Notification

OST Notification helps publish critical events using EventEmitter and RabbmitMQ. All events get published using node EventEmitter and, if configured, events are also published through RabbitMQ, using topic-based exchange.

Install

npm install @ostdotcom/notification --save

Examples:

Subscribe to events published through RabbitMQ:

• Basic example on how to listen a specific event. Arguments passed are:
  • Events [Array] (mandatory) - List of events to subscribe to
  • Options [object] (mandatory) -
    • queue [string] (optional) - Name of the queue on which you want to receive all your subscribed events. These queues and events, published in them, have TTL of 6 days. If a queue name is not passed, a queue with a unique name is created and is deleted when the subscriber gets disconnected.
    • ackRequired [number] - (optional) - The delivered message needs ack if passed 1 ( default 0 ). if 1 passed and ack not done, message will redeliver.
    • prefetch [number] (optional) - The number of messages released from queue in parallel. In case of ackRequired=1, queue will pause unless delivered messages are acknowledged.
  • Callback [function] (mandatory) - Callback method will be invoked whenever there is a new notification

// Config Strategy for OST Notification.
configStrategy = {
	"rabbitmq": {
        "username": "guest",
        "password": "guest",
        "host": "127.0.0.1",
        "port": "5672",
        "heartbeats": "30"
    }
};
// Import the notification module.
const OSTNotification = require('@ostdotcom/notification');
let unAckCount = 0; //Number of unacknowledged messages.

const subscribe = async function() {
  let ostNotificationInstance = await OSTNotification.getInstance(configStrategy);
  ostNotificationInstance.subscribeEvent.rabbit(
    ["event.ProposedBrandedToken"],
    {
      queue: 'myQueue',
      ackRequired: 1, // When set to 1, all delivered messages MUST get acknowledge.
      broadcastSubscription: 1, // When set to 1, it will subscribe to broadcast channel and receive all broadcasted messages. 
      prefetch:10
    }, 
    function(msgContent){
      // Please make sure to return promise in callback function. 
      // On resolving the promise, the message will get acknowledged.
      // On rejecting the promise, the message will be re-queued (noAck)
      return new Promise(async function(onResolve, onReject) {
        // Incrementing unacknowledged message count.
        unAckCount++;
        console.log('Consumed message -> ', msgContent);
        response = await processMessage(msgContent);
        
        // Complete the task and in the end of all tasks done
        if(response == success){
          // The message MUST be acknowledged here.
          // To acknowledge the message, call onResolve
          // Decrementing unacknowledged message count.
          unAckCount--;
          onResolve();   
        } else {
          //in case of failure to requeue same message.
          onReject();
        }
       
      })
    
    });
};
// Gracefully handle SIGINT, SIGTERM signals.
// Once SIGINT/SIGTERM signal is received, programme will stop consuming new messages. 
// But, the current process MUST handle unacknowledged queued messages.
process.on('SIGINT', function () {
  console.log('Received SIGINT, checking unAckCount.');
  const f = function(){
    if (unAckCount === 0) {
      process.exit(1);
    } else {
      console.log('waiting for open tasks to be done.');
      setTimeout(f, 1000);
    }
  };
  setTimeout(f, 1000);
});

function ostRmqError(err) {
  logger.info('ostRmqError occured.', err);
  process.emit('SIGINT');
}
// Event published from package in case of internal error.
process.on('ost_rmq_error', ostRmqError);
subscribe();

• Example on how to listen to multiple events with one subscriber.

// Config Strategy for OST Notification.
configStrategy = {
	"rabbitmq": {
        "username": "guest",
        "password": "guest",
        "host": "127.0.0.1",
        "port": "5672",
        "heartbeats": "30"
    }
};
// Import the notification module.
const OSTNotification = require('@ostdotcom/notification');
const subscribeMultiple = async function() {
  let ostNotificationInstance = await OSTNotification.getInstance(configStrategy);
  ostNotificationInstance.subscribeEvent.rabbit(
    ["event.ProposedBrandedToken", "obBoarding.registerBrandedToken"],
    {}, 
    function(msgContent){
      console.log('Consumed message -> ', msgContent)
    });
  };
subscribeMultiple();

Subscribe to local events published through EventEmitter:

• Basic example on how to listen a specific event. Arguments passed are:
  • Events (mandatory) - List of events to subscribe to
  • Callback (mandatory) - Callback method will be invoked whenever there is a new notification

// Config Strategy for OST Notification.
configStrategy = {
	"rabbitmq": {
        "username": "guest",
        "password": "guest",
        "host": "127.0.0.1",
        "port": "5672",
        "heartbeats": "30"
    }
};
// Import the notification module.
const OSTNotification = require('@ostdotcom/notification');
const subscribeLocal = async function() {
  let ostNotificationInstance = await OSTNotification.getInstance(configStrategy);
  ostNotificationInstance.subscribeEvent.local(["event.ProposedBrandedToken"], 
  function(msgContent){
    console.log('Consumed message -> ', msgContent)
  });
  };
subscribeLocal();

Publish Notifications:

• All events are by default published using EventEmitter and if configured, through RabbmitMQ as well.

// Config Strategy for OST Notification.
configStrategy = {
	"rabbitmq": {
        "username": "guest",
        "password": "guest",
        "host": "127.0.0.1",
        "port": "5672",
        "heartbeats": "30"
    }
};
// Import the notification module.
const OSTNotification = require('@ostdotcom/notification');
const publish = async function() {
  let ostNotificationInstance = await OSTNotification.getInstance(configStrategy);
  ostNotificationInstance.publishEvent.perform(
    {
      topics:["event.ProposedBrandedToken"],
      broadcast: 1, // When set to 1 message will be broadcasted to all channels. 'topics' parameter should not be sent.
      publishAfter: 1000, // message to be sent after milliseconds.
      publisher: 'MyPublisher',
      message: {
  	  kind: "event_received",
  	  payload: {
  		event_name: 'ProposedBrandedToken',
  		params: {
  		  //params of the event
  		},
          contract_address: 'contract address',
          chain_id: 'Chain id',
          chain_kind: 'kind of the chain'
  	  }
  	}
    });
};
publish();

Pause and Restart queue consumption:

• We also support pause and start queue consumption. According to your logical condition, you can fire below events from your process to cancel or restart consumption respectively.

// Config Strategy for OST Notification.
let configStrategy = {
	"rabbitmq": {
        "username": "guest",
        "password": "guest",
        "host": "127.0.0.1",
        "port": "5672",
        "heartbeats": "30"
    }
};
let queueConsumerTag = null;
// Import the notification module.
const OSTNotification = require('@ostdotcom/notification');
const subscribePauseRestartConsume = async function() {
  let ostNotificationInstance = await OSTNotification.getInstance(configStrategy);
  ostNotificationInstance.subscribeEvent.rabbit(
    ["event.ProposedBrandedToken", "obBoarding.registerBrandedToken"],
    {}, 
    function(msgContent){
      console.log('Consumed message -> ', msgContent);
      
      if(some_failure_condition){
        process.emit('CANCEL_CONSUME', queueConsumerTag);
      }
      
      if(failure_resolve_detected){
        process.emit('RESUME_CONSUME', queueConsumerTag);
      }
    },
    function(consumerTag) {
      queueConsumerTag = consumerTag;
    }
    );
  };
subscribePauseRestartConsume();