12 files changed, 1538 insertions, 0 deletions
diff --git a/weed/wdclient/net2/base_connection_pool.go b/weed/wdclient/net2/base_connection_pool.go
new file mode 100644
index 000000000..5cc037d0f
--- /dev/null
+++ b/weed/wdclient/net2/base_connection_pool.go
@@ -0,0 +1,159 @@
+package net2
+
+import (
+	"net"
+	"strings"
+	"time"
+
+	rp "github.com/chrislusf/seaweedfs/weed/wdclient/resource_pool"
+)
+
+const defaultDialTimeout = 1 * time.Second
+
+func defaultDialFunc(network string, address string) (net.Conn, error) {
+	return net.DialTimeout(network, address, defaultDialTimeout)
+}
+
+func parseResourceLocation(resourceLocation string) (
+	network string,
+	address string) {
+
+	idx := strings.Index(resourceLocation, " ")
+	if idx >= 0 {
+		return resourceLocation[:idx], resourceLocation[idx+1:]
+	}
+
+	return "", resourceLocation
+}
+
+// A thin wrapper around the underlying resource pool.
+type connectionPoolImpl struct {
+	options ConnectionOptions
+
+	pool rp.ResourcePool
+}
+
+// This returns a connection pool where all connections are connected
+// to the same (network, address)
+func newBaseConnectionPool(
+	options ConnectionOptions,
+	createPool func(rp.Options) rp.ResourcePool) ConnectionPool {
+
+	dial := options.Dial
+	if dial == nil {
+		dial = defaultDialFunc
+	}
+
+	openFunc := func(loc string) (interface{}, error) {
+		network, address := parseResourceLocation(loc)
+		return dial(network, address)
+	}
+
+	closeFunc := func(handle interface{}) error {
+		return handle.(net.Conn).Close()
+	}
+
+	poolOptions := rp.Options{
+		MaxActiveHandles:   options.MaxActiveConnections,
+		MaxIdleHandles:     options.MaxIdleConnections,
+		MaxIdleTime:        options.MaxIdleTime,
+		OpenMaxConcurrency: options.DialMaxConcurrency,
+		Open:               openFunc,
+		Close:              closeFunc,
+		NowFunc:            options.NowFunc,
+	}
+
+	return &connectionPoolImpl{
+		options: options,
+		pool:    createPool(poolOptions),
+	}
+}
+
+// This returns a connection pool where all connections are connected
+// to the same (network, address)
+func NewSimpleConnectionPool(options ConnectionOptions) ConnectionPool {
+	return newBaseConnectionPool(options, rp.NewSimpleResourcePool)
+}
+
+// This returns a connection pool that manages multiple (network, address)
+// entries.  The connections to each (network, address) entry acts
+// independently. For example ("tcp", "localhost:11211") could act as memcache
+// shard 0 and ("tcp", "localhost:11212") could act as memcache shard 1.
+func NewMultiConnectionPool(options ConnectionOptions) ConnectionPool {
+	return newBaseConnectionPool(
+		options,
+		func(poolOptions rp.Options) rp.ResourcePool {
+			return rp.NewMultiResourcePool(poolOptions, nil)
+		})
+}
+
+// See ConnectionPool for documentation.
+func (p *connectionPoolImpl) NumActive() int32 {
+	return p.pool.NumActive()
+}
+
+// See ConnectionPool for documentation.
+func (p *connectionPoolImpl) ActiveHighWaterMark() int32 {
+	return p.pool.ActiveHighWaterMark()
+}
+
+// This returns the number of alive idle connections.  This method is not part
+// of ConnectionPool's API.  It is used only for testing.
+func (p *connectionPoolImpl) NumIdle() int {
+	return p.pool.NumIdle()
+}
+
+// BaseConnectionPool can only register a single (network, address) entry.
+// Register should be call before any Get calls.
+func (p *connectionPoolImpl) Register(network string, address string) error {
+	return p.pool.Register(network + " " + address)
+}
+
+// BaseConnectionPool has nothing to do on Unregister.
+func (p *connectionPoolImpl) Unregister(network string, address string) error {
+	return nil
+}
+
+func (p *connectionPoolImpl) ListRegistered() []NetworkAddress {
+	result := make([]NetworkAddress, 0, 1)
+	for _, location := range p.pool.ListRegistered() {
+		network, address := parseResourceLocation(location)
+
+		result = append(
+			result,
+			NetworkAddress{
+				Network: network,
+				Address: address,
+			})
+	}
+	return result
+}
+
+// This gets an active connection from the connection pool.  Note that network
+// and address arguments are ignored (The connections with point to the
+// network/address provided by the first Register call).
+func (p *connectionPoolImpl) Get(
+	network string,
+	address string) (ManagedConn, error) {
+
+	handle, err := p.pool.Get(network + " " + address)
+	if err != nil {
+		return nil, err
+	}
+	return NewManagedConn(network, address, handle, p, p.options), nil
+}
+
+// See ConnectionPool for documentation.
+func (p *connectionPoolImpl) Release(conn ManagedConn) error {
+	return conn.ReleaseConnection()
+}
+
+// See ConnectionPool for documentation.
+func (p *connectionPoolImpl) Discard(conn ManagedConn) error {
+	return conn.DiscardConnection()
+}
+
+// See ConnectionPool for documentation.
+func (p *connectionPoolImpl) EnterLameDuckMode() {
+	p.pool.EnterLameDuckMode()
+}
diff --git a/weed/wdclient/net2/connection_pool.go b/weed/wdclient/net2/connection_pool.go
new file mode 100644
index 000000000..5b8d4d232
--- /dev/null
+++ b/weed/wdclient/net2/connection_pool.go
@@ -0,0 +1,97 @@
+package net2
+
+import (
+	"net"
+	"time"
+)
+
+type ConnectionOptions struct {
+	// The maximum number of connections that can be active per host at any
+	// given time (A non-positive value indicates the number of connections
+	// is unbounded).
+	MaxActiveConnections int32
+
+	// The maximum number of idle connections per host that are kept alive by
+	// the connection pool.
+	MaxIdleConnections uint32
+
+	// The maximum amount of time an idle connection can alive (if specified).
+	MaxIdleTime *time.Duration
+
+	// This limits the number of concurrent Dial calls (there's no limit when
+	// DialMaxConcurrency is non-positive).
