Getting Started with Modular Monoliths in .NET

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A modular monolith is a catchy name for a monolith system built from a few bounded contexts (modules) and following a set of principles to control coupling. Each module contains a cohesive set of functionalities and is isolated from other modules in the system. The isolation refers to database dependencies and inter-module communication. In this video, I'll explain the fundamentals of this architectural approach. You will also see a practical Modular Monolith implementation in .NET.
Check out my courses: bit.ly/3PupkOJ
What Is a Modular Monolith?
www.milanjovanovic.tech/blog/...
Monolith to Microservices: How a Modular Monolith Helps
www.milanjovanovic.tech/blog/...
Monolith to Microservices: How a Modular Monolith Helps:
www.milanjovanovic.tech/blog/...
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Chapters
0:00 What is a Modular Monolith?
1:28 Some architecture diagrams
3:45 Modular Monolith diagram
4:46 How to build a Modular Monolith
5:47 Module communication
8:11 Module data isolation
10:19 Demo: Modular Monolith in .NET

[@MilanJovanovicTech]

💡You can learn more about Modular Monolith Architecture here: bit.ly/3SXlzSt

[@davidpccode]

I took you CleanArchitecture course....it's the most clear and consise i've ever seen. Thank you Millan you are such a nice and professional person. Thank you very much

[@MilanJovanovicTech]

Wow, thank you! This course should be a very nice extension, but much more advanced.

[@theincredibleillmo9385]

Waiting for this course no matter what the clowns and haters say. Much love and mad respect from 🇲🇽 Mexico we! ❤❤❤

[@MilanJovanovicTech]

Apprecaite it, and returning the love back! 😁

[@user-rx2jr2nb8r]

Thank you for sharing this excellent video! It's clear, concise, and well done. Keep up the great work!

[@MilanJovanovicTech]

Glad it was helpful!

[@thiagomatu]

Looking forward for this next course Milan

[@MilanJovanovicTech]

About a ~month out from now, lots of work ahead

[@qamislo9740]

​@@MilanJovanovicTech Please hurry up with the course😊😊

[@dcuccia]

Should be a great course based on all the content presented so far. Looking forward to it.

[@MilanJovanovicTech]

Hope so! Starting work on it today 🤞😁

[@TechnicalVibs]

wonderfully explained in a short way

[@MilanJovanovicTech]

Glad you liked it!

[@saddamhossaindotnet]

Great video, my friend 🧡

[@MilanJovanovicTech]

Thank you 🤗

[@datadigger2432]

You are the best of the best and do not listen to silly comments Keep going ❤️❤️

[@MilanJovanovicTech]

Had to set the record straight 😁

[@datadigger2432]

Great video milan thanks

[@MilanJovanovicTech]

Glad you enjoyed it :)

[@supremedev4851]

Great video. Thanks 👍

[@MilanJovanovicTech]

Thank you very much! :)

[@EugeneZavalko]

Great explanation

[@MilanJovanovicTech]

Glad you liked it

[@umeshyeware1409]

Hi Milan, this is really cool implementation. Always love watching your videos. Just one help if you can explain how you are implementing InstallModule and InstallServices method with help of assembly loading in detail in your future video or shorts, it would be great help.

[@MilanJovanovicTech]

I talked about the general idea here: kzbin.info/www/bejne/qnyod2mulsqin8k You can see the IServiceInstaller concept. The IModuleInstaller is the same thing, just implemented for each module.

[@umeshyeware1409]

@@MilanJovanovicTech Thank you for quick reply

[@Kasiux]

Milan, the best .NET/Tech KZbinr

[@MilanJovanovicTech]

Thanks a lot, glad you think so 😁

[@pedrosilva1437]

This is good introduction to modular monoliths and interested in seeing more content about this. However, I'm not sure about the added complexity of using a message broker. It makes the modular monolith more complicated, and seems to only be beneficial if you're hedging you bets on moving to microservices later.

[@MilanJovanovicTech]

Yes, I agree there's a tradeoff to be made here

[@marklnz]

Thanks for the concise explanation. I'm even more convinced now that modular monoliths have no real use case, unfortunately. But thanks for confirming that!

[@MilanJovanovicTech]

Dumbest take ever 😂

[@marklnz]

@@MilanJovanovicTech You're entitled to your opinion, but I'm sticking to mine - modular monoliths are nonsense. Either break it up or stick with it, don't bother half-assing it, it's only going to make everyone's life a misery if you do.

