Severe performance drop with MongoDB Change Streams

Question

I want to get real-time updates about MongoDB database changes in Node.js.

A single MongoDB change stream sends update notifications almost instantly. But when I open multiple (10+) streams, there are massive delays (up to several minutes) between database writes and notification arrival.

That's how I set up a change stream:

let cursor = collection.watch([   {$match: {"fullDocument.room": roomId}}, ]); cursor.stream().on("data", doc => {...}); 

I tried an alternative way to set up a stream, but it's just as slow:

let cursor = collection.aggregate([   {$changeStream: {}},   {$match: {"fullDocument.room": roomId}}, ]); cursor.forEach(doc => {...}); 

An automated process inserts tiny documents into the collection while collecting performance data.

Some additional details:

• Open stream cursors count: 50
• Write speed: 100 docs/second (batches of 10 using insertMany)
• Runtime: 100 seconds
• Average delay: 7.1 seconds
• Largest delay: 205 seconds (not a typo, over three minutes)
• MongoDB version: 3.6.2
• Cluster setup #1: MongoDB Atlas M10 (3 replica set)
• Cluster setup #2: DigitalOcean Ubuntu box + single instance mongo cluster in Docker
• Node.js CPU usage: <1%

Both setups produce the same issue. What could be going on here?

Answer 1

The default connection pool size in the Node.js client for MongoDB is 5. Since each change stream cursor opens a new connection, the connection pool needs to be at least as large as the number of cursors.

In version 3.x of the Node Mongo Driver use 'poolSize':

const mongoConnection = await MongoClient.connect(URL, {poolSize: 100}); 

In version 4.x of the Node Mongo Driver use 'minPoolSize' and 'maxPoolSize':

const mongoConnection = await MongoClient.connect(URL, {minPoolSize: 100, maxPoolSize: 1000}); 

(Thanks to MongoDB Inc. for investigating this issue.)