4 files changed, 1085 insertions, 0 deletions
diff --git a/modules/caddyhttp/reverseproxy/healthchecker.go b/modules/caddyhttp/reverseproxy/healthchecker.go
new file mode 100755
index 0000000..add3aa0
--- /dev/null
+++ b/modules/caddyhttp/reverseproxy/healthchecker.go
@@ -0,0 +1,64 @@
+package reverseproxy
+
+import (
+	"net/http"
+	"time"
+)
+
+// Upstream represents the interface that must be satisfied to use the healthchecker.
+type Upstream interface {
+	SetHealthiness(bool)
+}
+
+// HealthChecker represents a worker that periodically evaluates if proxy upstream host is healthy.
+type HealthChecker struct {
+	upstream   Upstream
+	Ticker     *time.Ticker
+	HTTPClient *http.Client
+}
+
+// ScheduleChecks periodically runs health checks against an upstream host.
+func (h *HealthChecker) ScheduleChecks(url string) {
+	// check if a host is healthy on start vs waiting for timer
+	h.upstream.SetHealthiness(h.IsHealthy(url))
+
+	for {
+		select {
+		case <-h.Ticker.C:
+			h.upstream.SetHealthiness(h.IsHealthy(url))
+		}
+	}
+}
+
+// Stop stops the healthchecker from makeing further requests.
+func (h *HealthChecker) Stop() {
+	h.Ticker.Stop()
+}
+
+// IsHealthy attempts to check if a upstream host is healthy.
+func (h *HealthChecker) IsHealthy(url string) bool {
+	req, err := http.NewRequest(http.MethodGet, url, nil)
+	if err != nil {
+		return false
+	}
+
+	resp, err := h.HTTPClient.Do(req)
+	if err != nil {
+		return false
+	}
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 400 {
+		return false
+	}
+
+	return true
+}
+
+// NewHealthCheckWorker returns a new instance of a HealthChecker.
+func NewHealthCheckWorker(u Upstream, interval time.Duration, client *http.Client) *HealthChecker {
+	return &HealthChecker{
+		upstream:   u,
+		Ticker:     time.NewTicker(interval),
+		HTTPClient: client,
+	}
+}
diff --git a/modules/caddyhttp/reverseproxy/module.go b/modules/caddyhttp/reverseproxy/module.go
new file mode 100755
index 0000000..cc53bf5
--- /dev/null
+++ b/modules/caddyhttp/reverseproxy/module.go
@@ -0,0 +1,46 @@
+package reverseproxy
+
+import (
+	"fmt"
+
+	"bitbucket.org/lightcodelabs/caddy2"
+)
+
+// Register caddy module.
+func init() {
+	caddy2.RegisterModule(caddy2.Module{
+		Name: "http.responders.reverse_proxy",
+		New:  func() (interface{}, error) { return new(LoadBalanced), nil },
+		OnLoad: func(instances []interface{}, _ interface{}) (interface{}, error) {
+			// we don't need to do anything with prior state because healthcheckers are
+			// cleaned up in OnUnload.
+			s := &State{
+				HealthCheckers: []*HealthChecker{},
+			}
+
+			for _, i := range instances {
+				lb := i.(*LoadBalanced)
+
+				err := NewLoadBalancedReverseProxy(lb, s)
+				if err != nil {
+					return nil, err
+				}
+			}
+
+			return s, nil
+		},
+		OnUnload: func(state interface{}) error {
+			s, ok := state.(*State)
+			if !ok {
+				return fmt.Errorf("proxy OnLoad: prior state not expected proxy.State type")
+			}
+
+			// cleanup old healthcheckers
+			for _, hc := range s.HealthCheckers {
+				hc.Stop()
+			}
+
+			return nil
+		},
+	})
+}
diff --git a/modules/caddyhttp/reverseproxy/reverseproxy.go b/modules/caddyhttp/reverseproxy/reverseproxy.go
new file mode 100755
index 0000000..e5020e2
--- /dev/null
+++ b/modules/caddyhttp/reverseproxy/reverseproxy.go
@@ -0,0 +1,562 @@
+package reverseproxy
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"net/url"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/net/http/httpguts"
+)
+
+// ReverseProxy is an HTTP Handler that takes an incoming request and
+// sends it to another server, proxying the response back to the
+// client.
+type ReverseProxy struct {
+	// Director must be a function which modifies
+	// the request into a new request to be sent
+	// using Transport. Its response is then copied
+	// back to the original client unmodified.
+	// Director must not access the provided Request
+	// after returning.
+	Director func(*http.Request)
+
+	// The transport used to perform proxy requests.