+	DialMaxConcurrency int
+
+	// Dial specifies the dial function for creating network connections.
+	// If Dial is nil, net.DialTimeout is used, with timeout set to 1 second.
+	Dial func(network string, address string) (net.Conn, error)
+
+	// This specifies the now time function.  When the function is non-nil, the
+	// connection pool will use the specified function instead of time.Now to
+	// generate the current time.
+	NowFunc func() time.Time
+
+	// This specifies the timeout for any Read() operation.
+	// Note that setting this to 0 (i.e. not setting it) will make
+	// read operations block indefinitely.
+	ReadTimeout time.Duration
+
+	// This specifies the timeout for any Write() operation.
+	// Note that setting this to 0 (i.e. not setting it) will make
+	// write operations block indefinitely.
+	WriteTimeout time.Duration
+}
+
+func (o ConnectionOptions) getCurrentTime() time.Time {
+	if o.NowFunc == nil {
+		return time.Now()
+	} else {
+		return o.NowFunc()
+	}
+}
+
+// A generic interface for managed connection pool.  All connection pool
+// implementations must be threadsafe.
+type ConnectionPool interface {
+	// This returns the number of active connections that are on loan.
+	NumActive() int32
+
+	// This returns the highest number of active connections for the entire
+	// lifetime of the pool.
+	ActiveHighWaterMark() int32
+
+	// This returns the number of idle connections that are in the pool.
+	NumIdle() int
+
+	// This associates (network, address) to the connection pool; afterwhich,
+	// the user can get connections to (network, address).
+	Register(network string, address string) error
+
+	// This dissociate (network, address) from the connection pool;
+	// afterwhich, the user can no longer get connections to
+	// (network, address).
+	Unregister(network string, address string) error
+
+	// This returns the list of registered (network, address) entries.
+	ListRegistered() []NetworkAddress
+
+	// This gets an active connection from the connection pool.  The connection
+	// will remain active until one of the following is called:
+	//  1. conn.ReleaseConnection()
+	//  2. conn.DiscardConnection()
+	//  3. pool.Release(conn)
+	//  4. pool.Discard(conn)
+	Get(network string, address string) (ManagedConn, error)
+
+	// This releases an active connection back to the connection pool.
+	Release(conn ManagedConn) error
+
+	// This discards an active connection from the connection pool.
+	Discard(conn ManagedConn) error
+
+	// Enter the connection pool into lame duck mode.  The connection pool
+	// will no longer return connections, and all idle connections are closed
+	// immediately (including active connections that are released back to the
+	// pool afterward).
+	EnterLameDuckMode()
+}
diff --git a/weed/wdclient/net2/doc.go b/weed/wdclient/net2/doc.go
new file mode 100644
index 000000000..f4d6552e4
--- /dev/null
+++ b/weed/wdclient/net2/doc.go
@@ -0,0 +1,6 @@
+// net2 is a collection of functions meant to supplement the capabilities
+// provided by the standard "net" package.
+package net2
+
+// copied from https://github.com/dropbox/godropbox/tree/master/net2
+// removed other dependencies
+\ No newline at end of file
diff --git a/weed/wdclient/net2/ip.go b/weed/wdclient/net2/ip.go
new file mode 100644
index 000000000..60e46342f
--- /dev/null
+++ b/weed/wdclient/net2/ip.go
@@ -0,0 +1,177 @@
+package net2
+
+import (
+	"fmt"
+	"log"
+	"net"
+	"os"
+	"strings"
+	"sync"
+)
+
+var myHostname string
+var myHostnameOnce sync.Once
+
+// Like os.Hostname but caches first successful result, making it cheap to call it
+// over and over.
+// It will also crash whole process if fetching Hostname fails!
+func MyHostname() string {
+	myHostnameOnce.Do(func() {
+		var err error
+		myHostname, err = os.Hostname()
+		if err != nil {
+			log.Fatal(err)
+		}
+	})
+	return myHostname
+}
+
+var myIp4 *net.IPAddr
+var myIp4Once sync.Once
+
+// Resolves `MyHostname()` to an Ip4 address. Caches first successful result, making it
+// cheap to call it over and over.
+// It will also crash whole process if resolving the IP fails!
+func MyIp4() *net.IPAddr {
+	myIp4Once.Do(func() {
+		var err error
+		myIp4, err = net.ResolveIPAddr("ip4", MyHostname())
+		if err != nil {
+			log.Fatal(err)
+		}
+	})
+	return myIp4
+}
+
+var myIp6 *net.IPAddr
+var myIp6Once sync.Once
+
+// Resolves `MyHostname()` to an Ip6 address. Caches first successful result, making it
+// cheap to call it over and over.
+// It will also crash whole process if resolving the IP fails!
+func MyIp6() *net.IPAddr {
+	myIp6Once.Do(func() {
+		var err error
+		myIp6, err = net.ResolveIPAddr("ip6", MyHostname())
+		if err != nil {
+			log.Fatal(err)
+		}
+	})
+	return myIp6
+}
+
+// This returns the list of local ip addresses which other hosts can connect
+// to (NOTE: Loopback ip is ignored).
+// Also resolves Hostname to an address and adds it to the list too, so
+// IPs from /etc/hosts can work too.
+func GetLocalIPs() ([]*net.IP, error) {
+	hostname, err := os.Hostname()
+	if err != nil {
+		return nil, fmt.Errorf("Failed to lookup hostname: %v", err)
+	}
+	// Resolves IP Address from Hostname, this way overrides in /etc/hosts
+	// can work too for IP resolution.
+	ipInfo, err := net.ResolveIPAddr("ip4", hostname)
+	if err != nil {
+		return nil, fmt.Errorf("Failed to resolve ip: %v", err)
+	}
+	ips := []*net.IP{&ipInfo.IP}
+
+	// TODO(zviad): Is rest of the code really necessary?
