12 Sep 2025
JavaZone 2025 report
This is a summary of the JavaZone 2025 conference I attended in early September. It’s a conference focused on developers that I enjoyed and highly recommend.
Articles tagged with
kotlin
27 May 2025
How to call a REST API in integration tests
Have you ever struggled to identify which REST API is being tested in your integration tests? In this article, you’ll learn a clean and readable way to call REST APIs within your integration tests. The goal is to make the WHEN section of the test clearly show which API is called and in what context, while hiding all technical details.
04 Jun 2024
REST service client: design, testing, monitoring
The purpose of this article is to present how to design, test, and monitor a REST service client. The article includes a repository with clients written in Kotlin using various technologies such as WebClient, RestClient, Ktor Client, Retrofit. It demonstrates how to send and retrieve data from an external service, add a cache layer, and parse the received response into domain objects.
14 Sep 2023
Online MongoDB migration
MongoDB is the most popular database used at Allegro. We have hundreds of MongoDB databases running on our on—premise servers. In 2022 we decided that we need to migrate all our MongoDB databases from existing shared clusters to new MongoDB clusters hosted on Kubernetes pods with separated resources. To perform the migration of all databases we needed a tool for transfering all the data and keeping consistency between old and new databases. That’s how mongo-migration-stream project was born.
26 Sep 2022
Example of modularization in Allegro Pay Android application
Currently, in the Android world, the topic of modularization is very popular. Many bloggers describe their experiences with it and analyze what Google recommends. Our team started the modularization process before it was hot. I will describe our reasons, decisions, problems and give you some advice. We will see if modularization makes sense and what it brings to the table. I will also post some statistics showing what it looked like before and after the modularization process.
20 Jun 2022
Exploring GraphQL’s performance tradeoffs
At Allegro we decided to introduce GraphQL as our API Gateway for building several internal client systems. By building such a solution we’ve learnt a lot about this technology and we would like to share it with you in this article.
09 Mar 2022
Android UI testing in Allegro Pay organization
Will automatic UI tests be able to replace manual testers as artificial intelligence will try to replace programmers? I’ll show you how we write automatic UI tests on Android in Allegro Pay.
13 Dec 2021
Clean Architecture Story
The Clean Architecture concept has been around for some time and keeps surfacing in one place or another, yet it is not widely adopted. In this post I would like to introduce this topic in a less conventional way: starting with customer’s needs and going through various stages to present a solution that is clean enough to satisfy concepts from the aforementioned blog (or the book with the same name).
22 Nov 2021
Moving towards Micronaut
Micronaut is one of the new application frameworks that have recently sprung up. It promises low memory usage and faster application startup. At Allegro we decided to give it a try. In this article we’ll learn what came out of it and if it’s worth considering when creating microservices-based systems.
13 Apr 2021
Kotlin - a language for everyone and for everything. Even scripts.
According to Wikipedia there are approximately 700 computer languages available. Seven hundred. This is an unbelievable number and it’s the reason why programmers face the problem of choosing a programming language to work with, which frameworks to use and which tech stack to learn. All of them have pros and cons, but when looking for an all-purpose language you should take Kotlin into consideration and ask yourself the question: “Do I really need another programming language?”.
10 Feb 2020
Making Webflux code readable with Kotlin coroutines
Recently, our crucial microservice delivering listing data switched to Spring WebFlux. A non-blocking approach gave us the possibility to reduce the number of server worker threads compared to Spring WebMvc. The reactive approach helped us to effectively build a scalable solution. Also, we entered the world of functional programming where code becomes declarative: statements reduced to the minimum immutable data preferred no side effects quite a neat chaining of method calls causing it easier to understand.
23 May 2018
From Java to Kotlin and Back Again
Due to the high interest and controversy concerning this blog post, we believe that it is worth adding some context on how we work and make decisions at Allegro. Each of more than 50 development teams at Allegro has the freedom to choose technologies from those supported by our PaaS. We mainly code in Java, Kotlin, Python and Golang. The point of view presented in the article results from the author’s experience.
20 Jun 2016
Kotlin null safety and its performance considerations -- part 1
Kotlin may seem like a new kid on the block — itʼs been officially released only in February. Its history however dates a few years back and itʼs mature and stable enough to be used for developing solid reliable applications. Therefore at Allegro we decided to give it a chance — we built our new shiny server-side system using Kotlin as its primary language and we do not regret it.