+	// If nil, http.DefaultTransport is used.
+	Transport http.RoundTripper
+
+	// FlushInterval specifies the flush interval
+	// to flush to the client while copying the
+	// response body.
+	// If zero, no periodic flushing is done.
+	// A negative value means to flush immediately
+	// after each write to the client.
+	// The FlushInterval is ignored when ReverseProxy
+	// recognizes a response as a streaming response;
+	// for such responses, writes are flushed to the client
+	// immediately.
+	FlushInterval time.Duration
+
+	// ErrorLog specifies an optional logger for errors
+	// that occur when attempting to proxy the request.
+	// If nil, logging goes to os.Stderr via the log package's
+	// standard logger.
+	ErrorLog *log.Logger
+
+	// BufferPool optionally specifies a buffer pool to
+	// get byte slices for use by io.CopyBuffer when
+	// copying HTTP response bodies.
+	BufferPool BufferPool
+
+	// ModifyResponse is an optional function that modifies the
+	// Response from the backend. It is called if the backend
+	// returns a response at all, with any HTTP status code.
+	// If the backend is unreachable, the optional ErrorHandler is
+	// called without any call to ModifyResponse.
+	//
+	// If ModifyResponse returns an error, ErrorHandler is called
+	// with its error value. If ErrorHandler is nil, its default
+	// implementation is used.
+	ModifyResponse func(*http.Response) error
+
+	// ErrorHandler is an optional function that handles errors
+	// reaching the backend or errors from ModifyResponse.
+	//
+	// If nil, the default is to log the provided error and return
+	// a 502 Status Bad Gateway response.
+	ErrorHandler func(http.ResponseWriter, *http.Request, error)
+}
+
+// A BufferPool is an interface for getting and returning temporary
+// byte slices for use by io.CopyBuffer.
+type BufferPool interface {
+	Get() []byte
+	Put([]byte)
+}
+
+func singleJoiningSlash(a, b string) string {
+	aslash := strings.HasSuffix(a, "/")
+	bslash := strings.HasPrefix(b, "/")
+	switch {
+	case aslash && bslash:
+		return a + b[1:]
+	case !aslash && !bslash:
+		return a + "/" + b
+	}
+	return a + b
+}
+
+// NewSingleHostReverseProxy returns a new ReverseProxy that routes
+// URLs to the scheme, host, and base path provided in target. If the
+// target's path is "/base" and the incoming request was for "/dir",
+// the target request will be for /base/dir.
+// NewSingleHostReverseProxy does not rewrite the Host header.
+// To rewrite Host headers, use ReverseProxy directly with a custom
+// Director policy.
+func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
+	targetQuery := target.RawQuery
+	director := func(req *http.Request) {
+		req.URL.Scheme = target.Scheme
+		req.URL.Host = target.Host
+		req.URL.Path = singleJoiningSlash(target.Path, req.URL.Path)
+		if targetQuery == "" || req.URL.RawQuery == "" {
+			req.URL.RawQuery = targetQuery + req.URL.RawQuery
+		} else {
+			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
+		}
+		if _, ok := req.Header["User-Agent"]; !ok {
+			// explicitly disable User-Agent so it's not set to default value
+			req.Header.Set("User-Agent", "")
+		}
+	}
+	return &ReverseProxy{Director: director}
+}
+
+func copyHeader(dst, src http.Header) {
+	for k, vv := range src {
+		for _, v := range vv {
+			dst.Add(k, v)
+		}
+	}
+}
+
+func cloneHeader(h http.Header) http.Header {
+	h2 := make(http.Header, len(h))
+	for k, vv := range h {
+		vv2 := make([]string, len(vv))
+		copy(vv2, vv)
+		h2[k] = vv2
+	}
+	return h2
+}
+
+// Hop-by-hop headers. These are removed when sent to the backend.
+// As of RFC 7230, hop-by-hop headers are required to appear in the
+// Connection header field. These are the headers defined by the
+// obsoleted RFC 2616 (section 13.5.1) and are used for backward
+// compatibility.
