Lockstep

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Send orders, not armies

A strategy game can have hundreds of units moving at once. Sending every unit's position would flood a slow connection.

In lockstep, every machine runs the same simulation. Players send only their commands, like "these 20 archers, attack there", and each machine plays them out identically.

True story: 1500 archers on a modem

In 2001, Paul Bettner and Mark Terrano wrote up how Age of Empires played online over dial-up modems: "1500 Archers on a 28.8".

Each machine ran the same simulation with the same commands. The game ran in turns of about 200 ms, and commands were "sent out for execution two turns in the future", so they reached every machine in time.

Source: Bettner and Terrano: 1500 Archers on a 28.8 (2001), web.cs.wpi.edu

Quick check

In lockstep, what do players send to each other?

  1. Their commands
  2. Every unit's position
  3. A video of the screen
Show the answer

Their commands

Each machine works out the positions itself, from the same commands. That is why 1,500 archers fit through a modem.

Turns

A command given now waits a little before it runs, so every machine has it in time. A click in turn 10, with commands run two turns later, happens in turn 12.

Games hide that delay with instant feedback: the unit answers with a sound at once, and moves a moment later.

Do the sum

turns
turn length: 200 msrun commands: 2 turns laterclick: during turn 10

Kai clicks during turn 10. When do the archers start to move?

  1. In turn 12, about 400 ms later
  2. At once
  3. In turn 11, about 200 ms later
Show the answer

In turn 12, about 400 ms later

Two turns of 200 ms each is about 400 ms. Every machine has the command by then.

Out of sync

If any machine reaches a different result, the games no longer agree. In Age of Empires, a simulation that drifted was "tagged as 'out of sync' and the game stopped".

One tiny difference is enough, like one machine rounding a number differently.

Lockstep or snapshots?

Lockstep sends little data, but everyone waits for the slowest player, and one difference ends the game. Snapshots send more, but a slow player only hurts their own view.

Shard Garden lets players join a garden at any time, so Remy keeps snapshots.

Quick check

In lockstep, one slow player can hold up everyone.

Show the answer

True

Each turn needs everyone's commands. The whole game moves at the pace of the slowest connection.

Not everyone needs everything

With snapshots, the next question is how much each player needs to see.

Bolt says: Next: interest management.

Fact-checked 12 October 2026