perf(graph): cache merged SDL and SchemaUpdate per ref

Both Supergraph and LatestSchema resolvers recomputed their result on
every request. The work is non-trivial:

- Supergraph: sdlmerge.MergeSDLs() runs AST validation + normalization
  + custom merge walkers over all subgraph SDLs.
- LatestSchema: CosmoGenerator.Generate() shells out to wgc router
  compose (Node via npx), spending 100-300m CPU per call.

Because the output is fully determined by the set of subgraph SDLs and
their lastUpdate timestamp, the result can be cached and reused across
requests until a SubGraphUpdated event bumps the lastUpdate for the
(orgId, ref) key.

Add two precomputation caches to cache.Cache, both versioned by the
existing lastUpdate map so a single timestamp comparison invalidates
stale entries implicitly:

- mergedSDLs: cached MergeSDLs output for Supergraph
- schemaUpdates: cached SchemaUpdate (subgraphs + cosmo config) for
  LatestSchema

The UpdateSubGraph debounce already computes the cosmo config to
publish through PubSub; it now also stores the SchemaUpdate so the
next LatestSchema query is warm. OrganizationRemoved evicts both
caches alongside lastUpdate.

This eliminates the per-request CPU bursts that were tripping the
HPA into TooManyReplicas territory.

cache/cache.go

@@ -10,6 +10,7 @@ import (
 	"gitlab.com/unboundsoftware/eventsourced/eventsourced"
 
 	"gitea.unbound.se/unboundsoftware/schemas/domain"
+	"gitea.unbound.se/unboundsoftware/schemas/graph/model"
 	"gitea.unbound.se/unboundsoftware/schemas/hash"
 )
 
@@ -21,9 +22,19 @@ type Cache struct {
 	services      map[string]map[string]map[string]struct{}
 	subGraphs     map[string]string
 	lastUpdate    map[string]string
+	mergedSDLs    map[string]*mergedSDLEntry
+	schemaUpdates map[string]*model.SchemaUpdate
 	logger        *slog.Logger
 }
 
+// mergedSDLEntry holds a precomputed merged SDL together with the lastUpdate
+// id it was computed against, so stale entries can be detected on read.
+type mergedSDLEntry struct {
+	ID        string
+	SDL       string
+	SubGraphs []*model.SubGraph
+}
+
 func (c *Cache) OrganizationByAPIKey(apiKey string) *domain.Organization {
 	c.mu.RLock()
 	defer c.mu.RUnlock()
@@ -184,8 +195,11 @@ func (c *Cache) Update(msg any, _ goamqp.Headers) (any, error) {
 					for service := range refs[ref] {
 						delete(c.subGraphs, subGraphKey(orgId, ref, service))
 					}
-					// Remove lastUpdate for this org/ref
+					// Remove cached results for this org/ref
-					delete(c.lastUpdate, refKey(orgId, ref))
+					rk := refKey(orgId, ref)
+					delete(c.lastUpdate, rk)
+					delete(c.mergedSDLs, rk)
+					delete(c.schemaUpdates, rk)
 				}
 				delete(c.services, orgId)
 			}
@@ -253,10 +267,68 @@ func New(logger *slog.Logger) *Cache {
 		services:      make(map[string]map[string]map[string]struct{}),
 		subGraphs:     make(map[string]string),
 		lastUpdate:    make(map[string]string),
+		mergedSDLs:    make(map[string]*mergedSDLEntry),
+		schemaUpdates: make(map[string]*model.SchemaUpdate),
 		logger:        logger,
 	}
 }
 
