Foundations of Scalable Systems

Chapter 7. Asynchronous Messaging

Inevitably for a distributed systems book, I’ve spent a fair bit of time in the preceding chapters discussing communications issues. Communication is fundamental to distributed systems, and it is a major issue that architects need to incorporate into their system designs.

So far, these discussions have assumed a synchronous messaging style. A client sends a response and waits for a server to respond. This is how most distributed communications are designed to occur, as the client requires an instantaneous response to proceed.

Not all systems have this requirement. For example, when I return some goods I’ve purchased online, I take them to my local UPS or FedEx store. They scan my QR code, and I give them the package to process. I do not then wait in the store for confirmation that the product has been successfully received by the vendor and my payment returned. That would be dull and unproductive. I trust the shipping service to deliver my unwanted goods to the vendor and expect to get a message a few days later when it has been processed.

We can design our distributed systems to emulate this behavior. Using an asynchronous communications style, clients, known as producers, send their requests to an intermediary messaging service. This acts as a delivery mechanism to relay the request to the intended destination, known as the consumer, for processing. Producers “fire and forget” the requests they send. Once a request is delivered to the messaging ...