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401 lines
12 KiB
401 lines
12 KiB
// Package pubsub implements an event dispatching server with a single publisher
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// and multiple subscriber clients. Multiple goroutines can safely publish to a
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// single Server instance.
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//
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// Clients register subscriptions with a query to select which messages they
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// wish to receive. When messages are published, they are broadcast to all
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// clients whose subscription query matches that message. Queries are
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// constructed using the github.com/tendermint/tendermint/libs/pubsub/query
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// package.
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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.Events()
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// case <-subscription.Canceled():
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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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"fmt"
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"sync"
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"github.com/tendermint/tendermint/abci/types"
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"github.com/tendermint/tendermint/libs/service"
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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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// ErrServerStopped is returned when attempting to publish or subscribe to a
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// server that has been stopped.
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ErrServerStopped = errors.New("pubsub server is stopped")
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)
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// Query defines an interface for a query to be used for subscribing. A query
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// matches against a map of events. Each key in this map is a composite of the
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// even type and an attribute key (e.g. "{eventType}.{eventAttrKey}") and the
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// values are the event values that are contained under that relationship. This
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// allows event types to repeat themselves with the same set of keys and
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// different values.
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type Query interface {
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Matches(events []types.Event) (bool, error)
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String() string
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}
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// UnsubscribeArgs are the parameters to remove a subscription.
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// The subscriber ID must be populated, and at least one of the client ID or
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// the registered query.
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type UnsubscribeArgs struct {
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Subscriber string // subscriber ID chosen by the client (required)
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ID string // subscription ID (assigned by the server)
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Query Query // the query registered with the subscription
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}
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// Validate returns nil if args are valid to identify a subscription to remove.
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// Otherwise, it reports an error.
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func (args UnsubscribeArgs) Validate() error {
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if args.Subscriber == "" {
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return errors.New("must specify a subscriber")
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}
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if args.ID == "" && args.Query == nil {
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return fmt.Errorf("subscription is not fully defined [subscriber=%q]", args.Subscriber)
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}
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return nil
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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 events, and manages internal state.
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type Server struct {
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service.BaseService
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queue chan item
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done <-chan struct{} // closed when server should exit
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stop func() // signal the server to exit
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pubs sync.RWMutex // excl: shutdown; shared: active publisher
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exited chan struct{} // server exited
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// All subscriptions currently known.
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// Lock exclusive to add, remove, or cancel subscriptions.
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// Lock shared to look up or publish to subscriptions.
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subs struct {
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sync.RWMutex
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index *subIndex
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}
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// TODO(creachadair): Rework the options so that this does not need to live
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// as a field. It is not otherwise needed.
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queueCap int
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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 := new(Server)
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for _, opt := range options {
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opt(s)
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}
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s.BaseService = *service.NewBaseService(nil, "PubSub", s)
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// The queue receives items to be published.
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s.queue = make(chan item, s.queueCap)
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// The index tracks subscriptions by ID and query terms.
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s.subs.index = newSubIndex()
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return s
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}
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// BufferCapacity allows you to specify capacity for publisher's queue. This
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// is the number of messages that can be published without blocking. If no
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// buffer is specified, publishing is synchronous with delivery. This function
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// will panic if cap < 0.
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func BufferCapacity(cap int) Option {
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if cap < 0 {
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panic("negative buffer capacity")
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}
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return func(s *Server) { s.queueCap = cap }
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}
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// BufferCapacity returns capacity of the publication queue.
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func (s *Server) BufferCapacity() int { return cap(s.queue) }
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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(
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ctx context.Context,
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clientID string,
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query Query,
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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.subs.Lock()
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defer s.subs.Unlock()
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if s.subs.index == nil {
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return nil, ErrServerStopped
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} else if s.subs.index.contains(clientID, query.String()) {
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return nil, ErrAlreadySubscribed
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}
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sub, err := newSubscription(outCapacity)
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if err != nil {
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return nil, err
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}
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s.subs.index.add(&subInfo{
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clientID: clientID,
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query: query,
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subID: sub.id,
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sub: sub,
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})
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return sub, nil
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}
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// Unsubscribe removes the subscription for the given client and/or query. It
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// returns ErrSubscriptionNotFound if no such subscription exists.
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func (s *Server) Unsubscribe(ctx context.Context, args UnsubscribeArgs) error {
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if err := args.Validate(); err != nil {
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return err
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}
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s.subs.Lock()
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defer s.subs.Unlock()
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if s.subs.index == nil {
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return ErrServerStopped
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}
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// TODO(creachadair): Do we need to support unsubscription for an "empty"
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// query? I believe that case is not possible by the Query grammar, but we
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// should make sure.
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//
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// Revisit this logic once we are able to remove indexing by query.
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var evict subInfoSet
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if args.Subscriber != "" {
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evict = s.subs.index.findClientID(args.Subscriber)
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if args.Query != nil {
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evict = evict.withQuery(args.Query.String())
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}
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} else {
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evict = s.subs.index.findQuery(args.Query.String())
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}
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if len(evict) == 0 {
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return ErrSubscriptionNotFound
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}
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s.removeSubs(evict, ErrUnsubscribed)
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return nil
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}
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// UnsubscribeAll removes all subscriptions for the given client ID.
