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Google Cloud Pub/Sub

Google Cloud Pub/Sub is designed to provide reliable, many-to-many, asynchronous messaging between applications. Publisher applications can send messages to a "topic" and other applications can subscribe to that topic to receive the messages. By decoupling senders and receivers, Google Cloud Pub/Sub allows developers to communicate between independently written applications.

The goal of google-cloud is to provide an API that is comfortable to Rubyists. Your authentication credentials are detected automatically in Google Cloud Platform (GCP), including Google Compute Engine (GCE), Google Kubernetes Engine (GKE), Google App Engine (GAE), Google Cloud Functions (GCF) and Cloud Run. In other environments you can configure authentication easily, either directly in your code or via environment variables. Read more about the options for connecting in the Authentication Guide.

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

topic = pubsub.topic "my-topic"
topic.publish "task completed"

To learn more about Pub/Sub, read the Google Cloud Pub/Sub Overview .

Retrieving Topics

A Topic is a named resource to which messages are sent by publishers. A Topic is found by its name. (See Project#topic)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new
topic = pubsub.topic "my-topic"

Creating a Topic

A Topic is created from a Project. (See Project#create_topic)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new
topic = pubsub.create_topic "my-topic"

Retrieving Subscriptions

A Subscription is a named resource representing the stream of messages from a single, specific Topic, to be delivered to the subscribing application. A Subscription is found by its name. (See Topic#subscription)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

topic = pubsub.topic "my-topic"
subscription = topic.subscription "my-topic-subscription"
puts subscription.name

Creating a Subscription

A Subscription is created from a Topic. (See Topic#subscribe)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

topic = pubsub.topic "my-topic"
sub = topic.subscribe "my-topic-sub"
puts sub.name # => "my-topic-sub"

The subscription can be created that specifies the number of seconds to wait to be acknowledged as well as an endpoint URL to push the messages to:

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

topic = pubsub.topic "my-topic"
sub = topic.subscribe "my-topic-sub",
                      deadline: 120,
                      endpoint: "https://example.com/push"

Publishing Messages

Messages are published to a topic. Any message published to a topic without a subscription will be lost. Ensure the topic has a subscription before publishing. (See Topic#publish)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

topic = pubsub.topic "my-topic"
msg = topic.publish "task completed"

Messages can also be published with attributes:

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

topic = pubsub.topic "my-topic"
msg = topic.publish "task completed",
                    foo: :bar,
                    this: :that

Messages can also be published in batches asynchronously using publish_async. (See Topic#publish_async and AsyncPublisher)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

topic = pubsub.topic "my-topic"
topic.publish_async "task completed" do |result|
  if result.succeeded?
    log_publish_success result.data
  else
    log_publish_failure result.data, result.error
  end
end

topic.async_publisher.stop!

Or multiple messages can be published in batches at the same time by passing a block to publish. (See BatchPublisher)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

topic = pubsub.topic "my-topic"
msgs = topic.publish do |batch|
  batch.publish "task 1 completed", foo: :bar
  batch.publish "task 2 completed", foo: :baz
  batch.publish "task 3 completed", foo: :bif
end

Receiving Messages

Messages can be streamed from a subscription with a subscriber object that is created using listen. (See Subscription#listen and Subscriber)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

sub = pubsub.subscription "my-topic-sub"

# Create a subscriber to listen for available messages.
# By default, this block will be called on 8 concurrent threads
# but this can be tuned with the `threads` option.
# The `streams` and `inventory` parameters allow further tuning.
subscriber = sub.listen threads: { callback: 16 } do |received_message|
  # process message
  puts "Data: #{received_message.message.data}, published at #{received_message.message.published_at}"
  received_message.acknowledge!
end

# Handle exceptions from listener
subscriber.on_error do |exception|
  puts "Exception: #{exception.class} #{exception.message}"
end

# Gracefully shut down the subscriber on program exit, blocking until
# all received messages have been processed or 10 seconds have passed
at_exit do
  subscriber.stop!(10)
end

# Start background threads that will call the block passed to listen.
subscriber.start

# Block, letting processing threads continue in the background
sleep

Messages also can be pulled directly in a one-time operation. (See Subscription#pull)

The immediate: false option is recommended to avoid adverse impacts on the performance of pull operations.

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

sub = pubsub.subscription "my-topic-sub"
received_messages = sub.pull immediate: false

A maximum number of messages to pull can be specified:

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

sub = pubsub.subscription "my-topic-sub"
received_messages = sub.pull immediate: false, max: 10

Acknowledging a Message

Messages that are received can be acknowledged in Pub/Sub, marking the message to be removed so it cannot be pulled again.

A Message that can be acknowledged is called a ReceivedMessage. ReceivedMessages can be acknowledged one at a time: (See ReceivedMessage#acknowledge!)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

sub = pubsub.subscription "my-topic-sub"

subscriber = sub.listen do |received_message|
  # process message
  received_message.acknowledge!
end

# Start background threads that will call the block passed to listen.
subscriber.start

# Shut down the subscriber when ready to stop receiving messages.
subscriber.stop!

Or, multiple messages can be acknowledged in a single API call: (See Subscription#acknowledge)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

sub = pubsub.subscription "my-topic-sub"
received_messages = sub.pull immediate: false
sub.acknowledge received_messages

Modifying a Deadline

A message must be acknowledged after it is pulled, or Pub/Sub will mark the message for redelivery. The message acknowledgement deadline can delayed if more time is needed. This will allow more time to process the message before the message is marked for redelivery. (See ReceivedMessage#modify_ack_deadline!)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

sub = pubsub.subscription "my-topic-sub"
subscriber = sub.listen do |received_message|
  puts received_message.message.data

