Clean expired cache entries periodically

Regularly clean up of cache entries that have expired for a more efficient use of memory.
Introduce two new parameters to tune clean up frequency and threshold for forced FIFO eviction.

Fixes open-policy-agent#5320

Signed-off-by: Rudrakh Panigrahi <rudrakh97@gmail.com>

runtime/runtime.go

@@ -581,6 +581,8 @@ func (rt *Runtime) Serve(ctx context.Context) error {
 		})
 	}
 
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
 	rt.server, err = rt.server.Init(ctx)
 	if err != nil {
 		rt.logger.WithFields(map[string]interface{}{"err": err}).Error("Unable to initialize server.")

sdk/opa.go

@@ -212,7 +212,7 @@ func (opa *OPA) configure(ctx context.Context, bs []byte, ready chan struct{}, b
 
 	opa.state.manager = manager
 	opa.state.queryCache.Clear()
-	opa.state.interQueryBuiltinCache = cache.NewInterQueryCache(manager.InterQueryBuiltinCacheConfig())
+	opa.state.interQueryBuiltinCache = cache.NewInterQueryCacheWithContext(ctx, manager.InterQueryBuiltinCacheConfig())
 	opa.config = bs
 
 	return nil

server/server.go

@@ -184,7 +184,7 @@ func New() *Server {
 // Init initializes the server. This function MUST be called before starting any loops
 // from s.Listeners().
 func (s *Server) Init(ctx context.Context) (*Server, error) {
-	s.initRouters()
+	s.initRouters(ctx)
 
 	txn, err := s.store.NewTransaction(ctx, storage.WriteParams)
 	if err != nil {
@@ -755,7 +755,7 @@ func (s *Server) initHandlerCompression(handler http.Handler) (http.Handler, err
 	return compressHandler, nil
 }
 
-func (s *Server) initRouters() {
+func (s *Server) initRouters(ctx context.Context) {
 	mainRouter := s.router
 	if mainRouter == nil {
 		mainRouter = mux.NewRouter()
@@ -764,7 +764,7 @@ func (s *Server) initRouters() {
 	diagRouter := mux.NewRouter()
 
 	// authorizer, if configured, needs the iCache to be set up already
-	s.interQueryBuiltinCache = iCache.NewInterQueryCache(s.manager.InterQueryBuiltinCacheConfig())
+	s.interQueryBuiltinCache = iCache.NewInterQueryCacheWithContext(ctx, s.manager.InterQueryBuiltinCacheConfig())
 	s.manager.RegisterCacheTrigger(s.updateCacheConfig)
 
 	// Add authorization handler. This must come BEFORE authentication handler

topdown/cache/cache.go

@@ -7,15 +7,20 @@ package cache
 
 import (
 	"container/list"
+	"context"
+	"fmt"
+	"math"
 	"sync"
+	"time"
 
 	"github.com/open-policy-agent/opa/ast"
-
 	"github.com/open-policy-agent/opa/util"
 )
 
 const (
-	defaultMaxSizeBytes = int64(0) // unlimited
+	defaultMaxSizeBytes                      = int64(0)   // unlimited
+	defaultForcedEvictionThresholdPercentage = int64(100) // trigger at max_size_bytes
+	defaultStaleEntryEvictionPeriodSeconds   = int64(0)   // never
 )
 
 // Config represents the configuration of the inter-query cache.
@@ -24,16 +29,25 @@ type Config struct {
 }
 
 // InterQueryBuiltinCacheConfig represents the configuration of the inter-query cache that built-in functions can utilize.
+// MaxSizeBytes - max capacity of cache in bytes
+// ForcedEvictionThresholdPercentage - capacity usage in percentage after which forced FIFO eviction starts
+// StaleEntryEvictionPeriodSeconds - time period between end of previous and start of new stale entry eviction routine
 type InterQueryBuiltinCacheConfig struct {
-	MaxSizeBytes *int64 `json:"max_size_bytes,omitempty"`
+	MaxSizeBytes                      *int64 `json:"max_size_bytes,omitempty"`
+	ForcedEvictionThresholdPercentage *int64 `json:"forced_eviction_threshold_percentage,omitempty"`
+	StaleEntryEvictionPeriodSeconds   *int64 `json:"stale_entry_eviction_period_seconds,omitempty"`
 }
 
