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Open Sourcing our API Template

We'd like to help the community and share our best practices when coding an API in Scala.

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July 18th, 2021
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The Conduktor Engineers team

The Conduktor developer team is composed of a group of experienced and curious professionals. Scala features/enables a lot of characteristics we value, so that's the language we chose. Our DNA is our Kafka expertise and we believe that modern software architectures have to be built around streaming.

Functional Programming

We believe that FP is the best way to write robust software without surprises. Our team has the required knowledge to use this paradigm, and we of course want our applications to leverage FP.

Type Safety

We love Scala for its very good type system: It allows us to write very expressive code that is ready for changes and only needs limited amounts of testing to ensure it behaves as expected. Not convinced? Watch "Types vs Tests" by Julien Truffaut

We utilize the full potential of the Scala type system by defining value objects and banning primitive types as much as possible.

Domain-Driven Design

We like what DDD and its ecosystem brought to our field: Ubiquitous Language, Entity/Value Object dichotomy, Hexagonal Architecture, etc. We try to use these concepts whenever it makes sense for us, with support from our tools.


We are serious about software. We never want to rush to fix production bugs or give our customers buggy products so we do write extensive test suites.

Our applications go through tests at various layers:

  • domain logic tests

  • repository tests

  • APIs tests

  • various combinations of integration tests

  • end-to-end tests, from API calls to the database

We get a strong and fast test suite by writing tests respecting the famous pyramid of test principles.


As libs or services consumers, we like well-written documentation. Since we provide libs and services ourselves we always want to ensure the right level of information for our users.

Technologies we like

Now that we have gone through our principles, let's talk about how we reach these goals using technologies.

Effect System, FP, and type safety: ZIO to the rescue

We believe that ZIO is the best solution to write pure FP in Scala nowadays. At least it's what we are the most comfortable with.

To fulfill other requirements, we usually consider only libraries that fit well with ZIO whether they're ZIO native ones or have ZIO integrations.

Data persistence: welcome back SQL

During the NoSQL trend, SQL implementations got better and we learned that NoSQL databases drive development costs higher, thus more and more people are considering SQL databases again by default. So do we. This doesn't mean that we dislike NoSQL but to be honest, for most projects, a good old Postgres server will make your life so much easier.

However, performance is still a concern, as we don't want to use a blocking JDBC driver. We decided to go with Rob Norris's skunk library for that matter.

Finally, to manage our database schema, we picked flyway.

RESTful API: Code First

Everyone wants nice OpenAPI documentation but there are a lot of ways to build it. A very strong argument for using Scala and typing everything is: the code and the doc are actually so close that the generation from the code is really decent and efforts done to have good documentation also enhance the code directly.

It's why we decided to use tapir. Of course, it's also well integrated with ZIO, which makes our life easier.

The toolbox

There's always a bunch of tools that solve well-defined problems like JSON serialization, creation of zero-cost value objects, etc.

Here is our shopping list:

Why we built this template

Why build a template instead of just creating our project straight away? Well, there are many reasons.

Reaching a Team consensus

We all have different experiences and tastes. People could certainly prefer different sets of technologies or choose different test strategies. It's the kind of topic that is hard to debate while implementing a product. Why we decided to discuss these choices on a sample project that would be as small as possible while incorporating every aspect of our real-world products.

With this strategy, It was easy to benchmark two solutions by creating a PR on the project and reaching a team consensus.

Initial setup is often overlooked

As much as we try to pick ZIO-friendly libs, the cost of putting everything together and having a working prototype is far from small. We found that dedicating a project to concrete tasks is less stressful to manage than paying the price during the implementation of a feature.

We are growing fast

As the company is growing really fast and will continue that way, it's important to spread knowledge into the existing team and to make the knowledge easily accessible for new hires.

It's easier to learn from a project with a handful of classes with a trivial domain than having to learn a new domain and all the new technologies at the same time. Think of this template as guidelines defining how we code at Conduktor.

Many projects

We are not microservices fans, we will keep our domains in a modular monolith in a monorepo as long as it's doable.

But at some point, we'll need to have several projects with their own lifecycle.

This template will be kept up-to-date with our practices and technologies to ensure that new projects can start with the same strategies.

Experiments are cheaper on a small codebase

As time passes, we will want to update libs, replace them or include new ones. We will probably also want to migrate from one Scala version to another. Trying these changes on a small codebase using the same techs as our production apps will be a huge benefit. The cost involved in a proof-of-concept will be quite small, this will be our sandbox to try new things.

Be useful to our Scala community and get feedback

All this is hard work. To be honest, we read a lot of community template projects and code snippets to build this template, your hard work. We want to give back to the community, and we hope we have added value on some topics and that the result is not yet available as-is in the ecosystem.

Maybe it will be useful to others. Maybe it can inspire people. And maybe people will hate what we did, and it's ok too.

Whatever you want to say, we are eager to listen to your feedback because we want to learn from the community.


That's one of the best sections of every documentation: after reading all the marketing stuff, all good software developers go straight to the limitation section, right?

Here is a list of things you need to know about this template:

  • Postgres won't scale infinitely, don't use it for your 1M req/s project

  • It implements neither CQRS nor Event Sourcing: these patterns are awesome but require a lot of very specific knowledge that is rare in the industry

  • Skunk, our Postgres access library is still very young

  • We like Kafka but we don't include Kafka support in this template: we will add Kafka support in the future, we are just not there yet

  • There are no metrics yet, so not very ready for production


The code is here, if you consider writing a Scala application and you share some of our principles, it's probably a good idea to read it.

If you liked what you just read, maybe you will want to join us?

We aim to accelerate Kafka projects delivery by making developers and organizations more efficient with Kafka.