next.orly.dev/pkg/ratelimit/limiter.go

package ratelimit

import (
	"context"
	"sync"
	"sync/atomic"
	"time"

	"next.orly.dev/pkg/interfaces/loadmonitor"
)

// OperationType distinguishes between read and write operations
// for applying different rate limiting strategies.
type OperationType int

const (
	// Read operations (REQ queries)
	Read OperationType = iota
	// Write operations (EVENT saves, imports)
	Write
)

// String returns a human-readable name for the operation type.
func (o OperationType) String() string {
	switch o {
	case Read:
		return "read"
	case Write:
		return "write"
	default:
		return "unknown"
	}
}

// Config holds configuration for the adaptive rate limiter.
type Config struct {
	// Enabled controls whether rate limiting is active.
	Enabled bool

	// TargetMemoryMB is the target memory limit in megabytes.
	// Memory pressure is calculated relative to this target.
	TargetMemoryMB int

	// WriteSetpoint is the target process variable for writes (0.0-1.0).
	// Default: 0.85 (throttle when load exceeds 85%)
	WriteSetpoint float64

	// ReadSetpoint is the target process variable for reads (0.0-1.0).
	// Default: 0.90 (more tolerant for reads)
	ReadSetpoint float64

	// PID gains for writes
	WriteKp float64
	WriteKi float64
	WriteKd float64

	// PID gains for reads
	ReadKp float64
	ReadKi float64
	ReadKd float64

	// MaxWriteDelayMs is the maximum delay for write operations in milliseconds.
	MaxWriteDelayMs int

	// MaxReadDelayMs is the maximum delay for read operations in milliseconds.
	MaxReadDelayMs int

	// MetricUpdateInterval is how often to poll the load monitor.
	MetricUpdateInterval time.Duration

	// MemoryWeight is the weight given to memory pressure in process variable (0.0-1.0).
	// The remaining weight is given to the load metric.
	// Default: 0.7 (70% memory, 30% load)
	MemoryWeight float64
}

// DefaultConfig returns a default configuration for the rate limiter.
func DefaultConfig() Config {
	return Config{
		Enabled:              true,
		TargetMemoryMB:       1500, // 1.5GB target
		WriteSetpoint:        0.85,
		ReadSetpoint:         0.90,
		WriteKp:              0.5,
		WriteKi:              0.1,
		WriteKd:              0.05,
		ReadKp:               0.3,
		ReadKi:               0.05,
		ReadKd:               0.02,
		MaxWriteDelayMs:      1000, // 1 second max
		MaxReadDelayMs:       500,  // 500ms max
		MetricUpdateInterval: 100 * time.Millisecond,
		MemoryWeight:         0.7,
	}
}

// NewConfigFromValues creates a Config from individual configuration values.
// This is useful when loading configuration from environment variables.
func NewConfigFromValues(
	enabled bool,
	targetMB int,
	writeKp, writeKi, writeKd float64,
	readKp, readKi, readKd float64,
	maxWriteMs, maxReadMs int,
	writeTarget, readTarget float64,
) Config {
	return Config{
		Enabled:              enabled,
		TargetMemoryMB:       targetMB,
		WriteSetpoint:        writeTarget,
		ReadSetpoint:         readTarget,
		WriteKp:              writeKp,
		WriteKi:              writeKi,
		WriteKd:              writeKd,
		ReadKp:               readKp,
		ReadKi:               readKi,
		ReadKd:               readKd,
		MaxWriteDelayMs:      maxWriteMs,
		MaxReadDelayMs:       maxReadMs,
		MetricUpdateInterval: 100 * time.Millisecond,
		MemoryWeight:         0.7,
	}
}

// Limiter implements adaptive rate limiting using PID control.
// It monitors database load metrics and computes appropriate delays
// to keep the system within its target operating range.
type Limiter struct {
	config  Config
	monitor loadmonitor.Monitor

	// PID controllers for reads and writes
	writePID *PIDController
	readPID  *PIDController

	// Cached metrics (updated periodically)
	metricsLock    sync.RWMutex
	currentMetrics loadmonitor.Metrics

