Tower Web — Expanding the middleware stack

tower-web version 0.2.2 has been released. It comes with a number of new features, which I will talk about in this post. Primarily, the middleware story is starting to come together. I will be expanding some on how middleware fits into Tower and web in general.

First, a quick recap

Tower is a library for writing modular and reusable networking services (previously announced here). It does this by defining a simple trait representing asynchronous request / response based services. It then provides a number of components that add functionality such as retries, load balancing, logging, etc.. all based around the Service trait.

Tower Web is an HTTP web framework built using the tower stack (previously announced here). While it aims to be full featured, functional, and production ready, it also is an experiment driving improvements to the Tower stack. Soon (in software time estimation terms), it will be merged with Warp (more on that later).

Web services

Defining a web service with Tower Web is pretty straightforward. Here is a simple one:

The returned service is a value implementing the Tower Service trait. This trait takes the application logic provided by HelloWorld and exposes it as an asynchronous function of an HTTP request to an HTTP response. As a user, we can use the service value directly:

More importantly, it provides a simple, common interface for Rust HTTP server libraries to use. Any server library is able to take a value that implements Service and run it. Applications are able to implement their logic decoupled from the specific HTTP server implementation.

Middleware

One great feature that comes from having a standardized Service trait is the ability to define middleware. Middleware is used to decorate the application, providing additional functionality.

A middleware is a value that contains a Service and implements Service itself. The middleware’s Service implementation receives a request and passes the request to the inner service. This allows the implementation to inject logic either before sending the request to the inner service or after the inner service responds. The middleware can mutate the request or the response at either point.

Let’s update our service to add some middleware.

The returned service value still implements Tower Service and we can still use it the same way as before. Now, when a request is sent, the request will be logged and the response body will be compressed using zlib.

A simple implementation of LogService might look like this:

Here, the request is logged before passing it to the inner service. The actual implementation is more involved as we want to log the request once it has been processed so that we know things like if the response was successful and how long it took.

To implement DeflateMiddleware, we want to do two things:

  • Inject a Content-Encoding header in the response.
  • Compress the response body, ideally without buffering the entire body up front.

Injecting the header is easy enough, but compressing the body is trickier. The Tower Service trait does not set any restrictions on the request or response types and the http::Response type is generic over the body. We need to set some sort of bound on the body in order to work with it.

The BufStream trait

To solve the problem of “what should the request and response bodies be bound by?”, Tower Web introduces a new trait: BufStream. The trait is an asynchronous stream of values that implement Buf (such a creative name).

A Buf type already abstracts values that contain raw bytes, whether those bytes are in sequential memory or stored in fancier data structures like ropes. A BufStream describes a value that asynchronously produces any number of Buf instances, where each Buf instance represents a chunk of the stream.

Byte containers such as &static [u8], Vec<u8>, and Bytes, can all implement BufStream, themselves. They will just yield a single Buf containing all of their data.

We will also be able to implement BufStream for existing data types such as the Hyper Body or futures::sync::mpsc::Receiver<T> where T: Buf.

To implement the DeflateMiddleware, we can now use BufStream to implement CompressStream, a type that takes a T: BufStream and compresses it. Now, the deflate middleware just needs to modify the response to wrap the body stream with CompressStream.

The Middleware trait

As described above, a middleware is “just” a type that implements Service and dispatches the request to the inner service. We could build up our middleware stack like this:

But that is no fun. We want a pretty builder API:

To achieve this, there is one final missing piece: the Middleware trait.

The job of a Middleware implementation is to handle all of that annoying wrapping so that the end user doesn’t have to. For example, the LogMiddleware implementation takes the inner service S and returns LogService<S>.

Putting it all together, to implement a piece of logging middleware, the following is needed:

  • LogService — Handles the logging logging.
  • LogMiddleware — Adds logging to the middleware stack.
  • ResponseFuture — If the middleware needs to see the response or take an action when the request has been handled.

Looking at the middleware implementations to date, it takes a bit of boilerplate to get everything setup. Step one is to get everything working. Step two is to iterate and improve things. Already, Nikolay Kim (of Actix) has submitted a PR that should cut down on boilerplate. It adds combinators for combining service implementations. Hopefully, once all this shakes out, the only type that will be needed to implement by hand will be LogMiddleware.

The wrap function could look like:

Ok, it might take a few more Rust language features before we get to that, but we are heading in the right direction!

Other new features

Middleware isn’t the only new thing. There have been two other changes of note.

First @lnicola added support for the #[web(either)] annotation on derived response types. It is common for a handler to require returning one of multiple response types picked at run time. The way to handle this is by defining an enum. However, up until today, that then required manual boilerplate to define Response for the enum. Now, that boilerplate can be avoided. For example:

The enum is not limited to just two variants. So, go wild.

The second new feature of note, contributed by @manifest, is the ability to store arbitrary configuration data when defining a service. The configuration data can be retrieved in Extract implementations. This especially useful for authentication where the user must be validated using secrets set at configuration time. For example:

The issue had some good discussion.

Roadmap

There has been lots of good work to date and the work will continue. I wanted to close out this post with some thoughts on next steps.

Immediate work will focus on extracting BufStream into tokio-buf. There already is a PR open for that, though it requires more work. All the middleware infrastructure described in this post currently lives directly in the tower-web repository. It also needs to be extracted out.

The Middleware trait will most likely go in the tower-middleware crate. All HTTP specific concerns will move to tower-http.

Once these components are extracted, then we can start focusing on the plan for merging with Warp. We are already hitting some cases where we’d really like to integrate with the Warp filters. Once Middleware and BufStream are extracted, I would like to focus on writing up details on the merge plan.

Conclusion

I believe that the abstractions and implementations evolving out of Tower Web are promising and should help take the story for writing asynchronous services in Rust one step further. However, more validation is needed. We (the Rust community) must start pushing with real world applications.

I hope to see more people getting involved and join the Gitter channel. 😃

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