Replacing FastAPI with Rust: Part 4 - A Solution

The Rust mascot 'Ferris the Crab' holds the logos for FastAPI and Rust and is smooshing them together.

Cover image created by me using Ferris the Crab, the Rust logo, and the FastAPI logo.

This post is part of a series. If you haven’t already, you may want to read the previous post before continuing.

After investigating a few more options, I’ve finally decided on a framework to use. Here’s how I got there, step by step.


As planned, I started with rweb. This framework was ranked second of the three options I considered in part 2 of this blog series. As a short reminder, this framework is built around warp and seems to provide most of the features I’m looking for. My main reservations are about the maintenance and community around this project.

I started out by copying and pasting the example from the docs and trying to run it. I noticed right off the bat that the version included in the example is much older than the latest released version, so I went ahead and updated that. Attempting to run the example produced a series of issues:

  1. The example doesn’t include the tokio “macros” feature or serde as a dependency, both of which were required. Easy enough to fix.
  2. rweb requires rweb-macros which requires a different version of rweb-openapi than rweb itself does, so you get two different copies of that. I assume this was just a failure to update internal versions on release at some point. All three of these crates are in the same GitHub repository.
  3. CLion (my editor of choice) highlights a syntax error because of the way endpoints are registered. Basically, it seems like the macros transform the functions into something completely different. You then call the functions to register them, but CLion is expecting you to pass the input params (e.g. json body) to the function which you aren’t doing.
  4. The code doesn’t compile because it was using a module of syn which was documented as internal only and do not use. The maintainers of syn changed the name of that module which broke rweb. There are a couple of sub-issues here:
    1. rweb was using an internal module of syn which it should not have been. This seems to be a common issue though or syn would not have been changed to make the internal code more internal.
    2. Someone reported this as an issue, but the maintainer closed it without investigation. With additional prodding from both myself and the issue opener the maintainer fixed the problem.

All of this basically confirmed my fears about rweb. The maintenance and stability of the project are not consistent enough for me to be comfortable relying on it. By the time the maintainer fixed the issue, I’d already investigated two other frameworks and chosen one.


I received a DM on Twitter from @ayper recommending I take a look at this project. It’s certainly very interesting, though also very young. It seems like this project’s goals may align with my own though it would take some work to get there. Specifically, right now there is no way to make offline direct requests as I need for a serverless environment. It seems like even their test utilities (which is where you usually see offline invocation) use a local webserver. I’ll definitely be keeping my eyes on this in the future though to see where it goes.


Yes, Rocket. The framework I pretty much immediately discounted in part 2. I’m not too proud to admit that I was wrong. Between my frustrations with other frameworks and a glowing review on I had to at least try it. The results were… fantastic.

Okay so this solution doesn’t resolve either of my main misgivings about Rocket because it uses the released 0.4 version. The code still requires nightly rust and still does not support async, but both of these have already been resolved on Rocket’s “master” branch and therefore will be present in the 0.5.0 release. Then it’s just a matter of updating supporting packages.

Getting this all to work was a breath of fresh air compared to previous attempts. I started by copying the official JSON example and tweaking it to have the same pet echo behavior I used in my actix experiment. The code basically looked like this:

#![feature(proc_macro_hygiene, decl_macro)]

extern crate rocket;
extern crate serde_derive;

use rocket_contrib::json::Json;

#[derive(Serialize, Deserialize)]
struct Pet {
    name: String,
    id: Option<i64>,

#[post("/echo", format = "json", data = "<pet>")]
fn echo(pet: Json<Pet>) -> Json<Pet> {

fn main() {
    rocket::ignite().mount("/rocket", routes![echo]).launch();

Pretty simple. That code handles validation, serialization, and deserialization. Now I just needed to add in AWS Lambda support and OpenAPI documentation.

AWS Lambda Support

rocket_lamb uses the last release of the official AWS runtime which is based on an older version of Tokio, so it will take some work to update it for Rocket 0.5 once it’s ready. There was one issue I encountered while running this, but the maintainer responded within a day, and it turned out to be an issue with AWS SAM. So all I had to do to make my code work with sam invoke start-api was to:

  1. Downgrade to aws-sam-cli 1.12.0 to avoid the bug
  2. cargo add rocket_lamb
  3. Add use rocket_lamb::RocketExt; with the rest of my imports.
  4. Add .lambda() before .launch() in main.

That’s it! So easy! And this crate clearly has a responsive and helpful maintainer, great!

OpenAPI Documentation

okapi was clearly the way to go here and also happens to have the same maintainer as rocket_lamb. This process was also super simple:

  1. Add okapi, rocket_okapi, and schemars (0.7) as dependencies.
  2. Copy a couple of things from the example in the README
  3. There is no step three

Now I have a self-documenting API compatible with AWS Lambda with very minimal work on my part. okapi even comes with a functional (if a bit outdated) Swagger UI, just like FastAPI does!

Requirements Check

Let’s check in with our list of requirements for a FastAPI solution and see where we’re at.

  1. ”MUST be written in Rust.” Yup!
  2. ”MUST automatically produce an OpenAPI v3 document from the Rust code and comments.” Yes!
  3. ”MUST be easily deployable on AWS Lambda using some infrastructure as code tool (SAM, Serverless, etc.).” Actually yes, the only one so far!
  4. ”MUST perform at least as fast as an equivalent FastAPI application for common CRUD tasks.” Tentatively? TechEmpower has it showing a little better in some cases and worse in others, but switching to Tokio in 0.5 is bound change this one way or another.
  5. ”MUST interact with a relational database (MySQL or Postgres).” Yes, even though the method will likely change with 0.5.
  6. ”MUST have a simple way to test endpoints, comparable to pytest with FastAPI.” The basic tests were quite simple! Much better than what I was able to achieve with actix-web.
  7. ”MUST have great documentation.” I was able to get my simple experiment working by just reading the docs, so I’m giving this one a yes as well.
  8. ”MUST have stable, active maintenance.” The Rocket community is definitely active in developing 0.5, and the maintainer of rocket_lamb was quite responsive, so yes.
  9. ”SHOULD have automatically hosted documentation which allows direct interaction with the API.” Yes! The version of Swagger UI is a bit old but it works!

If you tally it up, this solution meets every single one of my framework requirements, even the optional one! There are a couple items that need more experimentation to be super confident in, but this is the best crate-stack I have tried yet by far.


I’m using Rocket! It was way too easy to get all of this working to pass it up. There will certainly be work to do to get rocket_lamb and okapi ready for Rocket 0.5, but my experience so far has left me very confident.

I’m not completely sure what the next step for me will be. I could try actually deploying a test function using Rocket to a real cloud provider. I could also dig into those two Rocket companion libraries and see how hard it is to update them to work with Rocket’s “master” branch. I do want to wire this thing up to a database and create an equivalent FastAPI function to run benchmarks against, but I’d rather wait until I have async since that will likely make a big impact. If you have a suggestion on what to do next, leave it in the ideas section of this blog’s GitHub repo.

By the way, if you want to check out the Rocket code I ended up with, it’s available in the experiments repo alongside the actix code from the previous post.

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