I started using Azure recently and It has been an overwhelming experience. I started experimenting with eventhubs and I'm basically following the official tutorials on how to send and receive messages from eventhubs using nodejs.
Everything worked perfectly so I built a small web app (static frontend app) and I connected it with a node backend, where the communication with eventhubs occurs. So basically my app is built like this:
frontend <----> node server <-----> eventhubs
As you can see it is very simple. The node server is fetching data from eventhubs and sending it forward to the frontend, where the values are shown. It is a cool experience and I'm enjoying MS Azure until this error occured:
azure.eventhub.common.EventHubError: ErrorCodes.ResourceLimitExceeded: Exceeded the maximum number of allowed receivers per partition in a consumer group which is 5. List of connected receivers - nil, nil, nil, nil, nil.
This error is really confusing. Im using the default consumer group and only one app. I never tried to access this consumer group from another app. It said the limit is 5, I'm using only one app so it should be fine or am I missing something? I'm not checking what is happening here.
I wasted too much time googling and researching about this but I didn't get it. At the end, I thought that maybe every time I deploy the app (my frontend and my node server) on azure, this would be counted as one consumer and since I deployed the app more than 5 times then this error is showing up. Am I right or this is nonsense?
I'm using websockets as a communication protocol between my app (frontend) and my node server (backend). The node server is using the default consumer group ( I didn't change nothing), I just followed this official example from Microsoft. I'm basically using the code from MS docs that's why I didn't post any code snippet from my node server and since the error happens in backend and not frontend then it will not be helpful if I posted any frontend code.
So to wrap up, I'm using websocket to connect front & backend. It works perfectly for a day or two and then this error starts to happen. Sometimes I open more than one client (for example a client from the browser and client from my smartphone).
I think I don't understand the concept of this consumer group. Like is every client a consumer? so if I open my app (the same app) in 5 different tabs in my browser, do I have 5 consumers then?
I didn't quite understand the answer below and what is meant by "pooling client", therefore, I will try to post code examples here to show you what I'm trying to do.
Here is the function I'm using on the server side to communicate with eventhubs and receive/consume a message
async function receiveEventhubMessage(socket, eventHubName, connectionString) {
const consumerClient = new EventHubConsumerClient(consumerGroup, connectionString, eventHubName);
const subscription = consumerClient.subscribe({
processEvents: async (events, context) => {
for (const event of events) {
console.log("[ consumer ] Message received : " + event.body);
io.emit('msg-received', event.body);
}
},
processError: async (err, context) => {
console.log(`Error : ${err}`);
}
}
);
If you notice, I'm giving the eventhub and connection string as an argument in order to be able to change that. Now in the frontend, I have a list of multiple topics and each topic have its own eventhubname but they have the same eventhub namespace.
Here is an example of two eventhubnames that I have:
{
"EventHubName": "eh-test-command"
"EventHubName": "eh-test-telemetry"
}
If the user chooses to send a command (from the frontend, I just have a list of buttons that the user can click to fire an event over websockets) then the CommandEventHubName will be sent from the frontend to the node server. The server will receive that eventhubname and switch the consumerClient in the function I posted above.
Here is the code where I'm calling that:
// io is a socket.io object
io.on('connection', socket => {
socket.on('onUserChoice', choice => {
// choice is an object sent from the frontend based on what the user choosed. e.g if the user choosed command then choice = {"EventhubName": "eh-test-command", "payload": "whatever"}
receiveEventhubMessage(socket, choice.EventHubName, choice.EventHubNameSpace)
.catch(err => console.log(`[ consumerClient ] Error while receiving eventhub messages: ${err}`));
}
}
The app I'm building will be extending in the future to a real use case in the automotive field, that's why this is important for me. Therefore, I'm trying to figure out how can I switch between eventhubs without creating a new consumerClient each time the eventhubname changes?
I must say that I didn't understand the example with the "pooling client". I am seeking more elaboration or, ideally, a minimal example just to put me on the way.
Based on the conversation in the issue, it would seem that the root cause of this is that your backend is creating a new EventHubConsumerClient for each request coming from your frontend. Because each client will open a dedicated connection to the service, if you have more than 5 requests for the same Event Hub instance using the same consumer group, you'll exceed the quota.
To get around this, you'll want to consider pooling your EventHubConsumerClient instances so that you're starting with one per Event Hub instance. You can safely use the pooled client to handle a request for your frontend by calling subscribe. This will allow you to share the connection amongst multiple frontend requests.
The key idea being that your consumerClient is not created for every request, but shares an instance among requests. Using your snippet to illustrate the simplest approach, you'd end up hoisting your client creation to outside the function to receive. It may look something like:
const consumerClient = new EventHubConsumerClient(consumerGroup, connectionString, eventHubName);
async function receiveEventhubMessage(socket, eventHubName, connectionString) {
const subscription = consumerClient.subscribe({
processEvents: async (events, context) => {
for (const event of events) {
console.log("[ consumer ] Message received : " + event.body);
io.emit('msg-received', event.body);
}
},
processError: async (err, context) => {
console.log(`Error : ${err}`);
}
}
);
That said, the above may not be adequate for your environment depending on the architecture of the application. If whatever is hosting receiveEventHubMessage is created dynamically for each request, nothing changes. In that case, you'd want to consider something like a singleton or dependency injection to help extend the lifespan.
If you end up having issues scaling to meet your requests, you can consider increasing the number of clients for each Event Hub and/or spreading requests out to different consumer groups.