In a multi-user game, the players move figures around a common scene. The state of the game is
replicated at the players’ workstations and at a server, which contains services controlling the
game overall, such as collision detection. Updates are multicast to all replicas.
(i) The figures may throw projectiles at one another and a hit debilitates the unfortunate
recipient for a limited time. What type of update ordering is required here? Hint: consider
the ‘throw’, ‘collide’ and ‘revive’ events.
(ii) The game incorporates magic devices which may be picked up by a player to assist them.
What type of ordering should be applied to the pick-up-device operation?

What will be an ideal response?

i) The event of the collision between the projectile and the figure, and the event of the player being
debilitated (which, we may assume, is represented graphically) should occur in causal order. Moreover,
changes in the velocity of the figure and the projectile chasing it should be causally ordered. Assume that the
workstation at which the projectile was launched regularly announces the projectile’s coordinates, and that the
workstation of the player corresponding to the figure regularly announces the figure’s coordinates and
announces the figure’s debilitation. These announcements should be processed in causal order. (The reader
may care to think of other ways of organising consistent views at the different workstations.)
ii) If two players move to pick up a piece at more-or-less the same time, only one should succeed and
the identity of the successful player should be agreed at all workstations. Therefore total ordering is required.
The most promising architecture for a game such as this is a peer group of game processes, one at each player’s
workstation. This is the architecture most likely to meet the real-time update propagation requirements; it also
is robust against the failure of any one workstation (assuming that at least two players are playing at the same
time).