In this chapter, we’re going to start with a fairly uncontroversial insight:
reads (queries) and writes (commands) are different, so they
should be treated differently (or have their responsibilities segregated, if you will). Then we’re going to push that insight as far
as we can.
If you’re anything like Harry, this will all seem extreme at first,
but hopefully we can make the argument that it’s not totally unreasonable.
The code for this chapter is in the
chapter_12_cqrs branch on GitHub.
git clone https://github.com/cosmicpython/code.git
cd code
git checkout chapter_12_cqrs
# or to code along, checkout the previous chapter:
git checkout chapter_11_external_events
First, though, why bother?
Figure 12-1. Separating reads from writes
Domain Models Are for Writing
We’ve spent a lot of time in this book talking about how to build software that
enforces the rules of our domain. These rules, or constraints, will be different
for every application, and they make up the interesting core of our systems.
In this book, we’ve set explicit constraints like “You can’t allocate more stock
than is available,” as well as implicit constraints like “Each order line is
allocated to a single batch.”
We wrote down these rules as unit tests at the beginning of the book:
Our basic domain tests ...
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