MeGraS

MeGraS, short for MediaGraph Store, is the data storage and query processing engine of the MediaGraph project which aims to elevate multimodal data to first-class citizens in Knowledge-Graphs. MegraS stores and processes multimodal knowledge graphs including its media components as an RDF graph.

Installation and Config

MeGraS is written in Kotlin and requires a Java runtime environment of version 8 or higher. For some operations on audio and video data, ffmpeg needs to be installed and added to the system path.

Building MeGraS

MeGraS uses Gradle as a build system. To build the application, simply run ./gradlew distZip and unpack the generated archive in build/distributions.

Configuring MeGraS

MeGraS uses an optional configuration file in JSON format. The file to be used can be passed as a parameter when starting the application. If no such parameter is provided, MeGraS will look for a config.json file in its root directory. If no such file is found, the default options are used. The configuration options look as follows:

{
  "objectStoreBase": "store", //directory to be used as base for the media object store
  "httpPort": 8080,           //port to listen to for HTTP connections
  "backend": "FILE",          //persistent backend to use for graph information
  "fileStore": {              //options to be used for 'FILE' backend
    "filename": "quads.tsv",  //filename to be used to store graph information in
    "compression": false      //store graph information in compressed form
  },
  "cottontailConnection": {   //options to be used for 'COTTONTAIL' or 'HYBRID' backend
    "host": "localhost",
    "port": 1865
  },
  "postgresConnection": {     //options to be used for the 'POSTGRES' or 'HYBRID' backend
    "host": "localhost",
    "port": 5432,
    "database": "megras",
    "user": "megras",
    "password": "megras"
  }
}

MeGraS supports several different backend implementations for storing the graph data. The backend to be used can be selected using the backend field in the configuration. The following graph storage backends are supported:

FILE: keeps all graph triples in memory and periodically dumps everything to a single file. Suitable for smaller graphs and for testing purposes.
COTTONTAIL: uses the Cottontail DB vector database. Supports all graph data types, including vector types. Suitable for medium-sized graphs of several 100k triples up to a few million triples.
POSTGRES: Uses PostgreSQL to store the graph. Does not support vector types and related operations. Suitable for larger graphs up to several tens of millions of triples.
HYBRID: Uses both PostgreSQL and Cottontail DB. The latter is used for vector types and operations, the former for everything else.

Using the CLI

MeGraS has a built-in command line interface for simple data management tasks. It enables adding media files as graph nodes and bulk-importing graph triples. Type help to see the available commands and their parameters.

Using the REST API

MeGraS offers a RESTful API for graph manipulation and querying. The OpenAPI specification of all available endpoints can be found in the docs directory or by accessing http://<your_host_and_port>/openapi.json. A Swagger UI is available via http://<your_host_and_port>/swagger-ui.

lucaro / MeGraS

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