+	addrs, err := net.InterfaceAddrs()
+	if err != nil {
+		return nil, fmt.Errorf( "Failed to get interface addresses: %v", err)
+	}
+	for _, addr := range addrs {
+		ipnet, ok := addr.(*net.IPNet)
+		if !ok {
+			continue
+		}
+
+		if ipnet.IP.IsLoopback() {
+			continue
+		}
+
+		ips = append(ips, &ipnet.IP)
+	}
+	return ips, nil
+}
+
+var localhostIPNets []*net.IPNet
+
+func init() {
+	for _, mask := range []string{"127.0.0.1/8", "::1/128"} {
+		_, ipnet, err := net.ParseCIDR(mask)
+		if err != nil {
+			panic(err)
+		}
+		localhostIPNets = append(localhostIPNets, ipnet)
+	}
+}
+
+func IsLocalhostIp(ipStr string) bool {
+	ip := net.ParseIP(ipStr)
+	if ip == nil {
+		return false
+	}
+	for _, ipnet := range localhostIPNets {
+		if ipnet.Contains(ip) {
+			return true
+		}
+	}
+	return false
+}
+
+// Given a host string, return true if the host is an ip (v4/v6) localhost.
+func IsLocalhost(host string) bool {
+	return IsLocalhostIp(host) ||
+		host == "localhost" ||
+		host == "ip6-localhost" ||
+		host == "ipv6-localhost"
+}
+
+// Resolves hostnames in addresses to actual IP4 addresses. Skips all invalid addresses
+// and all addresses that can't be resolved.
+// `addrs` are assumed to be of form: ["<hostname>:<port>", ...]
+// Returns an error in addition to resolved addresses if not all resolutions succeed.
+func ResolveIP4s(addrs []string) ([]string, error) {
+	resolvedAddrs := make([]string, 0, len(addrs))
+	var lastErr error
+
+	for _, server := range addrs {
+		hostPort := strings.Split(server, ":")
+		if len(hostPort) != 2 {
+			lastErr = fmt.Errorf("Skipping invalid address: %s", server)
+			continue
+		}
+
+		ip, err := net.ResolveIPAddr("ip4", hostPort[0])
+		if err != nil {
+			lastErr = err
+			continue
+		}
+		resolvedAddrs = append(resolvedAddrs, ip.IP.String()+":"+hostPort[1])
+	}
+	return resolvedAddrs, lastErr
+}
+
+func LookupValidAddrs() (map[string]bool, error) {
+	hostName, err := os.Hostname()
+	if err != nil {
+		return nil, err
+	}
+	addrs, err := net.LookupHost(hostName)
+	if err != nil {
+		return nil, err
+	}
+	validAddrs := make(map[string]bool)
+	validAddrs[hostName] = true
+	for _, addr := range addrs {
+		validAddrs[addr] = true
+	}
+	// Special case localhost/127.0.0.1 so that this works on devVMs. It should
+	// have no affect in production.
+	validAddrs["127.0.0.1"] = true
+	validAddrs["localhost"] = true
+	return validAddrs, nil
+}
diff --git a/weed/wdclient/net2/managed_connection.go b/weed/wdclient/net2/managed_connection.go
new file mode 100644
index 000000000..a886210d1
--- /dev/null
+++ b/weed/wdclient/net2/managed_connection.go
@@ -0,0 +1,185 @@
+package net2
+
+import (
+	"fmt"
+	"net"
+	"time"
+
+	"errors"
+	"github.com/chrislusf/seaweedfs/weed/wdclient/resource_pool"
+)
+
+// Dial's arguments.
+type NetworkAddress struct {
+	Network string
+	Address string
+}
+
+// A connection managed by a connection pool.  NOTE: SetDeadline,
+// SetReadDeadline and SetWriteDeadline are disabled for managed connections.
+// (The deadlines are set by the connection pool).
+type ManagedConn interface {
+	net.Conn
+
+	// This returns the original (network, address) entry used for creating
+	// the connection.
+	Key() NetworkAddress
+
+	// This returns the underlying net.Conn implementation.
+	RawConn() net.Conn
+
+	// This returns the connection pool which owns this connection.
+	Owner() ConnectionPool
+
+	// This indictes a user is done with the connection and releases the
+	// connection back to the connection pool.
+	ReleaseConnection() error
+
+	// This indicates the connection is an invalid state, and that the
+	// connection should be discarded from the connection pool.
+	DiscardConnection() error
+}
+
+// A physical implementation of ManagedConn
+type managedConnImpl struct {
+	addr    NetworkAddress
+	handle  resource_pool.ManagedHandle
+	pool    ConnectionPool
+	options ConnectionOptions
+}
+
+// This creates a managed connection wrapper.
+func NewManagedConn(
+	network string,
+	address string,
+	handle resource_pool.ManagedHandle,
+	pool ConnectionPool,
+	options ConnectionOptions) ManagedConn {
+
+	addr := NetworkAddress{
+		Network: network,
+		Address: address,
+	}
+
+	return &managedConnImpl{
+		addr:    addr,
+		handle:  handle,
+		pool:    pool,
+		options: options,
+	}
+}
+
+func (c *managedConnImpl) rawConn() (net.Conn, error) {
+	h, err := c.handle.Handle()
+	return h.(net.Conn), err
+}
+
+// See ManagedConn for documentation.
+func (c *managedConnImpl) RawConn() net.Conn {
+	h, _ := c.handle.Handle()
+	return h.(net.Conn)
+}
+
+// See ManagedConn for documentation.
+func (c *managedConnImpl) Key() NetworkAddress {
+	return c.addr
+}
+
+// See ManagedConn for documentation.
+func (c *managedConnImpl) Owner() ConnectionPool {
+	return c.pool
+}
+
+// See ManagedConn for documentation.
+func (c *managedConnImpl) ReleaseConnection() error {
+	return c.handle.Release()
+}
+
+// See ManagedConn for documentation.
