Connection count not responding to maxIdleTimeMS

[coleweinman]

Prerequisites

• I have written a descriptive issue title

Mongoose version

8.13.2

Node.js version

18

MongoDB version

8.0.8

Operating system

macOS

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

No response

Issue

I'm debugging an issue where my connection count increases over time like a memory leak. I'm using maxIdleTimeMS but after some testing it's not working the way I understand it should.

When I create a connection and run a query my connection count increases by a few (21 -> 26). Now, after maxIdleTimeMS has gone by, I continue to get the same count (26).

However, if I initiate another query on this connection, all of a sudden my connections drop (24). If I don't specify maxIdleTimeMS, the connection count remains the same (26).

At first I thought maybe I needed to adjust something like heartbeatFrequencyMS but that didn't change anything.

It looks like the "idle" connections aren't identifying themselves until a subsequent request is made to the database. Is this expected? Am I missing something? My issue is I need the connections to close automatically without subsequent requests.

const testC = await mongoose.connect(
      'mongodb+srv://....',
      {
        maxIdleTimeMS: 3000,
        heartbeatFrequencyMS: 3000,
        socketTimeoutMS: 3000,
      },
    );
    // 21
    const Question = testC.model('Question', QuestionSchema);
    await Question.find({});
    // 26
    await new Promise((resolve) => setTimeout(resolve, 5000));
    await Question.find({});
    // 24
    await testC.disconnect();
    // 21

[coleweinman]

Upon further testing I noticed that if I added minPoolSize it will work like I expected. Is this correct? Is there a way it can scale down to zero?

[vkarpov15]

This all appears to be expected with how the MongoDB Node driver works internally here: https://github.com/mongodb/node-mongodb-native/blob/28857b79898d6a6040b7d48f3781ebc2710f0279/src/cmap/connection_pool.ts

1. connectionIsIdle is the only place that uses maxIdleTimeMS
2. destroyConnectionIfPerished destroys idle connections based on connectionIsIdle
3. destroyConnectionIfPerished only runs in ensureMinPoolSize() (which is a no-op if minPoolSize === 0) or processWaitQueue(), which runs when a new operation runs. The minPoolSize === 0 check seems a bit fishy, I'll ask MongoDB about that.

"I'm debugging an issue where my connection count increases over time like a memory leak." <-- does your connection count go above your maxPoolSize, which is 100 by default? The MongoDB Node driver does grow/shrink the number of connections between minPoolSize and maxPoolSize based on how many operations you run.

[vkarpov15]

Looks like there is already a ticket in the MongoDB driver JIRA for the minPoolSize === 0 check here: https://jira.mongodb.org/browse/NODE-6589.