+var hopHeaders = []string{
+	"Connection",
+	"Proxy-Connection", // non-standard but still sent by libcurl and rejected by e.g. google
+	"Keep-Alive",
+	"Proxy-Authenticate",
+	"Proxy-Authorization",
+	"Te",      // canonicalized version of "TE"
+	"Trailer", // not Trailers per URL above; https://www.rfc-editor.org/errata_search.php?eid=4522
+	"Transfer-Encoding",
+	"Upgrade",
+}
+
+func (p *ReverseProxy) defaultErrorHandler(rw http.ResponseWriter, req *http.Request, err error) {
+	p.logf("http: proxy error: %v", err)
+	rw.WriteHeader(http.StatusBadGateway)
+}
+
+func (p *ReverseProxy) getErrorHandler() func(http.ResponseWriter, *http.Request, error) {
+	if p.ErrorHandler != nil {
+		return p.ErrorHandler
+	}
+	return p.defaultErrorHandler
+}
+
+// modifyResponse conditionally runs the optional ModifyResponse hook
+// and reports whether the request should proceed.
+func (p *ReverseProxy) modifyResponse(rw http.ResponseWriter, res *http.Response, req *http.Request) bool {
+	if p.ModifyResponse == nil {
+		return true
+	}
+	if err := p.ModifyResponse(res); err != nil {
+		res.Body.Close()
+		p.getErrorHandler()(rw, req, err)
+		return false
+	}
+	return true
+}
+
+func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) (*http.Response, error) {
+	transport := p.Transport
+	if transport == nil {
+		transport = http.DefaultTransport
+	}
+
+	ctx := req.Context()
+	if cn, ok := rw.(http.CloseNotifier); ok {
+		var cancel context.CancelFunc
+		ctx, cancel = context.WithCancel(ctx)
+		defer cancel()
+		notifyChan := cn.CloseNotify()
+		go func() {
+			select {
+			case <-notifyChan:
+				cancel()
+			case <-ctx.Done():
+			}
+		}()
+	}
+
+	outreq := req.WithContext(ctx) // includes shallow copies of maps, but okay
+	if req.ContentLength == 0 {
+		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
+	}
+
+	outreq.Header = cloneHeader(req.Header)
+
+	p.Director(outreq)
+	outreq.Close = false
+
+	reqUpType := upgradeType(outreq.Header)
+	removeConnectionHeaders(outreq.Header)
+
+	// Remove hop-by-hop headers to the backend. Especially
+	// important is "Connection" because we want a persistent
+	// connection, regardless of what the client sent to us.
+	for _, h := range hopHeaders {
+		hv := outreq.Header.Get(h)
+		if hv == "" {
+			continue
+		}
+		if h == "Te" && hv == "trailers" {
+			// Issue 21096: tell backend applications that
+			// care about trailer support that we support
+			// trailers. (We do, but we don't go out of
+			// our way to advertise that unless the
+			// incoming client request thought it was
+			// worth mentioning)
+			continue
+		}
+		outreq.Header.Del(h)
+	}
+
+	// After stripping all the hop-by-hop connection headers above, add back any
+	// necessary for protocol upgrades, such as for websockets.
+	if reqUpType != "" {
+		outreq.Header.Set("Connection", "Upgrade")
+		outreq.Header.Set("Upgrade", reqUpType)
+	}
+
+	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
+		// If we aren't the first proxy retain prior
+		// X-Forwarded-For information as a comma+space
+		// separated list and fold multiple headers into one.
+		if prior, ok := outreq.Header["X-Forwarded-For"]; ok {
+			clientIP = strings.Join(prior, ", ") + ", " + clientIP
+		}
+		outreq.Header.Set("X-Forwarded-For", clientIP)
+	}
+
+	res, err := transport.RoundTrip(outreq)
+	if err != nil {
+		p.getErrorHandler()(rw, outreq, err)
+		return nil, err
+	}
+
+	// Deal with 101 Switching Protocols responses: (WebSocket, h2c, etc)
+	if res.StatusCode == http.StatusSwitchingProtocols {
+		if !p.modifyResponse(rw, res, outreq) {
+			return res, nil
+		}
+
+		p.handleUpgradeResponse(rw, outreq, res)
+		return res, nil
+	}
+
+	removeConnectionHeaders(res.Header)
+
+	for _, h := range hopHeaders {
+		res.Header.Del(h)
+	}
+
+	if !p.modifyResponse(rw, res, outreq) {
+		return res, nil
+	}
+
+	copyHeader(rw.Header(), res.Header)
+
+	// The "Trailer" header isn't included in the Transport's response,
+	// at least for *http.Transport. Build it up from Trailer.
+	announcedTrailers := len(res.Trailer)
+	if announcedTrailers > 0 {
+		trailerKeys := make([]string, 0, len(res.Trailer))
+		for k := range res.Trailer {
+			trailerKeys = append(trailerKeys, k)
+		}
+		rw.Header().Add("Trailer", strings.Join(trailerKeys, ", "))
+	}
+
+	rw.WriteHeader(res.StatusCode)
+
+	err = p.copyResponse(rw, res.Body, p.flushInterval(req, res))
+	if err != nil {
+		defer res.Body.Close()
+		// Since we're streaming the response, if we run into an error all we can do
+		// is abort the request. Issue 23643: ReverseProxy should use ErrAbortHandler
+		// on read error while copying body.