+func (c *managedConnImpl) DiscardConnection() error {
+	return c.handle.Discard()
+}
+
+// See net.Conn for documentation
+func (c *managedConnImpl) Read(b []byte) (n int, err error) {
+	conn, err := c.rawConn()
+	if err != nil {
+		return 0, err
+	}
+
+	if c.options.ReadTimeout > 0 {
+		deadline := c.options.getCurrentTime().Add(c.options.ReadTimeout)
+		_ = conn.SetReadDeadline(deadline)
+	}
+	n, err = conn.Read(b)
+	if err != nil {
+		var localAddr string
+		if conn.LocalAddr() != nil {
+			localAddr = conn.LocalAddr().String()
+		} else {
+			localAddr = "(nil)"
+		}
+
+		var remoteAddr string
+		if conn.RemoteAddr() != nil {
+			remoteAddr = conn.RemoteAddr().String()
+		} else {
+			remoteAddr = "(nil)"
+		}
+		err = fmt.Errorf("Read error from host: %s <-> %s: %v", localAddr, remoteAddr, err)
+	}
+	return
+}
+
+// See net.Conn for documentation
+func (c *managedConnImpl) Write(b []byte) (n int, err error) {
+	conn, err := c.rawConn()
+	if err != nil {
+		return 0, err
+	}
+
+	if c.options.WriteTimeout > 0 {
+		deadline := c.options.getCurrentTime().Add(c.options.WriteTimeout)
+		_ = conn.SetWriteDeadline(deadline)
+	}
+	n, err = conn.Write(b)
+	if err != nil {
+		err = fmt.Errorf("Write error: %v", err)
+	}
+	return
+}
+
+// See net.Conn for documentation
+func (c *managedConnImpl) Close() error {
+	return c.handle.Discard()
+}
+
+// See net.Conn for documentation
+func (c *managedConnImpl) LocalAddr() net.Addr {
+	conn, _ := c.rawConn()
+	return conn.LocalAddr()
+}
+
+// See net.Conn for documentation
+func (c *managedConnImpl) RemoteAddr() net.Addr {
+	conn, _ := c.rawConn()
+	return conn.RemoteAddr()
+}
+
+// SetDeadline is disabled for managed connection (The deadline is set by
+// us, with respect to the read/write timeouts specified in ConnectionOptions).
+func (c *managedConnImpl) SetDeadline(t time.Time) error {
+	return errors.New("Cannot set deadline for managed connection")
+}
+
+// SetReadDeadline is disabled for managed connection (The deadline is set by
+// us with respect to the read timeout specified in ConnectionOptions).
+func (c *managedConnImpl) SetReadDeadline(t time.Time) error {
+	return errors.New("Cannot set read deadline for managed connection")
+}
+
+// SetWriteDeadline is disabled for managed connection (The deadline is set by
+// us with respect to the write timeout specified in ConnectionOptions).
+func (c *managedConnImpl) SetWriteDeadline(t time.Time) error {
+	return errors.New("Cannot set write deadline for managed connection")
+}
diff --git a/weed/wdclient/net2/port.go b/weed/wdclient/net2/port.go
new file mode 100644
index 000000000..f83adba28
--- /dev/null
+++ b/weed/wdclient/net2/port.go
@@ -0,0 +1,19 @@
+package net2
+
+import (
+	"net"
+	"strconv"
+)
+
+// Returns the port information.
+func GetPort(addr net.Addr) (int, error) {
+	_, lport, err := net.SplitHostPort(addr.String())
+	if err != nil {
+		return -1, err
+	}
+	lportInt, err := strconv.Atoi(lport)
+	if err != nil {
+		return -1, err
+	}
+	return lportInt, nil
+}
diff --git a/weed/wdclient/resource_pool/doc.go b/weed/wdclient/resource_pool/doc.go
new file mode 100644
index 000000000..c17b17c6c
--- /dev/null
+++ b/weed/wdclient/resource_pool/doc.go
@@ -0,0 +1,5 @@
+// A generic resource pool for managing resources such as network connections.
+package resource_pool
+
+// copied from https://github.com/dropbox/godropbox/tree/master/resource_pool
+// removed other dependencies
+\ No newline at end of file
diff --git a/weed/wdclient/resource_pool/managed_handle.go b/weed/wdclient/resource_pool/managed_handle.go
new file mode 100644
index 000000000..e1d82ca7b
--- /dev/null
+++ b/weed/wdclient/resource_pool/managed_handle.go
@@ -0,0 +1,97 @@
+package resource_pool
+
+import (
+	"sync/atomic"
+
+	"errors"
+)
+
+// A resource handle managed by a resource pool.
+type ManagedHandle interface {
+	// This returns the handle's resource location.
+	ResourceLocation() string
+
+	// This returns the underlying resource handle (or error if the handle
+	// is no longer active).
+	Handle() (interface{}, error)
+
+	// This returns the resource pool which owns this handle.
+	Owner() ResourcePool
+
+	// The releases the underlying resource handle to the caller and marks the
+	// managed handle as inactive.  The caller is responsible for cleaning up
+	// the released handle.  This returns nil if the managed handle no longer
+	// owns the resource.
+	ReleaseUnderlyingHandle() interface{}
+
+	// This indictes a user is done with the handle and releases the handle
+	// back to the resource pool.
+	Release() error
+
+	// This indicates the handle is an invalid state, and that the
+	// connection should be discarded from the connection pool.
+	Discard() error
+}
+
+// A physical implementation of ManagedHandle
+type managedHandleImpl struct {
+	location string
+	handle   interface{}
+	pool     ResourcePool
+	isActive int32 // atomic bool
+	options  Options
+}
+
+// This creates a managed handle wrapper.
+func NewManagedHandle(
+	resourceLocation string,
+	handle interface{},
+	pool ResourcePool,
+	options Options) ManagedHandle {
+
+	h := &managedHandleImpl{
+		location: resourceLocation,
+		handle:   handle,
+		pool:     pool,
+		options:  options,
+	}
+	atomic.StoreInt32(&h.isActive, 1)
+
+	return h
+}
+
+// See ManagedHandle for documentation.
+func (c *managedHandleImpl) ResourceLocation() string {
+	return c.location
+}
+
+// See ManagedHandle for documentation.
+func (c *managedHandleImpl) Handle() (interface{}, error) {
+	if atomic.LoadInt32(&c.isActive) == 0 {
+		return c.handle, errors.New("Resource handle is no longer valid")
+	}
+	return c.handle, nil
+}
+
+// See ManagedHandle for documentation.
+func (c *managedHandleImpl) Owner() ResourcePool {
+	return c.pool
+}
+
+// See ManagedHandle for documentation.
+func (c *managedHandleImpl) ReleaseUnderlyingHandle() interface{} {
+	if atomic.CompareAndSwapInt32(&c.isActive, 1, 0) {
+		return c.handle
+	}
+	return nil
+}
+
+// See ManagedHandle for documentation.
+func (c *managedHandleImpl) Release() error {
+	return c.pool.Release(c)
+}
+
+// See ManagedHandle for documentation.
