Chapter 10. Message flow
Now that you have established a hierarchy of encapsulated modules that represent an application, you need to orchestrate them and realize the solution. The key point developed throughout the previous chapters is that modules communicate only asynchronously by passing messages. They do not directly share mutable state. You have seen many advantages of this approach along the way, enabling scalability and resilience, especially in concert with location transparency. The alternative, shared-state concurrency, is hard to get right.
There is one further advantage: basing a distributed design exclusively on messages allows you to model and visualize the business processes within your application as message flows. This helps avoid limitations to scalability or resilience early in the planning process.
The fastest way for a message to travel from Alice via Bob to Charlie is if every station along the path sends the message onward as soon as the station receives it. The only delays in this process are due to the transmission of the message between the stations and the processing of the message within each station.