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- // Copyright 2016 Google LLC
- //
- // Licensed under the Apache License, Version 2.0 (the "License");
- // you may not use this file except in compliance with the License.
- // You may obtain a copy of the License at
- //
- // http://www.apache.org/licenses/LICENSE-2.0
- //
- // Unless required by applicable law or agreed to in writing, software
- // distributed under the License is distributed on an "AS IS" BASIS,
- // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- // See the License for the specific language governing permissions and
- // limitations under the License.
-
- /*
- Package pubsub provides an easy way to publish and receive Google Cloud Pub/Sub
- messages, hiding the the details of the underlying server RPCs. Google Cloud
- Pub/Sub is a many-to-many, asynchronous messaging system that decouples senders
- and receivers.
-
- More information about Google Cloud Pub/Sub is available at
- https://cloud.google.com/pubsub/docs
-
- See https://godoc.org/cloud.google.com/go for authentication, timeouts,
- connection pooling and similar aspects of this package.
-
-
- Publishing
-
- Google Cloud Pub/Sub messages are published to topics. Topics may be created
- using the pubsub package like so:
-
- topic, err := pubsubClient.CreateTopic(context.Background(), "topic-name")
-
- Messages may then be published to a topic:
-
- res := topic.Publish(ctx, &pubsub.Message{Data: []byte("payload")})
-
- Publish queues the message for publishing and returns immediately. When enough
- messages have accumulated, or enough time has elapsed, the batch of messages is
- sent to the Pub/Sub service.
-
- Publish returns a PublishResult, which behaves like a future: its Get method
- blocks until the message has been sent to the service.
-
- The first time you call Publish on a topic, goroutines are started in the
- background. To clean up these goroutines, call Stop:
-
- topic.Stop()
-
- Receiving
-
- To receive messages published to a topic, clients create subscriptions
- to the topic. There may be more than one subscription per topic; each message
- that is published to the topic will be delivered to all of its subscriptions.
-
- Subsciptions may be created like so:
-
- sub, err := pubsubClient.CreateSubscription(context.Background(), "sub-name",
- pubsub.SubscriptionConfig{Topic: topic})
-
- Messages are then consumed from a subscription via callback.
-
- err := sub.Receive(context.Background(), func(ctx context.Context, m *Message) {
- log.Printf("Got message: %s", m.Data)
- m.Ack()
- })
- if err != nil {
- // Handle error.
- }
-
- The callback is invoked concurrently by multiple goroutines, maximizing
- throughput. To terminate a call to Receive, cancel its context.
-
- Once client code has processed the message, it must call Message.Ack, otherwise
- the message will eventually be redelivered. As an optimization, if the client
- cannot or doesn't want to process the message, it can call Message.Nack to
- speed redelivery. For more information and configuration options, see
- "Deadlines" below.
-
- Note: It is possible for Messages to be redelivered, even if Message.Ack has
- been called. Client code must be robust to multiple deliveries of messages.
-
- Deadlines
-
- The default pubsub deadlines are suitable for most use cases, but may be
- overridden. This section describes the tradeoffs that should be considered
- when overriding the defaults.
-
- Behind the scenes, each message returned by the Pub/Sub server has an
- associated lease, known as an "ACK deadline".
- Unless a message is acknowledged within the ACK deadline, or the client requests that
- the ACK deadline be extended, the message will become elegible for redelivery.
- As a convenience, the pubsub package will automatically extend deadlines until
- either:
- * Message.Ack or Message.Nack is called, or
- * the "MaxExtension" period elapses from the time the message is fetched from the server.
-
- The initial ACK deadline given to each messages defaults to 10 seconds, but may
- be overridden during subscription creation. Selecting an ACK deadline is a
- tradeoff between message redelivery latency and RPC volume. If the pubsub
- package fails to acknowledge or extend a message (e.g. due to unexpected
- termination of the process), a shorter ACK deadline will generally result in
- faster message redelivery by the Pub/Sub system. However, a short ACK deadline
- may also increase the number of deadline extension RPCs that the pubsub package
- sends to the server.
-
- The default max extension period is DefaultReceiveSettings.MaxExtension, and can
- be overridden by setting Subscription.ReceiveSettings.MaxExtension. Selecting a
- max extension period is a tradeoff between the speed at which client code must
- process messages, and the redelivery delay if messages fail to be acknowledged
- (e.g. because client code neglects to do so). Using a large MaxExtension
- increases the available time for client code to process messages. However, if
- the client code neglects to call Message.Ack/Nack, a large MaxExtension will
- increase the delay before the message is redelivered.
-
- Slow Message Processing
-
- For use cases where message processing exceeds 30 minutes, we recommend using
- the base client in a pull model, since long-lived streams are periodically killed
- by firewalls. See the example at https://godoc.org/cloud.google.com/go/pubsub/apiv1#example-SubscriberClient-Pull-LengthyClientProcessing
- */
- package pubsub // import "cloud.google.com/go/pubsub"
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