121 lines
3.4 KiB
Go
121 lines
3.4 KiB
Go
/*
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*
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* Copyright 2023 gRPC authors.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*
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*/
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package grpcsync
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import (
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"context"
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"sync"
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)
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// Subscriber represents an entity that is subscribed to messages published on
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// a PubSub. It wraps the callback to be invoked by the PubSub when a new
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// message is published.
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type Subscriber interface {
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// OnMessage is invoked when a new message is published. Implementations
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// must not block in this method.
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OnMessage(msg any)
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}
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// PubSub is a simple one-to-many publish-subscribe system that supports
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// messages of arbitrary type. It guarantees that messages are delivered in
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// the same order in which they were published.
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//
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// Publisher invokes the Publish() method to publish new messages, while
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// subscribers interested in receiving these messages register a callback
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// via the Subscribe() method.
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//
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// Once a PubSub is stopped, no more messages can be published, but any pending
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// published messages will be delivered to the subscribers. Done may be used
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// to determine when all published messages have been delivered.
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type PubSub struct {
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cs *CallbackSerializer
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// Access to the below fields are guarded by this mutex.
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mu sync.Mutex
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msg any
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subscribers map[Subscriber]bool
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}
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// NewPubSub returns a new PubSub instance. Users should cancel the
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// provided context to shutdown the PubSub.
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func NewPubSub(ctx context.Context) *PubSub {
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return &PubSub{
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cs: NewCallbackSerializer(ctx),
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subscribers: map[Subscriber]bool{},
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}
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}
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// Subscribe registers the provided Subscriber to the PubSub.
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//
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// If the PubSub contains a previously published message, the Subscriber's
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// OnMessage() callback will be invoked asynchronously with the existing
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// message to begin with, and subsequently for every newly published message.
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//
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// The caller is responsible for invoking the returned cancel function to
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// unsubscribe itself from the PubSub.
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func (ps *PubSub) Subscribe(sub Subscriber) (cancel func()) {
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ps.mu.Lock()
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defer ps.mu.Unlock()
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ps.subscribers[sub] = true
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if ps.msg != nil {
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msg := ps.msg
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ps.cs.Schedule(func(context.Context) {
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ps.mu.Lock()
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defer ps.mu.Unlock()
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if !ps.subscribers[sub] {
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return
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}
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sub.OnMessage(msg)
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})
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}
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return func() {
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ps.mu.Lock()
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defer ps.mu.Unlock()
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delete(ps.subscribers, sub)
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}
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}
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// Publish publishes the provided message to the PubSub, and invokes
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// callbacks registered by subscribers asynchronously.
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func (ps *PubSub) Publish(msg any) {
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ps.mu.Lock()
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defer ps.mu.Unlock()
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ps.msg = msg
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for sub := range ps.subscribers {
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s := sub
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ps.cs.Schedule(func(context.Context) {
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ps.mu.Lock()
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defer ps.mu.Unlock()
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if !ps.subscribers[s] {
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return
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}
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s.OnMessage(msg)
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})
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}
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}
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// Done returns a channel that is closed after the context passed to NewPubSub
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// is canceled and all updates have been sent to subscribers.
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func (ps *PubSub) Done() <-chan struct{} {
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return ps.cs.Done()
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}
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