Google Pub/Sub API comes with a quota of 10,000 messages per second across up to 10,000 topics. At the nucleus of the Google Cloud Pub/Sub is Publish-Subscribe Channel. It delivers a single messages published to it to multiple subscribers. One of the advantages of this pattern is that adding subscribers is side-effect free. In reality, Google Cloud Pub/Sub API implements both the Publish-Subscribe Channel and Competing Consumer patterns.

Use cases for Pub/Sub API:

• Balancing workload
• Distributing event notifications
• Logging to multiple systems
• Data streaming

The Pub/Sub implementation separates the concepts of topic and subscription, only one of them will receive a message.

• Topics - represent the pub-sub aspect of the system: if multiple subscribers subscribe to the same topic, each of them will receive a copy of the message published to the topic.
• Subscriptions represent the competing consumers aspect of the system: if multiple subscribers pull messages from the same subscription, only one of them will receive the message.

Separating topics and messages is a way to provide both Publish-Subscribe Channel and Point-to-Point Channel semantics through a single API.

• Cloud Pub/Sub is accessed through a set of REST APIs. Resource names for topics and subscriptions must follow the following format: projects/project-identifier/collection/resource-name where collection can be topics or subscription. Following this scheme, listing defined subscriptions can be as simple as making the following REST call, assuming you have authenticated via OAuth and your app is called clarapubsub:

GET https://pubsub.googleapis.com/v1beta2/projects/clarapubsub/subscriptions

The response is in JSON structure:

{ "subscriptions": [ { "name": "projects/clarapubsub/subscriptions/mysubscription", "topic": "projects/clarapubsub/topics/sometopic", "pushConfig": { }, "ackDeadlineSeconds": 60 } ] }

Channels - Messaging applications transmit data through a Message Channel, a virtual pipe that connects a sender to a receiver.

Messages - A Message is an atomic packet of data that can be transmitted on a channel. To transmit data, a message must break the data into one or more packets, wrap each packet as a message, and then send the message on a channel. A receiver application receives a message and must extract the data from the message to process it. The message system will try repeatedly to deliver the message until it succeeds.

Pipe and Filters -

Messaging - One application publishes a message to a common message channel. Other applications can read the message from the channel at a later time. The applications must agree on a channel as well as on the format of the message. The communication is asyncronous.

Remote Procedure Invocation - One application exposes some of its functionality so that it can be accessed remotely by other applications as a remote procedure. The communication occurs in real time and synchronously.

A message is transmitted in five steps:

• 1. Create - The sender creates the message and populates it with data.
• 2. Send - The sender adds the message to a channel.
• 3. Deliver - The messaging system moves the message from the sender's computer to the receiver's computer, making it available to the receiver.
• 4. Receive - The receiver reads the message from the channel.
• 5. Process - The receiver extracts the data from the message.

Sending data to another computer is more complicated and requires data to be copied from one computer to another. This means that objects have to be "serialized" - they can be converted into a simple byte stream that can be sent across the network. Messaging takes care of this conversion so that the applications do not have to worry about it.

Coupling -