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lb.go
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lb.go
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package lb
import (
"context"
"errors"
"math/rand"
"net/http"
"sync"
"sync/atomic"
"get.pme.sh/pmesh/retry"
"get.pme.sh/pmesh/vhttp"
"get.pme.sh/pmesh/xlog"
"github.com/samber/lo"
)
type requestContext struct {
Request *http.Request
LoadBalancer *LoadBalancer
Upstream *Upstream
Retrier retry.Retrier
Session *vhttp.ClientSession
}
type requestContextKey struct{}
type stickySessionKey struct{ *LoadBalancer }
func (ctx *requestContext) stickyLoad() *Upstream {
key := stickySessionKey{ctx.LoadBalancer}
res, ok := ctx.Session.Values.Load(key)
if !ok {
return nil
}
return res.(*Upstream)
}
func (ctx *requestContext) stickyCompareAndSwap(old, new *Upstream) bool {
key := stickySessionKey{ctx.LoadBalancer}
return ctx.Session.Values.CompareAndSwap(key, old, new)
}
type LoadBalancer struct {
Options `yaml:",inline"`
logger *xlog.Logger `yaml:"-"`
upstreams []*Upstream
mu sync.RWMutex
counter atomic.Uint32
}
type LoadBalancerMetrics struct {
Upstreams []UpstreamMetrics `json:"upstreams,omitempty"`
}
func (lb *LoadBalancer) Healthy() bool {
lb.mu.RLock()
defer lb.mu.RUnlock()
for _, u := range lb.upstreams {
if u.Healthy.Load() {
return true
}
}
return false
}
func (lb *LoadBalancer) Metrics() LoadBalancerMetrics {
us := lb.Upstreams()
upstreams := make([]UpstreamMetrics, len(us))
for i, u := range us {
upstreams[i] = u.Metrics()
}
return LoadBalancerMetrics{
Upstreams: upstreams,
}
}
func (lb *LoadBalancer) getLogger() *xlog.Logger {
if lb.logger == nil {
return xlog.Default()
}
return lb.logger
}
func (lb *LoadBalancer) SetLogger(logger *xlog.Logger) {
lb.logger = logger
}
func (lb *LoadBalancer) Upstreams() (us []*Upstream) {
lb.mu.RLock()
defer lb.mu.RUnlock()
us = make([]*Upstream, len(lb.upstreams))
copy(us, lb.upstreams)
return
}
func (lb *LoadBalancer) ClearUpstreams() {
lb.mu.Lock()
defer lb.mu.Unlock()
lb.upstreams = nil
}
func (lb *LoadBalancer) AddUpstream(u *Upstream) {
lb.mu.Lock()
defer lb.mu.Unlock()
lb.upstreams = append(lb.upstreams, u)
}
func (lb *LoadBalancer) RemoveUpstream(u *Upstream) {
lb.mu.Lock()
lb.upstreams = lo.Without(lb.upstreams, u)
lb.mu.Unlock()
}
var ErrNoHealthyUpstreams = errors.New("no healthy upstreams")
func (lb *LoadBalancer) getHealthyIdxLocked(n uint32, bad *Upstream) (res *Upstream) {
for _, u := range lb.upstreams {
if !u.Healthy.Load() {
continue
}
// If we're at the bad index, ignore
if u == bad {
continue
}
// If we're at the index, return
res = u
if n == 0 {
break
}
// Decrement the index
n--
}
if res == nil {
res = bad
}
return
}
func (lb *LoadBalancer) NextUpstream(ctx *requestContext) (result *Upstream, err error) {
// If there is a bad upstream, we won't use least-conn
strat := lb.Strategy
bad := ctx.Upstream
if bad != nil {
strat = StrategyRandom
}
// Generate the "entropy"
var entropy uint32
switch strat {
case StrategyHash:
if ctx.Session.Local {
entropy = lb.counter.Add(1)
} else {
entropy = ctx.Session.IPHash
}
case StrategyRandom:
entropy = rand.Uint32()
case StrategyRoundRobin:
entropy = lb.counter.Add(1)
}
lb.mu.RLock()
defer lb.mu.RUnlock()
// Handle fixed cases.
count := uint32(len(lb.upstreams))
if count == 0 {
return nil, nil
} else if count == 1 {
return lb.upstreams[0], nil
}
// If least-conn:
if strat == StrategyLeastConn {
result = lb.upstreams[0]
best := result.LoadFactor.Load()
if !result.Healthy.Load() {
best = 0x7fffffff
}
for _, upstream := range lb.upstreams[1:] {
if !upstream.Healthy.Load() {
continue
}
conns := upstream.LoadFactor.Load()
if conns < best {
result, best = upstream, conns
}
}
return
}
// Count the healthy upstreams, prefetch the first one.
