Jans Blog

App API Rewrite in Rust 1



My two Android apps Metal Only and MensaUPB require some kind of online service to get their data from and - since I cannot force users to update apps easily - I wrote my own API as a wrapper to the upstream APIs provided by https://www.metal-only.de/ and https://studierendenwerk-pb.de/. That way, whenever one of these pages changes their API, I can just adapt the wrapper and keep my own API backwards-compatible so the apps themselves don’t require any updates.

The current version of the API was developed with Play! Scala. However, due to my interest in the Rust programming language I considered a rewrite. An additional advantage of switching to Rust would be the potential of switching to running the app on-demand instead of keeping it always running: a restart of the current Play! Scala API takes around 3~5 seconds.


The rewrite will be split in multiple phases due to limitted time.

The Experimental Phase will just consist of developing multiple proof of concepts to see how viable using Rust for the rewrite is. This entails querying upstream APIs and providing a REST API which can be used by my apps.

In the Cleanup Phase unused endpoints of the current API will be removed which is useful even without a rewrite. The fastest way to do this cleanup was quite simple:

The Rewrite Phase is most likely over-engineered and consists of two parts: an integration test and the actual apps. The integration test is used to ensure that the rewrite works as the currently existing app which is currently tested in production. The basic idea here is to replace the upstream APIs with simulated data (“mocks”). The first step here was to make the upstream API configurable.

Since there was never a requirement to replace the upstream API’s URL, there was no option to do so. The option was added as an environment variable and - if the environment variable is not set - defaults to the hard-coded URL (pseudo-code):

url = get_environment_variable("UPSTREAM_URL")
if is_not_set(url) {

Additionally, I wanted to use this rewrite to further my skills with Docker and docker-compose: the rewritten API, the current API, the integration test, and the mocked upstream APIs should all be started with a single command, i.e. docker-compose up app-api-rewrite.

Current State

The rewrite started in March 2023 and was paused for a few months. While experimenting and cleaning up were done quite fast, the rewrite itself is still in progress. One of the major factors is the deliberate over-engineering.