MongoError: connection 4 to cluster closed - node.js

Question

I have a function in NodeJS using Mongoose driver like below:
Pseudocode:
function someFn(someParams) {
// Step 1: a couple of very fast mongo queries (in milliseconds)
// Step 2: HUGE CPU processing - think millions of data grouped, mapped, etc. (takes about a minute)
// Step 3: another mongo query which inserts the results from Step 2 into a collection
}
At step 3, I get the following error:
MongoError: connection 4 to cluster closed
at Function.MongoError.create (/home/some-user/my-repo/node_modules/mongodb-core/lib/error.js:29:11)
at TLSSocket.<anonymous> (/home/some-user/my-repo/node_modules/mongodb-core/lib/connection/connection.js:202:22)
at Object.onceWrapper (events.js:293:19)
at emitOne (events.js:101:20)
at TLSSocket.emit (events.js:191:7)
at _handle.close (net.js:513:12)
at Socket.done (_tls_wrap.js:332:7)
at Object.onceWrapper (events.js:293:19)
at emitOne (events.js:101:20)
at Socket.emit (events.js:191:7)
at TCP._handle.close [as _onclose] (net.js:513:12)
My MongoDB connection params are as follows:
mongoose.connect(connStr, {
server: {
socketOptions: {
keepAlive: 300000,
connectTimeoutMS: 300000,
socketTimeoutMS: 300000,
auto_reconnect: true
}
}
});
I don't understand why I'm getting this error at Step 3. Can someone help me out with this, please?

Answer 1

Figured out the issue after hours of debugging. My Step 3 mongoose query had too many documents(in the order of millions, from Step 2). The error from mongoose gives no reason why the connection is closing. A message like Too many documents or Too large query would've gone a long way in saving a lot of time.