[@raycarlbrown-amory3509]

fantastic video

[@MilanJovanovicTech]

Thanks a lot! :)

[@yaminnather521]

How do you do dependency injection for classes that are the same in both the modules? For example, I have an Outbox class defined in a project that two modules share. But since you can't register two objects of the same type in Dependency Injection, i have to create wrapper classes for each modules, like ModuleAOutbox and ModuleBOutbox

[@MilanJovanovicTech]

They may have the same name, but if the namespaces is different it's not the same type. It's actually something I will touch on in a future video, where I will implement the Outbox pattern like that.

[@yaminnather521]

​@@MilanJovanovicTech That's true, but I'm sharing using an Outbox from a common package so I don't have to implement it per package, I can't register an Outbox per module since the namespace will be the same right? Is what I'm doing already the only way(creating wrappers per module)? It's very annoying do it like that lol. And there's another thing I'm confused about if you could spare some time, do you build all objects and their Dependencies in the Composition Root? I was considering another alternative since the Composition Root was getting too messy. Is using Inversion of Control and then constructing the object using a Factory which provides all the non dynamic parameters, and dynamic parameters as parameters in the build fuction, a nice approach? In the Composition Root I would then mostly pick the implementations I need for the abstractions. This way the smaller stuff like, for example, Mappers(for mapping between Domain and Database Mpdel) of a Repository dont' have to be added in the Composition Root, making it less cluttered imo. Any advice that you can give on these. Sorry for making you read this big block of text if you did read it and thanks :)

[@MilanJovanovicTech]

@@yaminnather521 I usually implement an extension method that configures DI per module. And then register all the modules from the composition root. The reason for this is decoupling the modules, so I can easily pull out one module and move it into a separate application. Does that make sense? It's quite common to duplicate stuff between modules, if you want to make them decoupled and independent.

[@AkosLukacs42]

​@@yaminnather521if you are calling the exact same DI registration code from two places, most likely you could just replace AddXY calls with TryAddXy, and should work, since the latter won't register it second time. Of course this only applies if the dependency registration is the same.

[@yaminnather521]

@@MilanJovanovicTech Yeah that makes sense. Thanks.

[@alexandrehando]

Excellent video! We are migrating our legacy application exactly in this context, how would you suggest to implement/adapt some high coupling entities, such as User, in new features or legacy features that are being migrated? Thanks!

[@MilanJovanovicTech]

Coupled stuff goes together, check my longer videos on Modular Monoliths.

[@HomeSlize]

Thanks for the content! I'm interested in your Modular Monolith course but can you share some details on the extra 2 hours of video content and the extra 7 lessons in the MMA + Community option? Thanks!

[@MilanJovanovicTech]

The 7 lessons are from the bonus course. It's a condensed version of the main one, from a slightly different angle.

[@alexandrehando]

Hey Milan, how would you suggest doing complex filtering data between modules? Like User has Teams, but in Company (another module) has only team and my filter would be Company that has X teams. Thanks!

[@MilanJovanovicTech]

Implement that functionality in a single module?

[@user-jk8ii8mt2y]

At some point is the artificial separations within the modular monolith more harmful than beneficial on the performance side? For instance, let’s say I am making 4 internal api calls to get the data from the necessary modules for a single external call from the ui. At what point should I just scrap it, write a view with a few joins, and get the data in one call to the db?

[@MilanJovanovicTech]

That's a pretty good signal that your module boundaries are probably wrong. You shouldn't have to talk to other modules so much that it impacts your performance negatively. Let's do a thought experiment. If that was a microservices system, what would you do?

[@alejomillo7744]

The EF Core feature of lazy loading and retrieiving the relations as part of the query is lost if using this modular monolith architecture right? Since from my users module for example I don't have access to the Orders entity neither does the database, right?

[@MilanJovanovicTech]

Yes

[@grzegorzr3621]

In Different Schema, how to check who placed the order from the orders level if the usernames are in users.*? Can I use users.* tables in the Orders module to take advantage of the advantages of a relational database?

[@iamprovidence-xj5wf]

I guess the same way you would do it with microservices. create users table in orders.* schema or access it through api of users module

[@MilanJovanovicTech]

I'll explain how in a future video. I recommend reading this in the meantime: www.milanjovanovic.tech/blog/modular-monolith-communication-patterns TL;DR 1) Query the Users module to get the data. This should be done through a public API 2) Duplicate the data in both modules somehow

[@grzegorzr3621]

ad1 What if I return a list of orders with different users? Should I ask about each one separately? ad2 Duplicate the data: and there is additional work to maintain current data between modules. I'm thinking that I want more information than just username when ordering. E.g. avatar, account creation date, etc. In the case of database queries, this is very simple. Does this separation introduce too much complexity?

