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@james-lawrence @abligh @slackpad @sean- @schmichael @rogpeppe
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package raft
import (
// Observation is sent along the given channel to observers when an event occurs.
type Observation struct {
// Raft holds the Raft instance generating the observation.
Raft *Raft
// Data holds observation-specific data. Possible types are
// *RequestVoteRequest
// RaftState
// PeerObservation
// LeaderObservation
Data interface{}
// LeaderObservation is used for the data when leadership changes.
type LeaderObservation struct {
leader ServerAddress
// PeerObservation is sent to observers when peers change.
type PeerObservation struct {
Removed bool
Peer Server
// nextObserverId is used to provide a unique ID for each observer to aid in
// deregistration.
var nextObserverID uint64
// FilterFn is a function that can be registered in order to filter observations.
// The function reports whether the observation should be included - if
// it returns false, the observation will be filtered out.
type FilterFn func(o *Observation) bool
// Observer describes what to do with a given observation.
type Observer struct {
// numObserved and numDropped are performance counters for this observer.
// 64 bit types must be 64 bit aligned to use with atomic operations on
// 32 bit platforms, so keep them at the top of the struct.
numObserved uint64
numDropped uint64
// channel receives observations.
channel chan Observation
// blocking, if true, will cause Raft to block when sending an observation
// to this observer. This should generally be set to false.
blocking bool
// filter will be called to determine if an observation should be sent to
// the channel.
filter FilterFn
// id is the ID of this observer in the Raft map.
id uint64
// NewObserver creates a new observer that can be registered
// to make observations on a Raft instance. Observations
// will be sent on the given channel if they satisfy the
// given filter.
// If blocking is true, the observer will block when it can't
// send on the channel, otherwise it may discard events.
func NewObserver(channel chan Observation, blocking bool, filter FilterFn) *Observer {
return &Observer{
channel: channel,
blocking: blocking,
filter: filter,
id: atomic.AddUint64(&nextObserverID, 1),
// GetNumObserved returns the number of observations.
func (or *Observer) GetNumObserved() uint64 {
return atomic.LoadUint64(&or.numObserved)
// GetNumDropped returns the number of dropped observations due to blocking.
func (or *Observer) GetNumDropped() uint64 {
return atomic.LoadUint64(&or.numDropped)
// RegisterObserver registers a new observer.
func (r *Raft) RegisterObserver(or *Observer) {
defer r.observersLock.Unlock()
r.observers[] = or
// DeregisterObserver deregisters an observer.
func (r *Raft) DeregisterObserver(or *Observer) {
defer r.observersLock.Unlock()
// observe sends an observation to every observer.
func (r *Raft) observe(o interface{}) {
// In general observers should not block. But in any case this isn't
// disastrous as we only hold a read lock, which merely prevents
// registration / deregistration of observers.
defer r.observersLock.RUnlock()
for _, or := range r.observers {
// It's wasteful to do this in the loop, but for the common case
// where there are no observers we won't create any objects.
ob := Observation{Raft: r, Data: o}
if or.filter != nil && !or.filter(&ob) {
if == nil {
if or.blocking { <- ob
atomic.AddUint64(&or.numObserved, 1)
} else {
select {
case <- ob:
atomic.AddUint64(&or.numObserved, 1)
atomic.AddUint64(&or.numDropped, 1)
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