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package pubsub
import (
var (
// ErrUnsubscribed is returned by Err when a client unsubscribes.
ErrUnsubscribed = errors.New("client unsubscribed")
// ErrOutOfCapacity is returned by Err when a client is not pulling messages
// fast enough. Note the client's subscription will be terminated.
ErrOutOfCapacity = errors.New("client is not pulling messages fast enough")
// A Subscription represents a client subscription for a particular query and
// consists of three things:
// 1) channel onto which messages and tags are published
// 2) channel which is closed if a client is too slow or choose to unsubscribe
// 3) err indicating the reason for (2)
type Subscription struct {
out chan Message
cancelled chan struct{}
mtx sync.RWMutex
err error
// NewSubscription returns a new subscription with the given outCapacity.
func NewSubscription(outCapacity int) *Subscription {
return &Subscription{
out: make(chan Message, outCapacity),
cancelled: make(chan struct{}),
// Out returns a channel onto which messages and tags are published.
// Unsubscribe/UnsubscribeAll does not close the channel to avoid clients from
// receiving a nil message.
func (s *Subscription) Out() <-chan Message {
return s.out
// Cancelled returns a channel that's closed when the subscription is
// terminated and supposed to be used in a select statement.
func (s *Subscription) Cancelled() <-chan struct{} {
return s.cancelled
// Err returns nil if the channel returned by Cancelled is not yet closed.
// If the channel is closed, Err returns a non-nil error explaining why:
// - ErrUnsubscribed if the subscriber choose to unsubscribe,
// - ErrOutOfCapacity if the subscriber is not pulling messages fast enough
// and the channel returned by Out became full,
// After Err returns a non-nil error, successive calls to Err return the same
// error.
func (s *Subscription) Err() error {
defer s.mtx.RUnlock()
return s.err
func (s *Subscription) cancel(err error) {
s.err = err
// Message glues data and tags together.
type Message struct {
data interface{}
events map[string][]string
func NewMessage(data interface{}, events map[string][]string) Message {
return Message{data, events}
// Data returns an original data published.
func (msg Message) Data() interface{} {
// Events returns events, which matched the client's query.
func (msg Message) Events() map[string][]string {
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