[@MilanJovanovicTech]

@@grzegorzr3621 Which of the two would you choose in a microservices system?

[@grzegorzr3621]

​@@MilanJovanovicTech I don't know. I don't know about this.

[@PinoyDevAko]

Hi Milan. Excited for your upcoming course. One little thing, can kindly find a way to make your "video player" fluid or a bit responsive? I maybe one of those few who finds your video player annoying because it is fixed to a maximum size. I bought other courses and they have quite similar player but their player is not fixed to maximum size.

[@MilanJovanovicTech]

It's a limitation of Teachable (the platform I'm using), so not much I can do about it :/

[@PinoyDevAko]

I see@@MilanJovanovicTech . So not an option to change the platform? :) Have you tried watching videos that has a small player and then you have to maximize the whole browser, not allowing you to multi-task or even code-along with you? :) I hope you understand.

[@MilanJovanovicTech]

@@PinoyDevAko Not an option to switch platforms. There is an option to see how I can increase the default size. It's something I'm looking into.

[@PinoyDevAko]

Thank you @@MilanJovanovicTech . Again you're wonderful and one of the best out there.

[@GuillermoValenzuela-Dev]

Excellent video Milan. One question here, with modular monolith would we lose the possibility of scaling each module according to demand?

[@MilanJovanovicTech]

You never had the ability for granular scalability with any monolith application. It's all or nothing.

[@KingOfBlades27]

When I create Azure function app that uses injected services does that count as modular monolith? Services are registered in start up and then used through interfaces which is a bit different than using api. But idea seems to be pretty much the same 🤔

[@MilanJovanovicTech]

I wouldn't consider that a modular monolith

[@nanvlad]

Is message broker running outside of the process of monolith app? Or is it just a logical module to communicate between layers like MediatR?

[@MilanJovanovicTech]

In this case, it's in-process behind a MassTransit abstraction. But because of this, I can just add a RabbitMQ instance, update MassTransit to connect to it, and all else works the same

[@marko95g]

hi, I have a question regarding message broker, how one module fetch data from another module? for example, order module requires user data, so it will send a request to message broker, and what should happen next? if I understand this, at the end, order module must wait for user data in order to continue execution, right? so basically, order module will use "sync" way to solve this problem?

[@MilanJovanovicTech]

Request-response is sync, however way you look at it. Async would imply processing messages outside of the normal request flow. You'd process a 'UserRegistered' event, and store that info in the other module as a local copy. Then, you can query from the local copy when you need user data.

[@marko95g]

@@MilanJovanovicTech this can be very very complex..thanks!

[@datadigger2432]

Can you please milan make a video about using grpc for microservices

[@MilanJovanovicTech]

Will do, at some point :)

[@cartongessow10]

what tool did you use for diagrams?

[@MilanJovanovicTech]

Excalidraw for the "shiny ones" and draw.io for the C4 diagrams (but you can do C4 diagrams in other tools)

[@DanielCipra]

What is up with the Bind and Tap methods? Is that from some nuget package?

[@MilanJovanovicTech]

Sorry 'bout that 😅 I was applying ROP on this project.

[@erradil]

Great thanks. could have access to the demo repository

[@MilanJovanovicTech]

It's shared on Patreon

[@bobek8030]

How you separate di containers for modules?

[@MilanJovanovicTech]

You don't necessarily have to. And the built-in DI doesn't support that. You have to use something like Autofac.

[@dannym817]

Can we see somewhere all code? Like in github?

[@MilanJovanovicTech]

I share all the source code on Patreon, and for some videos you can find it in the description

[@phw1009]

Master..... I will join the modular side of the force...

[@MilanJovanovicTech]

Come to the, erm..., modular side? 😂 lol

[@hugobarroca9538]

Good explanations and clear graphics! Good work on the video! 🙂

[@MilanJovanovicTech]

Thank you! 😃

[@Cornet435]

How can one module fetch data from another?

[@MilanJovanovicTech]

Read this: www.milanjovanovic.tech/blog/modular-monolith-communication-patterns I'll release a few more videos soon.

[@user-xm7sh3vw8o]

love

[@MilanJovanovicTech]

Thanks!

