CODEWITHSUNDEEP
javascriptnode.jsmongoose
realsenorloadenstein
realsenorloadenstein

Reputation: 91

What causes the Node.js EventEmitter warnings?

My node.js backend has a lot of mongoose.connection.useDb lines because it frequently needs to switch databases. Today I restarted my server and got this warnings:

MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 connected listeners added to [NativeConnection].

It took me some time to find a solution, and I found that by just adding mongoose.connection.removeAllListeners() the warning dissapears. What does this mean, and what exactly is an event? It feels like if I remove all listeners that the code that used those databases (like when I needed to create a model for a schema) shouldn't work anymore, but that isn't the case.

Upvotes: 1

Views: 449

Answers (1)

jfriend00
jfriend00

Reputation: 707258

What does this mean?

This warning indicates that you're registering 11 or more listeners for the same event or calling some code that is internally causing this to happen. The warning is because normal usage of an event emitter rarely needs 11 listeners for the same event.

Instead, this many listeners for the same event is usually an indication that you're registering for the same event over and over or repeatedly calling some function that causes the same listener over and over again. In your specific case it looks like you should perhaps use the noListener: true option when calling .useDb(). See the doc here and this troubleshooting report.

Failing to remove listeners that you are no longer using can create a regularly growing memory leak (just like the error message indicates) because nodejs can't tell that you no longer intend to use that listener. It's still registered for the event, the event can still occur and thus the listener (the ones you no longer intend to use) are still active. This takes additional memory inside the eventEmitter object and (perhaps more importantly), it may keep certain closure variables from getting garbage collected inside the listener callback function/scope.

So, whenever an eventEmitter object gets the 11th or more listener for the same event, it issues this warning.

If you show the actual code and context where this warning occurs, we could give you more specific advice about what you should/should not be doing in that code. The above is a generic description/advice because you didn't show your specific code and context.

What is an event?

This warning comes from an eventEmitter object which, given your explanation, is probably being used inside the database whenever you call .useDb(). Something you are doing with the database is causing it to register a listener for an event on an eventEmitter object. If you want to know more about events in the context of an eventEmitter object, I'd suggest reading the eventEmitter documentation.

In a nutshell, an eventEmitter object allows one to listen for events by calling obj.addListener("someEventName", fn) or the identical (but shorter) obj.on("someEventName, fn) where obj is an instance of the EventEmitter class. Then, other code can trigger events obj.emit("someEventName", someData). When an event is triggered, all registered listeners for that specific event name will have their callback called. Nodejs uses event objects and listeners like this in lots of places in its libraries. For example streams use listeners to listener for incoming data as in stream.on('data', myCallback).

Upvotes: 1

Related Questions