration:

$store = ARC2::getStore(array(
    'db_name' => 'testdb',
    'db_user' => 'root',
    'db_pwd'  => '',
    'db_host' => '127.0.0.1',
    // ...
    'cache_enabled' => true // <== activates cache
));

Per default it uses a file based cache, which stores items in the default temp folder of the system (in Linux its usually /tmp). But you can use another cache solution, such as memcached.

PSR-16 compatibility

Our cache solution is PSR-16 compatible, which means, that you can use your own cache instance. To do that, add the following to the database configuration:

$store = ARC2::getStore(array(
    'db_name' => 'testdb',
    'db_user' => 'root',
    'db_pwd'  => '',
    'db_host' => '127.0.0.1',
    // ...
    'cache_enabled' => true
    'cache_instance' => new ArrayCache() // <=== example Cache instance, managed by yourself
));

ARC2 uses Symfony Cache , which provides many connectors out of the box (Overview). This allows you to attach ARC2 to a Redis server, for instance.

Known problems/restrictions with database systems

In this section you find known problems with MariaDB or MySQL, regarding certain features. E.g. MySQL 5.5 doesn't allow FULLTEXT indexes in InnoDB. We try to encapsulate any differences in the DB adapters, so that you don't have to care about them. In case you run into problems, this section might be of help.

MySQL 8.0 and mysqli

Using mysqli with MySQL 8.0 as backend throws the following exception:

mysqli_connect(): The server requested authentication method unknown to the client [caching_sha2_password]

Based on this source, one has to change the my.cnf, adding the following entry:

[mysqld] default-authentication-plugin=mysql_native_password

Internal information for developers

Please have a look here to find information about maintaining and extending ARC2.

Docker setup

For ARC2 developers we recommend this following Docker setup. It provides a pre-configured set of software (for PHP, DBS etc.) and allows quick switches between different software versions.

For more information have a look here.