+		if !shouldPanicOnCopyError(req) {
+			p.logf("suppressing panic for copyResponse error in test; copy error: %v", err)
+			return nil, err
+		}
+
+		panic(http.ErrAbortHandler)
+	}
+	res.Body.Close() // close now, instead of defer, to populate res.Trailer
+
+	if len(res.Trailer) > 0 {
+		// Force chunking if we saw a response trailer.
+		// This prevents net/http from calculating the length for short
+		// bodies and adding a Content-Length.
+		if fl, ok := rw.(http.Flusher); ok {
+			fl.Flush()
+		}
+	}
+
+	if len(res.Trailer) == announcedTrailers {
+		copyHeader(rw.Header(), res.Trailer)
+		return res, nil
+	}
+
+	for k, vv := range res.Trailer {
+		k = http.TrailerPrefix + k
+		for _, v := range vv {
+			rw.Header().Add(k, v)
+		}
+	}
+
+	return res, nil
+}
+
+var inOurTests bool // whether we're in our own tests
+
+// shouldPanicOnCopyError reports whether the reverse proxy should
+// panic with http.ErrAbortHandler. This is the right thing to do by
+// default, but Go 1.10 and earlier did not, so existing unit tests
+// weren't expecting panics. Only panic in our own tests, or when
+// running under the HTTP server.
+func shouldPanicOnCopyError(req *http.Request) bool {
+	if inOurTests {
+		// Our tests know to handle this panic.
+		return true
+	}
+	if req.Context().Value(http.ServerContextKey) != nil {
+		// We seem to be running under an HTTP server, so
+		// it'll recover the panic.
+		return true
+	}
+	// Otherwise act like Go 1.10 and earlier to not break
+	// existing tests.
+	return false
+}
+
+// removeConnectionHeaders removes hop-by-hop headers listed in the "Connection" header of h.
+// See RFC 7230, section 6.1
+func removeConnectionHeaders(h http.Header) {
+	if c := h.Get("Connection"); c != "" {
+		for _, f := range strings.Split(c, ",") {
+			if f = strings.TrimSpace(f); f != "" {
+				h.Del(f)
+			}
+		}
+	}
+}
+
+// flushInterval returns the p.FlushInterval value, conditionally
+// overriding its value for a specific request/response.
+func (p *ReverseProxy) flushInterval(req *http.Request, res *http.Response) time.Duration {
+	resCT := res.Header.Get("Content-Type")
+
+	// For Server-Sent Events responses, flush immediately.
+	// The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
+	if resCT == "text/event-stream" {
+		return -1 // negative means immediately
+	}
+
+	// TODO: more specific cases? e.g. res.ContentLength == -1?
+	return p.FlushInterval
+}
+
+func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration) error {
+	if flushInterval != 0 {
+		if wf, ok := dst.(writeFlusher); ok {
+			mlw := &maxLatencyWriter{
+				dst:     wf,
+				latency: flushInterval,
+			}
+			defer mlw.stop()
+			dst = mlw
+		}
+	}
+
+	var buf []byte
+	if p.BufferPool != nil {
+		buf = p.BufferPool.Get()
+		defer p.BufferPool.Put(buf)
+	}
+	_, err := p.copyBuffer(dst, src, buf)
+	return err
+}
+
+// copyBuffer returns any write errors or non-EOF read errors, and the amount
+// of bytes written.
+func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
+	if len(buf) == 0 {
+		buf = make([]byte, 32*1024)
+	}
+	var written int64
+	for {
+		nr, rerr := src.Read(buf)
+		if rerr != nil && rerr != io.EOF && rerr != context.Canceled {
+			p.logf("httputil: ReverseProxy read error during body copy: %v", rerr)
+		}
+		if nr > 0 {
+			nw, werr := dst.Write(buf[:nr])
+			if nw > 0 {
+				written += int64(nw)
+			}
+			if werr != nil {
+				return written, werr
+			}
+			if nr != nw {
+				return written, io.ErrShortWrite
+			}
+		}
+		if rerr != nil {
+			if rerr == io.EOF {
+				rerr = nil
+			}
+			return written, rerr
+		}
+	}
+}
+
+func (p *ReverseProxy) logf(format string, args ...interface{}) {
+	if p.ErrorLog != nil {
+		p.ErrorLog.Printf(format, args...)
