Added ability to balance requests among multiple upstream hosts via HostClient

TODO

@@ -1,5 +1,4 @@
 - SessionClient with referer and cookies support.
-- Load balancing client for multiple upstream hosts.
 - Client with requests' pipelining support.
 - ProxyHandler similar to FSHandler.
 - WebSockets. See https://tools.ietf.org/html/rfc6455 .

client.go

@@ -99,7 +99,7 @@ type Client struct {
 
 	// Callback for establishing new connections to hosts.
 	//
-	// Default TCPDialer is used if not set.
+	// Default Dial is used if not set.
 	Dial DialFunc
 
 	// Attempt to connect to both ipv4 and ipv6 addresses if set to true.
@@ -301,7 +301,7 @@ func (c *Client) mCleaner(m map[string]*HostClient) {
 
 // Maximum number of concurrent connections http client may establish per host
 // by default.
-const DefaultMaxConnsPerHost = 4 * 1024
+const DefaultMaxConnsPerHost = 512
 
 // DialFunc must establish connection to addr.
 //
@@ -317,16 +317,18 @@ const DefaultMaxConnsPerHost = 4 * 1024
 //   - foobar.com:8080
 type DialFunc func(addr string) (net.Conn, error)
 
-// HostClient is a single-host http client. It can make http requests
-// to the given Addr only.
+// HostClient balances http requests among hosts listed in Addr.
+//
+// HostClient may be used for balancing load among multiple upstream hosts.
 //
 // It is forbidden copying HostClient instances. Create new instances instead.
 //
 // It is safe calling HostClient methods from concurrently running goroutines.
 type HostClient struct {
-	// HTTP server host address, which is passed to Dial.
+	// Comma-separated list of upstream HTTP server host addresses,
+	// which are passed to Dial in round-robin manner.
 	//
-	// The address may contain port if default dialer is used.
+	// Each address may contain port if default dialer is used.
 	// For example,
 	//
 	//    - foobar.com:80
@@ -339,7 +341,7 @@ type HostClient struct {
 
 	// Callback for establishing new connection to the host.
 	//
-	// Default TCPDialer is used if not set.
+	// Default Dial is used if not set.
 	Dial DialFunc
 
 	// Attempt to connect to both ipv4 and ipv6 host addresses
@@ -358,7 +360,8 @@ type HostClient struct {
 	// Optional TLS config.
 	TLSConfig *tls.Config
 
-	// Maximum number of connections to the host which may be established.
+	// Maximum number of connections which may be established to all hosts
+	// listed in Addr.
 	//
 	// DefaultMaxConnsPerHost is used if not set.
 	MaxConns int
@@ -399,12 +402,9 @@ type HostClient struct {
 	connsCount int
 	conns      []*clientConn
 
-	// dns caching stuff for default dialer.
-	tcpAddrsLock        sync.Mutex
-	tcpAddrs            []net.TCPAddr
-	tcpAddrsPending     bool
-	tcpAddrsResolveTime time.Time
-	tcpAddrsIdx         uint32
+	addrsLock sync.Mutex
+	addrs     []string
+	addrIdx   uint32
 
 	readerPool sync.Pool
 	writerPool sync.Pool
@@ -1044,9 +1044,23 @@ var defaultTLSConfig = &tls.Config{
 	InsecureSkipVerify: true,
 }
 
+func (c *HostClient) nextAddr() string {
+	c.addrsLock.Lock()
+	if c.addrs == nil {
+		c.addrs = strings.Split(c.Addr, ",")
+	}
+	addr := c.addrs[0]
+	if len(c.addrs) > 1 {
+		addr = c.addrs[c.addrIdx%uint32(len(c.addrs))]
+		c.addrIdx++
+	}
+	c.addrsLock.Unlock()
+	return addr
+}
+
 func (c *HostClient) dialHost() (net.Conn, error) {
 	dial := c.Dial
-	addr := c.Addr
+	addr := c.nextAddr()
 	if dial == nil {
 		if c.DialDualStack {
 			dial = DialDualStack

client_test.go

@@ -10,8 +10,68 @@ import (
 	"sync"
 	"testing"
 	"time"
+
+	"github.com/valyala/fasthttp/fasthttputil"
 )
 
+func TestHostClientMultipleAddrs(t *testing.T) {
+	ln := fasthttputil.NewInmemoryListener()
+
+	s := &Server{
+		Handler: func(ctx *RequestCtx) {
+			ctx.Write(ctx.Host())
+			ctx.SetConnectionClose()
+		},
+	}
+	serverStopCh := make(chan struct{})
+	go func() {
+		if err := s.Serve(ln); err != nil {
+			t.Fatalf("unexpected error: %s", err)
+		}
+		close(serverStopCh)
+	}()
+
+	dialsCount := make(map[string]int)
+	c := &HostClient{
+		Addr: "foo,bar,baz",
+		Dial: func(addr string) (net.Conn, error) {
+			dialsCount[addr]++
+			return ln.Dial()
+		},
+	}
+
+	for i := 0; i < 9; i++ {
+		statusCode, body, err := c.Get(nil, "http://foobar/baz/aaa?bbb=ddd")
+		if err != nil {
+			t.Fatalf("unexpected error: %s", err)
+		}
+		if statusCode != StatusOK {
+			t.Fatalf("unexpected status code %d. Expecting %d", statusCode, StatusOK)
+		}
+		if string(body) != "foobar" {
+			t.Fatalf("unexpected body %q. Expecting %q", body, "foobar")
+		}
+	}
+
+	if err := ln.Close(); err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+	select {
+	case <-serverStopCh:
+	case <-time.After(time.Second):
+		t.Fatalf("timeout")
+	}
+
+	if len(dialsCount) != 3 {
+		t.Fatalf("unexpected dialsCount size %d. Expecting 3", len(dialsCount))
+	}
+	for _, k := range []string{"foo", "bar", "baz"} {
+		if dialsCount[k] != 3 {
+			t.Fatalf("unexpected dialsCount for %q. Expecting 3", k)
+		}
+	}
+}
+
 func TestClientFollowRedirects(t *testing.T) {
 	addr := "127.0.0.1:55234"
 	s := &Server{