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Why Deliveroo decided to part ways from Ruby for the Dispatcher service?
The Logistics team at Deliveroo uses a service called Dispatcher. This service optimally offers an order to the rider, and it does this with the help of a timeline for each rider. This timeline helps in predicting where riders will be at a certain point of time. Knowing this information allows to efficiently suggest a rider for an order. Building these timelines requires a lot of computation. Though these computations are quick, they are a lot in number.
The Dispatcher service was first written in Ruby as it was the company’s preferred language in the beginning. Earlier, it was performing fine because the business was not as big it is now. With time, when Deliveroo started growing, the number of orders increased. This is why the Dispatch service started taking much longer than before.
Why they chose Rust as the replacement for Ruby?
Instead of writing the whole thing in Rust, the team decided to identify the bottlenecks that were slowing down the Dispatcher service and rewrite them in a different programming language (Rust). They concluded that it would be easier if they built some sort of native extension written in Rust and make it work with the current code written in Ruby.
The team chose Rust because it provides high performance than C and is memory safe. Rust also allowed them to build dynamic libraries, which can be later loaded into Ruby. Additionally, some of their team members also had experience with Rust and one part of the Dispatcher was already in Rust.
How they migrated from Ruby to Rust?
There are two options using which you can call Rust from Ruby. One, by writing a dynamic library in Rust with extern “C” interface and calling it using FFI. Second, writing a dynamic library, but using the Ruby API to register methods, so that you can call them from Ruby directly, just like any other Ruby code. The Deliveroo team chose the second approach of using Ruby API, as there are many libraries available to make it easier for them, for instance, ruru, rutie, and Helix. The team decided to use Rutie, which is a recent fork of Ruru and is under active development.
The team planned to gradually replace all parts of the Ruby Dispatcher with Rust. They began the migration by replacing with Rust classes which did not have any dependencies on other parts of the Dispatcher and adding feature flags. As the API of both Ruby and Rust classes implementation were quite similar, they were able to use the same tests.
With the help of Rust, the overall dispatch time was reduced significantly. For instance, in one of their larger zones, it dropped from ~4 sec to 0.8 sec. Out of these 0.8 seconds, the Rust part only consumed 0.2 seconds.
Read the post shared by Andrii Dmytrenko, a Software Engineer at Deliveroo, for more details.