June 6, 2021

deps.edn and monorepos III (Polylith)

Back in April, I talked about us dipping into Polylith at work in deps.edn and monorepos II, and also our planned migration away from clj-http. Since then, we've completed the migration to http-kit and we've also migrated away from clj-time (which is deprecated, because it is based on Joda Time). We've also started refactoring our subprojects into Polylith components. This is another periodic update on where we are in our journey.

While I've worked on a few new features for our online dating platform over the last month and a half, I've been fortunate enough to mostly work on eliminating "technical debt". I say "fortunate" because many companies do not formally recognize technical debt and do not value working on it. By contrast, at World Singles Networks, we are encouraged to create Jira tickets identifying any technical debt that we deliberately (or incidentally) take on and we are also encouraged to work on those tickets as part of our overall workload.

Part of that technical debt work for me, recently, was completing our migration from clj-http to http-kit and also migrating from clj-time to Java Time and starting to migrate away from date-clj (also to Java Time). Previously, it involved migrating from Cheshire to data.json.

In addition, I've been able to spend time reviewing some of our oldest code and refactoring it -- and finding out that some of it is no longer used and removing it! I have deleted several thousand lines of code over the last six weeks, both legacy non-Clojure code as well as some "legacy" Clojure code. Our Clojure codebase stretches back over a decade now and many of our early coding decisions were made while we were still learning Clojure and still figuring out reasonable approaches to architecture.

Another part of that technical debt work has been to start picking apart some of our older subprojects and figuring our better names for groups of functionality as Polylith components. Our first foray into Polylith had produced four components, one item in bases and one in projects. We now have sixteen components so I feel like I have a better handle on how Polylith is going to serve us going forward.

Per the high-level Polylith documentation, it is an architecture that focuses on simple, composable components ("bricks"). Refactoring our codebase to follow the Polylith architecture has produced the following benefits so far:

  • Naming,
  • Modularization,
  • Focusing on dependencies.


Polylith has made us think about naming more than we have before. Previously, as our monorepo has grown, code has been "organized" into very coarse-grained subprojects that either have fairly generic names (lowlevel, datamapper) or that represent entire applications (services) we build. Accordingly, these have accrued a lot of functionality over the years and some of them -- especially the more generic ones -- have become somewhat random "grab bags" of functionality.

Because Polylith favors much smaller components -- because it makes you think about individual units of reuse (a.k.a components) -- I've broken apart several of our generic subprojects and we've had discussions about meaningful names for the reusable pieces. So far, we have sixteen components that have mostly been extracted from three legacy subprojects.

Several of these new components have come from single namespaces within our old subprojects or even sometimes just a handful of functions from a single namespace. Extracting them out into standalone components has made us think more carefully about the names we should give each of them: sometimes their original namespace was reasonable but, with hindsight, several of them needed better names, especially where we've extracted just a subset of a single namespace. And by giving them better names -- often names we have now discussed as a team -- we are more likely to think before adding new functions: do they really belong in this well-named component or should they go in their own component?

This should make discoverability of functionality easier which in turn should reduce reimplementation of functionality (compared to figuring out when a function might already exist in the codebase in some more randomly-named location).


Following on from the deeper consideration of naming, Polylith components encourage you to separate out concerns. Even aside from Polylith's separation of projects (what you build and how), from bases (how you expose functionality -- APIs, command-lines, web apps), from components (the basic functionality -- domain logic etc -- of your systems), just the focus on small, well-named components means that you also tend to think more about improving the modularization of your code.

The mindfulness associated with identifying and constructing those components means that you think about narrowing your focus and teasing apart the code, decomplecting and simplifying what each "piece" does and separating it from its neighbors.

The very fact that three of our subprojects have been refactored to sixteen components speaks to the increased focus on modularization. We're very happy with that so far.

Focusing on Dependencies

Polylith components only specify the external libraries they depend on: they deliberately do not specify which other components they depend on. The reason for this is that projects assemble the "bill of materials" that goes into a finished, deployable artifact. Multiple components can implement the same (functional) interface so you can mix'n'match them, as needed, when you build an artifact.

In addition, the poly tool that supports the Polylith architecture can check that components only call into each others' interface namespace and don't try to access the implementation of any components.

This separation means that you also think carefully about any 3rd party library dependency you add to a component -- as well as considering, should you find yourself adding the same library to multiple components, whether a component should depend directly on that library or on another component that already uses that library. You tend to ask "Is this component wrapping the library? Or it is just using it an implementation detail? Is there useful commonality between the components that use this same library -- is there an abstraction I'm missing?".

When code is less modular, adding more external dependencies carries less semantic "weight": one more dependency "doesn't matter". With smaller pieces that have better names and are more modular, each new dependency becomes an important consideration.

Beyond Components

The increased focus on naming, modularization, and dependencies that Polylith has encouraged has spread beyond just "components": this new-found critical eye is being applied to our dev/test/build infrastructure as well.

Tags: clojure polylith monorepo