backend/internal/game/cache.go

@@ -63,6 +63,7 @@ type gameCache struct {
 
 type cachedGame struct {
 	game       *engine.Game
+	seats      []Seat
 	variant    string
 	lastAccess time.Time
 }
@@ -71,24 +72,27 @@ func newGameCache(ttl time.Duration, now func() time.Time) *gameCache {
 	return &gameCache{entries: make(map[uuid.UUID]*cachedGame), ttl: ttl, now: now}
 }
 
-// get returns the live game for id and refreshes its idle timer, or (nil, false).
+// get returns the live game and its immutable seat list for id and refreshes its idle
-func (c *gameCache) get(id uuid.UUID) (*engine.Game, bool) {
+// timer, or (nil, nil, false). The seats let a read check membership (and label seats)
+// without re-loading the game from the store, since seats never change after a game starts.
+func (c *gameCache) get(id uuid.UUID) (*engine.Game, []Seat, bool) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	e, ok := c.entries[id]
 	if !ok {
-		return nil, false
+		return nil, nil, false
 	}
 	e.lastAccess = c.now()
-	return e.game, true
+	return e.game, e.seats, true
 }
 
-// put stores g as the live game for id. variant labels the entry so the active-
+// put stores g as the live game for id together with its seat list. variant labels the
-// games gauge can report counts by variant without inspecting engine internals.
+// entry so the active-games gauge can report counts by variant without inspecting engine
-func (c *gameCache) put(id uuid.UUID, g *engine.Game, variant string) {
+// internals; seats are the game's immutable seat standings for the membership fast path.
+func (c *gameCache) put(id uuid.UUID, g *engine.Game, variant string, seats []Seat) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	c.entries[id] = &cachedGame{game: g, variant: variant, lastAccess: c.now()}
+	c.entries[id] = &cachedGame{game: g, seats: seats, variant: variant, lastAccess: c.now()}
 }
 
 // remove drops id from the cache (used on a finished game and after a failed

backend/internal/game/helpers_test.go

@@ -94,8 +94,8 @@ func TestGameCacheEviction(t *testing.T) {
 	cur := time.Unix(1_700_000_000, 0)
 	cache := newGameCache(time.Hour, func() time.Time { return cur })
 	id := uuid.New()
-	cache.put(id, nil, "scrabble_en")
+	cache.put(id, nil, "scrabble_en", nil)
-	if _, ok := cache.get(id); !ok {
+	if _, _, ok := cache.get(id); !ok {
 		t.Fatal("game must be resident after put")
 	}
 	cur = cur.Add(30 * time.Minute)
@@ -104,7 +104,7 @@ func TestGameCacheEviction(t *testing.T) {
 	if n := cache.sweep(); n != 1 {
 		t.Errorf("sweep evicted %d, want 1", n)
 	}
-	if _, ok := cache.get(id); ok {
+	if _, _, ok := cache.get(id); ok {
 		t.Error("game must be evicted after idle TTL")
 	}
 	if cache.size() != 0 {