[@vincentcifello4435]

The "Tight Coupling" slide depicts interactions within a monolith. These are classes calling each other. The arrows going back and forth show the coupling. Ok, fine. The next slide "Method Calls" shows Modules calling each other with a "clearly defined public interface". This presumably deals with the "Tight Coupling" previously shown. The problem, of course, is that these are exactly the same. Classes are modules, by definition. Classes have a clearly defined public interface. Naming these objects "Modules" does nothing to solve the problem. Presumably, the modules are calling each other in order to get data that is "isolated" (or not) in the other module so that the calling module can make some decision and perform an action. That means that the boundaries are incorrect and the modules are not really modules because they are not autonomous- ie they can not make independent decisions and can not be changed in isolation. I would also point out that temporal decoupling, while important from a performance and systems view, doesn't really decouple anything. If you are using events to propagate data between these modules, then that is still tight coupling. It is just asynchronous. Temporal coupling can not exist unless there is an actual form of coupling that underlies it. That is the error.

[@MilanJovanovicTech]

"Tight coupling" slide is supposed to show a 'mess' where any component can call any other component. Which introduces a lot of other problems I'm sure you're well aware of. The "Method calls" slide simply argues that instead of allowing components to freely call each other, we define a specific contract for how they communicate. The coupling is still there, but it's clearly defined and only in one place. This makes it much easier to control, expand, and manage second-level effects. I'm also unsure if you're confused by what I refer to as a Module. A module is not one class/object. A module is a logical concept inside the monolith, grouping together many components that provide the module's functionalities.

[@tuberklz]

would be nice to mingle with the example source code

[@MilanJovanovicTech]

Shared it on Patreon

[@tuberklz]

@@MilanJovanovicTech thanks

[@KarthikS30712]

Modular monoliths: Sounds like the industry's desparate cry to make developers do better OOPS and DI, PLUS relevant databases for different data models.

[@MilanJovanovicTech]

Desperate cry? We've been building modular monoliths way longer than we've been building microservices

[@tempusmagia486]

Are you stealing dometrain content? Or why is it that when they upload a course a few weeks later you upload the same topic. Is not blaming because I haven't seen this one and clean architecture with cqrs is pretty common. It is just too many coincidences

[@danilousuga410]

He had said some months ago that he would talk about this topic in the future. So I mean he had planned this video before that course you've pointed out.

[@yaminnather521]

Who cares though? Content creators of a similar type of content are obviously going to make videos on topics trending in their domain, right? They don't care if someone else covered a similar topic already, and why should they? It's not a big deal. The thing we should be worried about imo is whether they are making good content or not, and covering the topics well. But if the videos are the same and not just the idea of the video, then I wouldn't like it ig.

[@MilanJovanovicTech]

When throwing out serious accusations like that, I expect you to do your due diligence (which I'm sure you did, of course, why else would you accuse me of stealing without proof). So, here are a few references in case you missed them (I produce a lot of content, so I know it's easy to miss some). Here's an online talk about Modular Monoliths I held on Jan 31, 2023: kzbin.info/www/bejne/j3jFgGyaibt4bbs Here's another in-person talk I held in Norway (you'll never guess the topic) on Apr 26, 2023: kzbin.info/www/bejne/aGS0nX-tecaCe80 I have been writing about Modular Monoliths on my blog for months: - Modular Monolith Communication Patterns (August 5th, 2023): www.milanjovanovic.tech/blog/modular-monolith-communication-patterns - Monolith to Microservices: How a Modular Monolith Helps (September 23rd, 2023): www.milanjovanovic.tech/blog/monolith-to-microservices-how-a-modular-monolith-helps - Modular Monolith Data Isolation (December 9th, 2023): www.milanjovanovic.tech/blog/modular-monolith-data-isolation I've also been teasing a Modular Monolith course for months, so I have nothing to hide. Second, if you accuse someone of "stealing content" based on recency of releasing something: - My Clean Architecture course came out in August 2023 - Dometrain released two courses later that year So, who's copying who? I'm sure you'll agree this is a silly argument. I'm certainly not saying they're copying me, so why should the reverse hold true. I have a lot of practical experience using CA or MM, and I want to share my perspective. I'm confident the course will be very different from dometrain one. Third, and most important, it's a free market. If I want to release a Modular Monolith course, I can. Let the market decide which one is more valuable. Stay awesome. :)

[@flygonfiasco9751]

That’s a pretty serious accusation. Modular monoliths is a popular architecture right now. There’s going to be more than one content creator covering it.

[@tempusmagia486]

@@MilanJovanovicTech that's what I tried to say, I'm not blaming. I'm more like asking, maybe my words are not too clear, otherwise I would be directly reporting. I also pointed the fact that that there's no evidence