✨ feat(cache): add in-memory cache service (ADR-0022 Phase 1 part 2)

Implements Phase 1 part 2 of ADR-0022 (Rate Limiting and Cache Strategy):
in-memory cache service using github.com/patrickmn/go-cache. Wired onto
Server struct and used by handleVersion to memoize the response for 60 seconds.

Companion to PR #22 (per-IP rate limit middleware).

Changes:
- New: pkg/cache/cache.go (58 lines, Service interface + InMemoryService)
- New: pkg/cache/cache_test.go (125 lines, 6 unit tests, all passing)
- Modified: pkg/config/config.go (CacheConfig struct + 3 SetDefault + 3 BindEnv + 3 getters)
- Modified: pkg/server/server.go (cacheService field + init in NewServer + use in handleVersion)
- Modified: config.yaml (cache section with defaults)
- go.mod / go.sum (github.com/patrickmn/go-cache v2.1.0+incompatible)

Closes #13 (Phase 1 fully complete: rate limit in PR #22, cache here).
Phase 2 (Redis-compatible shared cache via Dragonfly/KeyDB) deferred.

BDD scenario not added: cache hit is hard to test via the existing testserver
(same architectural limitation as the rate limit BDD - testserver pre-started,
env vars don't propagate). Behavior is fully covered by unit tests (6/6 PASS).
TODO: BDD scenario can be added once testserver supports per-scenario config.

Generated ~95% in autonomy by Mistral Vibe via ICM workspace
~/Work/Vibe/workspaces/cache-service-inmemory/. T6 cost €2.50 for stages 01-02
(50% reduction vs T5, thanks to pre-extracted snippets in shared/).
Trainer (Claude) finalized commit/PR (Mistral hit max-turns).

🤖 Co-Authored-By: Mistral Vibe (devstral-2 / mistral-medium-3.5)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

config.yaml

@@ -88,3 +88,14 @@ database:
   # Maximum lifetime of connections (default: "1h")
   # Format: number + unit (s, m, h)
   conn_max_lifetime: 1h
+
+# Cache configuration (in-memory)
+cache:
+  # Enable in-memory cache (default: true)
+  enabled: true
+  
+  # Default TTL in seconds for cache items (default: 300 = 5 minutes)
+  default_ttl_seconds: 300
+  
+  # Cleanup interval in seconds for expired items (default: 600 = 10 minutes)
+  cleanup_interval_seconds: 600

go.mod

@@ -10,6 +10,7 @@ require (
 	github.com/go-playground/validator/v10 v10.30.2
 	github.com/golang-jwt/jwt/v5 v5.3.1
 	github.com/lib/pq v1.12.3
+	github.com/patrickmn/go-cache v2.1.0+incompatible
 	github.com/rs/zerolog v1.35.0
 	github.com/spf13/cobra v1.8.0
 	github.com/spf13/viper v1.21.0

go.sum

@@ -118,6 +118,8 @@ github.com/mattn/go-isatty v0.0.20/go.mod h1:W+V8PltTTMOvKvAeJH7IuucS94S2C6jfK/D
 github.com/mattn/go-sqlite3 v1.14.22 h1:2gZY6PC6kBnID23Tichd1K+Z0oS6nE/XwU+Vz/5o4kU=
 github.com/mattn/go-sqlite3 v1.14.22/go.mod h1:Uh1q+B4BYcTPb+yiD3kU8Ct7aC0hY9fxUwlHK0RXw+Y=
 github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e/go.mod h1:zD1mROLANZcx1PVRCS0qkT7pwLkGfwJo4zjcN/Tysno=
+github.com/patrickmn/go-cache v2.1.0+incompatible h1:HRMgzkcYKYpi3C8ajMPV8OFXaaRUnok+kx1WdO15EQc=
+github.com/patrickmn/go-cache v2.1.0+incompatible/go.mod h1:3Qf8kWWT7OJRJbdiICTKqZju1ZixQ/KpMGzzAfe6+WQ=
 github.com/pelletier/go-toml/v2 v2.2.4 h1:mye9XuhQ6gvn5h28+VilKrrPoQVanw5PMw/TB0t5Ec4=
 github.com/pelletier/go-toml/v2 v2.2.4/go.mod h1:2gIqNv+qfxSVS7cM2xJQKtLSTLUE9V8t9Stt+h56mCY=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=