+func (c *managedHandleImpl) Discard() error {
+	return c.pool.Discard(c)
+}
diff --git a/weed/wdclient/resource_pool/multi_resource_pool.go b/weed/wdclient/resource_pool/multi_resource_pool.go
new file mode 100644
index 000000000..9ac25526d
--- /dev/null
+++ b/weed/wdclient/resource_pool/multi_resource_pool.go
@@ -0,0 +1,200 @@
+package resource_pool
+
+import (
+	"fmt"
+	"sync"
+
+	"errors"
+)
+
+// A resource pool implementation that manages multiple resource location
+// entries.  The handles to each resource location entry acts independently.
+// For example "tcp localhost:11211" could act as memcache
+// shard 0 and "tcp localhost:11212" could act as memcache shard 1.
+type multiResourcePool struct {
+	options Options
+
+	createPool func(Options) ResourcePool
+
+	rwMutex    sync.RWMutex
+	isLameDuck bool // guarded by rwMutex
+	// NOTE: the locationPools is guarded by rwMutex, but the pool entries
+	// are not.
+	locationPools map[string]ResourcePool
+}
+
+// This returns a MultiResourcePool, which manages multiple
+// resource location entries.  The handles to each resource location
+// entry acts independently.
+//
+// When createPool is nil, NewSimpleResourcePool is used as default.
+func NewMultiResourcePool(
+	options Options,
+	createPool func(Options) ResourcePool) ResourcePool {
+
+	if createPool == nil {
+		createPool = NewSimpleResourcePool
+	}
+
+	return &multiResourcePool{
+		options:       options,
+		createPool:    createPool,
+		rwMutex:       sync.RWMutex{},
+		isLameDuck:    false,
+		locationPools: make(map[string]ResourcePool),
+	}
+}
+
+// See ResourcePool for documentation.
+func (p *multiResourcePool) NumActive() int32 {
+	total := int32(0)
+
+	p.rwMutex.RLock()
+	defer p.rwMutex.RUnlock()
+
+	for _, pool := range p.locationPools {
+		total += pool.NumActive()
+	}
+	return total
+}
+
+// See ResourcePool for documentation.
+func (p *multiResourcePool) ActiveHighWaterMark() int32 {
+	high := int32(0)
+
+	p.rwMutex.RLock()
+	defer p.rwMutex.RUnlock()
+
+	for _, pool := range p.locationPools {
+		val := pool.ActiveHighWaterMark()
+		if val > high {
+			high = val
+		}
+	}
+	return high
+}
+
+// See ResourcePool for documentation.
+func (p *multiResourcePool) NumIdle() int {
+	total := 0
+
+	p.rwMutex.RLock()
+	defer p.rwMutex.RUnlock()
+
+	for _, pool := range p.locationPools {
+		total += pool.NumIdle()
+	}
+	return total
+}
+
+// See ResourcePool for documentation.
+func (p *multiResourcePool) Register(resourceLocation string) error {
+	if resourceLocation == "" {
+		return errors.New("Registering invalid resource location")
+	}
+
+	p.rwMutex.Lock()
+	defer p.rwMutex.Unlock()
+
+	if p.isLameDuck {
+		return fmt.Errorf(
+			"Cannot register %s to lame duck resource pool",
+			resourceLocation)
+	}
+
+	if _, inMap := p.locationPools[resourceLocation]; inMap {
+		return nil
+	}
+
+	pool := p.createPool(p.options)
+	if err := pool.Register(resourceLocation); err != nil {
+		return err
+	}
+
+	p.locationPools[resourceLocation] = pool
+	return nil
+}
+
+// See ResourcePool for documentation.
+func (p *multiResourcePool) Unregister(resourceLocation string) error {
+	p.rwMutex.Lock()
+	defer p.rwMutex.Unlock()
+
+	if pool, inMap := p.locationPools[resourceLocation]; inMap {
+		_ = pool.Unregister("")
+		pool.EnterLameDuckMode()
+		delete(p.locationPools, resourceLocation)
+	}
+	return nil
+}
+
+func (p *multiResourcePool) ListRegistered() []string {
+	p.rwMutex.RLock()
+	defer p.rwMutex.RUnlock()
+
+	result := make([]string, 0, len(p.locationPools))
+	for key, _ := range p.locationPools {
+		result = append(result, key)
+	}
+
+	return result
+}
+
+// See ResourcePool for documentation.
+func (p *multiResourcePool) Get(
+	resourceLocation string) (ManagedHandle, error) {
+
+	pool := p.getPool(resourceLocation)
+	if pool == nil {
+		return nil, fmt.Errorf(
+			"%s is not registered in the resource pool",
+			resourceLocation)
+	}
+	return pool.Get(resourceLocation)
+}
+
+// See ResourcePool for documentation.
+func (p *multiResourcePool) Release(handle ManagedHandle) error {
+	pool := p.getPool(handle.ResourceLocation())
+	if pool == nil {
+		return errors.New(
+			"Resource pool cannot take control of a handle owned " +
+				"by another resource pool")
+	}
+
+	return pool.Release(handle)
+}
+
+// See ResourcePool for documentation.
+func (p *multiResourcePool) Discard(handle ManagedHandle) error {
+	pool := p.getPool(handle.ResourceLocation())
+	if pool == nil {
+		return errors.New(
+			"Resource pool cannot take control of a handle owned " +
+				"by another resource pool")
+	}
+
+	return pool.Discard(handle)
+}
+
+// See ResourcePool for documentation.
+func (p *multiResourcePool) EnterLameDuckMode() {
+	p.rwMutex.Lock()
+	defer p.rwMutex.Unlock()
+
+	p.isLameDuck = true
+
+	for _, pool := range p.locationPools {
+		pool.EnterLameDuckMode()
+	}
+}
+
+func (p *multiResourcePool) getPool(resourceLocation string) ResourcePool {
+	p.rwMutex.RLock()
+	defer p.rwMutex.RUnlock()
+
+	if pool, inMap := p.locationPools[resourceLocation]; inMap {
+		return pool
+	}
+	return nil
+}
diff --git a/weed/wdclient/resource_pool/resource_pool.go b/weed/wdclient/resource_pool/resource_pool.go
new file mode 100644
index 000000000..26c433f50
--- /dev/null
+++ b/weed/wdclient/resource_pool/resource_pool.go
@@ -0,0 +1,96 @@
+package resource_pool
+
+import (
+	"time"
+)
+
+type Options struct {
+	// The maximum number of active resource handles per resource location.  (A
+	// non-positive value indicates the number of active resource handles is
+	// unbounded).