+	} else {
+		log.Printf(format, args...)
+	}
+}
+
+type writeFlusher interface {
+	io.Writer
+	http.Flusher
+}
+
+type maxLatencyWriter struct {
+	dst     writeFlusher
+	latency time.Duration // non-zero; negative means to flush immediately
+
+	mu           sync.Mutex // protects t, flushPending, and dst.Flush
+	t            *time.Timer
+	flushPending bool
+}
+
+func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	n, err = m.dst.Write(p)
+	if m.latency < 0 {
+		m.dst.Flush()
+		return
+	}
+	if m.flushPending {
+		return
+	}
+	if m.t == nil {
+		m.t = time.AfterFunc(m.latency, m.delayedFlush)
+	} else {
+		m.t.Reset(m.latency)
+	}
+	m.flushPending = true
+	return
+}
+
+func (m *maxLatencyWriter) delayedFlush() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if !m.flushPending { // if stop was called but AfterFunc already started this goroutine
+		return
+	}
+	m.dst.Flush()
+	m.flushPending = false
+}
+
+func (m *maxLatencyWriter) stop() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.flushPending = false
+	if m.t != nil {
+		m.t.Stop()
+	}
+}
+
+func upgradeType(h http.Header) string {
+	if !httpguts.HeaderValuesContainsToken(h["Connection"], "Upgrade") {
+		return ""
+	}
+	return strings.ToLower(h.Get("Upgrade"))
+}
+
+func (p *ReverseProxy) handleUpgradeResponse(rw http.ResponseWriter, req *http.Request, res *http.Response) {
+	reqUpType := upgradeType(req.Header)
+	resUpType := upgradeType(res.Header)
+	if reqUpType != resUpType {
+		p.getErrorHandler()(rw, req, fmt.Errorf("backend tried to switch protocol %q when %q was requested", resUpType, reqUpType))
+		return
+	}
+
+	copyHeader(res.Header, rw.Header())
+
+	hj, ok := rw.(http.Hijacker)
+	if !ok {
+		p.getErrorHandler()(rw, req, fmt.Errorf("can't switch protocols using non-Hijacker ResponseWriter type %T", rw))
+		return
+	}
+	backConn, ok := res.Body.(io.ReadWriteCloser)
+	if !ok {
+		p.getErrorHandler()(rw, req, fmt.Errorf("internal error: 101 switching protocols response with non-writable body"))
+		return
+	}
+	defer backConn.Close()
+	conn, brw, err := hj.Hijack()
+	if err != nil {
+		p.getErrorHandler()(rw, req, fmt.Errorf("Hijack failed on protocol switch: %v", err))
+		return
+	}
+	defer conn.Close()
+	res.Body = nil // so res.Write only writes the headers; we have res.Body in backConn above
+	if err := res.Write(brw); err != nil {
+		p.getErrorHandler()(rw, req, fmt.Errorf("response write: %v", err))
+		return
+	}
+	if err := brw.Flush(); err != nil {
+		p.getErrorHandler()(rw, req, fmt.Errorf("response flush: %v", err))
+		return
+	}
+	errc := make(chan error, 1)
+	spc := switchProtocolCopier{user: conn, backend: backConn}
+	go spc.copyToBackend(errc)
+	go spc.copyFromBackend(errc)
+	<-errc
+	return
+}
+
+// switchProtocolCopier exists so goroutines proxying data back and
+// forth have nice names in stacks.
+type switchProtocolCopier struct {
+	user, backend io.ReadWriter
+}
+
+func (c switchProtocolCopier) copyFromBackend(errc chan<- error) {
+	_, err := io.Copy(c.user, c.backend)
+	errc <- err
+}
+
+func (c switchProtocolCopier) copyToBackend(errc chan<- error) {
+	_, err := io.Copy(c.backend, c.user)
+	errc <- err
+}
diff --git a/modules/caddyhttp/reverseproxy/upstream.go b/modules/caddyhttp/reverseproxy/upstream.go
new file mode 100755
index 0000000..b521d46
--- /dev/null
+++ b/modules/caddyhttp/reverseproxy/upstream.go
@@ -0,0 +1,413 @@
+// Package reverseproxy implements a load-balanced reverse proxy.
+package reverseproxy
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"math/rand"
+	"net"
+	"net/http"
+	"net/url"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"bitbucket.org/lightcodelabs/caddy2"
+)
+
+// State represents the global state of a loadbalancer. It is used to store
+// references to health checkers.
+type State struct {
+	HealthCheckers []*HealthChecker
+}
+
+// CircuitBreaker defines the functionality of a circuit breaker module.
