NodeJS How To Confirm MongoDB Driver Is Not Bkocking

I've asked some general questions around this topic before (node and blocking). This time the question is a little more specific.

Let's say I've got a node/express app which has a handle that is accepting HTTP requests (doesn't matter, say they're simple GETs).

And it has a separate handler which reads messages off of a RabbitMQ queue, as they arrive, and then does a read from Mongo (Mongo is on a different machine), followed by a write.

If Mongo was "very" busy, would/could that cause the HTTP handler to appear unavailable?

I'm using the Mongo native driver. I would think any blocking that is occurring while the Mongo driver waits for a response from the server would have Node happily accepting and handling HTTP requests, but I don't know for sure.

In a related scenario, swap-out a busy Mongo for a handler that reads a Rabbit message and PUTs a record into a "very" busy ElasticSearch. Will that cause issues with the HTTP handler?

I'd go straight to testing it, but that's a little tricky and gets expensive testing every time I'm not sure what the theory is. So I thought I'd ask.

Here's a (simplified) example of the code:

// HTTP handler...
app.post('/eventcapture/event', (req: express.Request, res: express.Response) => {
    var evt: eventDS.IEvent = ('TypeID' in req.body) ? req.body : JSON.parse(req.body);

    //create an id
    evt._id = uuid.v4();

    bus.Publish(evt)
        .then((success) => {
            res.jsonp(200, { success: true });
        })
        .catch((failReason:Error) => {
            console.error('[ERROR] - Failure writing event: %s,%s', failReason.name, failReason.message);
            logError(failReason, evt);

            res.jsonp(500, { success: false, reason: failReason });
        });
});

// We generically define additional handlers in an array, and then kick them off with a loop.
// Here we have one handler which reads an event, goes to mongo to get additional data which
// it adds into the event before publishing it back out.  And a second handler which will catch
// these "augmented" events and push them into Mongo
var processes = [
    {
        enabled: true,
        name: 'augmenter',
        inType: 'EventCapture:RawEvent',
        handler: (event: eventDS.IEvent) => {
            console.log('[LOG] - augment event: %s', event._id);

            Profile.FindOne({ _id: event.User.ProfileID })
                .then((profile) => {
                    if (profile) {
                        console.log('[LOG] - found Profile: %s', profile._id);
                        event.User.Email = profile.PersonalDetail.Email;
                        //other values also...

                        //change the TypeID for publishing
                        event.TypeID = 'EventCapture:AugmentedEvent';

                        return event;
                    }
                    else throw new Error(util.format('unable to find profile: %s', event.User.ProfileID));
                })
                .then((augmentedEvent) => bus.Publish(augmentedEvent)) //publish the event back out
                .catch((failReason:Error) => {
                    console.error('[ERROR] - failure publishing augmented event: %s, %s, %s', event._id, failReason.name, failReason.message);
                    logError(failReason, event);
                });
        }
    },
    {
        enabled: true,
        name: 'mongo',
        inType: 'EventCapture:AugmentedEvent',
        handler: (event: eventDS.IEvent) => {
            console.log('[LOG] - push to mongo: %s', event.User.ProfileID);

            Event.Save(event, { safe: true })
                .then((success) => console.log('[LOG] - pushed to mongo: %s', event._id))
                .catch((failReason:Error) => {
                    console.error('[ERROR] - failure pushing to mongo: %s, %s', event._id, failReason);
                    logError(failReason, event);
                });
        }
    }
];

processes.forEach((process, idx, allProcesses) => {
    if (process.enabled) {
        bus.Subscribe(process.name, process.inType, process.handler);
    }
});