adds middleware for rate limiting

.gitignore

@@ -5,3 +5,4 @@ vendor
 .idea
 *.iml
 *.out
+.vscode

go.mod

@@ -12,4 +12,5 @@ require (
 	golang.org/x/net v0.0.0-20200822124328-c89045814202
 	golang.org/x/sys v0.0.0-20200826173525-f9321e4c35a6 // indirect
 	golang.org/x/text v0.3.3 // indirect
+	golang.org/x/time v0.0.0-20201208040808-7e3f01d25324
 )

go.sum

@@ -46,6 +46,8 @@ golang.org/x/text v0.3.0 h1:g61tztE5qeGQ89tm6NTjjM9VPIm088od1l6aSorWRWg=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.3 h1:cokOdA+Jmi5PJGXLlLllQSgYigAEfHXJAERHVMaCc2k=
 golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/time v0.0.0-20201208040808-7e3f01d25324 h1:Hir2P/De0WpUhtrKGGjvSb2YxUgyZ7EFOSLIcSSpiwE=
+golang.org/x/time v0.0.0-20201208040808-7e3f01d25324/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=

middleware/rate_limiter.go

@@ -0,0 +1,268 @@
+package middleware
+
+import (
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/labstack/echo/v4"
+	"golang.org/x/time/rate"
+)
+
+type (
+	// RateLimiterStore is the interface to be implemented by custom stores.
+	RateLimiterStore interface {
+		// Stores for the rate limiter have to implement the Allow method
+		Allow(identifier string) (bool, error)
+	}
+)
+
+type (
+	// RateLimiterConfig defines the configuration for the rate limiter
+	RateLimiterConfig struct {
+		Skipper    Skipper
+		BeforeFunc BeforeFunc
+		// IdentifierExtractor uses echo.Context to extract the identifier for a visitor
+		IdentifierExtractor Extractor
+		// Store defines a store for the rate limiter
+		Store RateLimiterStore
+		// ErrorHandler provides a handler to be called when IdentifierExtractor returns an error
+		ErrorHandler func(context echo.Context, err error) error
+		// DenyHandler provides a handler to be called when RateLimiter denies access
+		DenyHandler func(context echo.Context, identifier string, err error) error
+	}
+	// Extractor is used to extract data from echo.Context
+	Extractor func(context echo.Context) (string, error)
+)
+
+// errors
+var (
+	// ErrRateLimitExceeded denotes an error raised when rate limit is exceeded
+	ErrRateLimitExceeded = echo.NewHTTPError(http.StatusTooManyRequests, "rate limit exceeded")
+	// ErrExtractorError denotes an error raised when extractor function is unsuccessful
+	ErrExtractorError = echo.NewHTTPError(http.StatusForbidden, "error while extracting identifier")
+)
+
+// DefaultRateLimiterConfig defines default values for RateLimiterConfig
+var DefaultRateLimiterConfig = RateLimiterConfig{
+	Skipper: DefaultSkipper,
+	IdentifierExtractor: func(ctx echo.Context) (string, error) {
+		id := ctx.RealIP()
+		return id, nil
+	},
+	ErrorHandler: func(context echo.Context, err error) error {
+		return &echo.HTTPError{
+			Code:     ErrExtractorError.Code,
+			Message:  ErrExtractorError.Message,
+			Internal: err,
+		}
+	},
+	DenyHandler: func(context echo.Context, identifier string, err error) error {
+		return &echo.HTTPError{
+			Code:     ErrRateLimitExceeded.Code,
+			Message:  ErrRateLimitExceeded.Message,
+			Internal: err,
+		}
+	},
+}
+
+/*
+RateLimiter returns a rate limiting middleware
+
+	e := echo.New()
+
+	limiterStore := middleware.NewRateLimiterMemoryStore(20)
+
+	e.GET("/rate-limited", func(c echo.Context) error {
+		return c.String(http.StatusOK, "test")
+	}, RateLimiter(limiterStore))
+*/
+func RateLimiter(store RateLimiterStore) echo.MiddlewareFunc {
+	config := DefaultRateLimiterConfig
+	config.Store = store
+
+	return RateLimiterWithConfig(config)
+}
+
+/*
+RateLimiterWithConfig returns a rate limiting middleware
+
+	e := echo.New()
+
+	config := middleware.RateLimiterConfig{
+		Skipper: DefaultSkipper,
+		Store: middleware.NewRateLimiterMemoryStore(
+			middleware.RateLimiterMemoryStoreConfig{Rate: 10, Burst: 30, ExpiresIn: 3 * time.Minute}
+		)
+		IdentifierExtractor: func(ctx echo.Context) (string, error) {
+			id := ctx.RealIP()
+			return id, nil
+		},
+		ErrorHandler: func(context echo.Context, err error) error {
+			return context.JSON(http.StatusTooManyRequests, nil)
+		},
+		DenyHandler: func(context echo.Context, identifier string) error {
+			return context.JSON(http.StatusForbidden, nil)
+		},
+	}
+
+	e.GET("/rate-limited", func(c echo.Context) error {
+		return c.String(http.StatusOK, "test")
+	}, middleware.RateLimiterWithConfig(config))
+*/
+func RateLimiterWithConfig(config RateLimiterConfig) echo.MiddlewareFunc {
+	if config.Skipper == nil {
+		config.Skipper = DefaultRateLimiterConfig.Skipper
+	}
+	if config.IdentifierExtractor == nil {
+		config.IdentifierExtractor = DefaultRateLimiterConfig.IdentifierExtractor
+	}
+	if config.ErrorHandler == nil {