	// Statistics
	totalWriteDelayMs atomic.Int64
	totalReadDelayMs  atomic.Int64
	writeThrottles    atomic.Int64
	readThrottles     atomic.Int64

	// Lifecycle
	ctx       context.Context
	cancel    context.CancelFunc
	stopOnce  sync.Once
	stopped   chan struct{}
	wg        sync.WaitGroup
}

// NewLimiter creates a new adaptive rate limiter.
// If monitor is nil, the limiter will be disabled.
func NewLimiter(config Config, monitor loadmonitor.Monitor) *Limiter {
	ctx, cancel := context.WithCancel(context.Background())

	l := &Limiter{
		config:  config,
		monitor: monitor,
		ctx:     ctx,
		cancel:  cancel,
		stopped: make(chan struct{}),
	}

	// Create PID controllers with configured gains
	l.writePID = NewPIDController(
		config.WriteKp, config.WriteKi, config.WriteKd,
		config.WriteSetpoint,
		0.2, // Strong filtering for writes
		-2.0, float64(config.MaxWriteDelayMs)/1000.0*2, // Anti-windup limits
		0, float64(config.MaxWriteDelayMs)/1000.0,
	)

	l.readPID = NewPIDController(
		config.ReadKp, config.ReadKi, config.ReadKd,
		config.ReadSetpoint,
		0.15, // Very strong filtering for reads
		-1.0, float64(config.MaxReadDelayMs)/1000.0*2,
		0, float64(config.MaxReadDelayMs)/1000.0,
	)

	// Set memory target on monitor
	if monitor != nil && config.TargetMemoryMB > 0 {
		monitor.SetMemoryTarget(uint64(config.TargetMemoryMB) * 1024 * 1024)
	}

	return l
}

// Start begins the rate limiter's background metric collection.
func (l *Limiter) Start() {
	if l.monitor == nil || !l.config.Enabled {
		return
	}

	// Start the monitor
	l.monitor.Start()

	// Start metric update loop
	l.wg.Add(1)
	go l.updateLoop()
}

// updateLoop periodically fetches metrics from the monitor.
func (l *Limiter) updateLoop() {
	defer l.wg.Done()

	ticker := time.NewTicker(l.config.MetricUpdateInterval)
	defer ticker.Stop()

	for {
		select {
		case <-l.ctx.Done():
			return
		case <-ticker.C:
			if l.monitor != nil {
				metrics := l.monitor.GetMetrics()
				l.metricsLock.Lock()
				l.currentMetrics = metrics
				l.metricsLock.Unlock()
			}
		}
	}
}

// Stop halts the rate limiter.
func (l *Limiter) Stop() {
	l.stopOnce.Do(func() {
		l.cancel()
		if l.monitor != nil {
			l.monitor.Stop()
		}
		l.wg.Wait()
		close(l.stopped)
	})
}

// Stopped returns a channel that closes when the limiter has stopped.
func (l *Limiter) Stopped() <-chan struct{} {
	return l.stopped
}

// Wait blocks until the rate limiter permits the operation to proceed.
// It returns the delay that was applied, or 0 if no delay was needed.
// If the context is cancelled, it returns immediately.
func (l *Limiter) Wait(ctx context.Context, opType OperationType) time.Duration {
	if !l.config.Enabled || l.monitor == nil {
		return 0
	}

	delay := l.ComputeDelay(opType)
	if delay <= 0 {
		return 0
	}

	// Apply the delay
	select {
	case <-ctx.Done():
		return 0
	case <-time.After(delay):
		return delay
	}
}

// ComputeDelay calculates the recommended delay for an operation.
// This can be used to check the delay without actually waiting.
func (l *Limiter) ComputeDelay(opType OperationType) time.Duration {
	if !l.config.Enabled || l.monitor == nil {
		return 0
	}

	// Get current metrics
	l.metricsLock.RLock()
	metrics := l.currentMetrics
	l.metricsLock.RUnlock()

	// Compute process variable as weighted combination of memory and load
	var loadMetric float64
	switch opType {
	case Write:
		loadMetric = metrics.WriteLoad
	case Read:
		loadMetric = metrics.ReadLoad
	}