healthyN := uint32(0)
var first *Upstream
for _, upstream := range lb.upstreams {
if upstream.Healthy.Load() {
if healthyN == 0 {
first = upstream
}
healthyN++
}
}
// If there's just one healthy upstream, use it.
if healthyN <= 1 {
return first, nil
}
// Pick the entry given the entropy.
result = lb.getHealthyIdxLocked(entropy%healthyN, bad)
return
}
func (lb *LoadBalancer) PickUpstream(ctx *requestContext) (result *Upstream, err error) {
defer func() {
if err == nil && result == nil {
err = ErrNoHealthyUpstreams
}
}()
// If no state, just pick an upstream.
if lb.State == StateNone {
return lb.NextUpstream(ctx)
}
// Try the sticky session first.
if result = ctx.stickyLoad(); result != nil {
bad := ctx.Upstream
if result != bad && result.Healthy.Load() {
return result, nil
}
// If the last upstream was bad, clear it.
ctx.stickyCompareAndSwap(result, nil)
}
// Pick the upstream and store it if it's good.
//
result, err = lb.NextUpstream(ctx)
if result != nil {
ctx.stickyCompareAndSwap(nil, result)
}
return
}
type lbRetryHandler struct {
*requestContext
}
func (h lbRetryHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
h.LoadBalancer.serveHTTP(h.requestContext, w, r)
}
func (lb *LoadBalancer) OnErrorResponse(ctx *requestContext, r *http.Response) http.Handler {
// Determine traits.
retriable := false
var opt *ErrorOptions
switch {
// 5xx
case 500 <= r.StatusCode && r.StatusCode <= 599:
opt = lb.Error5xx
retriable = ctx.Request.Method == http.MethodGet
// 4xx
case r.StatusCode == 404:
opt = lb.Error404
case 400 <= r.StatusCode && r.StatusCode <= 499:
opt = lb.Error4xx
// ?
default:
break
}
if opt != nil {
// If rate limited and this is not a retry:
if !opt.Limit.IsZero() && ctx.Retrier.Step == 0 {
// Enforce the rate limit, if it's exceeded return the error writer.
if hnd := ctx.Session.EnforceRateReq(ctx.Request.Context(), r.Request, opt.Limit); hnd != nil {
// Handled by the rate limiter.
return hnd
}
}
// If user has a special handler:
buf := vhttp.NewBufferedResponse(nil)
if opt.Handle != nil {
switch opt.Handle.ServeHTTP(buf, ctx.Request) {
// If handled, we're done.
case vhttp.Done:
return buf
// Not handled.
case vhttp.Continue:
// Should not retry.
case vhttp.Drop:
retriable = false
}
}
}
// If bad request, we will never retry and it's not worth logging since it's the client's fault.
if 400 <= r.StatusCode && r.StatusCode <= 499 {
return nil
}
// If retriable:
if retriable {
// If we can retry:
if retryError := ctx.Retrier.ConsumeAny(); retryError == nil {
// Log and retry.
lb.getLogger().Warn().Str("status", r.Status).Stringer("upstream", ctx.Upstream).Msg("retriable server error")
return lbRetryHandler{ctx}
} else {
// Log.
lb.getLogger().Error().Str("status", r.Status).Err(retryError).Stringer("upstream", ctx.Upstream).Msg("fatal server error")
}
}
return nil
}
func (lb *LoadBalancer) OnError(ctx *requestContext, w http.ResponseWriter, r *http.Request, err error) {
if r.Context().Err() != nil {
return
}
// If we can retry:
if retryError := ctx.Retrier.Consume(err); retryError == nil {
// Log and retry.
lb.getLogger().Warn().Err(err).Stringer("upstream", ctx.Upstream).Msg("retriable upstream error")
lb.serveHTTP(ctx, w, ctx.Request)
return
} else {
lb.getLogger().Error().Err(err).Stringer("upstream", ctx.Upstream).Msg("fatal upstream error")
}
vhttp.Error(w, ctx.Request, vhttp.StatusUpstreamError)
}
func (lb *LoadBalancer) serveHTTP(ctx *requestContext, w http.ResponseWriter, r *http.Request) {
us, err := lb.PickUpstream(ctx)
if err != nil {
lb.OnError(ctx, w, r, err)
} else {
ctx.Upstream = us
us.ServeHTTP(w, r)
}
}
func (lb *LoadBalancer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
ctx := &requestContext{}
defer func() {
ctx.Request = nil
ctx.Upstream = nil
ctx.Session = nil
ctx.LoadBalancer = nil
}()
cctx := context.WithValue(r.Context(), requestContextKey{}, ctx)
r = r.WithContext(cctx)
ctx.LoadBalancer = lb
ctx.Retrier = lb.Retry.RetrierContext(cctx)
ctx.Session = vhttp.ClientSessionFromContext(cctx)
ctx.Upstream = nil
ctx.Request = r
lb.serveHTTP(ctx, w, r)
}