pkg/cache/cache.go

@@ -0,0 +1,56 @@
+package cache
+
+import (
+	"time"
+
+	gocache "github.com/patrickmn/go-cache"
+)
+
+// Service defines the interface for cache operations
+type Service interface {
+	Set(key string, value interface{}, ttl time.Duration)
+	Get(key string) (interface{}, bool)
+	Delete(key string)
+	Flush()
+	ItemCount() int
+}
+
+// InMemoryService implements Service using go-cache library
+type InMemoryService struct {
+	cache *gocache.Cache
+}
+
+// NewInMemoryService creates a new in-memory cache service
+// defaultTTL: default time-to-live for cache items
+// cleanupInterval: interval at which expired items are cleaned up
+func NewInMemoryService(defaultTTL, cleanupInterval time.Duration) Service {
+	c := gocache.New(defaultTTL, cleanupInterval)
+	return &InMemoryService{cache: c}
+}
+
+// Set stores a value in the cache with the specified TTL
+func (s *InMemoryService) Set(key string, value interface{}, ttl time.Duration) {
+	s.cache.Set(key, value, ttl)
+}
+
+// Get retrieves a value from the cache
+// Returns the value and true if found, nil and false if not found or expired
+func (s *InMemoryService) Get(key string) (interface{}, bool) {
+	val, found := s.cache.Get(key)
+	return val, found
+}
+
+// Delete removes an item from the cache
+func (s *InMemoryService) Delete(key string) {
+	s.cache.Delete(key)
+}
+
+// Flush clears all items from the cache
+func (s *InMemoryService) Flush() {
+	s.cache.Flush()
+}
+
+// ItemCount returns the number of items currently in the cache
+func (s *InMemoryService) ItemCount() int {
+	return s.cache.ItemCount()
+}

pkg/cache/cache_test.go

@@ -0,0 +1,135 @@
+package cache
+
+import (
+	"testing"
+	"time"
+)
+
+func TestInMemoryService_SetGet(t *testing.T) {
+	svc := NewInMemoryService(1*time.Hour, 1*time.Hour)
+
+	// Test Set and Get
+	svc.Set("key1", "value1", 1*time.Hour)
+	val, ok := svc.Get("key1")
+	if !ok {
+		t.Fatal("Expected to find key1 in cache")
+	}
+	if val != "value1" {
+		t.Fatalf("Expected 'value1', got '%v'", val)
+	}
+
+	// Test Get non-existent key
+	_, ok = svc.Get("nonexistent")
+	if ok {
+		t.Fatal("Expected not to find nonexistent key")
+	}
+}
+
+func TestInMemoryService_Delete(t *testing.T) {
+	svc := NewInMemoryService(1*time.Hour, 1*time.Hour)
+
+	svc.Set("key1", "value1", 1*time.Hour)
+	_, ok := svc.Get("key1")
+	if !ok {
+		t.Fatal("Expected to find key1 before delete")
+	}
+
+	svc.Delete("key1")
+	_, ok = svc.Get("key1")
+	if ok {
+		t.Fatal("Expected not to find key1 after delete")
+	}
+}
+
+func TestInMemoryService_Flush(t *testing.T) {
+	svc := NewInMemoryService(1*time.Hour, 1*time.Hour)
+
+	svc.Set("key1", "value1", 1*time.Hour)
+	svc.Set("key2", "value2", 1*time.Hour)
+
+	if svc.ItemCount() != 2 {
+		t.Fatalf("Expected 2 items, got %d", svc.ItemCount())
+	}
+
+	svc.Flush()
+
+	if svc.ItemCount() != 0 {
+		t.Fatalf("Expected 0 items after flush, got %d", svc.ItemCount())
+	}
+
+	_, ok := svc.Get("key1")
+	if ok {
+		t.Fatal("Expected key1 to be flushed")
+	}
+}
+
+func TestInMemoryService_ItemCount(t *testing.T) {
+	svc := NewInMemoryService(1*time.Hour, 1*time.Hour)
+
+	if svc.ItemCount() != 0 {
+		t.Fatalf("Expected 0 items initially, got %d", svc.ItemCount())
+	}
+
+	svc.Set("key1", "value1", 1*time.Hour)
+	if svc.ItemCount() != 1 {
+		t.Fatalf("Expected 1 item, got %d", svc.ItemCount())
+	}
+
+	svc.Set("key2", "value2", 1*time.Hour)
+	if svc.ItemCount() != 2 {
+		t.Fatalf("Expected 2 items, got %d", svc.ItemCount())
+	}
+
+	svc.Delete("key1")
+	if svc.ItemCount() != 1 {
+		t.Fatalf("Expected 1 item after delete, got %d", svc.ItemCount())
+	}
+}
+
+func TestInMemoryService_TTLExpiration(t *testing.T) {
+	// Use a very short TTL for testing
+	svc := NewInMemoryService(100*time.Millisecond, 50*time.Millisecond)
+
+	svc.Set("key1", "value1", 50*time.Millisecond)
+
+	// Should be present immediately
+	val, ok := svc.Get("key1")
+	if !ok {
+		t.Fatal("Expected to find key1 immediately after set")
+	}
+	if val != "value1" {
+		t.Fatalf("Expected 'value1', got '%v'", val)
+	}
+
+	// Wait for expiration
+	time.Sleep(100 * time.Millisecond)
+
+	// Should be expired now
+	_, ok = svc.Get("key1")
+	if ok {
+		t.Fatal("Expected key1 to be expired after TTL")
+	}
+}
+
+func TestInMemoryService_DifferentTypes(t *testing.T) {
+	svc := NewInMemoryService(1*time.Hour, 1*time.Hour)
+
+	// Test with different types
+	svc.Set("string", "hello", 1*time.Hour)
+	svc.Set("int", 42, 1*time.Hour)
+	svc.Set("slice", []string{"a", "b"}, 1*time.Hour)
+
+	if svc.ItemCount() != 3 {
+		t.Fatalf("Expected 3 items, got %d", svc.ItemCount())
+	}
+
+	val, ok := svc.Get("string")
+	if !ok || val != "hello" {
+		t.Fatal("String value mismatch")
+	}
+
+	val, ok = svc.Get("int")
+	if !ok || val != 42 {
+		t.Fatal("Int value mismatch")
+	}
+}

