I want to open a discussion on the trade offs of two approaches of the query side (within a
single bounded context):
Have a single data warehouse which contains all of the view models for the bounded context.
Have each service within the bounded context maintain their own DBs.
Like all situations, the answer to what you should do is always: “it depends”, but I wondered
what the community’s opinions are on either option.
I am dealing with a situation where: (i) Axon is not currently being used, (ii) there is no
event sourcing (things are state stored in tables), (iii) there is a mix of ways of messages
being sent (HTTP/Kafka events) between services.
I am leaning towards 1 (having a single data warehouse) for now just because it will be easier
to do with my current constraints. Also it will avoid some of the data integrity issues that
will undoubtedly occur because there is no event sourcing.
If Axon was being used and things were event sourced would people reccomend 2?
Hi, I would opt for option 3 (unless I misunderstood your option 2)…
Tradionally, systems are very focused on data. This is why we build them on top of large relational data models that validate the data according to their constraints. When introducing an event-driven (or better: message-driven) approach to those systems, it is often unfeasible to immediately switch aware from this relational paradigm. Instead, you can embrace it and start modeling certain projections differently.
And this is where my “option 3” comes in. When designing projections, I prefer not to see them as part of a service. Instead, I see them as serving information to a specific target audience. It may be that this audience is clearly defined and overlaps with that of a certain service, but it may also be very different.
I like to see them as “isolated”, just listening to events in order to build a projection that is capable of answering the specific questions this target audience has. The deployment model of this projection can be chosen very independently of the other components that exist around it. It’s a question of trade-offs.