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package proxy
import (
"log"
"math/rand"
"sync/atomic"
)
// HostPool is a collection of UpstreamHosts.
type HostPool []*UpstreamHost
// Policy decides how a host will be selected from a pool. When all hosts are unhealthy, it is assumed the
// healthchecking failed. In this case each policy will *randomly* return a host from the pool to prevent
// no traffic to go through at all.
type Policy interface {
Select(pool HostPool) *UpstreamHost
}
func init() {
RegisterPolicy("random", func() Policy { return &Random{} })
RegisterPolicy("least_conn", func() Policy { return &LeastConn{} })
RegisterPolicy("round_robin", func() Policy { return &RoundRobin{} })
}
// Random is a policy that selects up hosts from a pool at random.
type Random struct{}
// Select selects an up host at random from the specified pool.
func (r *Random) Select(pool HostPool) *UpstreamHost {
// instead of just generating a random index
// this is done to prevent selecting a down host
var randHost *UpstreamHost
count := 0
for _, host := range pool {
if host.Down() {
continue
}
count++
if count == 1 {
randHost = host
} else {
r := rand.Int() % count
if r == (count - 1) {
randHost = host
}
}
}
return randHost
}
// Spray is a policy that selects a host from a pool at random. This should be used as a last ditch
// attempt to get a host when all hosts are reporting unhealthy.
type Spray struct{}
// Select selects an up host at random from the specified pool.
func (r *Spray) Select(pool HostPool) *UpstreamHost {
rnd := rand.Int() % len(pool)
randHost := pool[rnd]
log.Printf("[WARNING] All hosts reported as down, spraying to target: %s", randHost.Name)
return randHost
}
// LeastConn is a policy that selects the host with the least connections.
type LeastConn struct{}
// Select selects the up host with the least number of connections in the
// pool. If more than one host has the same least number of connections,
// one of the hosts is chosen at random.
func (r *LeastConn) Select(pool HostPool) *UpstreamHost {
var bestHost *UpstreamHost
count := 0
leastConn := int64(1<<63 - 1)
for _, host := range pool {
if host.Down() {
continue
}
hostConns := host.Conns
if hostConns < leastConn {
bestHost = host
leastConn = hostConns
count = 1
} else if hostConns == leastConn {
// randomly select host among hosts with least connections
count++
if count == 1 {
bestHost = host
} else {
r := rand.Int() % count
if r == (count - 1) {
bestHost = host
}
}
}
}
return bestHost
}
// RoundRobin is a policy that selects hosts based on round robin ordering.
type RoundRobin struct {
Robin uint32
}
// Select selects an up host from the pool using a round robin ordering scheme.
func (r *RoundRobin) Select(pool HostPool) *UpstreamHost {
poolLen := uint32(len(pool))
selection := atomic.AddUint32(&r.Robin, 1) % poolLen
host := pool[selection]
// if the currently selected host is down, just ffwd to up host
for i := uint32(1); host.Down() && i < poolLen; i++ {
host = pool[(selection+i)%poolLen]
}
return host
}
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