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Create your first LDP server

Purpose​

LDP means Linked Data Platform. It is a W3C Recommendation that defines a set of rules for HTTP operations on web resources to provide an architecture for read-write Linked Data on the web.

An LDP server is therefore useful for making HTTP requests to a semantic database (in which the Linked Data is stored) without operating directly on the database.

In this guide, you will :

  • Create a SemApps instance from a template using Moleculer;
  • Create a Jena Fuseki instance to store some data;
  • Add data to your semantic data base using your LDP server.

Prerequisites​

You need to have NodeJS installed on your computer (use NodeJS version 14).

You also need Docker and Docker-compose installed on your machine.

Initialize your LDP server​

Moleculer is a framework that facilitates the development of microservices that run by messages (and on different servers). It uses Node.js.

The moleculer-cli tool is a command line tool that allows you to initialize new Moleculer projects from templates.

First, you need to install the moleculer-cli tool globally. To do so, open a terminal and runs the following command:

npm install -g moleculer-cli

Then, initialize a new project based on a SemApps template with this command:

moleculer init assemblee-virtuelle/semapps-template-ldp my-project

Choose yes to all questions except the one about WebACL authorizations:

? Do you need a local instance of Jena Fuseki (with Docker)? Yes
? Do you need a read-only SPARQL endpoint? Yes
? Do you need WebACL (WAC) authorizations? No
Create 'my-project' folder...
? Would you like to run 'npm install'? Yes

Running 'npm install'...

You can now go to the newly-created directory:

cd my-project

Launch your local Jena Fuseki instance​

Jena Fuseki is a semantic triple store. It is where your app's data will be stored.

In the "my-project" directory, runs the following command :

docker-compose up
note

If you get this error message:

ERROR: Couldn't connect to Docker daemon at http+docker://localhost - is it running?
If it's at a non-standard location, specify the URL with the DOCKER_HOST environment variable.

Run the following command (taken from here):

sudo chown $USER /var/run/docker.sock

Jena Fuseki is now available at the URL http://localhost:3030.

By default the login is admin and the password is also admin.

There should be no dataset yet. It will be created when launching Moleculer.

Run Moleculer in dev mode​

Now that your triplestore is ready, you can launch you LDP server.

To do so, open another terminal and run the following command in your my-project directory:

npm run dev

Moleculer is in REPL mode. Once it has finished starting, you can enter the command services to check the available services.

If you go to http://localhost:3030, you should see a localData dataset has been created.

Success ! Your LDP server is running correctly. Let's test it now.

Testing your LDP server​

Now, it is time to test your LDP server, which means that you will try to update your database by using this LDP server and not Jena Fuseki.

The LDP service created a LDP container in the /notes path. Indeed, if you go to http://localhost:3000/notes, you should see this empty LDP container:

{ 
  "context": "http://localhost:3000/context.json",
  "id": "http://localhost:3000/users",
  "type": ["ldp:Container", "ldp:BasicContainer"],
  "ldp:contains": []
}

We recommend to use an extension like JSONView to your browser to see pretty-formatted JSON.

Add data to the LDP server​

Now, let's try to add some data to our container. Post an ActivityStreams Note with a tool like Insomnia, Postman or the RESTClient add-on for Firefox.

POST /notes HTTP/1.1
Host: localhost:3000
Content-Type: application/json
Accept: */*
Content-Length: 97
{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Note",
  "name": "My first note",
  "content": "Hello world !"
}

Retrieve the /notes LDP container:

GET /notes HTTP/1.1
Host: localhost:3000
Accept: application/ld+json

You should get this result:

{
  "context": "http://localhost:3000/context.json",
  "id": "http://localhost:3000/notes",
  "type": ["ldp:Container", "ldp:BasicContainer"],
  "ldp:contains": [
    {
      "id": "http://localhost:3000/notes/603288b18391d7738ebba0fe",
      "type": "Note",
      "name": "My first note",
      "content": "Hello world !"
    }
  ]
}

Verify changes on Jena Fuseki​

To test if our LDP server is really working, you should check if the data has been added to our Jena Fuseki instance.

Go to http://localhost:3030/dataset.html and make a SPARQL query to get all of your data.

SELECT ?subject ?predicate ?object
WHERE {
  ?subject ?predicate ?object
}

You should get the following result:

Your first note has been added to the database : the LDP server works!