 // ParseCachingConfig returns the config for the inter-query cache.
 func ParseCachingConfig(raw []byte) (*Config, error) {
 	if raw == nil {
 		maxSize := new(int64)
 		*maxSize = defaultMaxSizeBytes
-		return &Config{InterQueryBuiltinCache: InterQueryBuiltinCacheConfig{MaxSizeBytes: maxSize}}, nil
+		threshold := new(int64)
+		*threshold = defaultForcedEvictionThresholdPercentage
+		period := new(int64)
+		*period = defaultStaleEntryEvictionPeriodSeconds
+		return &Config{InterQueryBuiltinCache: InterQueryBuiltinCacheConfig{MaxSizeBytes: maxSize, ForcedEvictionThresholdPercentage: threshold, StaleEntryEvictionPeriodSeconds: period}}, nil
 	}
 
 	var config Config
@@ -55,6 +69,26 @@ func (c *Config) validateAndInjectDefaults() error {
 		*maxSize = defaultMaxSizeBytes
 		c.InterQueryBuiltinCache.MaxSizeBytes = maxSize
 	}
+	if c.InterQueryBuiltinCache.ForcedEvictionThresholdPercentage == nil {
+		threshold := new(int64)
+		*threshold = defaultForcedEvictionThresholdPercentage
+		c.InterQueryBuiltinCache.ForcedEvictionThresholdPercentage = threshold
+	} else {
+		threshold := *c.InterQueryBuiltinCache.ForcedEvictionThresholdPercentage
+		if threshold < 0 || threshold > 100 {
+			return fmt.Errorf("invalid forced_eviction_threshold_percentage %v", threshold)
+		}
+	}
+	if c.InterQueryBuiltinCache.StaleEntryEvictionPeriodSeconds == nil {
+		period := new(int64)
+		*period = defaultStaleEntryEvictionPeriodSeconds
+		c.InterQueryBuiltinCache.StaleEntryEvictionPeriodSeconds = period
+	} else {
+		period := *c.InterQueryBuiltinCache.StaleEntryEvictionPeriodSeconds
+		if period < 0 {
+			return fmt.Errorf("invalid stale_entry_eviction_period_seconds %v", period)
+		}
+	}
 	return nil
 }
 
@@ -68,23 +102,55 @@ type InterQueryCacheValue interface {
 type InterQueryCache interface {
 	Get(key ast.Value) (value InterQueryCacheValue, found bool)
 	Insert(key ast.Value, value InterQueryCacheValue) int
+	InsertWithExpiry(key ast.Value, value InterQueryCacheValue, expiresAt time.Time) int
 	Delete(key ast.Value)
 	UpdateConfig(config *Config)
 	Clone(value InterQueryCacheValue) (InterQueryCacheValue, error)
 }
 
 // NewInterQueryCache returns a new inter-query cache.
+// The cache uses a FIFO eviction policy when it reaches the forced eviction threshold.
+// Parameters:
+//
+//	config - to configure the InterQueryCache
 func NewInterQueryCache(config *Config) InterQueryCache {
-	return &cache{
-		items:  map[string]cacheItem{},
-		usage:  0,
-		config: config,
-		l:      list.New(),
+	return newCache(config)
+}
+
+// NewInterQueryCacheWithContext returns a new inter-query cache with context.
+// The cache uses a combination of FIFO eviction policy when it reaches the forced eviction threshold
+// and a periodic cleanup routine to remove stale entries that exceed their expiration time, if specified.
+// If configured with a zero stale_entry_eviction_period_seconds value, the stale entry cleanup routine is disabled.
+//
+// Parameters:
+//
+//	ctx - used to control lifecycle of the stale entry cleanup routine
+//	config - to configure the InterQueryCache
+func NewInterQueryCacheWithContext(ctx context.Context, config *Config) InterQueryCache {
+	iqCache := newCache(config)
+	if iqCache.staleEntryEvictionTimePeriodSeconds() > 0 {
+		cleanupTicker := time.NewTicker(time.Duration(iqCache.staleEntryEvictionTimePeriodSeconds()) * time.Second)
+		go func() {
+			for {
+				select {
+				case <-cleanupTicker.C:
+					cleanupTicker.Stop()
+					iqCache.cleanStaleValues()
+					cleanupTicker = time.NewTicker(time.Duration(iqCache.staleEntryEvictionTimePeriodSeconds()) * time.Second)
+				case <-ctx.Done():
+					cleanupTicker.Stop()
+					return
+				}
+			}
+		}()
 	}
+
+	return iqCache
 }
 
 type cacheItem struct {
 	value      InterQueryCacheValue
+	expiresAt  time.Time
 	keyElement *list.Element
 }
 
