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409 lines
10 KiB
409 lines
10 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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// Due to the blocking send implementation, a single subscriber can freeze an
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// entire server by not reading messages before it unsubscribes. To avoid such
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// scenario, subscribers must either:
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//
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// a) make sure they continue to read from the out channel until
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// Unsubscribe(All) is called
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//
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// s.Subscribe(ctx, sub, qry, out)
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// go func() {
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// for msg := range out {
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// // handle msg
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// // will exit automatically when out is closed by Unsubscribe(All)
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// }
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// }()
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// s.UnsubscribeAll(ctx, sub)
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//
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// b) drain the out channel before calling Unsubscribe(All)
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//
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// s.Subscribe(ctx, sub, qry, out)
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// defer func() {
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// // drain out to make sure we don't block
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// LOOP:
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// for {
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// select {
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// case <-out:
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// default:
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// break LOOP
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// }
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// }
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// s.UnsubscribeAll(ctx, sub)
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// }()
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// for msg := range out {
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// // handle msg
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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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//
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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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type cmd struct {
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op operation
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query Query
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ch chan<- interface{}
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clientID string
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msg interface{}
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tags TagMap
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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 TagMap) bool
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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]Query // subscriber -> query (string) -> Query
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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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// TagMap is used to associate tags to a message.
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// They can be queried by subscribers to choose messages they will received.
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type TagMap interface {
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// Get returns the value for a key, or nil if no value is present.
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// The ok result indicates whether value was found in the tags.
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Get(key string) (value string, ok bool)
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// Len returns the number of tags.
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Len() int
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}
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type tagMap map[string]string
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var _ TagMap = (*tagMap)(nil)
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// NewTagMap constructs a new immutable tag set from a map.
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func NewTagMap(data map[string]string) TagMap {
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return tagMap(data)
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}
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// Get returns the value for a key, or nil if no value is present.
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// The ok result indicates whether value was found in the tags.
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func (ts tagMap) Get(key string) (value string, ok bool) {
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value, ok = ts[key]
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return
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}
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// Len returns the number of tags.
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func (ts tagMap) Len() int {
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return len(ts)
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}
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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]Query),
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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. It accepts a channel
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// on which messages matching the given query can be received. An error will be
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// returned to the caller if the context is canceled or if subscription already
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// exist for pair clientID and query.
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func (s *Server) Subscribe(ctx context.Context, clientID string, query Query, out chan<- interface{}) 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 ErrAlreadySubscribed
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}
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select {
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case s.cmds <- cmd{op: sub, clientID: clientID, query: query, ch: out}:
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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]Query)
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}
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// preserve original query
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// see Unsubscribe
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s.subscriptions[clientID][query.String()] = query
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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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// 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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var origQuery Query
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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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origQuery, 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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// original query is used here because we're using pointers as map keys
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// ?
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select {
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case s.cmds <- cmd{op: unsub, clientID: clientID, query: origQuery}:
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s.mtx.Lock()
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// if its the only query left, should we also delete the client?
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delete(clientSubscriptions, query.String())
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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, NewTagMap(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 TagMap) 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 -> client -> ch
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queries map[Query]map[string]chan<- interface{}
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// client -> query -> struct{}
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clients map[string]map[Query]struct{}
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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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queries: make(map[Query]map[string]chan<- interface{}),
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clients: make(map[string]map[Query]struct{}),
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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)
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} else {
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state.removeAll(cmd.clientID)
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}
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case shutdown:
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for clientID := range state.clients {
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state.removeAll(clientID)
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}
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break loop
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case sub:
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state.add(cmd.clientID, cmd.query, cmd.ch)
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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, ch chan<- interface{}) {
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// initialize clientToChannelMap per query if needed
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if _, ok := state.queries[q]; !ok {
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state.queries[q] = make(map[string]chan<- interface{})
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}
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// create subscription
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state.queries[q][clientID] = ch
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// add client if needed
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if _, ok := state.clients[clientID]; !ok {
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state.clients[clientID] = make(map[Query]struct{})
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}
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state.clients[clientID][q] = struct{}{}
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}
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func (state *state) remove(clientID string, q Query) {
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clientToChannelMap, ok := state.queries[q]
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if !ok {
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return
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}
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ch, ok := clientToChannelMap[clientID]
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if !ok {
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return
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}
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close(ch)
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// remove the query from client map.
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// if client is not subscribed to anything else, remove it.
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delete(state.clients[clientID], q)
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if len(state.clients[clientID]) == 0 {
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delete(state.clients, clientID)
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}
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// remove the client from query map.
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// if query has no other clients subscribed, remove it.
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delete(state.queries[q], clientID)
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if len(state.queries[q]) == 0 {
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delete(state.queries, q)
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}
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}
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func (state *state) removeAll(clientID string) {
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queryMap, ok := state.clients[clientID]
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if !ok {
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return
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}
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for q := range queryMap {
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ch := state.queries[q][clientID]
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close(ch)
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// remove the client from query map.
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// if query has no other clients subscribed, remove it.
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delete(state.queries[q], clientID)
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if len(state.queries[q]) == 0 {
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delete(state.queries, q)
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}
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}
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// remove the client.
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delete(state.clients, clientID)
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}
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func (state *state) send(msg interface{}, tags TagMap) {
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for q, clientToChannelMap := range state.queries {
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if q.Matches(tags) {
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for _, ch := range clientToChannelMap {
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ch <- msg
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}
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}
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}
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}
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