backend/internal/game/service.go

@@ -287,12 +287,12 @@ func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, erro
 	if err := svc.store.CreateGame(ctx, ins, seats, seeding.draws); err != nil {
 		return Game{}, err
 	}
-	svc.cache.put(id, g, params.Variant.String())
 	svc.metrics.recordStarted(ctx, params.Variant, params.VsAI)
 	created, err := svc.store.GetGame(ctx, id)
 	if err != nil {
 		return Game{}, err
 	}
+	svc.cache.put(id, g, params.Variant.String(), created.Seats)
 	// Honest-AI game seated with a robot: if the robot moves first, reply at once
 	// (the periodic driver is the fallback). No-op for every human-only game.
 	svc.triggerAI(created)
@@ -890,26 +890,35 @@ func (svc *Service) timeoutGame(ctx context.Context, gameID uuid.UUID, now time.
 // EvaluatePlay previews a tentative play for a seated player against the current
 // board without committing it: whether it is legal and what it would score.
 func (svc *Service) EvaluatePlay(ctx context.Context, gameID, accountID uuid.UUID, tiles []engine.TileRecord) (EvalResult, error) {
-	pre, err := svc.store.GetGame(ctx, gameID)
-	if err != nil {
-		return EvalResult{}, err
-	}
-	if _, ok := pre.seatOf(accountID); !ok {
-		return EvalResult{}, ErrNotAPlayer
-	}
-	if pre.Status == StatusFinished {
-		return EvalResult{}, ErrFinished
-	}
-
 	unlock := svc.locks.lock(gameID)
 	defer unlock()
-	g, err := svc.liveGame(ctx, pre)
+
-	if err != nil {
+	// Hot path: an active game stays cached — the engine game is mutated in place across
-		return EvalResult{}, err
+	// moves and evicted only when it finishes — so on a hit the cached live game and its
+	// immutable seat list answer the membership check and the score with no DB read. This
+	// preview is fired on every tile placement, the hottest gameplay call at scale.
+	g, seats, ok := svc.cache.get(gameID)
+	if !ok {
+		// Cold path: load and validate from the store, then replay into the cache.
+		pre, err := svc.store.GetGame(ctx, gameID)
+		if err != nil {
+			return EvalResult{}, err
+		}
+		if pre.Status == StatusFinished {
+			return EvalResult{}, ErrFinished
+		}
+		if g, err = svc.liveGame(ctx, pre); err != nil {
+			return EvalResult{}, err
+		}
+		seats = pre.Seats
 	}
+	if !seatedIn(seats, accountID) {
+		return EvalResult{}, ErrNotAPlayer
+	}
+
 	validateStart := time.Now()
 	rec, err := g.EvaluatePlay(tiles)
-	svc.metrics.recordValidate(ctx, pre.Variant, validateStart)
+	svc.metrics.recordValidate(ctx, g.Variant(), validateStart)
 	if err != nil {
 		if errors.Is(err, engine.ErrIllegalPlay) {
 			return EvalResult{Valid: false}, nil
@@ -1359,7 +1368,7 @@ func (svc *Service) ExportGCG(ctx context.Context, gameID uuid.UUID) (string, er
 // liveGame returns the live engine.Game for pre, rebuilding it from the journal
 // on a cache miss. Callers must hold the per-game lock.
 func (svc *Service) liveGame(ctx context.Context, pre Game) (*engine.Game, error) {
-	if g, ok := svc.cache.get(pre.ID); ok {
+	if g, _, ok := svc.cache.get(pre.ID); ok {
 		return g, nil
 	}
 	g, err := svc.replay(ctx, pre)
@@ -1374,7 +1383,7 @@ func (svc *Service) liveGame(ctx context.Context, pre Game) (*engine.Game, error
 		}
 	}
 	if !g.Over() {
-		svc.cache.put(pre.ID, g, pre.Variant.String())
+		svc.cache.put(pre.ID, g, pre.Variant.String(), pre.Seats)
 	}
 	return g, nil
 }

backend/internal/game/store.go

@@ -355,27 +355,33 @@ func (s *Store) ExpiredOpen(ctx context.Context, now time.Time) ([]OpenGame, err
 // GetGame loads the games row joined with its seats (ordered by seat), or
 // ErrNotFound.
 func (s *Store) GetGame(ctx context.Context, id uuid.UUID) (Game, error) {
-	gstmt := postgres.SELECT(table.Games.AllColumns).
+	// One round-trip: the game joined with its seats. A LEFT JOIN keeps a (would-be)
-		FROM(table.Games).
+	// seatless game returning the game with no seats, exactly as the prior two-query
+	// version did; ORDER BY seat preserves seat order. The games columns repeat per seat
+	// row — cheap at 2-4 seats, and one round-trip instead of two, which matters because
+	// GetGame is the universal "load the game" step on every game operation.
+	stmt := postgres.SELECT(table.Games.AllColumns, table.GamePlayers.AllColumns).
+		FROM(table.Games.LEFT_JOIN(table.GamePlayers, table.GamePlayers.GameID.EQ(table.Games.GameID))).
 		WHERE(table.Games.GameID.EQ(postgres.UUID(id))).
-		LIMIT(1)
+		ORDER_BY(table.GamePlayers.Seat.ASC())
-	var grow model.Games
+	var rows []struct {
-	if err := gstmt.QueryContext(ctx, s.db, &grow); err != nil {
+		model.Games
-		if errors.Is(err, qrm.ErrNoRows) {
+		model.GamePlayers
-			return Game{}, ErrNotFound
+	}
-		}
+	if err := stmt.QueryContext(ctx, s.db, &rows); err != nil {
 		return Game{}, fmt.Errorf("game: get %s: %w", id, err)
 	}
-
+	if len(rows) == 0 {
-	sstmt := postgres.SELECT(table.GamePlayers.AllColumns).
+		return Game{}, ErrNotFound
-		FROM(table.GamePlayers).
-		WHERE(table.GamePlayers.GameID.EQ(postgres.UUID(id))).
-		ORDER_BY(table.GamePlayers.Seat.ASC())
-	var srows []model.GamePlayers
-	if err := sstmt.QueryContext(ctx, s.db, &srows); err != nil {
-		return Game{}, fmt.Errorf("game: get seats %s: %w", id, err)
 	}
-	return projectGame(grow, srows)
+	seats := make([]model.GamePlayers, 0, len(rows))
+	for i := range rows {
+		// Skip the phantom all-NULL seat row a LEFT JOIN yields for a seatless game.
+		if rows[i].GamePlayers.GameID == id {
+			seats = append(seats, rows[i].GamePlayers)
+		}
+	}
+	return projectGame(rows[0].Games, seats)
 }
 
