distribution/notifications/sinks.go
bin liu e0521d2d01 fix some typos in source comments
Signed-off-by: bin liu <liubin0329@gmail.com>
2015-04-17 12:39:52 +00:00

337 lines
8.4 KiB
Go

package notifications
import (
"container/list"
"fmt"
"sync"
"time"
"github.com/Sirupsen/logrus"
)
// NOTE(stevvooe): This file contains definitions for several utility sinks.
// Typically, the broadcaster is the only sink that should be required
// externally, but others are suitable for export if the need arises. Albeit,
// the tight integration with endpoint metrics should be removed.
// Broadcaster sends events to multiple, reliable Sinks. The goal of this
// component is to dispatch events to configured endpoints. Reliability can be
// provided by wrapping incoming sinks.
type Broadcaster struct {
sinks []Sink
events chan []Event
closed chan chan struct{}
}
// NewBroadcaster ...
// Add appends one or more sinks to the list of sinks. The broadcaster
// behavior will be affected by the properties of the sink. Generally, the
// sink should accept all messages and deal with reliability on its own. Use
// of EventQueue and RetryingSink should be used here.
func NewBroadcaster(sinks ...Sink) *Broadcaster {
b := Broadcaster{
sinks: sinks,
events: make(chan []Event),
closed: make(chan chan struct{}),
}
// Start the broadcaster
go b.run()
return &b
}
// Write accepts a block of events to be dispatched to all sinks. This method
// will never fail and should never block (hopefully!). The caller cedes the
// slice memory to the broadcaster and should not modify it after calling
// write.
func (b *Broadcaster) Write(events ...Event) error {
select {
case b.events <- events:
case <-b.closed:
return ErrSinkClosed
}
return nil
}
// Close the broadcaster, ensuring that all messages are flushed to the
// underlying sink before returning.
func (b *Broadcaster) Close() error {
logrus.Infof("broadcaster: closing")
select {
case <-b.closed:
// already closed
return fmt.Errorf("broadcaster: already closed")
default:
// do a little chan handoff dance to synchronize closing
closed := make(chan struct{})
b.closed <- closed
close(b.closed)
<-closed
return nil
}
}
// run is the main broadcast loop, started when the broadcaster is created.
// Under normal conditions, it waits for events on the event channel. After
// Close is called, this goroutine will exit.
func (b *Broadcaster) run() {
for {
select {
case block := <-b.events:
for _, sink := range b.sinks {
if err := sink.Write(block...); err != nil {
logrus.Errorf("broadcaster: error writing events to %v, these events will be lost: %v", sink, err)
}
}
case closing := <-b.closed:
// close all the underlying sinks
for _, sink := range b.sinks {
if err := sink.Close(); err != nil {
logrus.Errorf("broadcaster: error closing sink %v: %v", sink, err)
}
}
closing <- struct{}{}
logrus.Debugf("broadcaster: closed")
return
}
}
}
// eventQueue accepts all messages into a queue for asynchronous consumption
// by a sink. It is unbounded and thread safe but the sink must be reliable or
// events will be dropped.
type eventQueue struct {
sink Sink
events *list.List
listeners []eventQueueListener
cond *sync.Cond
mu sync.Mutex
closed bool
}
// eventQueueListener is called when various events happen on the queue.
type eventQueueListener interface {
ingress(events ...Event)
egress(events ...Event)
}
// newEventQueue returns a queue to the provided sink. If the updater is non-
// nil, it will be called to update pending metrics on ingress and egress.
func newEventQueue(sink Sink, listeners ...eventQueueListener) *eventQueue {
eq := eventQueue{
sink: sink,
events: list.New(),
listeners: listeners,
}
eq.cond = sync.NewCond(&eq.mu)
go eq.run()
return &eq
}
// Write accepts the events into the queue, only failing if the queue has
// beend closed.
func (eq *eventQueue) Write(events ...Event) error {
eq.mu.Lock()
defer eq.mu.Unlock()
if eq.closed {
return ErrSinkClosed
}
for _, listener := range eq.listeners {
listener.ingress(events...)
