scrabble-game/backend/internal/ratewatch/ratewatch.go

// Package ratewatch ingests the gateway's periodic rate-limiter rejection
// reports. It keeps an in-memory window of recent throttle episodes for
// the admin console's view and applies the conservative high-rate auto-flag:
// when one account's rejections within the rolling window cross the threshold,
// the account store stamps the soft, reversible flagged_high_rate_at marker
// (set-once; an operator clears it; never an automatic ban). Like the gateway's
// active_users gauge it is single-instance and resets on restart by design —
// the durable part is the account flag, not the episode window.
package ratewatch

import (
	"context"
	"fmt"
	"sort"
	"sync"
	"time"

	"github.com/google/uuid"
	"go.uber.org/zap"
)

// ClassUser is the limiter class whose keys are account ids — the only class
// the auto-flag applies to (the others are keyed by client IP).
const ClassUser = "user"

const (
	// maxSeries bounds the distinct (class, key) series kept for the console
	// view, so a key-spraying client cannot grow the map: past the bound the
	// least-recently-throttled series is evicted.
	maxSeries = 200
	// minRetention keeps an episode visible in the console for at least an hour
	// after its last rejection (longer when the flag window is longer).
	minRetention = time.Hour
)

// Config tunes the conservative high-rate auto-flag.
type Config struct {
	// FlagThreshold is the rejected-call count within FlagWindow past which a
	// user account is flagged.
	FlagThreshold int
	// FlagWindow is the rolling window the rejections accumulate over.
	FlagWindow time.Duration
}

// DefaultConfig returns the agreed conservative defaults — 1000 rejected calls
// within a rolling 10 minutes (~1.7/s sustained, far above the client's
// capped-backoff retry noise yet a fraction of an abusive loop).
func DefaultConfig() Config {
	return Config{FlagThreshold: 1000, FlagWindow: 10 * time.Minute}
}

// Validate reports whether the configuration values are acceptable.
func (c Config) Validate() error {
	if c.FlagThreshold <= 0 {
		return fmt.Errorf("ratewatch: flag threshold must be positive")
	}
	if c.FlagWindow <= 0 {
		return fmt.Errorf("ratewatch: flag window must be positive")
	}
	return nil
}

// Flagger stamps the account-level high-rate marker; account.Store satisfies it.
type Flagger interface {
	FlagHighRate(ctx context.Context, id uuid.UUID, at time.Time) (bool, error)
}

// Entry is one reported aggregate: the rejections of one limiter key within one
// gateway report window (the wire mirror of the gateway's rejection summary).
type Entry struct {
	Class    string
	Key      string
	Rejected int
}

// Episode is one key's recent-throttle aggregate for the admin view.
type Episode struct {
	Class     string
	Key       string
	Rejected  int
	FirstSeen time.Time
	LastSeen  time.Time
}

// Watch accumulates reports and applies the auto-flag rule.
type Watch struct {
	cfg     Config
	flagger Flagger
	log     *zap.Logger
	now     func() time.Time

	mu     sync.Mutex
	series map[seriesKey]*series
}

type seriesKey struct{ class, key string }

type series struct {
	points []point // ascending by time
}

type point struct {
	at time.Time
	n  int
}

// New constructs a Watch over flagger with cfg. A nil logger is replaced by a
// no-op one; a nil flagger disables the auto-flag (the view still works).
func New(cfg Config, flagger Flagger, log *zap.Logger) *Watch {
	if log == nil {
		log = zap.NewNop()
	}
	return &Watch{
		cfg:     cfg,
		flagger: flagger,
		log:     log,
		now:     time.Now,
		series:  make(map[seriesKey]*series),
	}
}

// Ingest records one gateway report. Entries with an empty class or key or a
// non-positive count are skipped. When a user-class series crosses the flag
// threshold within the flag window, the account is flagged (the store keeps it
// set-once, so a sustained episode costs one no-op UPDATE per report).
func (w *Watch) Ingest(ctx context.Context, entries []Entry) {
	if len(entries) == 0 {
		return
	}
	now := w.now()
	var flag []uuid.UUID
	w.mu.Lock()
	for _, e := range entries {
		if e.Class == "" || e.Key == "" || e.Rejected <= 0 {
			continue
		}
		k := seriesKey{class: e.Class, key: e.Key}
		s := w.series[k]
		if s == nil {
			s = &series{}
			w.series[k] = s
		}
		s.points = append(s.points, point{at: now, n: e.Rejected})
		if e.Class == ClassUser && s.sumSince(now.Add(-w.cfg.FlagWindow)) >= w.cfg.FlagThreshold {
			if id, err := uuid.Parse(e.Key); err == nil {
				flag = append(flag, id)
			}
		}
	}
	w.pruneLocked(now)
	w.mu.Unlock()

	if w.flagger == nil {
		return
	}
	for _, id := range flag {
		set, err := w.flagger.FlagHighRate(ctx, id, now)
		switch {
		case err != nil:
			w.log.Warn("high-rate flag failed", zap.String("account_id", id.String()), zap.Error(err))
		case set:
			w.log.Info("account flagged high-rate",
				zap.String("account_id", id.String()),
				zap.Int("threshold", w.cfg.FlagThreshold),
				zap.Duration("window", w.cfg.FlagWindow))
		}
	}
}

// Config returns the active auto-flag tuning (the admin console captions it).
func (w *Watch) Config() Config { return w.cfg }

// Recent returns the retained throttle episodes, most recently throttled first.
func (w *Watch) Recent() []Episode {
	w.mu.Lock()
	defer w.mu.Unlock()
	out := make([]Episode, 0, len(w.series))
	for k, s := range w.series {
		if len(s.points) == 0 {
			continue
		}
		ep := Episode{
			Class:     k.class,
			Key:       k.key,
			FirstSeen: s.points[0].at,
			LastSeen:  s.points[len(s.points)-1].at,
		}
		for _, p := range s.points {
			ep.Rejected += p.n
		}
		out = append(out, ep)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].LastSeen.After(out[j].LastSeen) })
	return out
}

// sumSince totals the points at or after cutoff.
func (s *series) sumSince(cutoff time.Time) int {
	sum := 0
	for i := len(s.points) - 1; i >= 0; i-- {
		if s.points[i].at.Before(cutoff) {
			break
		}
		sum += s.points[i].n
	}
	return sum
}

// pruneLocked drops points past retention, empty series, and — past maxSeries —
// the least-recently-throttled series. The caller holds w.mu.
func (w *Watch) pruneLocked(now time.Time) {
	cutoff := now.Add(-max(minRetention, w.cfg.FlagWindow))
	for k, s := range w.series {
		i := 0
		for i < len(s.points) && s.points[i].at.Before(cutoff) {
			i++
		}
		s.points = s.points[i:]
		if len(s.points) == 0 {
			delete(w.series, k)
		}
	}
	for len(w.series) > maxSeries {
		var oldest seriesKey
		var oldestAt time.Time
		first := true
		for k, s := range w.series {
			last := s.points[len(s.points)-1].at
			if first || last.Before(oldestAt) {
				oldest, oldestAt, first = k, last, false
			}
		}
		delete(w.series, oldest)
	}
}