EventHub with NodeJS SDK - All consumers in ConsumerGroup getting the message

Question

I hope someone can clarify this for me:

I have 2 consumers in the same ConsumerGroup, it is my understanding that they should coordinate between them, but I am having the issue that both consumers are getting all the messages. My code is pretty simple:

const connectionString =...";
const eventHubName = "my-hub-dev";
const consumerGroup = "processor"; 

async function main() {
  const consumerClient = new EventHubConsumerClient(consumerGroup, connectionString, eventHubName);
  const subscription = consumerClient.subscribe({
      processEvents: async (events, context) => {
        for (const event of events) {
          console.log(`Received event...`, event)
        }        
      },
    }
  );

If I run two instances of this consumer code and publish an event, both instances will receive the event.

So my questions are:

• Am I correct in my understanding that only 1 consumer should receive the message?
• Is there anything I am missing here?

Answer 1

The EventHubConsumerClient requires a CheckpointStore that facilitates coordination between multiple clients. You can pass this to the EventHubConsumerClient constructor when you instantiate it.

The @azure/eventhubs-checkpointstore-blob uses Azure Storage Blob to store the metadata and required to coordinate multiple consumers using the same consumer group. It also stores checkpoint data: you can call context.updateCheckpoint with an event and if you stop and start a new receiver, it will continue from the last checkpointed event in the partition that event was associated with.

There's a full sample using the @azure/eventhubs-checkpointstore-blob here: https://github.com/Azure/azure-sdk-for-js/blob/master/sdk/eventhub/eventhubs-checkpointstore-blob/samples/javascript/receiveEventsUsingCheckpointStore.js

Clarification: The Event Hubs service doesn't enforce a single owner for a partition when reading from a consumer group unless the client has specified an ownerLevel. The highest ownerLevel "wins". You can set this in the options bag you pass to subscribe, but if you want the CheckpointStore to handle coordination for you it's best not to set it.