perf(graph): warm schema cache on startup to kill cold-start spikes (#843)

cache/cache.go

@@ -102,6 +102,30 @@ func (c *Cache) Services(orgId, ref, lastUpdate string) ([]string, string) {
 	return services, c.lastUpdate[key]
 }
 
+// OrgRef identifies a single (organizationId, ref) pair that the cache
+// tracks subgraphs for.
+type OrgRef struct {
+	OrgId string
+	Ref   string
+}
+
+// AllOrgRefs returns every (orgId, ref) pair that currently has at least
+// one subgraph in the cache. Used by startup warmup to pre-compute the
+// merged SDL and SchemaUpdate for every known ref before the pod starts
+// serving traffic.
+func (c *Cache) AllOrgRefs() []OrgRef {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	var out []OrgRef
+	for orgId, refs := range c.services {
+		for ref := range refs {
+			out = append(out, OrgRef{OrgId: orgId, Ref: ref})
+		}
+	}
+	return out
+}
+
 func (c *Cache) SubGraphId(orgId, ref, service string) string {
 	c.mu.RLock()
 	defer c.mu.RUnlock()