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404 lines
11 KiB
404 lines
11 KiB
// Package pubsub implements a pub-sub model with a single publisher (Server)
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// and multiple subscribers (clients).
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//
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// Though you can have multiple publishers by sharing a pointer to a server or
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// by giving the same channel to each publisher and publishing messages from
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// that channel (fan-in).
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//
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// Clients subscribe for messages, which could be of any type, using a query.
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// When some message is published, we match it with all queries. If there is a
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// match, this message will be pushed to all clients, subscribed to that query.
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// See query subpackage for our implementation.
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//
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// Example:
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//
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// q, err := query.New("account.name='John'")
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// if err != nil {
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// return err
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// }
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// ctx, cancel := context.WithTimeout(context.Background(), 1 * time.Second)
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// defer cancel()
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// subscription, err := pubsub.Subscribe(ctx, "johns-transactions", q)
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// if err != nil {
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// return err
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// }
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//
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// for {
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// select {
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// case msg <- subscription.Out():
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// // handle msg.Data() and msg.Tags()
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// case <-subscription.Cancelled():
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// return subscription.Err()
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// }
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// }
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//
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package pubsub
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import (
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"context"
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"errors"
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"sync"
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cmn "github.com/tendermint/tendermint/libs/common"
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)
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type operation int
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const (
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sub operation = iota
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pub
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unsub
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shutdown
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)
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var (
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// ErrSubscriptionNotFound is returned when a client tries to unsubscribe
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// from not existing subscription.
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ErrSubscriptionNotFound = errors.New("subscription not found")
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// ErrAlreadySubscribed is returned when a client tries to subscribe twice or
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// more using the same query.
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ErrAlreadySubscribed = errors.New("already subscribed")
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)
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// Query defines an interface for a query to be used for subscribing.
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type Query interface {
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Matches(tags map[string]string) bool
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String() string
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}
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type cmd struct {
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op operation
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// subscribe, unsubscribe
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query Query
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subscription *Subscription
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clientID string
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// publish
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msg interface{}
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tags map[string]string
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}
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// Server allows clients to subscribe/unsubscribe for messages, publishing
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// messages with or without tags, and manages internal state.
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type Server struct {
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cmn.BaseService
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cmds chan cmd
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cmdsCap int
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mtx sync.RWMutex
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subscriptions map[string]map[string]struct{} // subscriber -> query (string) -> empty struct
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}
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// Option sets a parameter for the server.
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type Option func(*Server)
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// NewServer returns a new server. See the commentary on the Option functions
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// for a detailed description of how to configure buffering. If no options are
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// provided, the resulting server's queue is unbuffered.
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func NewServer(options ...Option) *Server {
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s := &Server{
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subscriptions: make(map[string]map[string]struct{}),
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}
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s.BaseService = *cmn.NewBaseService(nil, "PubSub", s)
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for _, option := range options {
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option(s)
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}
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// if BufferCapacity option was not set, the channel is unbuffered
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s.cmds = make(chan cmd, s.cmdsCap)
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return s
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}
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// BufferCapacity allows you to specify capacity for the internal server's
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// queue. Since the server, given Y subscribers, could only process X messages,
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// this option could be used to survive spikes (e.g. high amount of
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// transactions during peak hours).
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func BufferCapacity(cap int) Option {
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return func(s *Server) {
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if cap > 0 {
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s.cmdsCap = cap
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}
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}
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}
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// BufferCapacity returns capacity of the internal server's queue.
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func (s *Server) BufferCapacity() int {
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return s.cmdsCap
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}
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// Subscribe creates a subscription for the given client.
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//
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// An error will be returned to the caller if the context is canceled or if
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// subscription already exist for pair clientID and query.
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//
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// outCapacity can be used to set a capacity for Subscription#Out channel (1 by
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// default). Panics if outCapacity is less than or equal to zero. If you want
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// an unbuffered channel, use SubscribeUnbuffered.
