forked from TrueCloudLab/restic
120 lines
5 KiB
Go
120 lines
5 KiB
Go
// Copyright 2016 Google Inc. All Rights Reserved.
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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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Package pubsub provides an easy way to publish and receive Google Cloud Pub/Sub
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messages, hiding the the details of the underlying server RPCs. Google Cloud
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Pub/Sub is a many-to-many, asynchronous messaging system that decouples senders
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and receivers.
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Note: This package is in beta. Some backwards-incompatible changes may occur.
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More information about Google Cloud Pub/Sub is available at
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https://cloud.google.com/pubsub/docs
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Publishing
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Google Cloud Pub/Sub messages are published to topics. Topics may be created
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using the pubsub package like so:
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topic, err := pubsubClient.CreateTopic(context.Background(), "topic-name")
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Messages may then be published to a topic:
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res := topic.Publish(ctx, &pubsub.Message{Data: []byte("payload")})
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Publish queues the message for publishing and returns immediately. When enough
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messages have accumulated, or enough time has elapsed, the batch of messages is
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sent to the Pub/Sub service.
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Publish returns a PublishResult, which behaves like a future: its Get method
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blocks until the message has been sent to the service.
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The first time you call Publish on a topic, goroutines are started in the
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background. To clean up these goroutines, call Stop:
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topic.Stop()
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Receiving
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To receive messages published to a topic, clients create subscriptions
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to the topic. There may be more than one subscription per topic; each message
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that is published to the topic will be delivered to all of its subscriptions.
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Subsciptions may be created like so:
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sub, err := pubsubClient.CreateSubscription(context.Background(), "sub-name",
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pubsub.SubscriptionConfig{Topic: topic})
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Messages are then consumed from a subscription via callback.
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err := sub.Receive(context.Background(), func(ctx context.Context, m *Message) {
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log.Printf("Got message: %s", m.Data)
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m.Ack()
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})
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if err != nil {
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// Handle error.
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}
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The callback is invoked concurrently by multiple goroutines, maximizing
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throughput. To terminate a call to Receive, cancel its context.
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Once client code has processed the message, it must call Message.Ack, otherwise
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the message will eventually be redelivered. As an optimization, if the client
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cannot or doesn't want to process the message, it can call Message.Nack to
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speed redelivery. For more information and configuration options, see
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"Deadlines" below.
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Note: It is possible for Messages to be redelivered, even if Message.Ack has
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been called. Client code must be robust to multiple deliveries of messages.
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Deadlines
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The default pubsub deadlines are suitable for most use cases, but may be
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overridden. This section describes the tradeoffs that should be considered
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when overriding the defaults.
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Behind the scenes, each message returned by the Pub/Sub server has an
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associated lease, known as an "ACK deadline".
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Unless a message is acknowledged within the ACK deadline, or the client requests that
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the ACK deadline be extended, the message will become elegible for redelivery.
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As a convenience, the pubsub package will automatically extend deadlines until
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either:
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* Message.Ack or Message.Nack is called, or
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* the "MaxExtension" period elapses from the time the message is fetched from the server.
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The initial ACK deadline given to each messages defaults to 10 seconds, but may
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be overridden during subscription creation. Selecting an ACK deadline is a
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tradeoff between message redelivery latency and RPC volume. If the pubsub
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package fails to acknowledge or extend a message (e.g. due to unexpected
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termination of the process), a shorter ACK deadline will generally result in
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faster message redelivery by the Pub/Sub system. However, a short ACK deadline
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may also increase the number of deadline extension RPCs that the pubsub package
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sends to the server.
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The default max extension period is DefaultReceiveSettings.MaxExtension, and can
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be overridden by setting Subscription.ReceiveSettings.MaxExtension. Selecting a
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max extension period is a tradeoff between the speed at which client code must
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process messages, and the redelivery delay if messages fail to be acknowledged
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(e.g. because client code neglects to do so). Using a large MaxExtension
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increases the available time for client code to process messages. However, if
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the client code neglects to call Message.Ack/Nack, a large MaxExtension will
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increase the delay before the message is redelivered.
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Authentication
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See examples of authorization and authentication at
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https://godoc.org/cloud.google.com/go#pkg-examples.
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*/
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package pubsub // import "cloud.google.com/go/pubsub"
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