What is Pub/Sub?

Google Pub/Sub is a messaging service that uses the publish-subscribe pattern. Publishers and subscribers communicate asynchronously, with the Pub/Sub service handling all message storage and delivery. Publishers send messages to a topic, and Pub/Sub immediately delivers them to all attached subscriptions. Subscriptions can be configured to either push messages to a client automatically or to wait for a client to make a pull request. The owner of the topic sets the access rights that determine who is allowed to attach a subscription. Messages published to a topic prior to a subscription being created will not be available to the subscriber. By default, Pub/Sub messages are not ordered.