| // 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 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 or |
| message.Nack, otherwise the message will eventually be redelivered. 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. |
| |
| Note: This uses pubsub's streaming pull feature. This feature properties that |
| may be surprising. Please take a look at https://cloud.google.com/pubsub/docs/pull#streamingpull |
| for more details on how streaming pull behaves compared to the synchronous |
| pull method. |
| |
| |
| 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 eligible for redelivery. |
| |
| As a convenience, the pubsub client 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. |
| |
| ACK deadlines are extended periodically by the client. The initial ACK |
| deadline given to messages is 10s. The period between extensions, as well as the |
| length of the extension, automatically adjust depending on the time it takes to ack |
| messages, up to 10m. This has the effect that subscribers that process messages |
| quickly have their message ack deadlines extended for a short amount, whereas |
| subscribers that process message slowly have their message ack deadlines extended |
| for a large amount. The net effect is fewer RPCs sent from the client library. |
| |
| For example, consider a subscriber that takes 3 minutes to process each message. |
| Since the library has already recorded several 3 minute "time to ack"s in a |
| percentile distribution, future message extensions are sent with a value of 3 |
| minutes, every 3 minutes. Suppose the application crashes 5 seconds after the |
| library sends such an extension: the Pub/Sub server would wait the remaining |
| 2m55s before re-sending the messages out to other subscribers. |
| |
| Please note that the client library does not use the subscription's AckDeadline |
| by default. To enforce the subscription AckDeadline, set MaxExtension to the |
| subscription's AckDeadline: |
| |
| cfg, err := sub.Config(ctx) |
| if err != nil { |
| // TODO: handle err |
| } |
| |
| sub.ReceiveSettings.MaxExtension = cfg.AckDeadline |
| |
| |
| 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 |
| |
| Emulator |
| |
| To use an emulator with this library, you can set the PUBSUB_EMULATOR_HOST |
| environment variable to the address at which your emulator is running. This will |
| send requests to that address instead of to Cloud Pub/Sub. You can then create |
| and use a client as usual: |
| |
| // Set PUBSUB_EMULATOR_HOST environment variable. |
| err := os.Setenv("PUBSUB_EMULATOR_HOST", "localhost:9000") |
| if err != nil { |
| // TODO: Handle error. |
| } |
| // Create client as usual. |
| client, err := pubsub.NewClient(ctx, "my-project-id") |
| if err != nil { |
| // TODO: Handle error. |
| } |
| defer client.Close() |
| |
| |
| */ |
| package pubsub // import "cloud.google.com/go/pubsub" |