Matchmaking, queues and ratings

What it is#

A MatchTicket is a party waiting. A MatchPool is which of them a queue will consider. An IMatchFunction is the game's code that proposes matches, an IMatchEvaluator settles two proposals that want the same ticket, and a Matchmaker runs the cycle. IRatingModel ships twice: Elo and a TrueSkill-family Bayesian model.

IQueueGrain is the same cycle as a fleet-wide single writer, one grain per queue id, with QueueTicket and QueueMatch as its vocabulary.

What it is for#

Filling a battleground, an arena or a dungeon group. Doc 28 takes Open Match's separation of *filtering, proposing, evaluating and allocating* and leaves its Kubernetes deployment behind.

Using it#

Build a Matchmaker with a pool and your own IMatchFunction; Enqueue tickets; Cycle on a timer; handle Matched by calling doc 27's placement.

⚠ Allocation is not here. IMapGrain.Place is doc 27's, and a second allocator is what this must not become — two of them disagree about capacity and the disagreement is a shard nobody can join.

⚠ A ticket is a party, never a player. That is how "a party is never split" is a property of the types rather than a rule: nothing downstream is ever handed one member of a group.

⚠ default(MatchPool) admits nobody. A positional record struct's parameter defaults belong to its constructor, not to default — use MatchPool.Everybody or pass null.

⚠ The evaluator's oldest-first tie-break is what bounds a queue time, and so is the widening band, which uses the *wider* of two tickets' bands so a long-waiting player can still meet a newcomer.

⚠ Both rating models refuse a free-for-all rather than approximating one. Elo rates a team as the *mean* of its players, not the sum.

One queue, one writer, and the scoring stays a pure function#

Matchmaker is testable without a silo and IQueueGrain is the scheduling decision on top of it — the same relationship PlacementDirector has to IMapGrain. ⚠ A matchmaker only exercisable inside a silo is one nobody tests.

Four rules the grain adds, each for a failure the pure function cannot see:

⚠ Formed is not started. A roster still needs a shard and allocating one can fail, so the tickets are *held* and the caller confirms or abandons. It is a reservation, at a different scale from L2's.

⚠ Abandoning keeps the original enqueue time. Otherwise a ticket is punished for a failure that was the fleet's — you wait twenty minutes, the allocation fails, and you go to the back of the queue.

⚠ Backfill is preferred to a new match, and Cycle processes backfills first for that reason. A running game with an empty seat is a worse experience than one that has not started.

⚠ A ticket is a party and never a player, which is what makes *"never split a group"* a property of the types rather than a rule somebody has to remember in the match function.

Examples#

A queue with a filtered pool:

Handing an accepted match to placement:

Rating a result:

See also#

• PvP — one of its two callers.
• Instances — the other.
• Gameplay tags — what a pool filters on.