Since upgrading version to 6, disconnected event followed by unhandled promise rejection "mongoServerSelectionError"

[defields923]

Prerequisites

• I have written a descriptive issue title

Mongoose version

6.12.8

Node.js version

18.20.2

MongoDB version

4.17.2

Operating system

None

Operating system version (i.e. 20.04, 11.3, 10)

No response

Issue

Please provide some insight or direction on debugging this issue.

We run a number of Nodejs services in AWS Lambda. My team has migrated the Mongoose version to 6.12.8 about 7 months ago, and at the same time, we upgraded our Nodejs version from 14 to 18. At that time, we noticed an intermittent issue arising; the first lambda invocation will run successfully, then if there is 2-5 minutes before the next invocation, it will then immediately log the mongo 'disconnected' event followed by an 'error' event (both still logging within the context of the previous invocation judging by the apiRequestID) and an Unhandled Promise Rejection 'mongoServerSelectionError'. This causes the entire lambda instance to blow up and need to restart for the next invocation.

We do not know what is causing the serverSelectionError. We have observed the issue in many of our services and lambda handlers. We have tried to recreate the error both locally and within a simplified lambda environment, but we cannot intentionally reproduce it.

I have seen many other reported issues both here on github and elsewhere referring to a serverSelectionError after upgrading to version 6, but I don't believe those were specific to running within a lambda environment.

Here is one example of logs:

I believe the Unhandled Promise Rejection is originating from this piece of code in connections.js:825:
const serverSelectionError = new ServerSelectionError(); this.$initialConnection = promise. then(() => this). catch(err => { this.readyState = STATES.disconnected; if (err != null && err.name === 'MongoServerSelectionError') { err = serverSelectionError.assimilaateError(err); } if (this.listeners('error').length > 0) { immediate(() => this.emit('error', err)); } **throw err;** });

Here is how we are creating the connection:
const connection = await mongoose.createConnection(modelConnectionMap[domain], { maxPoolSize: (options && options.maxPoolSize) ? options.maxPoolSize : 2, minPoolSize: (options && options.minPoolSize) ? options.minPoolSize : 1, connectTimeoutMS: 300000, socketTimeoutMS: 7200000, });

We have a connection per domain, which is only created once when any domain is first referenced.

[defields923]

I have one more finding:

The code I showed in connection.js is definitely what is causing our handler to blow up. That catch block is immediately invoked when the lambda handler is called, preventing us from attempting to reconnect. If I deploy the code with that "throw err" removed, I still see the disconnected event but our code automatically handles the reconnect and it functions fine from there.

I see now that the createConnection method isn't returning a promise by default, so our try/catch isn't handling the promise rejection. I will see if fixing this solves our issue.

[github-actions]

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[billouboq]

@defields923 Did you find a solution to it ?

[defields923]

@defields923 Did you find a solution to it ?

Yes, our code was calling "createConnection" without using "asPromise." On previous version of Mongoose, this did not cause same issue, but after upgrading, we needed to use it. Thanks.