@@ -190,7 +190,8 @@ func (r *Header) Select(pool HostPool, request *http.Request) *UpstreamHost {
 	}
 	val := request.Header.Get(r.Name)
 	if val == "" {
-		return nil
+		// fallback to RoundRobin policy in case no Header in request
+		return (&RoundRobin{}).Select(pool, request)
 	}
 	return hostByHashing(pool, val)
 }
// Copyright 2015 Light Code Labs, LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package proxy

import (
	"hash/fnv"
	"math"
	"math/rand"
	"net"
	"net/http"
	"sync"
)

// HostPool is a collection of UpstreamHosts.
type HostPool []*UpstreamHost

// Policy decides how a host will be selected from a pool.
type Policy interface {
	Select(pool HostPool, r *http.Request) *UpstreamHost
}

func init() {
	RegisterPolicy("random", func(arg string) Policy { return &Random{} })
	RegisterPolicy("least_conn", func(arg string) Policy { return &LeastConn{} })
	RegisterPolicy("round_robin", func(arg string) Policy { return &RoundRobin{} })
	RegisterPolicy("ip_hash", func(arg string) Policy { return &IPHash{} })
	RegisterPolicy("first", func(arg string) Policy { return &First{} })
	RegisterPolicy("uri_hash", func(arg string) Policy { return &URIHash{} })
	RegisterPolicy("header", func(arg string) Policy { return &Header{arg} })
}

// 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, request *http.Request) *UpstreamHost {

	// Because the number of available hosts isn't known
	// up front, the host is selected via reservoir sampling
	// https://en.wikipedia.org/wiki/Reservoir_sampling
	var randHost *UpstreamHost
	count := 0
	for _, host := range pool {
		if !host.Available() {
			continue
		}

		// (n % 1 == 0) holds for all n, therefore randHost
		// will always get assigned a value if there is
		// at least 1 available host
		count++
		if (rand.Int() % count) == 0 {
			randHost = host
		}
	}
	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, request *http.Request) *UpstreamHost {
	var bestHost *UpstreamHost
	count := 0
	leastConn := int64(math.MaxInt64)
	for _, host := range pool {
		if !host.Available() {
			continue
		}

		if host.Conns < leastConn {
			leastConn = host.Conns
			count = 0
		}

		// Among hosts with same least connections, perform a reservoir
		// sample: https://en.wikipedia.org/wiki/Reservoir_sampling
		if host.Conns == leastConn {
			count++
			if (rand.Int() % count) == 0 {
				bestHost = host
			}
		}
	}
	return bestHost
}

// RoundRobin is a policy that selects hosts based on round-robin ordering.
type RoundRobin struct {
	robin uint32
	mutex sync.Mutex
}

// Select selects an up host from the pool using a round-robin ordering scheme.
func (r *RoundRobin) Select(pool HostPool, request *http.Request) *UpstreamHost {
	poolLen := uint32(len(pool))
	r.mutex.Lock()
	defer r.mutex.Unlock()
	// Return next available host
	for i := uint32(0); i < poolLen; i++ {
		r.robin++
		host := pool[r.robin%poolLen]
		if host.Available() {
			return host
		}
	}
	return nil
}

// hostByHashing returns an available host from pool based on a hashable string
func hostByHashing(pool HostPool, s string) *UpstreamHost {
	poolLen := uint32(len(pool))
	index := hash(s) % poolLen
	for i := uint32(0); i < poolLen; i++ {
		index += i
		host := pool[index%poolLen]
		if host.Available() {
			return host
		}
	}
	return nil
}

// hash calculates a hash based on string s
func hash(s string) uint32 {
	h := fnv.New32a()
	h.Write([]byte(s))
	return h.Sum32()
}

// IPHash is a policy that selects hosts based on hashing the request IP
type IPHash struct{}

// Select selects an up host from the pool based on hashing the request IP
func (r *IPHash) Select(pool HostPool, request *http.Request) *UpstreamHost {
	clientIP, _, err := net.SplitHostPort(request.RemoteAddr)
	if err != nil {
		clientIP = request.RemoteAddr
	}
	return hostByHashing(pool, clientIP)
}

// URIHash is a policy that selects the host based on hashing the request URI
type URIHash struct{}

// Select selects the host based on hashing the URI
func (r *URIHash) Select(pool HostPool, request *http.Request) *UpstreamHost {
	return hostByHashing(pool, request.RequestURI)
}

// First is a policy that selects the first available host
type First struct{}

// Select selects the first available host from the pool
func (r *First) Select(pool HostPool, request *http.Request) *UpstreamHost {
	for _, host := range pool {
		if host.Available() {
			return host
		}
	}
	return nil
}

// Header is a policy that selects based on a hash of the given header
type Header struct {
	// The name of the request header, the value of which will determine
	// how the request is routed
	Name string
}

// Select selects the host based on hashing the header value
func (r *Header) Select(pool HostPool, request *http.Request) *UpstreamHost {
	if r.Name == "" {
		return nil
	}
	val := request.Header.Get(r.Name)
	if val == "" {
		return nil
	}
	return hostByHashing(pool, val)
}

I think this will always choose the same host, since a new RoundRobin is made every time, thus losing the state it creates. We should use the same round robin for all requests. I'm not too particular how this is done, even if it's an unexported variable that's used specifically for this purpose.