+	MaxActiveHandles int32
+
+	// The maximum number of idle resource handles per resource location that
+	// are kept alive by the resource pool.
+	MaxIdleHandles uint32
+
+	// The maximum amount of time an idle resource handle can remain alive (if
+	// specified).
+	MaxIdleTime *time.Duration
+
+	// This limits the number of concurrent Open calls (there's no limit when
+	// OpenMaxConcurrency is non-positive).
+	OpenMaxConcurrency int
+
+	// This function creates a resource handle (e.g., a connection) for a
+	// resource location.  The function must be thread-safe.
+	Open func(resourceLocation string) (
+		handle interface{},
+		err error)
+
+	// This function destroys a resource handle and performs the necessary
+	// cleanup to free up resources.  The function must be thread-safe.
+	Close func(handle interface{}) error
+
+	// This specifies the now time function.  When the function is non-nil, the
+	// resource pool will use the specified function instead of time.Now to
+	// generate the current time.
+	NowFunc func() time.Time
+}
+
+func (o Options) getCurrentTime() time.Time {
+	if o.NowFunc == nil {
+		return time.Now()
+	} else {
+		return o.NowFunc()
+	}
+}
+
+// A generic interface for managed resource pool.  All resource pool
+// implementations must be threadsafe.
+type ResourcePool interface {
+	// This returns the number of active resource handles.
+	NumActive() int32
+
+	// This returns the highest number of actives handles for the entire
+	// lifetime of the pool.  If the pool contains multiple sub-pools, the
+	// high water mark is the max of the sub-pools' high water marks.
+	ActiveHighWaterMark() int32
+
+	// This returns the number of alive idle handles.  NOTE: This is only used
+	// for testing.
+	NumIdle() int
+
+	// This associates a resource location to the resource pool; afterwhich,
+	// the user can get resource handles for the resource location.
+	Register(resourceLocation string) error
+
+	// This dissociates a resource location from the resource pool; afterwhich,
+	// the user can no longer get resource handles for the resource location.
+	// If the given resource location corresponds to a sub-pool, the unregistered
+	// sub-pool will enter lame duck mode.
+	Unregister(resourceLocation string) error
+
+	// This returns the list of registered resource location entries.
+	ListRegistered() []string
+
+	// This gets an active resource handle from the resource pool.  The
+	// handle will remain active until one of the following is called:
+	//  1. handle.Release()
+	//  2. handle.Discard()
+	//  3. pool.Release(handle)
+	//  4. pool.Discard(handle)
+	Get(key string) (ManagedHandle, error)
+
+	// This releases an active resource handle back to the resource pool.
+	Release(handle ManagedHandle) error
+
+	// This discards an active resource from the resource pool.
+	Discard(handle ManagedHandle) error
+
+	// Enter the resource pool into lame duck mode.  The resource pool
+	// will no longer return resource handles, and all idle resource handles
+	// are closed immediately (including active resource handles that are
+	// released back to the pool afterward).
+	EnterLameDuckMode()
+}
diff --git a/weed/wdclient/resource_pool/semaphore.go b/weed/wdclient/resource_pool/semaphore.go
new file mode 100644
index 000000000..ff35d5bc5
--- /dev/null
+++ b/weed/wdclient/resource_pool/semaphore.go
@@ -0,0 +1,154 @@
+package resource_pool
+
+import (
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+type Semaphore interface {
+	// Increment the semaphore counter by one.
+	Release()
+
+	// Decrement the semaphore counter by one, and block if counter < 0
+	Acquire()
+
+	// Decrement the semaphore counter by one, and block if counter < 0
+	// Wait for up to the given duration.  Returns true if did not timeout
+	TryAcquire(timeout time.Duration) bool
+}
+
+// A simple counting Semaphore.
+type boundedSemaphore struct {
+	slots chan struct{}
+}
+
+// Create a bounded semaphore. The count parameter must be a positive number.
+// NOTE: The bounded semaphore will panic if the user tries to Release
+// beyond the specified count.
+func NewBoundedSemaphore(count uint) Semaphore {
+	sem := &boundedSemaphore{
+		slots: make(chan struct{}, int(count)),
+	}
+	for i := 0; i < cap(sem.slots); i++ {
+		sem.slots <- struct{}{}
+	}
+	return sem
+}
+
+// Acquire returns on successful acquisition.
+func (sem *boundedSemaphore) Acquire() {
+	<-sem.slots
+}
+
+// TryAcquire returns true if it acquires a resource slot within the
+// timeout, false otherwise.
+func (sem *boundedSemaphore) TryAcquire(timeout time.Duration) bool {
+	if timeout > 0 {
+		// Wait until we get a slot or timeout expires.
+		tm := time.NewTimer(timeout)
+		defer tm.Stop()
+		select {
+		case <-sem.slots:
+			return true
+		case <-tm.C:
+			// Timeout expired. In very rare cases this might happen even if
+			// there is a slot available, e.g. GC pause after we create the timer
+			// and select randomly picked this one out of the two available channels.
+			// We should do one final immediate check below.
+		}
+	}
+
+	// Return true if we have a slot available immediately and false otherwise.
+	select {
+	case <-sem.slots:
+		return true
+	default:
+		return false
+	}
+}
+
+// Release the acquired semaphore. You must not release more than you
+// have acquired.
+func (sem *boundedSemaphore) Release() {
+	select {
+	case sem.slots <- struct{}{}:
+	default:
+		// slots is buffered. If a send blocks, it indicates a programming
+		// error.
+		panic(fmt.Errorf("too many releases for boundedSemaphore"))
+	}
+}
+
+// This returns an unbound counting semaphore with the specified initial count.
+// The semaphore counter can be arbitrary large (i.e., Release can be called
+// unlimited amount of times).
+//
+// NOTE: In general, users should use bounded semaphore since it is more
+// efficient than unbounded semaphore.