+		config.ErrorHandler = DefaultRateLimiterConfig.ErrorHandler
+	}
+	if config.DenyHandler == nil {
+		config.DenyHandler = DefaultRateLimiterConfig.DenyHandler
+	}
+	if config.Store == nil {
+		panic("Store configuration must be provided")
+	}
+	return func(next echo.HandlerFunc) echo.HandlerFunc {
+		return func(c echo.Context) error {
+			if config.Skipper(c) {
+				return next(c)
+			}
+			if config.BeforeFunc != nil {
+				config.BeforeFunc(c)
+			}
+
+			identifier, err := config.IdentifierExtractor(c)
+			if err != nil {
+				c.Error(config.ErrorHandler(c, err))
+				return nil
+			}
+
+			if allow, err := config.Store.Allow(identifier); !allow {
+				c.Error(config.DenyHandler(c, identifier, err))
+				return nil
+			}
+			return next(c)
+		}
+	}
+}
+
+type (
+	// RateLimiterMemoryStore is the built-in store implementation for RateLimiter
+	RateLimiterMemoryStore struct {
+		visitors    map[string]*Visitor
+		mutex       sync.Mutex
+		rate        rate.Limit
+		burst       int
+		expiresIn   time.Duration
+		lastCleanup time.Time
+	}
+	// Visitor signifies a unique user's limiter details
+	Visitor struct {
+		*rate.Limiter
+		lastSeen time.Time
+	}
+)
+
+/*
+NewRateLimiterMemoryStore returns an instance of RateLimiterMemoryStore with
+the provided rate (as req/s). Burst and ExpiresIn will be set to default values.
+
+Example (with 20 requests/sec):
+
+	limiterStore := middleware.NewRateLimiterMemoryStore(20)
+
+*/
+func NewRateLimiterMemoryStore(rate rate.Limit) (store *RateLimiterMemoryStore) {
+	return NewRateLimiterMemoryStoreWithConfig(RateLimiterMemoryStoreConfig{
+		Rate: rate,
+	})
+}
+
+/*
+NewRateLimiterMemoryStoreWithConfig returns an instance of RateLimiterMemoryStore
+with the provided configuration. Rate must be provided. Burst will be set to the value of
+the configured rate if not provided or set to 0.
+
+The build-in memory store is usually capable for modest loads. For higher loads other
+store implementations should be considered.
+
+Characteristics:
+* Concurrency above 100 parallel requests may causes measurable lock contention
+* A high number of different IP addresses (above 16000) may be impacted by the internally used Go map
+* A high number of requests from a single IP address may cause lock contention
+
+Example:
+
+	limiterStore := middleware.NewRateLimiterMemoryStoreWithConfig(
+		middleware.RateLimiterMemoryStoreConfig{Rate: 50, Burst: 200, ExpiresIn: 5 * time.Minutes},
+	)
+*/
+func NewRateLimiterMemoryStoreWithConfig(config RateLimiterMemoryStoreConfig) (store *RateLimiterMemoryStore) {
+	store = &RateLimiterMemoryStore{}
+
+	store.rate = config.Rate
+	store.burst = config.Burst
+	store.expiresIn = config.ExpiresIn
+	if config.ExpiresIn == 0 {
+		store.expiresIn = DefaultRateLimiterMemoryStoreConfig.ExpiresIn
+	}
+	if config.Burst == 0 {
+		store.burst = int(config.Rate)
+	}
+	store.visitors = make(map[string]*Visitor)
+	store.lastCleanup = now()
+	return
+}
+
+// RateLimiterMemoryStoreConfig represents configuration for RateLimiterMemoryStore
+type RateLimiterMemoryStoreConfig struct {
+	Rate      rate.Limit    // Rate of requests allowed to pass as req/s
+	Burst     int           // Burst additionally allows a number of requests to pass when rate limit is reached
+	ExpiresIn time.Duration // ExpiresIn is the duration after that a rate limiter is cleaned up
+}
+
+// DefaultRateLimiterMemoryStoreConfig provides default configuration values for RateLimiterMemoryStore
+var DefaultRateLimiterMemoryStoreConfig = RateLimiterMemoryStoreConfig{
+	ExpiresIn: 3 * time.Minute,
+}
+
+// Allow implements RateLimiterStore.Allow
+func (store *RateLimiterMemoryStore) Allow(identifier string) (bool, error) {
+	store.mutex.Lock()
+	limiter, exists := store.visitors[identifier]
+	if !exists {
+		limiter = new(Visitor)
+		limiter.Limiter = rate.NewLimiter(store.rate, store.burst)
+		store.visitors[identifier] = limiter
+	}
+	limiter.lastSeen = now()
+	if now().Sub(store.lastCleanup) > store.expiresIn {
+		store.cleanupStaleVisitors()
+	}
+	store.mutex.Unlock()
+	return limiter.AllowN(now(), 1), nil
+}
+
+/*
+cleanupStaleVisitors helps manage the size of the visitors map by removing stale records
+of users who haven't visited again after the configured expiry time has elapsed
+*/
+func (store *RateLimiterMemoryStore) cleanupStaleVisitors() {
+	for id, visitor := range store.visitors {
+		if now().Sub(visitor.lastSeen) > store.expiresIn {
+			delete(store.visitors, id)
+		}
+	}
+	store.lastCleanup = now()
+}
+
+/*
+actual time method which is mocked in test file
+*/
+var now = func() time.Time {
+	return time.Now()
+}