relaxdiego (Mark Maglana's Technical Blog)

A Case for Monorepos

Jul 2, 2023
Est. read: 3 minutes

For some time now, we’ve been living in the era of microservices. An era where a strict separation of concerns is the norm. Where it is difficult to circumvent the rule by way of putting each microservice in its own repository and execution environment. This has, in many ways, permitted us to be more thoughtful about our system designs, allowing us to avoid the “big ball of mud” problem to a significant extent. However, this era of isolation has also presented a unique set of challenges, specifically in the aspect of code reuse.

Code Re-use Across Microservices

Conventional wisdom recommends the use of libraries in addressing the issue of code reuse. Libraries shine when the aim is to encourage code reuse across diverse groups or teams that have little interaction with each other. However, libraries that are exclusively used by the same team can be more of an unnecessary burden and, oftentimes, ineffective method of reducing code duplication.

Because when you think about it in that context, libraries are just “code copypasta” with extra steps. Bear with me for a moment:

Code reuse via a library

  1. Write common code in the library
  2. Commit the change
  3. Push to the repo
  4. Wait for CI/CD pipeline
  5. Update the version dependency in consuming microservice(s)
  6. Import the module
  7. Use the module/class/method

Compare that with:

Code reuse via copypasta

  1. Copy code from microservice A
  2. Paste code to microservice B

No wonder copypasta is such an alluring, hard habit to break!

“But, Mark!” you might exclaim, “What about code maintenance? Wouldn’t libraries make that easier because you only have to change one place?”

That’s true, but even then you’d still have to go through the same 7 steps to distribute that change across your microservices. In some cases that won’t even happen just once for every fix/update. Sometimes, you’ll introduce a change that breaks one microservice so you’ll likely have to go through the dreaded 7 steps–maybe multiple times–until all is well.

“You just need discipline,” you might respond. Maybe, but when you’re thinking of multiple aspects of the code–more than likely, even in the case of microservices–dealing with the 7 steps just to get your code to run becomes the least of your concerns. So the path of least resistance wins and you just promise yourself that you’ll move the code to the library as soon as you have time to spare.

def foo():
    # TODO: Put this in the library (yeah, right!)

Monorepo For Microservices

Using a monorepo project structure for a microservices deployment architecture can address this problem. By locating each microservice’s directoryalongside each other in the same repo, you get the benefits of code reusability without the arduous 7 steps.

├── Makefile
├── common
│   ├── pyproject.toml
│   └── ...
├── microservice_a
│   ├── pyproject.toml
│   └── ...
├── microservice_b
│   ├── pyproject.toml
│   └── ...
└── microservice_c
    ├── pyproject.toml
    └── ...

In this setup, you could have a top-level Makefile that takes care of installing each microservice and the common library in editable mode in your local machine so that changing the code is just a matter of saving and then running the tests. Then, once you are happy with your changes, just push it upstream and the CI/CD pipeline can test and deploy each microservice and the common code.

In conclusion, the microservice era, despite its challenges, continues to evolve. Traditional methods of code reuse, such as libraries, have proven cumbersome within the same team, leading to the exploration of other approaches like monorepos. Monorepos, with their streamlined workflow and simplified code maintenance, represent a promising solution that leverages the benefits of code reusability without the toil of the traditional steps. As we venture further into this era, embracing such innovative strategies will help us to continue reaping the benefits of microservices while effectively managing the complexities they bring.