+func NewUnboundedSemaphore(initialCount int) Semaphore {
+	res := &unboundedSemaphore{
+		counter: int64(initialCount),
+	}
+	res.cond.L = &res.lock
+	return res
+}
+
+type unboundedSemaphore struct {
+	lock    sync.Mutex
+	cond    sync.Cond
+	counter int64
+}
+
+func (s *unboundedSemaphore) Release() {
+	s.lock.Lock()
+	s.counter += 1
+	if s.counter > 0 {
+		// Not broadcasting here since it's unlike we can satify all waiting
+		// goroutines.  Instead, we will Signal again if there are left over
+		// quota after Acquire, in case of lost wakeups.
+		s.cond.Signal()
+	}
+	s.lock.Unlock()
+}
+
+func (s *unboundedSemaphore) Acquire() {
+	s.lock.Lock()
+	for s.counter < 1 {
+		s.cond.Wait()
+	}
+	s.counter -= 1
+	if s.counter > 0 {
+		s.cond.Signal()
+	}
+	s.lock.Unlock()
+}
+
+func (s *unboundedSemaphore) TryAcquire(timeout time.Duration) bool {
+	done := make(chan bool, 1)
+	// Gate used to communicate between the threads and decide what the result
+	// is.  If the main thread decides, we have timed out, otherwise we succeed.
+	decided := new(int32)
+	atomic.StoreInt32(decided, 0)
+	go func() {
+		s.Acquire()
+		if atomic.SwapInt32(decided, 1) == 0 {
+			// Acquire won the race
+			done <- true
+		} else {
+			// If we already decided the result, and this thread did not win
+			s.Release()
+		}
+	}()
+	select {
+	case <-done:
+		return true
+	case <-time.After(timeout):
+		if atomic.SwapInt32(decided, 1) == 1 {
+			// The other thread already decided the result
+			return true
+		}
+		return false
+	}
+}
diff --git a/weed/wdclient/resource_pool/simple_resource_pool.go b/weed/wdclient/resource_pool/simple_resource_pool.go
new file mode 100644
index 000000000..b0c539100
--- /dev/null
+++ b/weed/wdclient/resource_pool/simple_resource_pool.go
@@ -0,0 +1,343 @@
+package resource_pool
+
+import (
+	"errors"
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+type idleHandle struct {
+	handle    interface{}
+	keepUntil *time.Time
+}
+
+type TooManyHandles struct {
+	location string
+}
+
+func (t TooManyHandles) Error() string {
+	return fmt.Sprintf("Too many handles to %s", t.location)
+}
+
+type OpenHandleError struct {
+	location string
+	err      error
+}
+
+func (o OpenHandleError) Error() string {
+	return fmt.Sprintf("Failed to open resource handle: %s (%v)", o.location, o.err)
+}
+
+// A resource pool implementation where all handles are associated to the
+// same resource location.
+type simpleResourcePool struct {
+	options Options
+
+	numActive *int32 // atomic counter
+
+	activeHighWaterMark *int32 // atomic / monotonically increasing value
+
+	openTokens Semaphore
+
+	mutex       sync.Mutex
+	location    string        // guard by mutex
+	idleHandles []*idleHandle // guarded by mutex
+	isLameDuck  bool          // guarded by mutex
+}
+
+// This returns a SimpleResourcePool, where all handles are associated to a
+// single resource location.
+func NewSimpleResourcePool(options Options) ResourcePool {
+	numActive := new(int32)
+	atomic.StoreInt32(numActive, 0)
+
+	activeHighWaterMark := new(int32)
+	atomic.StoreInt32(activeHighWaterMark, 0)
+
+	var tokens Semaphore
+	if options.OpenMaxConcurrency > 0 {
+		tokens = NewBoundedSemaphore(uint(options.OpenMaxConcurrency))
+	}
+
+	return &simpleResourcePool{
+		location:            "",
+		options:             options,
+		numActive:           numActive,
+		activeHighWaterMark: activeHighWaterMark,
+		openTokens:          tokens,
+		mutex:               sync.Mutex{},
+		idleHandles:         make([]*idleHandle, 0, 0),
+		isLameDuck:          false,
+	}
+}
+
+// See ResourcePool for documentation.
+func (p *simpleResourcePool) NumActive() int32 {
+	return atomic.LoadInt32(p.numActive)
+}
+
+// See ResourcePool for documentation.
+func (p *simpleResourcePool) ActiveHighWaterMark() int32 {
+	return atomic.LoadInt32(p.activeHighWaterMark)
+}
+
+// See ResourcePool for documentation.
+func (p *simpleResourcePool) NumIdle() int {
+	p.mutex.Lock()
+	defer p.mutex.Unlock()
+	return len(p.idleHandles)
+}
+
+// SimpleResourcePool can only register a single (network, address) entry.
+// Register should be call before any Get calls.
+func (p *simpleResourcePool) Register(resourceLocation string) error {
+	if resourceLocation == "" {
+		return errors.New("Invalid resource location")
+	}
+
+	p.mutex.Lock()
+	defer p.mutex.Unlock()
+
+	if p.isLameDuck {
+		return fmt.Errorf(
+			"cannot register %s to lame duck resource pool",
+			resourceLocation)
+	}
+
+	if p.location == "" {
+		p.location = resourceLocation
+		return nil
+	}
+	return errors.New("SimpleResourcePool can only register one location")
+}
+
+// SimpleResourcePool will enter lame duck mode upon calling Unregister.
+func (p *simpleResourcePool) Unregister(resourceLocation string) error {
+	p.EnterLameDuckMode()
+	return nil
+}
+
+func (p *simpleResourcePool) ListRegistered() []string {
+	p.mutex.Lock()
+	defer p.mutex.Unlock()
+
+	if p.location != "" {
+		return []string{p.location}
+	}
+	return []string{}
+}
+
+func (p *simpleResourcePool) getLocation() (string, error) {
+	p.mutex.Lock()
+	defer p.mutex.Unlock()
+
+	if p.location == "" {
+		return "", fmt.Errorf(
+			"resource location is not set for SimpleResourcePool")
+	}
+
+	if p.isLameDuck {
+		return "", fmt.Errorf(
+			"lame duck resource pool cannot return handles to %s",
+			p.location)
+	}
+
+	return p.location, nil
+}
+
+// This gets an active resource from the resource pool.  Note that the
+// resourceLocation argument is ignored (The handles are associated to the
+// resource location provided by the first Register call).
