Getting Started

This guide describes the necessary steps to create your first drasyl node and how to integrate it into your application.

Once the node is set up, it and therefore your application can participate in the drasyl Overlay Network and communicate with other nodes and applications.

Add Dependency

Create a new maven project and add the dependency to your pom.xml:

Maven:

pom.xml

<dependency>
    <groupId>org.drasyl</groupId>
    <artifactId>drasyl-node</artifactId>
    <version>0.8.0</version>
</dependency>

Other Dependency Managers:

Gradle : compile "org.drasyl:drasyl-node:0.8.0" // build.gradle 
   Ivy : <dependency org="org.drasyl" name="drasyl-node" rev="0.8.0" conf="build" /> // ivy.xml
   SBT : libraryDependencies += "org.drasyl" % "drasyl-node" % "0.8.0" // build.sbt

Implementing DrasylNode

Next, you can create your own drasyl node by implementing DrasylNode.

This class contains the following methods that are now relevant for you:

• send(...): allows your application to send arbitrary messages to other drasyl nodes.
• onEvent(...): allows your application to react to certain events (e.g. process received messages, connection to the network established/lost). This method must be implemented.
• start(): starts the node, which will then automatically connect to the drasyl network.
• shutdown(): disconnects from the drasyl network and shuts down the node.

Here you can see a minimal working example of a node that forwards all received events to System.out:

GettingStarted.java

import org.drasyl.node.*;
import org.drasyl.node.event.Event;

public class GettingStarted {
    public static void main(final String[] args) throws DrasylException {
        final DrasylNode node = new DrasylNode() {
            @Override
            public void onEvent(final Event event) {
                System.out.println("Event received: " + event);
            }
        };
    }
}

Node Events

As mentioned before, different events are received by the application. These provide information about the state of your node, received messages or connections to other nodes. It is therefore important that you become familiar with the definitions and implications of the different event types.

For example, you should listen for NodeOnlineEvent before start sending messages, and pause when NodeOfflineEvent has been received.

Sending Messages

Every message that is to be sent requires a recipient address. Each drasyl node creates an identity at its first startup consisting of a cryptographic public-private key pair. From the public key, a 10 hex digit address is derived, by which each node can be uniquely identified.

The send() method needs the recipient as first argument and the message payload as second argument.

GettingStarted.java

import org.drasyl.node.*;
import org.drasyl.node.event.*;
import java.util.concurrent.*;

public class GettingStarted {
    public static void main(final String[] args) throws DrasylException {
        final CompletableFuture<Void> online = new CompletableFuture<>();

        final DrasylNode node = new DrasylNode() {
            @Override
            public void onEvent(final Event event) {
                if (event instanceof NodeOnlineEvent) {
                    online.complete(null);
                }
            }
        };

        // wait for node to come online
        online.join();

        final String recipient = "0229041b273dd5ee1c2bef2d77ae17dbd00d2f0a2e939e22d42ef1c4bf05147ea9";
        final String payload = "Hello World";
        final CompletionStage<Void> sent = node.send(recipient, payload);
    }
}

The returned Future does not give any feedback on whether the message could be delivered. The future is successfully completed if the local node could deliver the message itself or forward it to a super peer that acts as a default gateway. Otherwise the future is completed exceptionally.

Advanced References

The addresses of recipient nodes must be known, as drasyl-core has no function for querying available addresses. The Groups Plugin can be used to get automatically notified about other nodes joining and leaving the network.

Receiving Messages

Each received message is announced by an MessageEvent to the application. The event contains a getters for the message's sender and payload.

GettingStarted.java

import org.drasyl.node.*;
import org.drasyl.node.event.*;

public class GettingStarted {
    public static void main(final String[] args) throws DrasylException {
        final DrasylNode node = new DrasylNode() {
            @Override
            public void onEvent(final Event event) {
                if (event instanceof MessageEvent) {
                    System.out.println("Message received from " + message.getSender() + " with payload " + message.getPayload());
                }
            }
        };
    }
}

Starting & Stopping the drasyl Node

Before you can use the drasyl node, you must start it using node.start(). For communication with other nodes in the local network, the node starts a server listening on port 22527. Make sure that the port is available. After the node has been successfully started, it emits an NodeUpEvent to the application. Then, once it has successfully connected to the overlay network, an NodeOnlineEvent is emitted.

If the node is temporarily or permanently no longer needed, it can be shut down using node.shutdown(). A NodeDownEvent is emitted immediately after this call. The application should now no longer attempt to send messages. As soon as the connection to the drasyl network is terminated, an NodeOfflineEvent is emitted. A NodeNormalTerminationEvent is emitted when the shutdown is done.

Summary

In this chapter, we had a quick tour of drasyl with a demonstration on how to create a node, start and stop it, send messages, and process emitted events.

Have a look at our examples to see how drasyl can be used for different scenarios.