Tower Async

Tower Async is a library of modular and reusable components for building robust networking clients and servers.

Fork

Tower Async is a fork of https://github.com/tower-rs/tower and makes use of async traits to simplify things and make it more easier to integrate async functions into middleware.

If you want to see a prime example of how much simpler using tower_async is versus tower, you can see an example here:

A delay service using tower: https://github.com/plabayo/tower-async/blob/4ae0c4747fac6cc69b27c87a7ea5cacdd8bab3fb/tower-async-bridge/src/into_async/async_layer.rs#L91-L169.

That same service can be written in tower_async as follows:

#[derive(Debug)]
struct DelayService<S> {
    inner: S,
    delay: std::time::Duration,
}

impl<S> DelayService<S> {
    fn new(inner: S, delay: std::time::Duration) -> Self {
        Self { inner, delay }
    }
}

impl<S, Request> tower_async_service::Service<Request> for DelayService<S>
where
    S: tower_async_service::Service<Request>,
{
    type Response = S::Response;
    type Error = S::Error;

    async fn call(&self, request: Request) -> Result<Self::Response, Self::Error> {
        tokio::time::sleep(self.delay).await;
        self.inner.call(request)
    }
}

If you compare that with the linked tower version you can probably agree that things are a lot simpler if you do not have to hand write future state machines yourself, which is the reason why we have the async sugar in the first place.

Of course I do acknowledge that if you make use of amazing utilities provided by crates such as https://docs.rs/futures-util-preview/latest/futures_util/future/index.html That you an write pretty much the same code without having to handwrite the future yourself.

This is however not always possible, and it does mean that you need to (1) know about this and (2) pull it as a dependency and all that it brings with it. While in reallity you really just want to be able write your middleware in an async manner.

We fully acknowledge that tower had to use the approach it used as for many years this was simply the only sensible thing to do, unless you want to force your users to make use of the https://docs.rs/async-trait/latest/async_trait/, a choice not everyone is willing to make, and sometimes they might not even have the luxary to do so.

Come join us at discord on the #tower-async public channel at Discord or tag @glendc at Tokio's Tower discord instead.

Where suitable we'll keep in sync (manually) with Tower and if the opportunity arises we'll contribute back "upstream" as well. Given however how big the diversange we aren't sure how likely that is.

This set of libraries is best suited in an ecosystem of its own, that is to say, making use only of tower-async libraries and dependents on it. At the very least it is desired that tower-async is the puppeteer with where needed making use of tower (classic) (middleware) layers.

For an example on how to operate purely within a tower-async environment you can explore the Rama codebase, a proxy framework, written purely with a tower-async mindset, and the main motivator to start this fork.

You can however also bridge tower and tower-async in any other way. Please consult the "Bridging to Tokio's official Tower Ecosystem" chapter for more information on how to do that.

Overview

Tower Async aims to make it as easy as possible to build robust networking clients and servers. It is protocol agnostic, but is designed around a request / response pattern. If your protocol is entirely stream based, Tower may not be a good fit.

Tower Async provides a simple core abstraction, the Service trait, which represents an asynchronous function taking a request and returning either a response or an error. This abstraction can be used to model both clients and servers.

Generic components, like timeouts, can be modeled as Services that wrap some inner service and apply additional behavior before or after the inner service is called. This allows implementing these components in a protocol-agnostic, composable way. Typically, such services are referred to as middleware.

An additional abstraction, the Layer trait, is used to compose middleware with Services. If a Service can be thought of as an asynchronous function from a request type to a response type, a Layer is a function taking a Service of one type and returning a Service of a different type. The ServiceBuilder type is used to add middleware to a service by composing it with multiple Layers.

Difference with Tokio's official Tower Ecosystem?

• Make use of Async Traits (RFC-3185: Static async fn in traits) instead of requiring the user to manually implement Futures;
  • Which in fact forces users to Box Services that rely on futures which cannot be named, e.g. those returned by async functions that the user might have to face by using common utility functions from the wider Tokio ecosystem;
• Drop the notion of poll_ready (See the FAQ).
• Use &self for Service::call instead of &mut self:
  • this to simplify its usage;
  • makes it clear that the user is responsible for proper state sharing;
  • makes it more compatible with the ecosystem (e.g. hyper (v1) also takes services by &self);

Bridging to Tokio's official Tower Ecosystem

You can make use of the tower-async-bridge crate as found in this repo in the ./tower-async-bridge directory, and published at crates.io under the same name.

At a high level it allows you to:

• Turn a tower::Service into a tower_async::Service (requires the into_async feature);
• Turn a tower_async::Service into a tower::Service;
• Use a tower_async::Layer within a tower environment (e.g. tower::ServiceBuilder);
• Use a tower::Layer within a tower_async environment (e.g. tower_async::ServiceBuilder) (requires the into_async feature);

Please check the crate's unit tests and examples to see specifically how to use the crate in order to achieve this.

Furthermore we also urge you to only use this kind of approach for transition purposes and not as a permanent way of life. Best in our opinion is to use one or the other and not to combine the two. But if you do absolutely must use one combined with the other, tower-async-bridge should allow you to do exactly that.

The Tower Async Ecosystem

Tower Async is made up of the following crates:

• tower-async (this crate)
• tower-async-bridge
• tower-async-service
• tower-async-layer
• tower-async-test
• tower-async-http

Since the Service and Layer traits are important integration points for all libraries using Tower, they are kept as stable as possible, and breaking changes are made rarely. Therefore, they are defined in separate crates, tower-async-service and tower-async-layer. This crate contains re-exports of those core traits, implementations of commonly-used middleware, and utilities for working with Services and Layers.

tower-async-bridge is there to bridge Tokio's official Tower ecosystem with this (Aync Trait) version (Fork).

