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.

Gcloud's goal is to provide a API that is familiar and comfortable to Rubyists. Authentication is handled by Gcloud#pubsub. You can provide the project and credential information to connect to the Pub/Sub service, or if you are running on Google Compute Engine this configuration is taken care of for you.

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

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 Gcloud::Pubsub::Project#topic)

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub
topic = pubsub.topic "my-topic"

Creating a Topic¶ ↑

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

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub
topic = pubsub.create_topic "my-topic"

Publishing Messages¶ ↑

Messages are published to a topic. (See Gcloud::Pubsub::Topic#publish)

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

topic = pubsub.topic "my-topic"
msg = topic.publish "new-message"

Messages can also be published with attributes:

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

topic = pubsub.topic "my-topic"
msg = topic.publish "new-message",
                    foo: :bar,
                    this: :that

Multiple messages can be published at the same time by passing a block:

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

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

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 Gcloud::Pubsub::Topic#subscription)

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

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 Gcloud::Pubsub::Topic#subscribe)

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

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 "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

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

Working Across Projects¶ ↑

All calls to the Pub/Sub service use the same project and credentials provided to the Gcloud#pubsub 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 "gcloud"

gcloud = Gcloud.new # my-project-id
pubsub = gcloud.pubsub

# Get a topic in the current project
my_topic = pubsub.topic "my-topic"
my_topic.name #=> "projects/my-project-id/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 "gcloud"

gcloud = Gcloud.new # my-project-id
pubsub = gcloud.pubsub

# 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-id/subscriptions/my-sub"
sub.topic.name #=> "projects/other-project-id/topics/other-topic"

Pulling Messages¶ ↑

Messages are pulled from a Subscription. (See Gcloud::Pubsub::Subscription#pull)

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

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

A maximum number of messages returned can also be specified:

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

sub = pubsub.subscription "my-topic-sub", max: 10
msgs = sub.pull

The request for messages can also block until messages are available. (See Gcloud::Pubsub::Subscription#wait_for_messages)

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

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

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 Gcloud::Pubsub::ReceivedMessage#acknowledge!)

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

sub = pubsub.subscription "my-topic-sub"
sub.pull.each { |msg| msg.acknowledge! }

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

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

sub = pubsub.subscription "my-topic-sub"
received_messages = sub.pull
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 Gcloud::Pubsub::ReceivedMessage#delay!)

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

sub = pubsub.subscription "my-topic-sub"
received_message = sub.pull.first
if received_message
  puts received_message.message.data
  # Delay for 2 minutes
  received_message.delay! 120
end

The message can also be made available for immediate redelivery:

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

sub = pubsub.subscription "my-topic-sub"
received_message = sub.pull.first
if received_message
  puts received_message.message.data
  # Mark for redelivery by setting the deadline to now
  received_message.delay! 0
end

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

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

sub = pubsub.subscription "my-topic-sub"
received_messages = sub.pull
sub.delay 120, received_messages

Listening for Messages¶ ↑

Long running workers are easy to create with listen, which runs an infinitely blocking loop to process messages as they are received. (See Gcloud::Pubsub::Subscription#listen)

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

sub = pubsub.subscription "my-topic-sub"
sub.listen do |msg|
  # process msg
end

Messages are retrieved in batches for efficiency. The number of messages pulled per batch can be limited with the max option:

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

sub = pubsub.subscription "my-topic-sub"
sub.listen max: 20 do |msg|
  # process msg
end

When processing time and the acknowledgement deadline are a concern, messages can be automatically acknowledged as they are pulled with the autoack option:

require "gcloud"

gcloud = Gcloud.new
pubsub = gcloud.pubsub

sub = pubsub.subscription "my-topic-sub"
sub.listen autoack: true do |msg|
  # process msg
end