# Linked Data Fragments Server
[](https://travis-ci.org/LinkedDataFragments/Server.js)
[](https://www.npmjs.com/package/ldf-server)
[](https://hub.docker.com/r/linkeddatafragments/server.js/)
[](https://zenodo.org/badge/latestdoi/16891600)
On today's Web, Linked Data is published in different ways,
which include [data dumps](http://downloads.dbpedia.org/3.9/en/),
[subject pages](http://dbpedia.org/page/Linked_data),
and [results of SPARQL queries](http://dbpedia.org/sparql?default-graph-uri=http%3A%2F%2Fdbpedia.org&query=CONSTRUCT+%7B+%3Fp+a+dbpedia-owl%3AArtist+%7D%0D%0AWHERE+%7B+%3Fp+a+dbpedia-owl%3AArtist+%7D&format=text%2Fturtle).
We call each such part a [**Linked Data Fragment**](http://linkeddatafragments.org/).
The issue with the current Linked Data Fragments
is that they are either so powerful that their servers suffer from low availability rates
([as is the case with SPARQL](http://sw.deri.org/~aidanh/docs/epmonitorISWC.pdf)),
or either don't allow efficient querying.
Instead, this server offers **[Triple Pattern Fragments](http://www.hydra-cg.com/spec/latest/triple-pattern-fragments/)**.
Each Triple Pattern Fragment offers:
- **data** that corresponds to a _triple pattern_
_([example](http://data.linkeddatafragments.org/dbpedia?subject=&predicate=rdf%3Atype&object=dbpedia-owl%3ARestaurant))_.
- **metadata** that consists of the (approximate) total triple count
_([example](http://data.linkeddatafragments.org/dbpedia?subject=&predicate=rdf%3Atype&object=))_.
- **controls** that lead to all other fragments of the same dataset
_([example](http://data.linkeddatafragments.org/dbpedia?subject=&predicate=&object=%22John%22%40en))_.
An example server is available at [data.linkeddatafragments.org](http://data.linkeddatafragments.org/).
## Install the server
This server requires [Node.js](http://nodejs.org/) 4.0 or higher
and is tested on OSX and Linux.
To install, execute:
```bash
$ [sudo] npm install -g ldf-server
```
## Use the server
### Configure the data sources
First, create a configuration file `config.json` similar to `config/config-example.json`,
in which you detail your data sources.
For example, this configuration uses an [HDT file](http://www.rdfhdt.org/)
and a [SPARQL endpoint](http://www.w3.org/TR/sparql11-protocol/) as sources:
```json
{
"title": "My Linked Data Fragments server",
"datasources": {
"dbpedia": {
"title": "DBpedia 2014",
"type": "HdtDatasource",
"description": "DBpedia 2014 with an HDT back-end",
"settings": { "file": "data/dbpedia2014.hdt" }
},
"dbpedia-sparql": {
"title": "DBpedia 3.9 (Virtuoso)",
"type": "SparqlDatasource",
"description": "DBpedia 3.9 with a Virtuoso back-end",
"settings": { "endpoint": "http://dbpedia.restdesc.org/", "defaultGraph": "http://dbpedia.org" }
}
}
}
```
The following sources are supported out of the box:
- HDT files ([`HdtDatasource`](https://github.com/LinkedDataFragments/Server.js/blob/master/lib/datasources/HdtDatasource.js) with `file` setting)
- N-Triples documents ([`TurtleDatasource`](https://github.com/LinkedDataFragments/Server.js/blob/master/lib/datasources/TurtleDatasource.js) with `url` setting)
- Turtle documents ([`TurtleDatasource`](https://github.com/LinkedDataFragments/Server.js/blob/master/lib/datasources/TurtleDatasource.js) with `url` setting)
- JSON-LD documents ([`JsonLdDatasource`](https://github.com/LinkedDataFragments/Server.js/blob/master/lib/datasources/JsonLdDatasource.js) with `url` setting)
- SPARQL endpoints ([`SparqlDatasource`](https://github.com/LinkedDataFragments/Server.js/blob/master/lib/datasources/SparqlDatasource.js) with `endpoint` and optionally `defaultGraph` settings)
Support for new sources is possible by implementing the [`Datasource`](https://github.com/LinkedDataFragments/Server.js/blob/master/lib/datasources/Datasource.js) interface.