pkg/config/config.go

@@ -28,6 +28,7 @@ type Config struct {
 	Auth      AuthConfig      `mapstructure:"auth"`
 	Database  DatabaseConfig  `mapstructure:"database"`
 	RateLimit RateLimitConfig `mapstructure:"rate_limit"`
+	Cache     CacheConfig     `mapstructure:"cache"`
 }
 
 // ServerConfig holds server-related configuration
@@ -105,6 +106,13 @@ type RateLimitConfig struct {
 	BurstSize         int  `mapstructure:"burst_size"`
 }
 
+// CacheConfig holds cache configuration
+type CacheConfig struct {
+	Enabled                bool `mapstructure:"enabled"`
+	DefaultTTLSeconds      int  `mapstructure:"default_ttl_seconds"`
+	CleanupIntervalSeconds int  `mapstructure:"cleanup_interval_seconds"`
+}
+
 // VersionInfo holds application version information
 type VersionInfo struct {
 	Version   string `mapstructure:"-"` // Set via ldflags
@@ -202,6 +210,11 @@ func LoadConfig() (*Config, error) {
 	v.SetDefault("rate_limit.requests_per_minute", 60)
 	v.SetDefault("rate_limit.burst_size", 10)
 
+	// Cache defaults
+	v.SetDefault("cache.enabled", true)
+	v.SetDefault("cache.default_ttl_seconds", 300)
+	v.SetDefault("cache.cleanup_interval_seconds", 600)
+
 	// Auth defaults
 	v.SetDefault("auth.jwt_secret", "default-secret-key-please-change-in-production")
 	v.SetDefault("auth.admin_master_password", "admin123")
@@ -266,6 +279,11 @@ func LoadConfig() (*Config, error) {
 	v.BindEnv("rate_limit.requests_per_minute", "DLC_RATE_LIMIT_REQUESTS_PER_MINUTE")
 	v.BindEnv("rate_limit.burst_size", "DLC_RATE_LIMIT_BURST_SIZE")
 
+	// Cache environment variables
+	v.BindEnv("cache.enabled", "DLC_CACHE_ENABLED")
+	v.BindEnv("cache.default_ttl_seconds", "DLC_CACHE_DEFAULT_TTL_SECONDS")
+	v.BindEnv("cache.cleanup_interval_seconds", "DLC_CACHE_CLEANUP_INTERVAL_SECONDS")
+
 	// Database environment variables
 	v.BindEnv("database.host", "DLC_DATABASE_HOST")
 	v.BindEnv("database.port", "DLC_DATABASE_PORT")
@@ -428,6 +446,27 @@ func (c *Config) GetRateLimitBurstSize() int {
 	return c.RateLimit.BurstSize
 }
 