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func (s *Server) Subscribe(ctx context.Context, clientID string, query Query, outCapacity ...int) (*Subscription, error) {
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outCap := 1
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if len(outCapacity) > 0 {
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if outCapacity[0] <= 0 {
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panic("Negative or zero capacity. Use SubscribeUnbuffered if you want an unbuffered channel")
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}
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outCap = outCapacity[0]
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}
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return s.subscribe(ctx, clientID, query, outCap)
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}
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// SubscribeUnbuffered does the same as Subscribe, except it returns a
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// subscription with unbuffered channel. Use with caution as it can freeze the
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// server.
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func (s *Server) SubscribeUnbuffered(ctx context.Context, clientID string, query Query) (*Subscription, error) {
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return s.subscribe(ctx, clientID, query, 0)
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}
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func (s *Server) subscribe(ctx context.Context, clientID string, query Query, outCapacity int) (*Subscription, error) {
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s.mtx.RLock()
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clientSubscriptions, ok := s.subscriptions[clientID]
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if ok {
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_, ok = clientSubscriptions[query.String()]
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}
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s.mtx.RUnlock()
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if ok {
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return nil, ErrAlreadySubscribed
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}
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subscription := &Subscription{
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out: make(chan Message, outCapacity),
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cancelled: make(chan struct{}),
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}
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select {
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case s.cmds <- cmd{op: sub, clientID: clientID, query: query, subscription: subscription}:
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s.mtx.Lock()
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if _, ok = s.subscriptions[clientID]; !ok {
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s.subscriptions[clientID] = make(map[string]struct{})
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}
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s.subscriptions[clientID][query.String()] = struct{}{}
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s.mtx.Unlock()
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return subscription, nil
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case <-ctx.Done():
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return nil, ctx.Err()
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case <-s.Quit():
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return nil, nil
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}
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}
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// Unsubscribe removes the subscription on the given query. An error will be
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// returned to the caller if the context is canceled or if subscription does
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// not exist.
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func (s *Server) Unsubscribe(ctx context.Context, clientID string, query Query) error {
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s.mtx.RLock()
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clientSubscriptions, ok := s.subscriptions[clientID]
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if ok {
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_, ok = clientSubscriptions[query.String()]
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}
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s.mtx.RUnlock()
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if !ok {
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return ErrSubscriptionNotFound
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}
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select {
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case s.cmds <- cmd{op: unsub, clientID: clientID, query: query}:
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s.mtx.Lock()
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delete(clientSubscriptions, query.String())
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if len(clientSubscriptions) == 0 {
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delete(s.subscriptions, clientID)
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}
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s.mtx.Unlock()
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return nil
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case <-ctx.Done():
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return ctx.Err()
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case <-s.Quit():
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return nil
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}
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}
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// UnsubscribeAll removes all client subscriptions. An error will be returned
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// to the caller if the context is canceled or if subscription does not exist.
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func (s *Server) UnsubscribeAll(ctx context.Context, clientID string) error {
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s.mtx.RLock()
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_, ok := s.subscriptions[clientID]
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s.mtx.RUnlock()
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if !ok {
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return ErrSubscriptionNotFound
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}
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select {
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case s.cmds <- cmd{op: unsub, clientID: clientID}:
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s.mtx.Lock()
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delete(s.subscriptions, clientID)
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s.mtx.Unlock()
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return nil
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case <-ctx.Done():
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return ctx.Err()
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case <-s.Quit():
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return nil
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}
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}
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// Publish publishes the given message. An error will be returned to the caller
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// if the context is canceled.
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func (s *Server) Publish(ctx context.Context, msg interface{}) error {
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return s.PublishWithTags(ctx, msg, make(map[string]string))
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}
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// PublishWithTags publishes the given message with the set of tags. The set is
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// matched with clients queries. If there is a match, the message is sent to
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// the client.
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func (s *Server) PublishWithTags(ctx context.Context, msg interface{}, tags map[string]string) error {
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select {
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case s.cmds <- cmd{op: pub, msg: msg, tags: tags}:
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return nil
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case <-ctx.Done():
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return ctx.Err()
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case <-s.Quit():
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return nil
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}
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}
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// OnStop implements Service.OnStop by shutting down the server.
