Skip to main content
Ben Nadel at CFUNITED 2010 (Landsdown, VA) with: Tim Cunningham
Ben Nadel at CFUNITED 2010 (Landsdown, VA) with: Tim Cunningham ( @TimCunningham71 )

Layla Porter On Modular Monolith Boundaries

By on

Yesterday, I listened to episode 1,856 of the dotnet rocks podcast with guest Layla Porter, Developer Advocate at VMWare. The show discussed "Modular Monoliths"; and, was one of those exciting moments in which I learned that the mental model I've constructed about an idea is completely wrong. That is to say, a "modular monolith" is not at all what I thought it was.

When I first heard the term "modular monolith", I assumed that it simply meant a monolithic codebase with well-defined boundaries. And, more to the point, I assumed that those boundaries were drawn around domain models. As such, I figured that a modular monolith would have separate modules for "Users" and "Products" and "Invoices" and "Preferences" and so on.

To be clear, I had no reason to assume any of this - it's just how my brain decided to fill-in my gaps in understanding.

On the podcast, Layla did not discuss modularity in terms of "features" or "domain models" (the way I had initially considered it). Instead, she discussed it in terms of "synchronicity": which areas of the application had to block-and-wait in order to produce the response for the user; and, which areas of the application could take place asynchronously without creating a poor user experience (UX).

The way I interpreted this is that everything required to construct the user response should be in the same module. That is, everything with a synchronous dependency should be grouped together. The only code that can reside in "other modules" is the code that can be executed in the background at a later time.

ASIDE: Layla also advocated for using some form of "messaging" to communicate between modules, even they are located within the same monolith.

When I reflect back on previous conversations about "modular monoliths", this new understanding makes so much more sense. Consider the concept of evolving a modular monolith into a distributed microservices architecture: If different modules were synchronously dependent, breaking them apart would lead to synchronously dependent microservices. Which — we now know after years of fumbling through distributed systems — is really just a distributed monolith.

Which is, of course, the worst of all possible outcomes: all the complexity of a distributed system architecture combined with all the complexity of a monolith.

If, on the other hand, monolithic module boundaries are draw around synchronicity constraints, then splitting a modular monolith into a distributed services architecture would have no real bearing on how the application operated. Asynchronous communication would remain asynchronous; and, the user's experience would never degrade (since all synchronous dependencies were still collocated within the same module / service).

This isn't the first time I've been totally misguided by my own assumptions. And, I'm sure it won't be my last. But, at least I now have a more clear understanding of what a modular monolith is. Which, in turn, helps me better understand what a beneficial microservices architecture could look like. Thanks Layla Porter!

Epilogue on Choosing Microservices

On the podcast, Layla talked about helping people understand whether or not a microservices architecture would make sense for them. She summed it up as follows:

If you can't answer YES right away, then the answer is No.

I like it!

Reader Comments


If you can't answer YES right away, then the answer is No.

In other words — If it's not a "HELL YES" then it's a "HELL NAH"

We're migrating to a microservice-oriented-architecture and so far, I am not a fan. The complexity just skyrocketed. Troubleshooting is more difficult. Lots of finger pointing between the various domain owners. Maybe we're just doing it wrong, but this is a "Hell Nah"



Yeah, we've had a very similar experience. Progress took a massive hit as did the ability to see and debug issues. It's one of my big regrets. I wish you luck! 👍

Post A Comment — I'd Love To Hear From You!

Post a Comment

I believe in love. I believe in compassion. I believe in human rights. I believe that we can afford to give more of these gifts to the world around us because it costs us nothing to be decent and kind and understanding. And, I want you to know that when you land on this site, you are accepted for who you are, no matter how you identify, what truths you live, or whatever kind of goofy shit makes you feel alive! Rock on with your bad self!
Ben Nadel