MPG are Hard II: Sync Conflicts

We are making a multiplayer game and there is some data that is shared between clients.

The clients are running concurrently.

Even if our data sync was somehow instant we’d still have race conditions.

But our data sync is not instant. At internet scale, we can expect it to take 50 to 150 milliseconds for data to travel from one client to another. As quick as that might be for sending data around the world, its still pretty long. Certainly noticeable to humans, and computers can make millions of decisions in that amount of time! It’s also a good idea to think “it might take minutes”, as this helps you think clearly and code defensively.

So, we have multiple clients that share data and need to coordinate despite delays.

What if...

• start: x = 10

  client A: x = 9

  client B: x = 11

• start: x = 10

  client A: x = 11

  client B: x = 11

• start: a = 1, b = 1

  client A: a = 2, b = 1

  client B: a = 1, b = 2

• boss 1 email: paint the wall red

  boss 2 email: paint the wall blue

When two clients try to change the data, even different parts of the data, at the same time we don’t know what to do.

At the same time?

What does “at the same time” really mean?

• If a client had received all changes from all the other clients when it tried to make a change, then its okay.
• But a client can’t know! It won’t know that a change is incoming until it gets the change!

(If we had a magic system that could instantly notify about incoming data, well, we’d use it to send the data!)

So what the heck do we do?

The data sharing system that powers p5.party is called deepstream.io.