 // GetGameVariant reads just a game's variant — a cheap single-column lookup the edge uses

backend/internal/game/types.go

@@ -184,6 +184,18 @@ func (g Game) seatOf(accountID uuid.UUID) (int, bool) {
 	return 0, false
 }
 
+// seatedIn reports whether accountID holds a seat in seats. It backs the read-side
+// membership check against the cached, immutable seat list, so a hot read can skip
+// loading the game from the store.
+func seatedIn(seats []Seat, accountID uuid.UUID) bool {
+	for _, s := range seats {
+		if s.AccountID == accountID {
+			return true
+		}
+	}
+	return false
+}
+
 // MoveResult is the outcome of a committed transition: the decoded move and the
 // post-move game, plus the actor's own refilled rack and the bag size after the draw
 // (Rack/BagLen), so the mover renders the next state from the response without a

backend/internal/inttest/game_test.go

@@ -543,6 +543,12 @@ func TestEvaluatePlayPreview(t *testing.T) {
 	if bad.Valid {
 		t.Error("disconnected play must be invalid")
 	}
+
+	// A non-seated account cannot preview: with the game warm in the live cache, the
+	// membership check runs against the cached seat list (the hot path that skips GetGame).
+	if _, err := svc.EvaluatePlay(ctx, g.ID, provisionAccount(t), hint.Tiles); !errors.Is(err, game.ErrNotAPlayer) {
+		t.Errorf("evaluate by a non-player = %v, want ErrNotAPlayer", err)
+	}
 }
 
 // TestConcurrentSubmitSerialized confirms the per-game lock lets only one of two

loadtest/REPORT.md

@@ -64,10 +64,10 @@ limiter probe):
 
 - **Volume:** 802 200 total edge calls (1 084 req/s incl. the hammer; ~377 req/s of real
   gameplay). `stream errors: 0`. Live events: 11 199 `opponent_moved`, 4 153 `your_turn`.
-- **`game.evaluate` is now the dominant gameplay write-path call** at ~116 req/s — second
+- **`game.evaluate` is the dominant gameplay write-path call** at ~116 req/s — second only
-  only to the `game.state` poll — and it is cheap: p50 1 ms, p99 200 ms, effectively zero
+  to the `game.state` poll — and it is cheap: p50 1 ms, effectively zero errors. The backend
-  errors. The backend serves it from the in-memory live-game cache plus a single
+  serves it straight from the in-memory live-game cache; on a warm hit it skips the database
-  `GetGame` read.
+  entirely (see *Postgres read path* below, which halved its p99 to 100 ms).
 - **Latency stayed healthy** under the heavier evaluate load: every gameplay op p99 ≤ 200 ms.
 - **The limiter holds** unchanged: 99.97 % of the hammer rejected at p99 2 ms.
 
@@ -149,15 +149,25 @@ The gateway's compose limit can drop well below its old 3 cores; it is now conne
 bound, not connection-CPU bound. Memory was never the constraint. Disk is still dominated
 by observability retention (Tempo, Prometheus) + DB growth — unchanged from before.
 
-## Next optimisation (noted, not done)
+## Postgres read path (warm-cache optimization)
 
-`game.evaluate` reads `GetGame` from Postgres on **every** call (to re-check seat
+Following this pass, `game.evaluate` no longer reads the database on the hot path. An
-membership and status) before validating against the cached live game. At ~116 evaluate
+active game is already resident in the in-memory live-game cache (mutated in place across
-req/s on top of the `game.state` / `game.history` reads, that is the bulk of the postgres
+moves, evicted only on finish), so the preview answers its seat-membership check from the
-load now. Caching the game metadata alongside the live engine game in
+cached immutable seat list and scores against the cached engine game — **no `GetGame` on a
-`backend/internal/game` would cut it, but it touches persistence/cache coherency (a higher
+warm hit**. `GetGame` itself was also folded from two round-trips (game, then seats) into a
-blast-radius change) and postgres still has headroom, so it is left as a deliberate
+single `LEFT JOIN`. Measured at 500 players, **`game.evaluate` p99 halved (200 → 100 ms)**
-follow-up rather than bundled here.
+and the per-operation query count dropped.
+
+It did **not** cut postgres CPU, and the measurement says why: postgres is **write-bound**,
+not read-bound. `pg_stat_user_tables` puts the cost in the per-move `CommitMove`
+transaction (a `game_moves` insert plus `games` / `game_players` updates), the debounced
+`game_drafts` upserts (~60 k in one run), and the journal replays — not the cheap, indexed,
+fully-cached `GetGame` lookups this change removed (one re-run even committed 28 % more
+plays, whose extra writes masked the saved reads). Postgres also runs with headroom
+(~1.5 of 2 cores), and the gateway fix freed ~3 cores on the box, so the lever if postgres
+ever caps is **more cores** (it is CPU-bound, not I/O), not riskier write-path surgery. So
+this change is a latency / query-volume win, deliberately not a DB-CPU one.
 
 ## Caveat — harness fidelity