}
eq.events.PushBack(events)
eq.cond.Signal() // signal waiters
return nil
}
// Close shutsdown the event queue, flushing
func (eq *eventQueue) Close() error {
eq.mu.Lock()
defer eq.mu.Unlock()
if eq.closed {
return fmt.Errorf("eventqueue: already closed")
}
// set closed flag
eq.closed = true
eq.cond.Signal() // signal flushes queue
eq.cond.Wait() // wait for signal from last flush
return eq.sink.Close()
}
// run is the main goroutine to flush events to the target sink.
func (eq *eventQueue) run() {
for {
block := eq.next()
if block == nil {
return // nil block means event queue is closed.
}
if err := eq.sink.Write(block...); err != nil {
logrus.Warnf("eventqueue: error writing events to %v, these events will be lost: %v", eq.sink, err)
}
for _, listener := range eq.listeners {
listener.egress(block...)
}
}
}
// next encompasses the critical section of the run loop. When the queue is
// empty, it will block on the condition. If new data arrives, it will wake
// and return a block. When closed, a nil slice will be returned.
func (eq *eventQueue) next() []Event {
eq.mu.Lock()
defer eq.mu.Unlock()
for eq.events.Len() < 1 {
if eq.closed {
eq.cond.Broadcast()
return nil
}
eq.cond.Wait()
}
front := eq.events.Front()
block := front.Value.([]Event)
eq.events.Remove(front)
return block
}
// retryingSink retries the write until success or an ErrSinkClosed is
// returned. Underlying sink must have p > 0 of succeeding or the sink will
// block. Internally, it is a circuit breaker retries to manage reset.
// Concurrent calls to a retrying sink are serialized through the sink,
// meaning that if one is in-flight, another will not proceed.
type retryingSink struct {
mu sync.Mutex
sink Sink
closed bool
// circuit breaker heuristics
failures struct {
threshold int
recent int
last time.Time
backoff time.Duration // time after which we retry after failure.
}
}
type retryingSinkListener interface {
active(events ...Event)
retry(events ...Event)
}
// TODO(stevvooe): We are using circuit break here, which actually doesn't
// make a whole lot of sense for this use case, since we always retry. Move
// this to use bounded exponential backoff.
// newRetryingSink returns a sink that will retry writes to a sink, backing
// off on failure. Parameters threshold and backoff adjust the behavior of the
// circuit breaker.
func newRetryingSink(sink Sink, threshold int, backoff time.Duration) *retryingSink {
rs := &retryingSink{
sink: sink,
}
rs.failures.threshold = threshold
rs.failures.backoff = backoff
return rs
}
// Write attempts to flush the events to the downstream sink until it succeeds
// or the sink is closed.
func (rs *retryingSink) Write(events ...Event) error {
rs.mu.Lock()
defer rs.mu.Unlock()
retry:
if rs.closed {
return ErrSinkClosed
}
if !rs.proceed() {
logrus.Warnf("%v encountered too many errors, backing off", rs.sink)
rs.wait(rs.failures.backoff)
goto retry
}
if err := rs.write(events...); err != nil {
if err == ErrSinkClosed {
// terminal!
return err
}
logrus.Errorf("retryingsink: error writing events: %v, retrying", err)
goto retry
}
return nil
}
// Close closes the sink and the underlying sink.
func (rs *retryingSink) Close() error {
rs.mu.Lock()
defer rs.mu.Unlock()
if rs.closed {
return fmt.Errorf("retryingsink: already closed")
}
rs.closed = true
return rs.sink.Close()
}
// write provides a helper that dispatches failure and success properly. Used
// by write as the single-flight write call.
func (rs *retryingSink) write(events ...Event) error {
if err := rs.sink.Write(events...); err != nil {
rs.failure()
return err
}
rs.reset()
return nil
}
// wait backoff time against the sink, unlocking so others can proceed. Should
// only be called by methods that currently have the mutex.
func (rs *retryingSink) wait(backoff time.Duration) {
rs.mu.Unlock()
defer rs.mu.Lock()
// backoff here
time.Sleep(backoff)
}
// reset marks a successful call.
func (rs *retryingSink) reset() {
rs.failures.recent = 0
rs.failures.last = time.Time{}
}
// failure records a failure.
func (rs *retryingSink) failure() {
rs.failures.recent++
rs.failures.last = time.Now().UTC()
}
// proceed returns true if the call should proceed based on circuit breaker
// heuristics.
func (rs *retryingSink) proceed() bool {
return rs.failures.recent < rs.failures.threshold ||
time.Now().UTC().After(rs.failures.last.Add(rs.failures.backoff))
}