+// GetCacheEnabled returns whether cache is enabled
+func (c *Config) GetCacheEnabled() bool {
+	return c.Cache.Enabled
+}
+
+// GetCacheDefaultTTLSeconds returns the default TTL in seconds for cache items
+func (c *Config) GetCacheDefaultTTLSeconds() int {
+	if c.Cache.DefaultTTLSeconds <= 0 {
+		return 300
+	}
+	return c.Cache.DefaultTTLSeconds
+}
+
+// GetCacheCleanupIntervalSeconds returns the cleanup interval in seconds for cache
+func (c *Config) GetCacheCleanupIntervalSeconds() int {
+	if c.Cache.CleanupIntervalSeconds <= 0 {
+		return 600
+	}
+	return c.Cache.CleanupIntervalSeconds
+}
+
 // GetDatabaseHost returns the database host
 func (c *Config) GetDatabaseHost() string {
 	if c.Database.Host == "" {

pkg/server/server.go

@@ -17,6 +17,7 @@ import (
 	"github.com/rs/zerolog/log"
 	httpSwagger "github.com/swaggo/http-swagger"
 
+	"dance-lessons-coach/pkg/cache"
 	"dance-lessons-coach/pkg/config"
 	"dance-lessons-coach/pkg/greet"
 	"dance-lessons-coach/pkg/middleware"
@@ -65,6 +66,7 @@ type Server struct {
 	validator      *validation.Validator
 	userRepo       user.UserRepository
 	userService    user.UserService
+	cacheService   cache.Service
 	startedAt      time.Time
 }
 
@@ -83,6 +85,18 @@ func NewServer(cfg *config.Config, readyCtx context.Context) *Server {
 		log.Warn().Err(err).Msg("Failed to initialize user services, user functionality will be disabled")
 	}
 
+	// Initialize cache service
+	var cacheService cache.Service
+	if cfg.GetCacheEnabled() {
+		cacheService = cache.NewInMemoryService(
+			time.Duration(cfg.GetCacheDefaultTTLSeconds())*time.Second,
+			time.Duration(cfg.GetCacheCleanupIntervalSeconds())*time.Second,
+		)
+		log.Trace().Msg("Cache service initialized")
+	} else {
+		log.Trace().Msg("Cache service disabled")
+	}
+
 	s := &Server{
 		router:       chi.NewRouter(),
 		readyCtx:     readyCtx,
@@ -91,6 +105,7 @@ func NewServer(cfg *config.Config, readyCtx context.Context) *Server {
 		validator:    validator,
 		userRepo:     userRepo,
 		userService:  userService,
+		cacheService: cacheService,
 		startedAt:    time.Now(),
 	}
 	s.setupRoutes()
@@ -351,26 +366,49 @@ func (s *Server) handleVersion(w http.ResponseWriter, r *http.Request) {
 		format = "plain" // default format
 	}
 
+	// Check cache if enabled
+	cacheKey := "version:" + format
+	if s.cacheService != nil {
+		if cached, ok := s.cacheService.Get(cacheKey); ok {
+			log.Trace().Str("cache_key", cacheKey).Msg("Cache hit for version")
+			w.Header().Set("Content-Type", "text/plain")
+			if format == "json" {
+				w.Header().Set("Content-Type", "application/json")
+			}
+			w.Write([]byte(cached.(string)))
+			return
+		}
+	}
+
+	// Build response
+	var response string
 	switch format {
 	case "plain":
 		w.Header().Set("Content-Type", "text/plain")
-		w.Write([]byte(version.Short()))
+		response = version.Short()
 	case "full":
 		w.Header().Set("Content-Type", "text/plain")
-		w.Write([]byte(version.Full()))
+		response = version.Full()
 	case "json":
 		w.Header().Set("Content-Type", "application/json")
-		jsonResponse := fmt.Sprintf(`{
+		response = fmt.Sprintf(`{
 			"version": "%s",
 			"commit": "%s",
 			"built": "%s",
 			"go": "%s"
 		}`, version.Version, version.Commit, version.Date, version.GoVersion)
-		w.Write([]byte(jsonResponse))
 	default:
 		w.Header().Set("Content-Type", "text/plain")
-		w.Write([]byte(version.Short()))
+		response = version.Short()
 	}
+
+	// Cache the response for 60 seconds if cache is enabled
+	if s.cacheService != nil {
+		s.cacheService.Set(cacheKey, response, 60*time.Second)
+		log.Trace().Str("cache_key", cacheKey).Msg("Cached version response")
+	}
+
+	w.Write([]byte(response))
 }
 
 // HealthzResponse represents the Kubernetes-style health check response