+type CircuitBreaker interface {
+	Ok() bool
+	RecordMetric(statusCode int, latency time.Duration)
+}
+
+type noopCircuitBreaker struct{}
+
+func (ncb noopCircuitBreaker) RecordMetric(statusCode int, latency time.Duration) {}
+func (ncb noopCircuitBreaker) Ok() bool {
+	return true
+}
+
+const (
+	// TypeBalanceRoundRobin represents the value to use for configuring a load balanced reverse proxy to use round robin load balancing.
+	TypeBalanceRoundRobin = iota
+
+	// TypeBalanceRandom represents the value to use for configuring a load balanced reverse proxy to use random load balancing.
+	TypeBalanceRandom
+
+	// msgNoHealthyUpstreams is returned if there are no upstreams that are healthy to proxy a request to
+	msgNoHealthyUpstreams = "No healthy upstreams."
+
+	// by default perform health checks every 30 seconds
+	defaultHealthCheckDur = time.Second * 30
+
+	// used when an upstream is unhealthy, health checks can be configured to perform at a faster rate
+	defaultFastHealthCheckDur = time.Second * 1
+)
+
+var (
+	// defaultTransport is the default transport to use for the reverse proxy.
+	defaultTransport = &http.Transport{
+		Dial: (&net.Dialer{
+			Timeout: 5 * time.Second,
+		}).Dial,
+		TLSHandshakeTimeout: 5 * time.Second,
+	}
+
+	// defaultHTTPClient is the default http client to use for the healthchecker.
+	defaultHTTPClient = &http.Client{
+		Timeout:   time.Second * 10,
+		Transport: defaultTransport,
+	}
+
+	// typeMap maps caddy load balance configuration to the internal representation of the loadbalance algorithm type.
+	typeMap = map[string]int{
+		"round_robin": TypeBalanceRoundRobin,
+		"random":      TypeBalanceRandom,
+	}
+)
+
+// NewLoadBalancedReverseProxy returns a collection of Upstreams that are to be loadbalanced.
+func NewLoadBalancedReverseProxy(lb *LoadBalanced, state *State) error {
+	// set defaults
+	if lb.NoHealthyUpstreamsMessage == "" {
+		lb.NoHealthyUpstreamsMessage = msgNoHealthyUpstreams
+	}
+
+	if lb.TryInterval == "" {
+		lb.TryInterval = "20s"
+	}
+
+	// set request retry interval
+	ti, err := time.ParseDuration(lb.TryInterval)
+	if err != nil {
+		return fmt.Errorf("NewLoadBalancedReverseProxy: %v", err.Error())
+	}
+	lb.tryInterval = ti
+
+	// set load balance algorithm
+	t, ok := typeMap[lb.LoadBalanceType]
+	if !ok {
+		t = TypeBalanceRandom
+	}
+	lb.loadBalanceType = t
+
+	// setup each upstream
+	var us []*upstream
+	for _, uc := range lb.Upstreams {
+		// pass the upstream decr and incr methods to keep track of unhealthy nodes
+		nu, err := newUpstream(uc, lb.decrUnhealthy, lb.incrUnhealthy)
+		if err != nil {
+			return err
+		}
+
+		// setup any configured circuit breakers
+		var cbModule = "http.responders.reverse_proxy.circuit_breaker"
+		var cb CircuitBreaker
+
+		if uc.CircuitBreaker != nil {
+			if _, err := caddy2.GetModule(cbModule); err == nil {
+				val, err := caddy2.LoadModule(cbModule, uc.CircuitBreaker)
+				if err == nil {
+					cbv, ok := val.(CircuitBreaker)
+					if ok {
+						cb = cbv
+					} else {
+						fmt.Printf("\nerr: %v; cannot load circuit_breaker, using noop", err.Error())
+						cb = noopCircuitBreaker{}
+					}
+				} else {
+					fmt.Printf("\nerr: %v; cannot load circuit_breaker, using noop", err.Error())
+					cb = noopCircuitBreaker{}
+				}
+			} else {
+				fmt.Println("circuit_breaker module not loaded, using noop")
+				cb = noopCircuitBreaker{}
+			}
+		} else {
+			cb = noopCircuitBreaker{}
+		}
+		nu.CB = cb
+
+		// start a healthcheck worker which will periodically check to see if an upstream is healthy
+		// to proxy requests to.
+		nu.healthChecker = NewHealthCheckWorker(nu, defaultHealthCheckDur, defaultHTTPClient)
+
+		// TODO :- if path is empty why does this empty the entire Target?