### Start the server
After creating a configuration file, execute
```bash
$ ldf-server config.json 5000 4
```
Here, `5000` is the HTTP port on which the server will listen,
and `4` the number of worker processes.
Now visit `http://localhost:5000/` in your browser.
### Reload running server
You can reload the server without any downtime
in order to load a new configuration or version.
In order to do this, you need the process ID of the server master process.
One possibility to obtain this are the server logs:
```bash
$ bin/ldf-server config.json
Master 28106 running.
Worker 28107 running on http://localhost:3000/.
```
If you send the server a `SIGHUP` signal:
```bash
$ kill -s SIGHUP 28106
```
it will reload by replacing its workers.
Note that crashed or killed workers are always replaced automatically.
### _(Optional)_ Set up a reverse proxy
A typical Linked Data Fragments server will be exposed
on a public domain or subdomain along with other applications.
Therefore, you need to configure the server to run behind an HTTP reverse proxy.
To set this up, configure the server's public URL in your server's `config.json`:
```json
{
"title": "My Linked Data Fragments server",
"baseURL": "http://data.example.org/",
"datasources": { … }
}
```
Then configure your reverse proxy to pass requests to your server.
Here's an example for [nginx](http://nginx.org/):
```nginx
server {
server_name data.example.org;
location / {
proxy_pass http://127.0.0.1:3000$request_uri;
proxy_set_header Host $http_host;
proxy_pass_header Server;
}
}
```
Change the value `3000` into the port on which your Linked Data Fragments server runs.
If you would like to proxy the data in a subfolder such as `http://example.org/my/data`,
modify the `baseURL` in your `config.json` to `"http://example.org/my/data"`
and change `location` from `/` to `/my/data` (excluding a trailing slash).
### _(Optional)_ Running under HTTPS
HTTPS can be enabled in two ways: natively by the server, or through a proxy (explained above).
With native HTTPS, the server will establish the SSL layer. Set the following values in your config file to enable this:
{
"protocol": "https",
"ssl": {
"keys" : {
"key": "./private-key-server.key.pem",
"ca": ["./root-ca.crt.pem"],
"cert": "./server-certificate.crt.pem"
}
}
}
If `protocol`is not specified, it will derive the protocol from the `baseURL`. Hence, HTTPS can also be enabled as such:
{
"baseURL": "https://data.example.org/",
"ssl": {
"keys" : {
"key": "./private-key-server.key.pem",
"ca": ["./root-ca.crt.pem"],
"cert": "./server-certificate.crt.pem"
}
}
}
If you decide to let a proxy handle HTTPS, use this configuration to run the server as `http`, but construct links as `https` (so clients don't break):
{
"protocol": "http",
"baseURL": "https://data.example.org/",
}
### _(Optional)_ Running in a Docker container
If you want to rapidly deploy the server as a microservice, you can build a [Docker](https://www.docker.com/) container as follows:
```bash
$ docker build -t ldf-server .
```
After that, you can run your newly created container:
```bash
$ docker run -p 3000:3000 -t -i --rm -v $(pwd)/config.json:/tmp/config.json ldf-server /tmp/config.json
```
### _(Optional)_ Host historical version of datasets
You can [enable the Memento protocol](https://github.com/LinkedDataFragments/Server.js/wiki/Configuring-Memento) to offer different versions of an evolving dataset.
## License
The Linked Data Fragments server is written by [Ruben Verborgh](http://ruben.verborgh.org/).
## License
The Linked Data Fragments client is written by [Ruben Verborgh](http://ruben.verborgh.org/) and colleagues.
This code is copyrighted by [Ghent University – imec](http://idlab.ugent.be/)
and released under the [MIT license](http://opensource.org/licenses/MIT).