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# 7. Topic Generation and Discovery

A DDS Topic represents the unit for information that can be produced or consumed by a DDS application. Topics are defined by a name, a type, and a set of QoS policies.

The Node.js DCPS API provides several ways of generating Node.js classes to represent DDS topics.

> - over the wire discovery
> - dynamic generation of Node.js classes using parameters IDL file and topic name

Note

- The Examples section provides the examples directory location, example descriptions and running instructions.

## 7.1. Over the Wire Discovery

Node.js topic classes can be generated for existing DDS topics in the DDS system. These topics are “discovered over the wire”.

The Node.js classes are generated when the topic is requested by name.

A code snippet is provided from findTopicExample.js. This example finds a topic registered by another process, and writes a sample to that topic.

Example

    ...
    const dds = require('vortexdds');

    console.log('Connecting to DDS domain...');
    const participant = new dds.Participant();

    console.log('Finding topic...');
    let topic = participant.findTopic('HelloWorldData_Msg');

    console.log('Creating writer and sample data to write...');
    let writer = participant.createWriter(topic);
    let sample = { userID: 4, message: 7 };

    console.log('Writing sample data...');
    let status = writer.write(sample);
    ...

## 7.2. Dynamic Generation of Node.js Topic Classes Using IDL and Name

The Node.js DCPS API supports generation of Node.js topic classes from IDL. This section describes the details of the IDL-Node.js binding.

### 7.2.1. Dynamic Generation

The Node.js DCPS API provides an asynchronous function that returns a Map of `TypeSupport` objects.

A `TypeSupport` object includes the topic typename, keys and descriptor.

The structure type representation of a topic is created by the TypeSupport object. However, the usage of the structure type is internal to the Node.js DCPS API.

In order to create a topic, a topic name and a `TypeSupport` are passed into the `Participant` createTopicFor function. (qos and listener parameters are optional)

The code snippet below is taken from the ‘IoTTopicHelper.js’ file referred in IoTData example.

    const dds = require('vortexdds');
    const path = require('path');

    //asynchronous function to create the topic
    module.exports.create = async function(participant) {

      const topicName = 'IoTData';
      const idlName = 'dds_IoTData.idl';
      const idlPath = path.resolve(idlName);

      //wait for dds.getTopicTypeSupportsForIDL to return a map of typeSupports
      let typeSupports = await dds.getTopicTypeSupportsForIDL(idlPath);

      //idl contains 1 topic.
      let typeSupport = typeSupports.get('DDS::IoT::IoTData');

      return participant.createTopicFor(topicName, typeSupport);
    };

### 7.2.2. Generated Artifacts

The following table defines the Node.js artifacts generated from IDL concepts:

IDL Concept | Node.js Concept
--- | ---
module | N/A
enum | enum from npm ‘enum’ package
enum value | enum value
struct | object
field | object property
union | object (IoTValue from dds_IoTData.idl is the only supported union)

Datatype mappings

The following table shows the Node.js equivalents to IDL primitive types:

IDL Type | Node.js Type
--- | ---
boolean | Boolean
char | Number
octet | Number
short | Number
ushort | Number
long | Number
ulong | Number
long long | Number
ulong long | Number
float | Number
double | Number
string | String
wchar | Unsupported
wstring | Unsupported
any | Unsupported
long double | Unsupported

Implementing Arrays and Sequences in Node.js

Both IDL arrays and IDL sequences are mapped to JavaScript arrays.

## 7.3. Limitations of Node.js Support

The Node.js binding has the following limitations:

> - Listeners are not supported
> - Only the IoTValue union from dds_IoTData.idl is supported in the beta
> - JavaScript does not currently include standard support for 64-bit integer values. 64-bit integers with more than 53 bits of data are represented by String values to avoid loss of precision. If the value will fit inside a JavaScript Number without losing precision, a Number can be used, otherwise use a String. (Refer to IoTData example which demonstrates the usage and ranges for the unsigned and signed 64 bit integers within nodejs.)

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