CNS-Dapr Client Examples

Table of Contents

• About
• Installing
• Usage
• Maintainers
• License
• Copyright Notice

About

This repository contains various examples that talk to the CNS Dapr Sidecar, written in Node.js and using the Dapr SDK. The examples are used in conjunction with CNS Dapr and it is assumed this is already installed and running (See the CNS Dapr repository for details).

The examples demonstrate the services provided by CNS Dapr.

Installing

To install or update the application, you should fetch the latest version from this Git repository. To do that, you may either download and unpack the repo zip file, or clone the repo using:

git clone https://github.com/cnscp/cns-dapr-client.git

Either method should get you a copy of the latest version. It is recommended (but not compulsory) to place the repo in the ~/cns-dapr-client project directory. Go to the project directory and install Node.js dependancies with:

npm install

Your application should now be ready to rock.

Usage

Once installed, run the application with:

npm run start

To shut down the application, hit ctrl-c.

Environment Variables

The example uses the following environment variables to configure itself:

Variable	Description	Default
CNS_CONTEXT	CNS Broker context	Must be set
CNS_DAPR	CNS Dapr application	'cns-dapr'
CNS_DAPR_HOST	CNS Dapr host	'localhost'
CNS_DAPR_PORT	CNS Dapr port	'3500'
CNS_PUBSUB	CNS Dapr PUBSUB component	'cns-pubsub'
CNS_SERVER_HOST	CNS Client server host	'localhost'
CNS_SERVER_PORT	CNS Client server port	'3100'

Alternatively, variables can be stored in a .env file in the project directory.

Walkthrough

The main example code is held in index.js and uses the Dapr SDK to communicate with the CNS Dapr Sidecar. The SDK is included near the top of the file with the line:

const dapr = require('@dapr/dapr');

The Dapr SDK exposes two object classes: DaprClient and DaprServer. The client allows you to communicate with a Dapr Sidecar and access its client facing features, while the server allows you to receive communication from a Dapr Sidecar and access its server facing features.

Now the example creates an instance of both client and server objects using the values held in the environment variables outlined above. These values should have been set before running the app (or you may use the defaults).

const client = new dapr.DaprClient({ ... });
const server = new dapr.DaprServer({ ... });

Skipping to the bottom of the file, you should see the start function. This is called when the application starts up. The first thing it does is to start the DaprClient created previously.

await client.start();

Once the client is started, it invokes a GET /<context> request to the CNS Dapr Sidecar. Any communication error thrown by the Dapr SDK is also caught and displayed.

res = await client.invoker.invoke(
  config.CNS_DAPR,
  context,
  dapr.HttpMethod.GET);

Now it checks the response from CNS Dapr for errors.

if (res.error !== undefined)

If CNS Dapr does not understand or cannot fulfil the request, it is sets the error property of the response to a string indicating the error that occurred. If no error exists, it passes the response data property to the display function:

display(res.data);

For a full rundown of methods and what is returned in the data object, see the CNS Dapr documentation. Essentially, it is all information about the context and its connections that CNS Dapr is configured for.

Now the example sets up a subscription to CNS Dapr, listening for changes to the context topic. When changes arrive, these also get sent to the display function.

server.pubsub.subscribe(
  config.CNS_PUBSUB,
  context,
  display);

Once the subscription has been setup, the application starts the DaprServer.

await server.start();

The example now sits displaying context topic changes until the application is closed with ctrl-c.

Other Examples

Example	Description	Source
npm run start	Main example application	index.js
npm run subscribe	Subscribe to context	subscribe.js
npm run publish [data]	Publish to context	publish.js
npm run get [path]	Invoke a GET method	get.js
npm run post [path] [data]	Invoke a POST method	post.js
npm run profile [name]	Show profile definition	profile.js
npm run conns	Show active connections	conns.js

Maintainers

License

See LICENSE.md.

Copyright Notice

See COPYRIGHT.md.