	// Combine memory pressure and load
	// Process variable = memoryWeight * memoryPressure + (1-memoryWeight) * loadMetric
	pv := l.config.MemoryWeight*metrics.MemoryPressure + (1-l.config.MemoryWeight)*loadMetric

	// Select the appropriate PID controller
	var delaySec float64
	switch opType {
	case Write:
		delaySec = l.writePID.Update(pv)
		if delaySec > 0 {
			l.writeThrottles.Add(1)
			l.totalWriteDelayMs.Add(int64(delaySec * 1000))
		}
	case Read:
		delaySec = l.readPID.Update(pv)
		if delaySec > 0 {
			l.readThrottles.Add(1)
			l.totalReadDelayMs.Add(int64(delaySec * 1000))
		}
	}

	if delaySec <= 0 {
		return 0
	}

	return time.Duration(delaySec * float64(time.Second))
}

// RecordLatency records an operation latency for the monitor.
func (l *Limiter) RecordLatency(opType OperationType, latency time.Duration) {
	if l.monitor == nil {
		return
	}

	switch opType {
	case Write:
		l.monitor.RecordWriteLatency(latency)
	case Read:
		l.monitor.RecordQueryLatency(latency)
	}
}

// Stats returns rate limiter statistics.
type Stats struct {
	WriteThrottles    int64
	ReadThrottles     int64
	TotalWriteDelayMs int64
	TotalReadDelayMs  int64
	CurrentMetrics    loadmonitor.Metrics
	WritePIDState     PIDState
	ReadPIDState      PIDState
}

// PIDState contains the internal state of a PID controller.
type PIDState struct {
	Integral          float64
	PrevError         float64
	PrevFilteredError float64
}

// GetStats returns current rate limiter statistics.
func (l *Limiter) GetStats() Stats {
	l.metricsLock.RLock()
	metrics := l.currentMetrics
	l.metricsLock.RUnlock()

	wIntegral, wPrevErr, wPrevFiltered := l.writePID.GetState()
	rIntegral, rPrevErr, rPrevFiltered := l.readPID.GetState()

	return Stats{
		WriteThrottles:    l.writeThrottles.Load(),
		ReadThrottles:     l.readThrottles.Load(),
		TotalWriteDelayMs: l.totalWriteDelayMs.Load(),
		TotalReadDelayMs:  l.totalReadDelayMs.Load(),
		CurrentMetrics:    metrics,
		WritePIDState: PIDState{
			Integral:          wIntegral,
			PrevError:         wPrevErr,
			PrevFilteredError: wPrevFiltered,
		},
		ReadPIDState: PIDState{
			Integral:          rIntegral,
			PrevError:         rPrevErr,
			PrevFilteredError: rPrevFiltered,
		},
	}
}

// Reset clears all PID controller state and statistics.
func (l *Limiter) Reset() {
	l.writePID.Reset()
	l.readPID.Reset()
	l.writeThrottles.Store(0)
	l.readThrottles.Store(0)
	l.totalWriteDelayMs.Store(0)
	l.totalReadDelayMs.Store(0)
}

// IsEnabled returns whether rate limiting is active.
func (l *Limiter) IsEnabled() bool {
	return l.config.Enabled && l.monitor != nil
}

// UpdateConfig updates the rate limiter configuration.
// This is useful for dynamic tuning.
func (l *Limiter) UpdateConfig(config Config) {
	l.config = config

	// Update PID controllers
	l.writePID.SetSetpoint(config.WriteSetpoint)
	l.writePID.SetGains(config.WriteKp, config.WriteKi, config.WriteKd)
	l.writePID.OutputMax = float64(config.MaxWriteDelayMs) / 1000.0

	l.readPID.SetSetpoint(config.ReadSetpoint)
	l.readPID.SetGains(config.ReadKp, config.ReadKi, config.ReadKd)
	l.readPID.OutputMax = float64(config.MaxReadDelayMs) / 1000.0

	// Update memory target
	if l.monitor != nil && config.TargetMemoryMB > 0 {
		l.monitor.SetMemoryTarget(uint64(config.TargetMemoryMB) * 1024 * 1024)
	}
}