🎉 Celebrating 25 Years of GameDev.net! 🎉

Not many can claim 25 years on the Internet! Join us in celebrating this milestone. Learn more about our history, and thank you for being a part of our community!

Back to Networking and Multiplayer

Entities interaction

Gabriele Farina · 2015-09-02T21:21:42

hello, this weekend I went back to an old multiplayer prototype I was working on. For now, I render the world state 100ms back in time, and interpolate the server states to compensate for lag and updating the server-controlled entities smoothly. When the client performs an action, I send the action to the server, roll back the server state to match more or less how the client saw it when performing the action, and execute it. Than I send the response back to the client. Everything seems to work fine, but I have issue when simulating two players interacting with the same entity. Let's assume the entity is a monster, and both players will level up if they kill it. Player A has a very slow connection, Player B a very fast one. A attacks, and a few milliseconds later B does the same. As long as B has a faster connection, B gets its event to the server earlier, get it processed, kills the monster, get the response and levels up. While this is happending, the event from A arrives: the state goes back to when the moster was still alive, the monster gets killed again, and the response is sent back to A and B. A levels up (correctly because he killed the moster first), but than B is in this weird state where he leveled up, but the new server state says it never did and I need to somehow rollback. How do you guys solve normally situation like this one? I could execute the events as they arrive to the server, without rolling back. But than nothing works decently 'cause my game is an action-packed game and I have to keep lag in consideration. Any suggestion? Thanks!

Networking and Multiplayer Programming

Started by gabrielefarina August 31, 2015 12:00 PM

11 comments, last by hplus0603 8 years, 10 months ago

Mussi

4,453

September 01, 2015 10:25 PM

For movement in my game, I allow for some rewind by the server, but actions only happen when they arrive at the server and when the server says it's okay. This prevents the scenario you described in your OP. It gets a little bit tricky with actions that affect movement, but that's manageable.

Wouldn't this create collision detection issues potentially? The player will shoot at the place where he sees the monster (up to 100ms back in time) and then will get the action executed when the monster is probably at a different location.

Yes, but you have to give in somewhere, everything is a trade-off. You need to design your game around these trade-offs. Is shooting in your ARPG really that precise? How often will that scenario present itself / do monsters move about erratically?

gabrielefarina

204

Author

September 02, 2015 06:43 AM

You need to design your game around these trade-offs. Is shooting in your ARPG really that precise? How often will that scenario present itself / do monsters move about erratically?

I think you are right. I don't need shooting to be super precise, so I might solve the issue by having hitboxes big enough to compensate for the fact that the event is being executed later. I might even enlarge these hitboxes only when an entity is moving, thus still having fairly precise collision detection when an entity is standing.

How often will that scenario present itself / do monsters move about erratically?

I think it is another fair point: enemies at the moment tend to not move too much, as long as most of them have ranged attacks, and the ones who don't usually come straight to the player and stop when close enough.

PvP will still be an issue with the approach above, but I think that I could design the PvP-related networking system in a slightly different way, so that it is more precise and compensates for lag.

hplus0603

11,920

September 02, 2015 09:21 PM

how do games that show instant death deal with this kind of situation?

The thing died in either case. You can show the death right away. Crediting the death (as in, giving XP, looting rights, or whatever) can happen a little bit later, after the server knows the "who" is fully determined.

If you want to support armor spells or such, that would change the outcome of the death blow, then some player will end up seeing a thing start its death animation, but then the armor spell kicks in and the thing has hitpoints again -- a "lag induced glitch," which is the draw-back of showing actions right away. If you can wait for the server round-trip for each action (such as RTS-es do, and some RPG-s) then you totally avoid that problem; instead you have control lag.

enum Bool { True, False, FileNotFound };