Tower Web — A new web framework for Rust

I previously announced Tower and mentioned that a web framework was in the works. It took longer than I had hoped (as it sometimes does with software), but today, I am opening up Tower Web.

Tower Web is an asynchronous HTTP web framework that focuses on removing boilerplate. It is built on top of Tokio, Hyper, and of course Tower. It works today on stable Rust.

Here is “hello world”.

Tower Web uses a macro based approach. The impl_web! macro looks at the impl blocks and generates the necessary glue code to serve HelloWorld as an HTTP service.

The @get("/") doc comment is meaningful. This tells Tower Web that GET requests to / should be mapped to the hello_world method. Tower Web opted to use doc comments as annotations to enable supporting stable Rust today. Once attribute macros land on the stable channel, Tower Web will switch to attribute macros.

This hello world example is cool, but it doesn’t illustrate where Tower Web really shines.

Decoupling HTTP and application logic

Tower Web strives to enable applications to be written without including any HTTP related concerns. That means, methods should be written using only “plain old Rust types” (PORT?). To illustrate, lets look at an example that uses a path capture.

This example extracts a request path segment and uses that as part of the response. What is interesting is that the greet method is implemented without any special types. It just takes a String as the argument.

Tower Web uses convention over configuration. The convention is that the argument identifier will match the capture identifier. In this case, both the argument and capture are named name. Because of this, Tower Web knows that the name argument should be populated with the value captured from the path.

Because the method is written without any special types, testing also becomes very easy.

When it comes to more complicated types, such as JSON request bodies, Tower Web also has you covered.

The #[derive(Extract)] generates an extract implementation for the type. It uses Serde under the hood, so all the various Serde annotations work here too. When used as an argument named body, Tower Web will deserialize the request body into the struct. Once again, the method has been implemented without any HTTP specific types.

The following curl request will succeed:

curl --vv --request POST -H 'content-type: application/json' --data '{"foo":1,"bar":"baz"}' http://localhost:8080/data

Tower Web is able to do validation on the type as well. The foo field is defined to be numeric and required. Omitting it results in an HTTP 400, bad request, response.

Responding

A resource method can return any type that implements Response. String is one of these types. Some others include serde_json::Value, http::Response, and tokio::fs::File (for serving a file from disk). Tower Web also provides a #derive(Response) macro enabling custom return types.

In this example, we respond with a custom type. The resource method is also annotated with @content_type("json"). This indicates that MyData should be serialized using the JSON format.

Also note that MyData has an attribute: #[web(status = "201")]. This tells Tower Web to set the HTTP response status code to 201. There are a number of other ways to customize the response with #[derive(Response)] and these are documented in the API docs and examples.

Asynchronous

So far, all the requests have been processed synchronously. However, resource methods may return futures as well. Here is an example using impl Future.

Note that impl Future is bound with Send. Tower Web uses Hyper to serve the application and Hyper requires the response future to be bound by Send.

Just a taste

This is just a very quick tour of what Tower Web has to offer (I didn’t even touch on middleware). To get started, check out the examples and API docs found at the Tower Web repository. Also, there are a lot more features planned, so try things out, provide feedback, and get your hands dirty with some PRs.

Warp — the future of asynchronous web for Rust

As you might have seen, my esteemed colleague, seanmonstar recently announced Warp, a framework he has been working on. We both have been working in parallel and have been experimenting with different approaches to define web services in Rust.

The goal is to avoid fragmentation and rally behind a single crate. There is no need to duplicate effort and re-implement the world each time.

In a month or so, Tower Web will become a part of Warp. Warp will then provide both the filter based API and the macro API offered by Tower Web. This will let us focus on a single library.

Merging with Warp will also enable mixing and matching the various API styles. For example, you could build a single service using both filters and macros. Or, you could respond from a filter with a #[derive(Response)] struct.

In the short term, while I gather feedback from the initial release, feature development will happen in the Tower Web repository.

Acknowledgements

I want to call out Jake Goulding (shepmaster). Jake provided a lot of valuable early feedback and has already contributed quite a good number of PRs ❤️.

Conclusion

Give Tower Web a shot, provide feedback and PRs. Also, I will be blogging again soon to talk more about the Tower / Web stack!