+func (p *simpleResourcePool) Get(unused string) (ManagedHandle, error) {
+	activeCount := atomic.AddInt32(p.numActive, 1)
+	if p.options.MaxActiveHandles > 0 &&
+		activeCount > p.options.MaxActiveHandles {
+
+		atomic.AddInt32(p.numActive, -1)
+		return nil, TooManyHandles{p.location}
+	}
+
+	highest := atomic.LoadInt32(p.activeHighWaterMark)
+	for activeCount > highest &&
+		!atomic.CompareAndSwapInt32(
+			p.activeHighWaterMark,
+			highest,
+			activeCount) {
+
+		highest = atomic.LoadInt32(p.activeHighWaterMark)
+	}
+
+	if h := p.getIdleHandle(); h != nil {
+		return h, nil
+	}
+
+	location, err := p.getLocation()
+	if err != nil {
+		atomic.AddInt32(p.numActive, -1)
+		return nil, err
+	}
+
+	if p.openTokens != nil {
+		// Current implementation does not wait for tokens to become available.
+		// If that causes availability hits, we could increase the wait,
+		// similar to simple_pool.go.
+		if p.openTokens.TryAcquire(0) {
+			defer p.openTokens.Release()
+		} else {
+			// We could not immediately acquire a token.
+			// Instead of waiting
+			atomic.AddInt32(p.numActive, -1)
+			return nil, OpenHandleError{
+				p.location, errors.New("Open Error: reached OpenMaxConcurrency")}
+		}
+	}
+
+	handle, err := p.options.Open(location)
+	if err != nil {
+		atomic.AddInt32(p.numActive, -1)
+		return nil, OpenHandleError{p.location, err}
+	}
+
+	return NewManagedHandle(p.location, handle, p, p.options), nil
+}
+
+// See ResourcePool for documentation.
+func (p *simpleResourcePool) Release(handle ManagedHandle) error {
+	if pool, ok := handle.Owner().(*simpleResourcePool); !ok || pool != p {
+		return errors.New(
+			"Resource pool cannot take control of a handle owned " +
+				"by another resource pool")
+	}
+
+	h := handle.ReleaseUnderlyingHandle()
+	if h != nil {
+		// We can unref either before or after queuing the idle handle.
+		// The advantage of unref-ing before queuing is that there is
+		// a higher chance of successful Get when number of active handles
+		// is close to the limit (but potentially more handle creation).
+		// The advantage of queuing before unref-ing is that there's a
+		// higher chance of reusing handle (but potentially more Get failures).
+		atomic.AddInt32(p.numActive, -1)
+		p.queueIdleHandles(h)
+	}
+
+	return nil
+}
+
+// See ResourcePool for documentation.
+func (p *simpleResourcePool) Discard(handle ManagedHandle) error {
+	if pool, ok := handle.Owner().(*simpleResourcePool); !ok || pool != p {
+		return errors.New(
+			"Resource pool cannot take control of a handle owned " +
+				"by another resource pool")
+	}
+
+	h := handle.ReleaseUnderlyingHandle()
+	if h != nil {
+		atomic.AddInt32(p.numActive, -1)
+		if err := p.options.Close(h); err != nil {
+			return fmt.Errorf("failed to close resource handle: %v", err)
+		}
+	}
+	return nil
+}
+
+// See ResourcePool for documentation.
+func (p *simpleResourcePool) EnterLameDuckMode() {
+	p.mutex.Lock()
+
+	toClose := p.idleHandles
+	p.isLameDuck = true
+	p.idleHandles = []*idleHandle{}
+
+	p.mutex.Unlock()
+
+	p.closeHandles(toClose)
+}
+
+// This returns an idle resource, if there is one.
+func (p *simpleResourcePool) getIdleHandle() ManagedHandle {
+	var toClose []*idleHandle
+	defer func() {
+		// NOTE: Must keep the closure around to late bind the toClose slice.
+		p.closeHandles(toClose)
+	}()
+
+	now := p.options.getCurrentTime()
+
+	p.mutex.Lock()
+	defer p.mutex.Unlock()
+
+	var i int
+	for i = 0; i < len(p.idleHandles); i++ {
+		idle := p.idleHandles[i]
+		if idle.keepUntil == nil || now.Before(*idle.keepUntil) {
+			break
+		}
+	}
+	if i > 0 {
+		toClose = p.idleHandles[0:i]
+	}
+
+	if i < len(p.idleHandles) {
+		idle := p.idleHandles[i]
+		p.idleHandles = p.idleHandles[i+1:]
+		return NewManagedHandle(p.location, idle.handle, p, p.options)
+	}
+
+	if len(p.idleHandles) > 0 {
+		p.idleHandles = []*idleHandle{}
+	}
+	return nil
+}
+
+// This adds an idle resource to the pool.
+func (p *simpleResourcePool) queueIdleHandles(handle interface{}) {
+	var toClose []*idleHandle
+	defer func() {
+		// NOTE: Must keep the closure around to late bind the toClose slice.
+		p.closeHandles(toClose)
+	}()
+
+	now := p.options.getCurrentTime()
+	var keepUntil *time.Time
+	if p.options.MaxIdleTime != nil {
+		// NOTE: Assign to temp variable first to work around compiler bug
+		x := now.Add(*p.options.MaxIdleTime)
+		keepUntil = &x
+	}
+
+	p.mutex.Lock()
+	defer p.mutex.Unlock()
+
+	if p.isLameDuck {
+		toClose = []*idleHandle{
+			{handle: handle},
+		}
+		return
+	}
+
+	p.idleHandles = append(
+		p.idleHandles,
+		&idleHandle{
+			handle:    handle,
+			keepUntil: keepUntil,
+		})
+
+	nIdleHandles := uint32(len(p.idleHandles))
+	if nIdleHandles > p.options.MaxIdleHandles {
+		handlesToClose := nIdleHandles - p.options.MaxIdleHandles
+		toClose = p.idleHandles[0:handlesToClose]
+		p.idleHandles = p.idleHandles[handlesToClose:nIdleHandles]
+	}
+}
+
+// Closes resources, at this point it is assumed that this resources
+// are no longer referenced from the main idleHandles slice.
+func (p *simpleResourcePool) closeHandles(handles []*idleHandle) {
+	for _, handle := range handles {
+		_ = p.options.Close(handle.handle)
+	}
+}