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subscriber.go
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subscriber.go
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// SPDX-License-Identifier: Apache-2.0
/*
Package dnssrv defines some implementations for a dns based service discovery
*/
package dnssrv
import (
"fmt"
"net"
"sort"
"sync"
"time"
"github.com/luraproject/lura/v2/config"
"github.com/luraproject/lura/v2/sd"
)
// Namespace is the key for the dns sd module
const Namespace = "dns"
const DefaultTTL = 30 * time.Second
const MinTTL = time.Second
// Register registers the dns sd subscriber factory under the name defined by Namespace
func Register() error {
return sd.GetRegister().Register(Namespace, SubscriberFactory)
}
// TTL is the duration of the cached data
var TTL = DefaultTTL
func SetTTL(d time.Duration) {
if d < MinTTL {
// in case the TTL is less than the minimum, we leave what is
// already set.
return
}
TTL = d
}
// DefaultLookup is the function used for the DNS resolution
var DefaultLookup = net.LookupSRV
// SubscriberFactory builds a DNS_SRV Subscriber with the received config
func SubscriberFactory(cfg *config.Backend) sd.Subscriber {
return NewDetailedWithScheme(cfg.Host[0], DefaultLookup, TTL, cfg.SDScheme)
}
// New creates a DNS subscriber with the default values
func New(name string) sd.Subscriber {
return NewDetailed(name, DefaultLookup, TTL)
}
// NewDetailed creates a DNS subscriber with the received values
func NewDetailed(name string, lookup lookup, ttl time.Duration) sd.Subscriber {
return NewDetailedWithScheme(name, lookup, ttl, "http")
}
// NewDetailedWithScheme creates a DNS subscriber with the received values and the scheme to use
// for the fetched server entries.
func NewDetailedWithScheme(name string, lookup lookup, ttl time.Duration, scheme string) sd.Subscriber {
if scheme == "" {
scheme = "http"
}
s := subscriber{
name: name,
cache: &sd.FixedSubscriber{},
mutex: &sync.RWMutex{},
ttl: ttl,
lookup: lookup,
scheme: scheme,
}
s.update()
go func() {
for {
<-time.After(s.ttl)
s.update()
}
}()
return s
}
type lookup func(service, proto, name string) (cname string, addrs []*net.SRV, err error)
type subscriber struct {
name string
cache *sd.FixedSubscriber
mutex *sync.RWMutex
ttl time.Duration
lookup lookup
scheme string
}
// Hosts returns a copy of the cached set of hosts. It is safe to call it concurrently
func (s subscriber) Hosts() ([]string, error) {
s.mutex.RLock()
defer s.mutex.RUnlock()
hs, err := s.cache.Hosts()
if err != nil {
return []string{}, err
}
res := make([]string, len(hs))
copy(res, hs)
return res, nil
}
func (s subscriber) update() {
instances, err := s.resolve()
if err != nil {
return
}
s.mutex.Lock()
defer s.mutex.Unlock()
if len(instances) > 100 {
*(s.cache) = sd.NewRandomFixedSubscriber(instances)
} else {
*(s.cache) = sd.FixedSubscriber(instances)
}
}
func (s subscriber) resolve() ([]string, error) {
_, srvs, err := s.lookup("", "", s.name)
if err != nil {
return []string{}, err
}
sort.Slice(
srvs,
func(i, j int) bool {
if srvs[i].Priority == srvs[j].Priority {
if srvs[i].Weight == srvs[j].Weight {
if srvs[i].Target == srvs[j].Target {
return srvs[i].Port < srvs[j].Port
}
return srvs[i].Target < srvs[j].Target
}
return srvs[i].Weight > srvs[j].Weight
}
return srvs[i].Priority < srvs[j].Priority
},
)
ws := make([]uint16, 0, len(srvs))
host := make([]string, 0, len(srvs))
for _, a := range srvs {
if a.Priority > srvs[0].Priority {
break
}
ws = append(ws, a.Weight)
host = append(host, s.scheme+"://"+net.JoinHostPort(a.Target, fmt.Sprint(a.Port)))
}
instances := make([]string, 0, len(ws))
for i, times := range compact(ws) {
for j := uint16(0); j < times; j++ {
instances = append(instances, host[i])
}
}
return instances, nil
}
func compact(ws []uint16) []uint16 {
tmp := normalize(ws)
div := gcd(tmp)
if div < 2 {
return tmp
}
res := make([]uint16, len(tmp))
for i, w := range tmp {
res[i] = w / div
}
return res
}
func normalize(ws []uint16) []uint16 {
scale := 100
if l := len(ws); l > scale {
scale = l
}
var sum int64
for _, w := range ws {
sum += int64(w)
}
if sum <= int64(scale) {
return ws
}
res := make([]uint16, len(ws))
for i, w := range ws {
res[i] = uint16(int64(w) * int64(scale) / sum)
}
return res
}
func gcd(ws []uint16) uint16 {
if len(ws) == 0 {
return 0
}
localGCD := func(a uint16, b uint16) uint16 {
for b > 0 {
a, b = b, a%b
}
return a
}
result := ws[0]
for _, i := range ws[1:] {
result = localGCD(result, i)
}
return result
}