How to communicate between modules (avoiding circular dependency with good architecture)

[carmona44]

Hello 👋 First of all, I want to express my gratitude for this fantastic repository. It is extremely useful.

I have encountered issues with circular dependencies multiple times while working with NestJS, which led me to seek answers here. To give an example of what I am trying to understand:

1. There are three modules: user, book, and user_book.
2. Each of these modules has its own database schema.
3. user_book is the relational table between user and book, with each record representing a book read by a user.

With this in mind, consider a website with a page displaying a list of books. If a book has been read by the logged-in user, the book should have a caption indicating that the user has already read it.

Given the architecture of this repository, how can a flag (e.g. "hasRead") be returned by a book query to indicate that a book has been read by the logged-in user?

[Sairyss]

I'm not exactly sure from your explanation how you got circular dependencies and how hasRead flag is related to circular dependencies that you get, will need more context.

how can a flag (e.g. "hasRead") be returned by a book query to indicate that a book has been read by the logged-in user

You can have one endpoint (command) to set book as read, and another one like getBooks (query) that returns a list of books where you join book and user_book and return it.

Also, book and user_book are part of the same bounded context and can be a part of a single module, I don't see much sense separating them. If you have 3 tables it doesn't mean you need 3 modules, you are just creating modules that are tightly coupled. Instead they should be self-contained.

Anyway, need more context to give a better answer.

[carmona44]

You are correct. I had originally provided an incomplete example. It is still difficult for me to think of bounded contexts, but you have correctly understood what I meant.

Now, I need to review my thoughts to see if I can come up with a concrete example. What I was trying to say is, if data from another bounded context is needed, how can they communicate with each other without having to import modules or services?

It is probably corrected by understanding the bounded context well and performing joins when other data is needed.

[Sairyss]

To communicate between modules you can use messages (commands/queries/events), as described in modules section. If you need data from other module you can just query it using a query bus. But keep in mind that querying between modules should be a last resort, usually you should design your modules as independent as possible.

Alternatively you can make one module emit an event when something happen (for example BookWasReadEvent), subscribe to this event from other modules and change the state in a db accordingly.

[carmona44]

Ok, so just to clarify, following the books example, if in the books list we need to show the name of the book's author(authors are in other BC) the best approach is to do a join query in the BC use case of books or, in the worst case, do one of the options described in modules sections?

[Sairyss]

There is no right answer. Everything depends on application that you're building.

Books example is so simple that for such kind of apps I wouldn't even bother and just do simple joins.

For apps with complex business logic and microservices I would use events and messages.

There is no single one size fits all approach.

[carmona44]

Ok, I get it.

Thank you so much for your time and your patient! 💯