DataDog and StatsD Metrics Support

* Added support for DataDog and StatsD monitoring
* Added documentation

vendor/github.com/go-kit/kit/util/conn/manager.go

@@ -0,0 +1,145 @@
+package conn
+
+import (
+	"errors"
+	"net"
+	"time"
+
+	"github.com/go-kit/kit/log"
+)
+
+// Dialer imitates net.Dial. Dialer is assumed to yield connections that are
+// safe for use by multiple concurrent goroutines.
+type Dialer func(network, address string) (net.Conn, error)
+
+// AfterFunc imitates time.After.
+type AfterFunc func(time.Duration) <-chan time.Time
+
+// Manager manages a net.Conn.
+//
+// Clients provide a way to create the connection with a Dialer, network, and
+// address. Clients should Take the connection when they want to use it, and Put
+// back whatever error they receive from its use. When a non-nil error is Put,
+// the connection is invalidated, and a new connection is established.
+// Connection failures are retried after an exponential backoff.
+type Manager struct {
+	dialer  Dialer
+	network string
+	address string
+	after   AfterFunc
+	logger  log.Logger
+
+	takec chan net.Conn
+	putc  chan error
+}
+
+// NewManager returns a connection manager using the passed Dialer, network, and
+// address. The AfterFunc is used to control exponential backoff and retries.
+// The logger is used to log errors; pass a log.NopLogger if you don't care to
+// receive them. For normal use, prefer NewDefaultManager.
+func NewManager(d Dialer, network, address string, after AfterFunc, logger log.Logger) *Manager {
+	m := &Manager{
+		dialer:  d,
+		network: network,
+		address: address,
+		after:   after,
+		logger:  logger,
+
+		takec: make(chan net.Conn),
+		putc:  make(chan error),
+	}
+	go m.loop()
+	return m
+}
+
+// NewDefaultManager is a helper constructor, suitable for most normal use in
+// real (non-test) code. It uses the real net.Dial and time.After functions.
+func NewDefaultManager(network, address string, logger log.Logger) *Manager {
+	return NewManager(net.Dial, network, address, time.After, logger)
+}
+
+// Take yields the current connection. It may be nil.
+func (m *Manager) Take() net.Conn {
+	return <-m.takec
+}
+
+// Put accepts an error that came from a previously yielded connection. If the
+// error is non-nil, the manager will invalidate the current connection and try
+// to reconnect, with exponential backoff. Putting a nil error is a no-op.
+func (m *Manager) Put(err error) {
+	m.putc <- err
+}
+
+// Write writes the passed data to the connection in a single Take/Put cycle.
+func (m *Manager) Write(b []byte) (int, error) {
+	conn := m.Take()
+	if conn == nil {
+		return 0, ErrConnectionUnavailable
+	}
+	n, err := conn.Write(b)
+	defer m.Put(err)
+	return n, err
+}
+
+func (m *Manager) loop() {
+	var (
+		conn       = dial(m.dialer, m.network, m.address, m.logger) // may block slightly
+		connc      = make(chan net.Conn, 1)
+		reconnectc <-chan time.Time // initially nil
+		backoff    = time.Second
+	)
+
+	// If the initial dial fails, we need to trigger a reconnect via the loop
+	// body, below. If we did this in a goroutine, we would race on the conn
+	// variable. So we use a buffered chan instead.
+	connc <- conn
+
+	for {
+		select {
+		case <-reconnectc:
+			reconnectc = nil // one-shot
+			go func() { connc <- dial(m.dialer, m.network, m.address, m.logger) }()
+
+		case conn = <-connc:
+			if conn == nil {
+				// didn't work
+				backoff = exponential(backoff) // wait longer
+				reconnectc = m.after(backoff)  // try again
+			} else {
+				// worked!
+				backoff = time.Second // reset wait time
+				reconnectc = nil      // no retry necessary
+			}
+
+		case m.takec <- conn:
+
+		case err := <-m.putc:
+			if err != nil && conn != nil {
+				m.logger.Log("err", err)
+				conn = nil                            // connection is bad
+				reconnectc = m.after(time.Nanosecond) // trigger immediately
+			}
+		}
+	}
+}
+
+func dial(d Dialer, network, address string, logger log.Logger) net.Conn {
+	conn, err := d(network, address)
+	if err != nil {
+		logger.Log("err", err)
+		conn = nil // just to be sure
+	}
+	return conn
+}
+
+func exponential(d time.Duration) time.Duration {
+	d *= 2
+	if d > time.Minute {
+		d = time.Minute
+	}
+	return d
+}
+
+// ErrConnectionUnavailable is returned by the Manager's Write method when the
+// manager cannot yield a good connection.
+var ErrConnectionUnavailable = errors.New("connection unavailable")