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func (s *Server) OnStop() {
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s.cmds <- cmd{op: shutdown}
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}
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// NOTE: not goroutine safe
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type state struct {
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// query string -> client -> subscription
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subscriptions map[string]map[string]*Subscription
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// query string -> queryPlusRefCount
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queries map[string]*queryPlusRefCount
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}
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// queryPlusRefCount holds a pointer to a query and reference counter. When
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// refCount is zero, query will be removed.
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type queryPlusRefCount struct {
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q Query
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refCount int
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}
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// OnStart implements Service.OnStart by starting the server.
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func (s *Server) OnStart() error {
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go s.loop(state{
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subscriptions: make(map[string]map[string]*Subscription),
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queries: make(map[string]*queryPlusRefCount),
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})
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return nil
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}
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// OnReset implements Service.OnReset
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func (s *Server) OnReset() error {
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return nil
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}
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func (s *Server) loop(state state) {
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loop:
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for cmd := range s.cmds {
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switch cmd.op {
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case unsub:
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if cmd.query != nil {
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state.remove(cmd.clientID, cmd.query.String(), ErrUnsubscribed)
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} else {
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state.removeClient(cmd.clientID, ErrUnsubscribed)
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}
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case shutdown:
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state.removeAll(nil)
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break loop
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case sub:
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state.add(cmd.clientID, cmd.query, cmd.subscription)
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case pub:
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state.send(cmd.msg, cmd.tags)
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}
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}
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}
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func (state *state) add(clientID string, q Query, subscription *Subscription) {
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qStr := q.String()
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// initialize subscription for this client per query if needed
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if _, ok := state.subscriptions[qStr]; !ok {
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state.subscriptions[qStr] = make(map[string]*Subscription)
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}
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// create subscription
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state.subscriptions[qStr][clientID] = subscription
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// initialize query if needed
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if _, ok := state.queries[qStr]; !ok {
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state.queries[qStr] = &queryPlusRefCount{q: q, refCount: 0}
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}
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// increment reference counter
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state.queries[qStr].refCount++
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}
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func (state *state) remove(clientID string, qStr string, reason error) {
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clientSubscriptions, ok := state.subscriptions[qStr]
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if !ok {
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return
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}
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subscription, ok := clientSubscriptions[clientID]
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if !ok {
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return
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}
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subscription.mtx.Lock()
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subscription.err = reason
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subscription.mtx.Unlock()
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close(subscription.cancelled)
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// remove client from query map.
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// if query has no other clients subscribed, remove it.
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delete(state.subscriptions[qStr], clientID)
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if len(state.subscriptions[qStr]) == 0 {
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delete(state.subscriptions, qStr)
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}
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// decrease ref counter in queries
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state.queries[qStr].refCount--
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// remove the query if nobody else is using it
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if state.queries[qStr].refCount == 0 {
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delete(state.queries, qStr)
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}
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}
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func (state *state) removeClient(clientID string, reason error) {
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for qStr, clientSubscriptions := range state.subscriptions {
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if _, ok := clientSubscriptions[clientID]; ok {
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state.remove(clientID, qStr, reason)
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}
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}
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}
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func (state *state) removeAll(reason error) {
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for qStr, clientSubscriptions := range state.subscriptions {
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for clientID := range clientSubscriptions {
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state.remove(clientID, qStr, reason)
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}
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}
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}
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func (state *state) send(msg interface{}, tags map[string]string) {
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for qStr, clientSubscriptions := range state.subscriptions {
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q := state.queries[qStr].q
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if q.Matches(tags) {
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for clientID, subscription := range clientSubscriptions {
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if cap(subscription.out) == 0 {
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// block on unbuffered channel
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subscription.out <- Message{msg, tags}
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} else {
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// don't block on buffered channels
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select {
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case subscription.out <- Message{msg, tags}:
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default:
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state.remove(clientID, qStr, ErrOutOfCapacity)
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}
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}
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}
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}
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}
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}
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