I guess this could go into the Monster AI sub, but not sure,

Long story short, I have a web browser based game similar to Diablo 2. Everything is server-side and synced with nodejs using websockets.

The game is instanced based and every time a player moves, their position is updated and sent to everyone in that current game (real time).

Here is a simple dev screen to show what I'm trying to talk about:

That monster is piped from the gameserver into this users game. I am at the point where I can run a global game loop that runs every 1.3 seconds and check if the monster is next to the player (to start attacking it), but that's boring! I want monster AI where the mob moves around and attacks the player.

I can obviously do that client side within JavaScript. But, my problem is how do I do that serverside? For example, if the monster starts to agro towards the player, there is no way my server can handle every single request of that monster moving to validate it. It would be thousands of requests. There has to be some type of Monster / Client Prediction right? That's where I'm stuck. Not looking for code, just some design philosophy insights.

First, you shouldn't path for enemies that are far away from players (like, way beyond the screen range). Next, you need to share movement information between clients instead of recalculating it for each user's session. You calculate the movement, and send that to each client within range. You shouldn't be getting thousands of re-validations for movement unless you have thousands of monsters interacting with players.

Why update hundreds of enemies when there are only a few onscreen?

Only update what can be seen by the player, this also prevents some cheating.

Thanks everyone for the responses. I do only update the monsters when a player is in range and currently do what @SeanMiddleditch said using a AI ticker.

I cannot do the peer to peer topology as I'm using Websockets (Browser based). But I wish I could.

To iterate a bit about my problem, I just don't know how to track the monster movements.

For example, if a player get's in range of a mob (The global Monster AI ticker will detect this on the server and start dealing damage to the player). (Runs every 1300 ms)

But, how do I go about having the monster inch its way closer and closer to the player? (agro) If I want the server to be authoritative the ticker would have to be running almost instantly and as @hplus said running that would eat up nodes resources quite fast. Especially when there's more than 1 game open. Would that be the only way? That's why I'm kind of looking for monster AI prediction / movement but somehow synced with node? I mean, when a player uses a skill I check for cooldowns, and the distance to a monster server-side to prevent naughty stuff, but just not sure how to make the server authoritative to track monster movements and AI. I'm a bit lost, but that's kind of where I'm coming from, I'll take a look at the archers link.