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Eyevinn Open Source Cloud Dev Blog profile image Jonas Birmé
Jonas Birmé for Eyevinn Open Source Cloud Dev Blog

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Publish-subscribe messaging with open web services

#pubsub #opensource #node #javascript

In this blog post we will show an example of pub/sub messaging using open web services available in Eyevinn Open Source Cloud. Publish-subscribe messaging, or pub/sub messaging, is an asynchronous communication model that is central for a scalable and distributed system of modules or services.

Pub sub messaging

Based on the open web service Valkey in Eyevinn Open Source Cloud we can build an application using the pub/sub messaging model.

In this guide

  1. Get an API Access Token and setup project
  2. Develop a subscriber example application
  3. Develop a publisher example application

Prerequisites

Get an API Access Token and setup project

Navigate to Settings / API in the Eyevinn Open Source Cloud web console.

Access token

Copy this token and store in your shell's environment in the environment variable OSC_ACCESS_TOKEN.

% export OSC_ACCESS_TOKEN=<access-token-copied-above>
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Setup a NodeJS project.

% mkdir pubsub % cd pubsub % npm init
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Install the Javascript client SDK.

% npm install --save @osaas/client-core @osaas/client-services
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Subscriber application

We will start by developing the subscriber application that will register (subscribe) to topics of interest.

Create a file called subscriber.js. Then add the following code to setup the Valkey instance that will be managing all topics and messages. Valkey is an open source, in-memory data store that is API compatible with a Redis client.

const { Context, getPortsForInstance } = require('@osaas/client-core'); const { createValkeyIoValkeyInstance, getValkeyIoValkeyInstance } = require('@osaas/client-services'); async function setup() { const ctx = new Context(); let valkey = await getValkeyIoValkeyInstance(ctx, 'example'); if (!valkey) { valkey = await createValkeyIoValkeyInstance(ctx, { name: 'example', Password: 'secret' }); } const token = await ctx.getServiceAccessToken('valkey-io-valkey'); const ports = await getPortsForInstance( ctx, 'valkey-io-valkey', 'example', token ); const port = ports.find((p) => p.internalPort === 6379); if (port) { return `redis://:${valkey.Password}@${port.externalIp}:${port.externalPort}`; } throw new Error('No redis port found'); }
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The setup function will return a Redis URL to the Valkey instance.

Next step is to start waiting for messages and first we need to install a Redis client.

% npm install --save ioredis
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We will add the following to the subscriber.js file to subscribe on a topic called "messages".

const Redis = require('ioredis'); async function main() { const redisUrl = await setup(); const client = new Redis(redisUrl); try { await client.subscribe('messages'); console.log('Waiting for messages...'); client.on('message', (channel, message) => { console.log(`Received message: ${message} from ${channel}`); }); } catch (err) { console.error('Error:', err); } } main();
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Now running this application we will get.

% node subscriber.js Waiting for messages...
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Publisher application

Now open a new terminal and create a file called publisher.js. Copy the setup function we created in the subscriber application and in addition add the following to this file.

async function publishMessage(topic, message) { const redisUrl = await setup(); const redisClient = new Redis(redisUrl); try { const receivedCount = await redisClient.publish(topic, message); console.log( `Message "${message}" published to channel "${topic}". Received by ${receivedCount} subscribers.` ); } catch (err) { console.error('Error publishing message', err); } finally { await redisClient.quit(); } } if (!process.argv[2]) { console.error('Please a message to publish'); process.exit(1); } publishMessage('messages', process.argv[2]);
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Now run the publisher application and publish a Hello world message.

% node publisher.js "Hello world" Message "Hello world" published to channel "messages". Received by 1 subscribers.
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And we will see that the subscriber application will output the message.

Waiting for messages... Received message: Hello world from messages
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Conclusion

You have now successfully created a pub/sub messaging application using the open source Valkey in-memory data store for the messaging. With Valkey as an open web service in Eyevinn Open Source Cloud you could immediately start building your pub/sub messaging application without having to host the Valkey instance yourself. You also avoided to be locked in with a specific cloud vendor as it is based on open source.

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