Which approach to choose for messenger architecture? - node.js

Question

We are writing an instant messenger for private use in our application. The planned load of the current version is hundreds of users, maybe a thousand or two.
As a database, I use Mongo. Messages are written in messages (linked via chats_id with user_chats and uid with an external table in the users database). The arrival of a message on the client creates a flurry of events in response (mark read). It is clear that I need to make a queue. How to do it better? Are there methods to prioritize queues?
How many users should the spherical process of a node in a vacuum withstand? How much memory should it use? I'm using pm2 for process management and node-ipc for broadcast.
Where to turn, who to ask to optimize requests to the database. Now there is a quick written construction of the kind illustrated below. This example requests all chats, for each chat it selects the last message, the number of unread messages and the last users seen. In MySQL I would do it with one big request. How is it better to do this in Mongo? My level with Mongo is still in the middle.
The architecture itself. Now the action from the client comes and goes to the handler who understands what to do with it and who needs to be notified about this action. Usually the handler notifies the corresponding manager, and it already writes to the database and, if necessary, notifies the other processes.
const loadUserChatsStatistic = async (chatIds, userId) => {
const startExecTime = new Date();
const userChats = await db()
.collection("user_chats")
.aggregate([
{ $match: { uid: userId, chats_id: { $in: chatIds.map(ObjectID) } } },
{
$lookup: {
from: "chats",
localField: "chats_id",
foreignField: "_id",
as: "chat"
}
},
{ $unwind: "$chat" },
{
$lookup: {
from: "user_last_seen",
localField: "chats_id",
foreignField: "chats_id",
as: "last_seens"
}
}
])
.toArray();
const chats = await Promise.all(
userChats.map(async userChat => {
const usersCount = await db()
.collection("user_chats")
.find({ chats_id: userChat.chats_id })
.count();
let unreadCountQuery = {};
if (userChat.last_read) {
unreadCountQuery = { _id: { $gt: userChat.last_read } };
} else {
unreadCountQuery = { _id: { $gt: userChat._id } };
}
const unreadCount = await db()
.collection("messages")
.find({ ...unreadCountQuery, chats_id: userChat.chats_id })
.count();
const message =
(await db()
.collection("messages")
.findOne({ chats_id: userChat.chats_id }, { sort: { _id: -1 } })) ||
{};
return { ...userChat, usersCount, unreadCount, message };
})
);
console.log(
"Execution time for loadUserChatsStatistic",
new Date() - startExecTime
);
return chats.sort((a, b) => {
if (!a.message._id) {
return true;
}
if (!b.message._id) {
return false;
}
return (
new Date(ObjectID(a.message._id).getTimestamp()) <
new Date(ObjectID(b.message._id).getTimestamp())
);
});
};