+		// nu.Target.Path = uc.HealthCheckPath
+
+		go nu.healthChecker.ScheduleChecks(nu.Target.String())
+		state.HealthCheckers = append(state.HealthCheckers, nu.healthChecker)
+
+		us = append(us, nu)
+	}
+
+	lb.upstreams = us
+
+	return nil
+}
+
+// LoadBalanced represents a collection of upstream hosts that are loadbalanced. It
+// contains multiple features like health checking and circuit breaking functionality
+// for upstreams.
+type LoadBalanced struct {
+	mu              sync.Mutex
+	numUnhealthy    int32
+	selectedServer  int // used during round robin load balancing
+	loadBalanceType int
+	tryInterval     time.Duration
+	upstreams       []*upstream
+
+	// The following struct fields are set by caddy configuration.
+	// TryInterval is the max duration for which request retrys will be performed for a request.
+	TryInterval string `json:"try_interval"`
+
+	// Upstreams are the configs for upstream hosts
+	Upstreams []*UpstreamConfig `json:"upstreams"`
+
+	// LoadBalanceType is the string representation of what loadbalancing algorithm to use. i.e. "random" or "round_robin".
+	LoadBalanceType string `json:"load_balance_type"`
+
+	// NoHealthyUpstreamsMessage is returned as a response when there are no healthy upstreams to loadbalance to.
+	NoHealthyUpstreamsMessage string `json:"no_healthy_upstreams_message"`
+}
+
+// ServeHTTP implements the http.Handler interface to dispatch an http request to the proper
+// server.
+func (lb *LoadBalanced) ServeHTTP(w http.ResponseWriter, r *http.Request) error {
+	// ensure requests don't hang if an upstream does not respond or is not eventually healthy
+	var u *upstream
+	var done bool
+
+	retryTimer := time.NewTicker(lb.tryInterval)
+	defer retryTimer.Stop()
+
+	go func() {
+		select {
+		case <-retryTimer.C:
+			done = true
+		}
+	}()
+
+	// keep trying to get an available upstream to process the request
+	for {
+		switch lb.loadBalanceType {
+		case TypeBalanceRandom:
+			u = lb.random()
+		case TypeBalanceRoundRobin:
+			u = lb.roundRobin()
+		}
+
+		// if we can't get an upstream and our retry interval has ended return an error response
+		if u == nil && done {
+			w.WriteHeader(http.StatusBadGateway)
+			fmt.Fprint(w, lb.NoHealthyUpstreamsMessage)
+
+			return fmt.Errorf(msgNoHealthyUpstreams)
+		}
+
+		// attempt to get an available upstream
+		if u == nil {
+			continue
+		}
+
+		start := time.Now()
+
+		// if we get an error retry until we get a healthy upstream
+		res, err := u.ReverseProxy.ServeHTTP(w, r)
+		if err != nil {
+			if err == context.Canceled {
+				return nil
+			}
+
+			continue
+		}
+
+		// record circuit breaker metrics
+		go u.CB.RecordMetric(res.StatusCode, time.Now().Sub(start))
+
+		return nil
+	}
+}
+
+// incrUnhealthy increments the amount of unhealthy nodes in a loadbalancer.
+func (lb *LoadBalanced) incrUnhealthy() {
+	atomic.AddInt32(&lb.numUnhealthy, 1)
+}
+
+// decrUnhealthy decrements the amount of unhealthy nodes in a loadbalancer.
+func (lb *LoadBalanced) decrUnhealthy() {
+	atomic.AddInt32(&lb.numUnhealthy, -1)
+}
+
+// roundRobin implements a round robin load balancing algorithm to select
+// which server to forward requests to.
+func (lb *LoadBalanced) roundRobin() *upstream {
+	if atomic.LoadInt32(&lb.numUnhealthy) == int32(len(lb.upstreams)) {
+		return nil
+	}
+
+	selected := lb.upstreams[lb.selectedServer]
+
+	lb.mu.Lock()
+	lb.selectedServer++
+	if lb.selectedServer >= len(lb.upstreams) {
+		lb.selectedServer = 0
+	}
+	lb.mu.Unlock()
+
+	if selected.IsHealthy() && selected.CB.Ok() {
+		return selected
+	}
+
+	return nil
+}
+
+// random implements a random server selector for load balancing.
+func (lb *LoadBalanced) random() *upstream {
+	if atomic.LoadInt32(&lb.numUnhealthy) == int32(len(lb.upstreams)) {
+		return nil
+	}
+
+	n := rand.Int() % len(lb.upstreams)
+	selected := lb.upstreams[n]
+
+	if selected.IsHealthy() && selected.CB.Ok() {
+		return selected
+	}
+
+	return nil
+}
+
+// UpstreamConfig represents the config of an upstream.