@@ -96,11 +162,26 @@ type cache struct {
 	mtx    sync.Mutex
 }
 
-// Insert inserts a key k into the cache with value v.
-func (c *cache) Insert(k ast.Value, v InterQueryCacheValue) (dropped int) {
+func newCache(config *Config) *cache {
+	return &cache{
+		items:  map[string]cacheItem{},
+		usage:  0,
+		config: config,
+		l:      list.New(),
+	}
+}
+
+// InsertWithExpiry inserts a key k into the cache with value v with an expiration time expiresAt.
+// A zero time value for expiresAt indicates no expiry
+func (c *cache) InsertWithExpiry(k ast.Value, v InterQueryCacheValue, expiresAt time.Time) (dropped int) {
 	c.mtx.Lock()
 	defer c.mtx.Unlock()
-	return c.unsafeInsert(k, v)
+	return c.unsafeInsert(k, v, expiresAt)
+}
+
+// Insert inserts a key k into the cache with value v with no expiration time.
+func (c *cache) Insert(k ast.Value, v InterQueryCacheValue) (dropped int) {
+	return c.InsertWithExpiry(k, v, time.Time{})
 }
 
 // Get returns the value in the cache for k.
@@ -137,10 +218,9 @@ func (c *cache) Clone(value InterQueryCacheValue) (InterQueryCacheValue, error)
 	return c.unsafeClone(value)
 }
 
-func (c *cache) unsafeInsert(k ast.Value, v InterQueryCacheValue) (dropped int) {
+func (c *cache) unsafeInsert(k ast.Value, v InterQueryCacheValue, expiresAt time.Time) (dropped int) {
 	size := v.SizeInBytes()
-	limit := c.maxSizeBytes()
-
+	limit := int64(math.Ceil(float64(c.forcedEvictionThresholdPercentage())/100.0) * (float64(c.maxSizeBytes())))
 	if limit > 0 {
 		if size > limit {
 			dropped++
@@ -159,6 +239,7 @@ func (c *cache) unsafeInsert(k ast.Value, v InterQueryCacheValue) (dropped int)
 
 	c.items[k.String()] = cacheItem{
 		value:      v,
+		expiresAt:  expiresAt,
 		keyElement: c.l.PushBack(k),
 	}
 	c.usage += size
@@ -191,3 +272,32 @@ func (c *cache) maxSizeBytes() int64 {
 	}
 	return *c.config.InterQueryBuiltinCache.MaxSizeBytes
 }
+
+func (c *cache) forcedEvictionThresholdPercentage() int64 {
+	if c.config == nil {
+		return defaultForcedEvictionThresholdPercentage
+	}
+	return *c.config.InterQueryBuiltinCache.ForcedEvictionThresholdPercentage
+}
+
+func (c *cache) staleEntryEvictionTimePeriodSeconds() int64 {
+	if c.config == nil {
+		return defaultStaleEntryEvictionPeriodSeconds
+	}
+	return *c.config.InterQueryBuiltinCache.StaleEntryEvictionPeriodSeconds
+}
+
+func (c *cache) cleanStaleValues() (dropped int) {
+	c.mtx.Lock()
+	defer c.mtx.Unlock()
+	for key := c.l.Front(); key != nil; {
+		nextKey := key.Next()
+		// if expiresAt is zero, the item doesn't have an expiry
+		if ea := c.items[(key.Value.(ast.Value)).String()].expiresAt; !ea.IsZero() && ea.Before(time.Now()) {
+			c.unsafeDelete(key.Value.(ast.Value))
+			dropped++
+		}
+		key = nextKey
+	}
+	return dropped
+}