Testing Layers can be done with unit tests very easily suing tower-async-test.

Finally in case you are using tower-async for HTTP purposes (e.g. an HTTP web server), then you might find it useful to also make use of tower-async-http as it provides you with builder extensions and middleware specifically tailored for http purposes.

Usage

Tower (Async) provides an abstraction layer, and generic implementations of various middleware. This means that the tower-async crate on its own does not provide a working implementation of a network client or server. Instead, Tower's Service trait provides an integration point between application code, libraries providing middleware implementations, and libraries that implement servers and/or clients for various network protocols.

Depending on your particular use case, you might use Tower in several ways:

• Implementing application logic for a networked program. You might use the Service trait to model your application's behavior, and use the middleware provided by this crate and by other libraries to add functionality to clients and servers provided by one or more protocol implementations.

• Implementing middleware to add custom behavior to network clients and servers in a reusable manner. This might be general-purpose middleware (and if it is, please consider releasing your middleware as a library for other Tower users!) or application-specific behavior that needs to be shared between multiple clients or servers.

• Implementing a network protocol. Libraries that implement network protocols (such as HTTP) can depend on tower-async-service to use the Service trait as an integration point between the protocol and user code. For example, a client for some protocol might implement Service, allowing users to add arbitrary Tower middleware to those clients. Similarly, a server might be created from a user-provided Service.

  Additionally, when a network protocol requires functionality already provided by existing Tower middleware, a protocol implementation might use Tower middleware internally, as well as an integration point.

Library Support

Following are some libraries that make use of Tower Async (instead of Tower) and the Service trait:

• rama: A proxy framework to anonymise your network traffic.

If you're the maintainer of a crate that supports Tower Async, we'd love to add your crate to this list! Please open a PR adding a brief description of your library!

Getting Started

The various middleware implementations provided by this crate are feature flagged, so that users can only compile the parts of Tower they need. By default, all the optional middleware are disabled.

To get started using all of Tower's optional middleware, add this to your Cargo.toml:

tower-async = { version = "0.2", features = ["full"] }

Alternatively, you can only enable some features. For example, to enable only the timeout middleware, write:

tower-async = { version = "0.2", features = ["timeout"] }

See here for a complete list of all middleware provided by Tower.

Browse the examples at tower-async-http/examples to see some examples on how to use tower-async and its sibling crates. While these are focussed on http examples, note that:

• tower-async can work for any request-response flow (akin to tower);
• you can also use tower-async with http web services without making use of the tower-async-http crate, it only is there to provide extra middleware for http-specific purposes, but this is all optional.

The documentation also contains some smaller examples and of course the codebase can be read as well, together with its unit tests.

Supported Rust Versions

Tower Async requires nightly Rust for the time being and has no backwards compatibility promises for the time being.

Once async traits are stabilized we'll start supporting stable rust once again, and we can start working towards backwards compatibility.

Read https://blog.rust-lang.org/inside-rust/2023/05/03/stabilizing-async-fn-in-trait.html for more information on this roadmap by the Rust Language Core Team.

Sponsorship

Regular and onetime sponsors alike help us to pay the development and service costs done in function of all Plabayo's Free and Open Source work.

We're also a monthly sponsor of Tokio ourselves, to give back to all the great work done and continued effort being put in by them.

You can find more about Plabayo Sponsorship at https://github.com/sponsors/plabayo.

One time sponsorships (the so called "buy me a coffee", but then via GitHub Sponsors payments), are welcome as much as regular sponsors. Not everybody have the financial means to sponsor, so feel free to contribute in any other way that you can think of.

FAQ

Where is the poll_ready method from Tower's Service?

This has been removed for the purpose of simplification and because the authors of this fork consider it a problem out of scope:

• most Tower services / layers do not ever need the poll_ready method, and simply call the inner service for that;
• for some backpressure purposes you do want to know the request to know how to handle it, so poll_ready wouldn't work for these;

poll_ready was also used for load balancing services but this is considered out of scope:

• load balancing incoming network streams is in our humble opinion more something to be handled by your network infrastructure surrounding your service (using a... load balancer);
• and again... if you do want to load balance within a service it might be because you actually require context from the request to know what to do, in which case poll_ready wouldn't work for you;

Where you do still want to apply some kind of rate limiting, back pressure or load balancing within a Tower (Async) Service you are to do it within the call function instead.

This fork is however still in its early days, so feel free to start a discussion if you feel different about this topic. The authors of this library are always open for feedback but retain the reservation to deny any request they wish.

Where is my favourite Tower Utility?

As all the tower code has to be manually ported, there might be some features missing. The tower ecosystem also continues to thrive and live happy, so there might still be new features added there as well. Feel free to chat with us or open a ticket on GitHub in case you wish to add/port such feature(s).

Note that some features are not supported on purpose:

1. all the 'ready' related functionality was removed on purpose as we believe it to be out of scope

• as such also all utilities that rely on this or build on top of this aren't supported

See the previous FAQ point to get our point of view related to load balancing and the like.

License

This project is licensed under the MIT license.

Big thanks and credits go towards the original Tower authors which licensed their code under the same License type.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Tower Async by you, shall be licensed as MIT, without any additional terms or conditions.