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// It returns ErrSubscriptionNotFound if no subscriptions exist for that client.
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func (s *Server) UnsubscribeAll(ctx context.Context, clientID string) error {
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s.subs.Lock()
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defer s.subs.Unlock()
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evict := s.subs.index.findClientID(clientID)
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if len(evict) == 0 {
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return ErrSubscriptionNotFound
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}
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s.removeSubs(evict, ErrUnsubscribed)
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return nil
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}
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// NumClients returns the number of clients.
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func (s *Server) NumClients() int {
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s.subs.RLock()
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defer s.subs.RUnlock()
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return len(s.subs.index.byClient)
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}
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// NumClientSubscriptions returns the number of subscriptions the client has.
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func (s *Server) NumClientSubscriptions(clientID string) int {
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s.subs.RLock()
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defer s.subs.RUnlock()
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return len(s.subs.index.findClientID(clientID))
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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.publish(ctx, msg, []types.Event{})
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}
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// PublishWithEvents publishes the given message with the set of events. The set
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// is 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) PublishWithEvents(ctx context.Context, msg interface{}, events []types.Event) error {
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return s.publish(ctx, msg, events)
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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() { s.stop() }
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// Wait implements Service.Wait by blocking until the server has exited, then
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// yielding to the base service wait.
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func (s *Server) Wait() { <-s.exited; s.BaseService.Wait() }
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// OnStart implements Service.OnStart by starting the server.
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func (s *Server) OnStart() error { s.run(); return nil }
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// OnReset implements Service.OnReset. It has no effect for this service.
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func (s *Server) OnReset() error { return nil }
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func (s *Server) publish(ctx context.Context, data interface{}, events []types.Event) error {
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s.pubs.RLock()
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defer s.pubs.RUnlock()
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select {
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case <-s.done:
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return ErrServerStopped
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case <-ctx.Done():
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return ctx.Err()
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case s.queue <- item{
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Data: data,
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Events: events,
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}:
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return nil
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}
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}
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func (s *Server) run() {
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// The server runs until ctx is canceled.
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ctx, cancel := context.WithCancel(context.Background())
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s.done = ctx.Done()
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s.stop = cancel
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// Shutdown monitor: When the context ends, wait for any active publish
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// calls to exit, then close the queue to signal the sender to exit.
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go func() {
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<-ctx.Done()
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s.pubs.Lock()
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defer s.pubs.Unlock()
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close(s.queue)
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}()
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s.exited = make(chan struct{})
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go func() {
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defer close(s.exited)
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// Sender: Service the queue and forward messages to subscribers.
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for it := range s.queue {
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if err := s.send(it.Data, it.Events); err != nil {
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s.Logger.Error("Error sending event", "err", err)
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}
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}
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// Terminate all subscribers without error before exit.
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s.subs.Lock()
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defer s.subs.Unlock()
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for si := range s.subs.index.all {
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si.sub.cancel(nil)
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}
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s.subs.index = nil
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}()
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}
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// removeSubs cancels and removes all the subscriptions in evict with the given
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// error. The caller must hold the s.subs lock.
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func (s *Server) removeSubs(evict subInfoSet, reason error) {
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for si := range evict {
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si.sub.cancel(reason)
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}
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s.subs.index.removeAll(evict)
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}
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// send delivers the given message to all matching subscribers. An error in
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// query matching stops transmission and is returned.
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func (s *Server) send(data interface{}, events []types.Event) error {
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// At exit, evict any subscriptions that were too slow.
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evict := make(subInfoSet)
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defer func() {
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if len(evict) != 0 {
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s.subs.Lock()
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defer s.subs.Unlock()
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s.removeSubs(evict, ErrOutOfCapacity)
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}
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}()
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// N.B. Order is important here. We must acquire and defer the lock release
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// AFTER deferring the eviction cleanup: The cleanup must happen after the
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// reader lock has released, or it will deadlock.
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s.subs.RLock()
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defer s.subs.RUnlock()
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for si := range s.subs.index.all {
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match, err := si.query.Matches(events)
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if err != nil {
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return fmt.Errorf("match failed against query: %w", err)
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// TODO(creachadair): Should we evict this subscription?
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} else if !match {
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continue
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}
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// Subscriptions may be buffered or unbuffered. Unbuffered subscriptions
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// are intended for internal use such as indexing, where we don't want to
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// penalize a slow reader. Buffered subscribers must keep up with their
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// queue, or they will be terminated.
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//
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// TODO(creachadair): Unbuffered subscriptions used by the event indexer
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// to avoid losing events if it happens to be slow. Rework this so that
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// use case doesn't require this affordance, and then remove unbuffered
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// subscriptions.
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msg := NewMessage(si.sub.id, data, events)
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if err := si.sub.putMessage(msg); err != nil {
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// The subscriber was too slow, cancel them.
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evict.add(si)
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}
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}
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return nil
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}
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