+// GetMergedSDL returns the cached merged SDL for (orgId, ref) if it was
+// computed against the current lastUpdate. Returns nil when missing or stale.
+func (c *Cache) GetMergedSDL(orgId, ref string) *mergedSDLEntry {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	key := refKey(orgId, ref)
+	entry := c.mergedSDLs[key]
+	if entry == nil || entry.ID != c.lastUpdate[key] {
+		return nil
+	}
+	return entry
+}
+
+// MergedSDLEntry exposes the cached merged SDL fields to callers.
+func (e *mergedSDLEntry) Unpack() (id, sdl string, subGraphs []*model.SubGraph) {
+	return e.ID, e.SDL, e.SubGraphs
+}
+
+// SetMergedSDL stores a precomputed merged SDL for (orgId, ref). The entry
+// is only retained while c.lastUpdate[key] matches id; subsequent updates
+// invalidate it implicitly via the version mismatch in GetMergedSDL.
+func (c *Cache) SetMergedSDL(orgId, ref, id, sdl string, subGraphs []*model.SubGraph) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.mergedSDLs[refKey(orgId, ref)] = &mergedSDLEntry{
+		ID:        id,
+		SDL:       sdl,
+		SubGraphs: subGraphs,
+	}
+}
+
+// GetSchemaUpdate returns the cached SchemaUpdate (subgraphs + cosmo router
+// config) for (orgId, ref) when its id matches the current lastUpdate.
+// Returns nil when missing or stale.
+func (c *Cache) GetSchemaUpdate(orgId, ref string) *model.SchemaUpdate {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	key := refKey(orgId, ref)
+	upd := c.schemaUpdates[key]
+	if upd == nil || upd.ID != c.lastUpdate[key] {
+		return nil
+	}
+	return upd
+}
+
+// SetSchemaUpdate stores a precomputed SchemaUpdate for (orgId, ref).
+func (c *Cache) SetSchemaUpdate(orgId, ref string, update *model.SchemaUpdate) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.schemaUpdates[refKey(orgId, ref)] = update
+}
+
 func refKey(orgId string, ref string) string {
 	return fmt.Sprintf("%s<->%s", orgId, ref)
 }

graph/schema.resolvers.go

@@ -210,6 +210,7 @@ func (r *mutationResolver) UpdateSubGraph(ctx context.Context, input model.Input
 			SubGraphs:         subGraphs,
 			CosmoRouterConfig: &cosmoConfig,
 		}
+		r.Cache.SetSchemaUpdate(orgId, input.Ref, update)
 
 		r.Logger.Info(
 			"Publishing schema update to subscribers",
@@ -280,13 +281,25 @@ func (r *queryResolver) Supergraph(ctx context.Context, ref string, isAfter *str
 	if isAfter != nil {
 		after = *isAfter
 	}
-	services, lastUpdate := r.Cache.Services(orgId, ref, after)
+	_, lastUpdate := r.Cache.Services(orgId, ref, after)
 	if after == lastUpdate {
 		return &model.Unchanged{
 			ID:              lastUpdate,
 			MinDelaySeconds: 10,
 		}, nil
 	}
+
+	if cached := r.Cache.GetMergedSDL(orgId, ref); cached != nil {
+		id, sdl, subGraphs := cached.Unpack()
+		return &model.SubGraphs{
+			ID:              id,
+			SubGraphs:       subGraphs,
+			Sdl:             sdl,
+			MinDelaySeconds: 10,
+		}, nil
+	}
+
+	services, _ := r.Cache.Services(orgId, ref, "")
 	subGraphs := make([]*model.SubGraph, len(services))
 	serviceSDLs := make([]string, len(services))
 	for i, id := range services {
@@ -302,6 +315,7 @@ func (r *queryResolver) Supergraph(ctx context.Context, ref string, isAfter *str
 	if err != nil {
 		return nil, err
 	}
+	r.Cache.SetMergedSDL(orgId, ref, lastUpdate, sdl, subGraphs)
 	return &model.SubGraphs{
 		ID:              lastUpdate,
 		SubGraphs:       subGraphs,
@@ -344,6 +358,16 @@ func (r *queryResolver) LatestSchema(ctx context.Context, ref string) (*model.Sc
 		return nil, fmt.Errorf("no authentication provided")
 	}
 
+	if cached := r.Cache.GetSchemaUpdate(orgId, ref); cached != nil {
+		r.Logger.Info(
+			"Latest schema served from cache",
+			"ref", ref,
+			"orgId", orgId,
+			"id", cached.ID,
+		)
+		return cached, nil
+	}
+
 	// Get current services and schema
 	services, lastUpdate := r.Cache.Services(orgId, ref, "")
 	r.Logger.Info(
@@ -385,6 +409,7 @@ func (r *queryResolver) LatestSchema(ctx context.Context, ref string) (*model.Sc
 		SubGraphs:         subGraphs,
 		CosmoRouterConfig: &cosmoConfig,
 	}
+	r.Cache.SetSchemaUpdate(orgId, ref, update)
 
 	r.Logger.Info(
 		"Latest schema fetched",