MongoDb Node.js slow concurrent queries - node.js

Question

When I run concurrent MongoDb queries using Node.js, the second query always takes ~2 seconds to return. Using explain(), executionTimeMillis always returns 0ms, which is absolutely normal as my test collection has only 2 entries. Here's my reduced testcase:
'use strict'
const { MongoClient } = require('mongodb')
const main = async () => {
const client = new MongoClient('mongodb://admin:123456#localhost:27017/', {
useNewUrlParser: true,
useUnifiedTopology: true,
})
await client.connect()
const db = client.db('test')
const numbers = db.collection('numbers')
const promises = []
console.time()
for (let i = 0; i < 3; i++) {
promises.push(numbers.find({ number: i }).explain())
}
for (const promise of promises) {
console.log(await promise)
console.timeLog()
}
console.timeEnd()
await client.close()
}
main()
Output:
{
queryPlanner: {
plannerVersion: 1,
namespace: 'test.numbers',
indexFilterSet: false,
parsedQuery: { number: [Object] },
winningPlan: { stage: 'FETCH', inputStage: [Object] },
rejectedPlans: []
},
executionStats: {
executionSuccess: true,
nReturned: 1,
executionTimeMillis: 0,
totalKeysExamined: 1,
totalDocsExamined: 1,
executionStages: {
stage: 'FETCH',
nReturned: 1,
executionTimeMillisEstimate: 0,
works: 2,
advanced: 1,
needTime: 0,
needYield: 0,
saveState: 0,
restoreState: 0,
isEOF: 1,
invalidates: 0,
docsExamined: 1,
alreadyHasObj: 0,
inputStage: [Object]
},
allPlansExecution: []
},
serverInfo: {
host: 'DESKTOP-C7CAL9N',
port: 27017,
version: '4.0.10',
gitVersion: 'c389e7f69f637f7a1ac3cc9fae843b635f20b766'
},
ok: 1
}
default: 32.252ms
{
queryPlanner: {
plannerVersion: 1,
namespace: 'test.numbers',
indexFilterSet: false,
parsedQuery: { number: [Object] },
winningPlan: { stage: 'FETCH', inputStage: [Object] },
rejectedPlans: []
},
executionStats: {
executionSuccess: true,
nReturned: 1,
executionTimeMillis: 0,
totalKeysExamined: 1,
totalDocsExamined: 1,
executionStages: {
stage: 'FETCH',
nReturned: 1,
executionTimeMillisEstimate: 0,
works: 2,
advanced: 1,
needTime: 0,
needYield: 0,
saveState: 0,
restoreState: 0,
isEOF: 1,
invalidates: 0,
docsExamined: 1,
alreadyHasObj: 0,
inputStage: [Object]
},
allPlansExecution: []
},
serverInfo: {
host: 'DESKTOP-C7CAL9N',
port: 27017,
version: '4.0.10',
gitVersion: 'c389e7f69f637f7a1ac3cc9fae843b635f20b766'
},
ok: 1
}
default: 2042.929ms
{
queryPlanner: {
plannerVersion: 1,
namespace: 'test.numbers',
indexFilterSet: false,
parsedQuery: { number: [Object] },
winningPlan: { stage: 'FETCH', inputStage: [Object] },
rejectedPlans: []
},
executionStats: {
executionSuccess: true,
nReturned: 0,
executionTimeMillis: 0,
totalKeysExamined: 0,
totalDocsExamined: 0,
executionStages: {
stage: 'FETCH',
nReturned: 0,
executionTimeMillisEstimate: 0,
works: 1,
advanced: 0,
needTime: 0,
needYield: 0,
saveState: 0,
restoreState: 0,
isEOF: 1,
invalidates: 0,
docsExamined: 0,
alreadyHasObj: 0,
inputStage: [Object]
},
allPlansExecution: []
},
serverInfo: {
host: 'DESKTOP-C7CAL9N',
port: 27017,
version: '4.0.10',
gitVersion: 'c389e7f69f637f7a1ac3cc9fae843b635f20b766'
},
ok: 1
}
default: 2062.851ms
default: 2063.513ms
If I run queries consequentially, each query takes only some milliseconds to return. Then why is the 2 seconds response time?
Edit:
In the first for loop, I made/ran "concurrent" queries promises.push(numbers.find({ number: i }).explain()). In the second for loop, I wait for promises to resolve one after another but that doesn't mean that a promise must wait till the previous one resolved to begin its job.
To avoid misunderstandings, I've made a little changes to my code, replacing the two for loops with this:
for (let i = 0; i < 3; i++) {
promises.push(
numbers
.find({ number: i })
.explain()
.then(result => {
// console.log(result)
console.log('query index:', i)
console.timeLog()
})
)
}
await Promise.all(promises)
Output:
query index: 0
default: 22.040ms
query index: 2
default: 2032.921ms
query index: 1
default: 2034.682ms
default: 2035.260ms
Edit 2:
For further clarification, I use labels to denote timers.
for (let i = 0; i < 3; i++) {
console.time(`query index: ${ i }`)
promises.push(
numbers
.find({ number: i })
.explain()
.then(result => {
// console.log(result)
console.timeEnd(`query index: ${ i }`)
})
)
}
await Promise.all(promises)
Output:
query index: 0: 12.692ms
query index: 1: 2015.143ms
query index: 2: 2015.310ms

Answer 1

Set MongoClient's poolSize to 1.