  # Delay for 2 minutes
  received_message.modify_ack_deadline! 120
end

# Start background threads that will call the block passed to listen.
subscriber.start

# Shut down the subscriber when ready to stop receiving messages.
subscriber.stop!

The message can also be made available for immediate redelivery:

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

sub = pubsub.subscription "my-topic-sub"
subscriber = sub.listen do |received_message|
  puts received_message.message.data

  # Mark for redelivery
  received_message.reject!
end

# Start background threads that will call the block passed to listen.
subscriber.start

# Shut down the subscriber when ready to stop receiving messages.
subscriber.stop!

Multiple messages can be delayed or made available for immediate redelivery: (See Subscription#modify_ack_deadline)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

sub = pubsub.subscription "my-topic-sub"
received_messages = sub.pull immediate: false
sub.modify_ack_deadline 120, received_messages

Using Ordering Keys

Google Cloud Pub/Sub ordering keys provide the ability to ensure related messages are sent to subscribers in the order in which they were published. Messages can be tagged with an ordering key, a string that identifies related messages for which publish order should be respected. The service guarantees that, for a given ordering key and publisher, messages are sent to subscribers in the order in which they were published. Ordering does not require sacrificing high throughput or scalability, as the service automatically distributes messages for different ordering keys across subscribers.

Note: At the time of this release, ordering keys are not yet publicly enabled and requires special project enablements.

Publishing Ordered Messages

To use ordering keys when publishing messages, a call to Topic#enable_message_ordering! must be made and the ordering_key argument must be provided when calling Topic#publish_async.

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

topic = pubsub.topic "my-ordered-topic"

# Ensure that message ordering is enabled.
topic.enable_message_ordering!

# Publish an ordered message with an ordering key.
topic.publish_async "task completed",
                    ordering_key: "task-key"

# Shut down the publisher when ready to stop publishing messages.
topic.async_publisher.stop!

Handling errors with Ordered Keys

Ordered messages that fail to publish to the Pub/Sub API due to error will put the ordering_key in a failed state, and future calls to Topic#publish_async with the ordering_key will raise OrderingKeyError. To allow future messages with the ordering_key to be published, the ordering_key must be passed to Topic#resume_publish.

Receiving Ordered Messages

To use ordering keys when subscribing to messages, the subscription must be created with message ordering enabled (See Topic#subscribe and Subscription#message_ordering?) before calling Subscription#listen. When enabled, the subscriber will deliver messages with the same ordering_key in the order they were published.

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

sub = pubsub.subscription "my-ordered-topic-sub"
sub.message_ordering? #=> true

subscriber = sub.listen do |received_message|
  # Messsages with the same ordering_key are received
  # in the order in which they were published.
  received_message.acknowledge!
end

# Start background threads that will call block passed to listen.
subscriber.start

# Shut down the subscriber when ready to stop receiving messages.
subscriber.stop!

Minimizing API calls before receiving and acknowledging messages

A subscription object can be created without making any API calls by providing the skip_lookup argument to Project#subscription or Topic#subscription. A subscriber object can also be created without an API call by providing the deadline optional argument to Subscription#listen:

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

# No API call is made to retrieve the subscription resource.
sub = pubsub.subscription "my-topic-sub", skip_lookup: true

# No API call is made to retrieve the subscription deadline.
subscriber = sub.listen deadline: 60 do |received_message|
  # process message
  received_message.acknowledge!
end

# Start background threads that will call block passed to listen.
subscriber.start

# Shut down the subscriber when ready to stop receiving messages.
subscriber.stop!

Skipping API calls may be used to avoid Google::Cloud::PermissionDeniedError if your account has limited access to the Pub/Sub API. In particular, the role roles/pubsub.subscriber does not have the permission pubsub.subscriptions.get, which is required to retrieve a subscription resource. See Access Control - Roles for the complete list of Pub/Sub roles and permissions.

Creating a snapshot and using seek

You can create a snapshot to retain the existing backlog on a subscription. The snapshot will hold the messages in the subscription's backlog that are unacknowledged upon the successful completion of the create_snapshot operation.

Later, you can use seek to reset the subscription's backlog to the snapshot.

(See Subscription#create_snapshot and Subscription#seek)

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new

sub = pubsub.subscription "my-topic-sub"

snapshot = sub.create_snapshot

received_messages = sub.pull immediate: false
sub.acknowledge received_messages

sub.seek snapshot

Working Across Projects

All calls to the Pub/Sub service use the same project and credentials provided to the PubSub.new method. However, it is common to reference topics or subscriptions in other projects, which can be achieved by using the project option. The main credentials must have permissions to the topics and subscriptions in other projects.

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new # my-project

# Get a topic in the current project
my_topic = pubsub.topic "my-topic"
my_topic.name #=> "projects/my-project/topics/my-topic"
# Get a topic in another project
other_topic = pubsub.topic "other-topic", project: "other-project-id"
other_topic.name #=> "projects/other-project-id/topics/other-topic"

It is possible to create a subscription in the current project that pulls from a topic in another project:

require "google/cloud/pubsub"

pubsub = Google::Cloud::PubSub.new # my-project

# Get a topic in another project
topic = pubsub.topic "other-topic", project: "other-project-id"
# Create a subscription in the current project that pulls from
# the topic in another project
sub = topic.subscribe "my-sub"
sub.name #=> "projects/my-project/subscriptions/my-sub"
sub.topic.name #=> "projects/other-project-id/topics/other-topic"

Additional information

Google Cloud Pub/Sub can be configured to use an emulator or to enable gRPC's logging. To learn more, see the Emulator guide and Logging guide.