+type UpstreamConfig struct {
+	// Host is the host name of the upstream server.
+	Host string `json:"host"`
+
+	// FastHealthCheckDuration is the duration for which a health check is performed when a node is considered unhealthy.
+	FastHealthCheckDuration string `json:"fast_health_check_duration"`
+
+	CircuitBreaker json.RawMessage `json:"circuit_breaker"`
+
+	// // CircuitBreakerConfig is the config passed to setup a circuit breaker.
+	// CircuitBreakerConfig *circuitbreaker.Config `json:"circuit_breaker"`
+	circuitbreaker CircuitBreaker
+
+	// HealthCheckDuration is the default duration for which a health check is performed.
+	HealthCheckDuration string `json:"health_check_duration"`
+
+	// HealthCheckPath is the path at the upstream host to use for healthchecks.
+	HealthCheckPath string `json:"health_check_path"`
+}
+
+// upstream represents an upstream host.
+type upstream struct {
+	Healthy            int32 // 0 = false, 1 = true
+	Target             *url.URL
+	ReverseProxy       *ReverseProxy
+	Incr               func()
+	Decr               func()
+	CB                 CircuitBreaker
+	healthChecker      *HealthChecker
+	healthCheckDur     time.Duration
+	fastHealthCheckDur time.Duration
+}
+
+// newUpstream returns a new upstream.
+func newUpstream(uc *UpstreamConfig, d func(), i func()) (*upstream, error) {
+	host := strings.TrimSpace(uc.Host)
+	protoIdx := strings.Index(host, "://")
+	if protoIdx == -1 || len(host[:protoIdx]) == 0 {
+		return nil, fmt.Errorf("protocol is required for host")
+	}
+
+	hostURL, err := url.Parse(host)
+	if err != nil {
+		return nil, err
+	}
+
+	// parse healthcheck durations
+	hcd, err := time.ParseDuration(uc.HealthCheckDuration)
+	if err != nil {
+		hcd = defaultHealthCheckDur
+	}
+
+	fhcd, err := time.ParseDuration(uc.FastHealthCheckDuration)
+	if err != nil {
+		fhcd = defaultFastHealthCheckDur
+	}
+
+	u := upstream{
+		healthCheckDur:     hcd,
+		fastHealthCheckDur: fhcd,
+		Target:             hostURL,
+		Decr:               d,
+		Incr:               i,
+		Healthy:            int32(0), // assume is unhealthy on start
+	}
+
+	u.ReverseProxy = newReverseProxy(hostURL, u.SetHealthiness)
+	return &u, nil
+}
+
+// SetHealthiness sets whether an upstream is healthy or not. The health check worker is updated to
+// perform checks faster if a node is unhealthy.
+func (u *upstream) SetHealthiness(ok bool) {
+	h := atomic.LoadInt32(&u.Healthy)
+	var wasHealthy bool
+	if h == 1 {
+		wasHealthy = true
+	} else {
+		wasHealthy = false
+	}
+
+	if ok {
+		u.healthChecker.Ticker = time.NewTicker(u.healthCheckDur)
+
+		if !wasHealthy {
+			atomic.AddInt32(&u.Healthy, 1)
+			u.Decr()
+		}
+	} else {
+		u.healthChecker.Ticker = time.NewTicker(u.fastHealthCheckDur)
+
+		if wasHealthy {
+			atomic.AddInt32(&u.Healthy, -1)
+			u.Incr()
+		}
+	}
+}
+
+// IsHealthy returns whether an Upstream is healthy or not.
+func (u *upstream) IsHealthy() bool {
+	i := atomic.LoadInt32(&u.Healthy)
+	if i == 1 {
+		return true
+	}
+
+	return false
+}
+
+// newReverseProxy returns a new reverse proxy handler.
+func newReverseProxy(target *url.URL, setHealthiness func(bool)) *ReverseProxy {
+	errorHandler := func(w http.ResponseWriter, r *http.Request, err error) {
+		// we don't need to worry about cancelled contexts since this doesn't necessarilly mean that
+		// the upstream is unhealthy.
+		if err != context.Canceled {
+			setHealthiness(false)
+		}
+	}
+
+	rp := NewSingleHostReverseProxy(target)
+	rp.ErrorHandler = errorHandler
+	rp.Transport = defaultTransport // use default transport that times out in 5 seconds
+	return rp
+}