Building Modular Rails Applications: A Deep Dive into Rails Engines
15 comments
·July 14, 2025mrinterweb
giovapanasiti
This is exactly my experience. Most of the time people go to microservices for the wrong reason and they will regret that for years
whstl
Different sections of an app can use different databases, if the bottleneck is in the database.
Different routes can be served by different servers, if the bottleneck is in CPU usage.
Different async tasks can run on different task runner services, if the problem is tasks competing with each other.
Different test suites can run for different sections of the app, if the problem is with tests taking too long to run.
Github and others even allow specific subfolders to be "owned" by different teams.
What else is there? Even slowness of compilation and/or initialization can be alleviated, depending on the language or framework.
nurettin
I use multiple services for resilience. Example: With multiple services that have clear separation of concerns, you can debug and fix your processing layer without stopping the collection layer. You can update a distributor while workers wait and vice versa. This way I never have downtime anxiety. No regrets.
zrail
Separation of services is orthogonal to separation of concerns. There's nothing stopping you from having multiple entry points into the same monolith. I.e. web servers run `puma` and workers run `sidekiq` but both are running the same codebase. This is, in fact, the way that every production Rails app that I've worked with is structured in terms of services.
Concerns (in the broad sense, not ActiveSupport::Concern) can be separated any number of ways. The important part is delineating and formalizing the boundaries between them. For example, a worker running in Puma might instantiate and call three or four or a dozen different service objects all within different engines to accomplish what it needs, but all of that runs in the same Sidekiq thread.
Inserting HTTP or gRPC requests between layers might enforce clean logical boundaries but often what you end up with is a distributed ball of mud that is harder to reason about than a single codebase.
helle253
I love Rails Engines, it's a very slick feature.
I recently migrated a featureset from one Rails project into another, as a mounted engine, and ensuring isolation (but not requiring it!) has been tremendously helpful.
capevace
The Filament package for Laravel lets you build similarly encapsulated „plugins“, that are basically mini Laravel apps, that can be easily added to existing apps.
The plugins can rely on all of the Laravelisms (auth, storage etc) and Filament allows them to easily draw app/admin UI.
hk1337
I have been looking at using Rails Engines recently playing around with trying to get an idea off the ground.
GGO
Rails engines are one of the most underrated features that everyone should be using more.
henning
This blog post just shows how libraries and frameworks often solve one problem but create another. This leads to the emission of ridiculous sentences like `One of the trickiest aspects of building engines is handling routing correctly` which would be a non-issue if you just wrote simple code to solve the problem in front of you instead of doing a bunch of "modular" "engine" framework-y compiler-y nonsense that adds boatloads of complexity just to accomplish one basic thing like handling file uploads.
adenta
What problem was created?
pqdbr
"one basic thing like handling file uploads" - say no more.
Actually, the article isn't even about handling file uploads - it's about deliberately creating a modular admin panel for dealing with file uploads.
It's not modularity for "framework-y" sake, but to easily deploy that admin panel in other applications with literally a one-liner.
giovapanasiti
I couldn't have written this comment better myself. Thank you this is exactly the point
One of the reasons microservice architecture originally became popular was to break apart monolithic applications. In many cases, I bet a big driver was a lack of separation of concerns, and a more modular design was desired. There are many ways to put up walls in software to help make software more modular and self-contained. Rails engines are a good way to make more a rails app more modular. The number of times I've seen microservices created for the purpose of modularity (not scaling concerns), and the complexity that has brought has really soured me on microservices.