So what is Google Cloud Pub/Sub? The simple definition is that it's a scalable message queuing service. Now, when I say message, I'm not talking about something like email or chat. Cloud Pub/Sub is built for communication between different applications or services. While you certainly could use Pub/Sub to build a chat application, it's really designed for keeping track of a lot of different changes, and then communicating those updates with different systems.

When I said queuing, I meant that Pub/Sub doesn't directly deliver messages. It's not like app A sends a message to app B. Instead, app A can post updates throughout the day, and Pub/Sub will store them. Later, app B can ask Pub/Sub to get a list of the most recent updates. So it works less like email and more like a blog or an internet forum. When you send messages, you don't specify any recipients. Instead, you publish messages to Pub/Sup and then anyone else interested can read them later if they wish.

Just like a blog, it supports one-to-many communication, that is one author and many recipients, but it also can support many-to-one, that is many authors and one recipient. It can support many-to-many or even one-to-one. And finally, I also mentioned that it was scalable. Now by that, I meant that it can handle a staggering amount of data. Google products, including Ads, Search and Gmail currently use Pub/Sub to send over five million messages per second. That's over one terabyte of data. And because Cloud Pub/Sub is a global service, messages are delivered to wherever your services are located with consistent latency and no manual replication required.

To summarize, Cloud Pub/Sub is designed to provide scalable, reliable, asynchronous messaging between applications. It's typically used for communicating change events between microservices. Senders can reach hundreds or thousands of recipients without having to keep track of them all. And recipients only have to receive the messages they are interested in.