pool settings ignored with mongoDB

[edorgeville]

Describe the bug
I am trying to limit the number of concurrent connections. I used the pool parameter for MikroORM.init, but it doesn't seem to be used. Instead, 11 connections are opened. I am guessing one is the main em, the 10 others being each RequestContext, but then those connections get reused (?).

To Reproduce

import { MikroORM, RequestContext } from '@mikro-orm/core'
import { MongoDriver } from '@mikro-orm/mongodb'
import { User } from './entities/User'
import { name, random } from 'faker'
import delay from 'delay'

const entrypoint = async () => {
  const orm = await MikroORM.init<MongoDriver>({
    entities: [User],
    dbName: 'main',
    type: 'mongo',
    implicitTransactions: false,
    forceEntityConstructor: true,
    pool: {
      min: 1,
      max: 1
    }
  })

  // Cleanup
  await orm.em.nativeDelete(User, {})


  console.log('Create a user in 500 setImmediate')
  for (let i = 0; i < 500; i++) {
    setImmediate(async () => {
      await RequestContext.createAsync(orm.em, async () => {
        const user = new User()
        user.name = name.findName()
        console.log(i, user)
        await RequestContext.getEntityManager().persistAndFlush(user)
      })
    })
  }

  await delay(100)
  await orm.close()
}


entrypoint()

MongoDB logs

I  NETWORK  [listener] connection accepted from 172.27.0.1:59734 #1 (1 connection now open)
I  NETWORK  [conn1] received client metadata from 172.27.0.1:59734 conn1: { driver: { name: "nodejs", version: "3.6.6" }, os: { type: "Windows_NT", name: "win32", architecture: "x64", version: "10.0.19042" }, platform: "'Node.js v14.17.3, LE (unified)" }      
I  NETWORK  [listener] connection accepted from 172.27.0.1:59738 #2 (2 connections now open)
I  NETWORK  [conn2] received client metadata from 172.27.0.1:59738 conn2: { driver: { name: "nodejs", version: "3.6.6" }, os: { type: "Windows_NT", name: "win32", architecture: "x64", version: "10.0.19042" }, platform: "'Node.js v14.17.3, LE (unified)" }      
I  SHARDING [conn2] Marking collection main.user as collection version: <unsharded>
I  NETWORK  [listener] connection accepted from 172.27.0.1:59742 #3 (3 connections now open)
I  NETWORK  [listener] connection accepted from 172.27.0.1:59746 #4 (4 connections now open)
I  NETWORK  [listener] connection accepted from 172.27.0.1:59750 #5 (5 connections now open)
I  NETWORK  [listener] connection accepted from 172.27.0.1:59754 #6 (6 connections now open)
I  NETWORK  [listener] connection accepted from 172.27.0.1:59758 #7 (7 connections now open)
I  NETWORK  [listener] connection accepted from 172.27.0.1:59762 #8 (8 connections now open)
I  NETWORK  [listener] connection accepted from 172.27.0.1:59768 #9 (9 connections now open)
I  NETWORK  [listener] connection accepted from 172.27.0.1:59770 #10 (10 connections now open)
I  NETWORK  [listener] connection accepted from 172.27.0.1:59774 #11 (11 connections now open)
I  NETWORK  [conn1] end connection 172.27.0.1:59734 (10 connections now open)
I  NETWORK  [conn3] end connection 172.27.0.1:59742 (9 connections now open)
I  NETWORK  [conn4] end connection 172.27.0.1:59746 (8 connections now open)
I  NETWORK  [conn8] end connection 172.27.0.1:59762 (7 connections now open)
I  NETWORK  [conn5] end connection 172.27.0.1:59750 (6 connections now open)
I  NETWORK  [conn11] end connection 172.27.0.1:59774 (5 connections now open)
I  NETWORK  [conn7] end connection 172.27.0.1:59758 (4 connections now open)
I  NETWORK  [conn6] end connection 172.27.0.1:59754 (3 connections now open)
I  NETWORK  [conn9] end connection 172.27.0.1:59768 (2 connections now open)
I  NETWORK  [conn10] end connection 172.27.0.1:59770 (1 connection now open)
I  NETWORK  [conn2] end connection 172.27.0.1:59738 (0 connections now open)

Expected behavior
I expected a single connection to be created and used.

Versions

Dependency	Version
node	14.17.3
typescript	4.3.4
@mikro-orm/core	4.5.6
@mikro-orm/mongodb	4.5.6

[B4nan]

That is a knex configuration (better say tarn configuration), not used in mongo driver at all. Based on the mongodb typings, it looks like the options are called poolSize and minSize (or maxPoolSize and minPoolSize, not sure what is the real difference for us).

You can use driverOptions to set anything that is not directly supported in the ORM configuration.

But based on the docs, the default pool size should be 5, so it sounds weird you would be getting 11 connections. But even if that is not working, it would be an upstream issue.

[edorgeville]

As always, thanks for the blazing fast answer! 🚀
Adding driverOptions: { poolSize: n } to the MikroORM.init<MongoDriver> options seems to work. Oddly, there seems to always be n+1 connections. I'll look deeper into this and report back with my findings. Might just be user error 😅

[B4nan]

Let's keep this open, we can definitely map the pool: {min, max} to the mongo connection settings. Can you please verify whether we want the poolSize and minSize, or the maxPoolSize and minPoolSize? I guess it should be safe to set both of them at the same time?

[edorgeville]

I can confirm that driverOptions: { minPoolSize: n, maxPoolSize: n } work identically. Still have the n+1 issue but let's put this aside for now.
minPoolSize and maxPoolSize seem to be documented properly, but I can't find minSize nor poolSize. Possibly deprecated?