cqrs | opinionated CQRS and event sourcing framework | Microservice library

When reading about CQRS it is often mentioned that the write model should not depend on any read model (assuming there is one write model and up to N read models). This makes a lot of sense, especially since read models usually only become eventually consistent with the write model. Also, we should be able to change or replace read models without breaking the write model.

However, read models might contain valuable information that is aggregated across many entities of the write model. These aggregations might even contain non-trivial business rules. One can easily imagine a business policy that evaluates a piece of information that a read model possesses, and in reaction to that changes one or many entities via the write model. But where should this policy be located/implemented? Isn't this critical business logic that tightly couples information coming from one particular read model with the write model?

When I want to implement said policy without coupling the write model to the read model, I can imagine the following strategy: Include a materialized view in the write model that gets updated synchronously whenever a relevant part of the involved entities changes (when using DDD, this could be done via domain events). However, this denormalizes the write model, and is effectively a special read model embedded in the write model itself.

I can imagine that DDD purists would say that such a policy should not exist, because it represents a business invariant/rule that encompasses multiple entities (a.k.a. aggregates). I could probably agree in theory, but in practice, I often encounter such requirements anyway.

Finally, my question is simply: How do you deal with requirements that change data in reaction to certain conditions whose evaluation requires a read model?

Before posting this, I referred many sites and learning platforms but saw similar pattern of developing CQRS with event sourcing. Again to have proper events you need to follow DDD pattern. I have below questions.

1. Can we keep read and write DB in sync just by publishing event from write model and consume it at read model using event handler and updating read database
2. Why Event-Sourcing and replay of events needed if my requirement is to only see latest data
3. I can manage audit of data as and when events reaches at event handler
4. I can version messages based on timestamp in case if race condition.
5. Please explain by doing steps 1,3 and 4, am I still following CQRS pattern?

FYI, I am using .NetCore 3.1, AWS Lambda services, MassTransit as a MessageBus and SQS as a transport.

Thanks in advance.

I'm scaffolding my backend application and I want to use CQRS and rabbitmq with it (I'm pretty new to rabbitmq). For that, I have specified different vhosts for my prod and dev environments, but I'm not sure how to use exchanges and queues for command, events and query buses.

Should I use just one exchange, named for example CQRS and three different queues for commands, queries and events?

Or maybe should I use three different exchanges (named query_bus, command_bus and event_bus) and inside each one map one queue to every possible command query and event using routing keys?

Thanks!

I'm having an odd case while thinking about a solution for my problem.

A quick recap: I'm using an event store with CQRS, and i have 2 aggregates called 'Group' and 'User'.

Basically a User defines some characteristics like his region, age, and a couple of interests.

He then can choose to 'match' with a Group that is in the same region, around the same age and same interests.

Now here's the case: the 'matchmaking' part should happen completely on the backend, it can be a long running process, but for the client it's just 1 call to the endpoint and the end result should be him matching with a group.

So for this case, I have to query the groups which have the same region, the same age slice, the interests don't really matter in my query. I know have a list of groups, and the match maker is going to give each group a rating based on the common interests between the group and the user. The group with the best rating will be joined.

So again, using CQRS and ES, and my problem is that this case seems a mix between queries and a command, and mixing queries into a match command seems to go against the purpose of CQRS.

Querying multiple groups and filtering them against my write side, the event store, also is a bad idea as the aggregates have to be rebuilt and loaded in memory before being able to filter them out.

So I:m kind of stuck here, something is telling me that a long running process / saga could be an answer to my problem, but I don't see how I would still not break the mix of query and commands in my saga, as a saga is basically a chain of commands/events.

How do I tackle this specific case ? No real code is needed, a conceptual solution to get me going is perfect.

I'm in the process of learning about Event Sourcing and CQRS in distributed systems and I'm having some trouble trying to work out when is the best time to perform validation ... before, or after, the event has been stored? I've done a heap of searching and reading on the subject but I just can't seem to find an answer/suggestion that addresses this question.

For example (simple example), if I have a Web API request to withdraw some money from a bank account, I might perform the following validation:

1. Does the bank account exist?
2. Does the bank account have enough funds to withdraw?

When the request comes in, do I save the event before performing the above validation (and risk storing invalid events) or after the validation (and risk something going wrong part-way through the process, like the service going down, and not storing the event at all)? In the case of CQRS, is the event stored before the Command is executed or as part of the Command (in the Command handler)?

I can appreciate some validation would be performed before even making the request (e.g. valid amount to withdraw) but there might be a situation where some validation can't be done before making the request.

This also leads to working out how I can return an error (e.g. Bank Account is not valid) in the response of the Web API call?

My understanding of this subject may be all wrong, but as I mentioned before, I'm just learning this subject and I'm hoping someone either has an answer, or can point me to some posts/articles, that will help my understanding.

I am coding a new application usign CQRS+ES architecture with Event Store DB. In my app, I have the following streams:

• user-1
• user-2
• user-3
• ...

Each stream contains all events regarding a given user.

I am now creating a projection called user-account, which consists in basic data regarding my user's account (like first name, email, and others)

What is the optimal way to design that projection?

I should have a single projection for each user, creating projections called:

• user-account-1
• user-account-2
• user-account-3
• ...

Or a single projection for all user-accounts? Being it a key-value pair record (that may store millions of keys in the future)

I'm making a custom enumeration class along the lines of the Microsoft recommendation but struggling to make a version that supports flag-style enums.

The problem occurs when trying to bitwise or together two instances to create a new instance that doesn't exist.

As I have been able to verify, in MassTransit with Azure Service Bus, each type of object consumed by a "Consumer" generates a Topic for that type regardless of whether it is only consumed in a specific "receive endpoint" (queue). When sending a message of this type with the "Send()" method, the message is sent directly to the "receive endpoint" (queue) without going through the topic. If this same message is published with the "Publish()" method, it is published in the Topic, and is forwarded to the receive endpoint (queue) from the corresponding subscriber.

My application uses a CQRS pattern where the messages are divided into commands and events. Commands use the send-receive pattern and are therefore always dispatched in MassTransit with the "Send()" method. The events, however, are based on the publish-subscribe pattern, and therefore are always dispatched in MassTransit with the "Publish()" method. As a result, a large number of topics are created on the bus that are never used (one for each type of command), since the messages belonging to these topics are sent directly to the receiver's queue.

For all these reasons, the question I ask is whether it is possible to configure MassTransit so that it does not automatically create the topics of some types of messages consumed because they will only be sent using the "Send()" method? Does this make sense in MassTransit or is it not possible/recommended? Thank you!

Regards

Edited 16/04/2021

After doing some testing, I edit this topic to clarify that the intention is to configure MassTransit so that it does not automatically create the topics of some types of messages consumed, all of them received on the same receive endpoint. That is, the intention is to configure (dynamically if possible, through the type of object) which types of messages consumed create a topic and which do not in the same receive endpoint. Let's imagine that we have a receive endpoint (a queue) associated with a service, and this service is capable of consuming both commands and events, since the commands are only dispatched through Send(), it is not necessary to create the topic for them, however the events that are dispatched via Publish(), they need their topic (and their subscribers) to exist in order to deliver the message and be consumed.

Thanks in advance

I am following a basic tutorial for a project with CQRS and EventSourcing. In the source code of the full tutorial project there is only one configuration class to configure Axon framework for the project, the code is as follows: