Crate tower_async

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async fn(Request) -> Result<Response, Error>

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.

This fork is made entirely with the needs of the author in mind, and thus might not yet contain all features you might need.

Come join us at discord at https://discord.com/channels/1114459060050333696/1123537825929900113 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.

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, [rate limiting], and [load balancing], 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:

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:

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 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.

Modules

  • builder
    Builder types to compose layers and services
  • filterfilter
    Conditionally dispatch requests to the inner service based on the result of a predicate.
  • layer
    A collection of Layer based tower services
  • limitlimit
    A middleware that limits the number of in-flight requests.
  • makemake
    Trait aliases for Services that produce specific types of Responses.
  • retryretry
    Middleware for retrying “failed” requests.
  • timeouttimeout
    Middleware that applies a timeout to requests.
  • utilutil
    Various utility types and functions that are generally used with Tower.

Structs

Traits

  • Layer
    Decorates a Service, transforming either the request or the response.
  • MakeServicemake
    Creates new Service values.
  • Service
    An asynchronous function from a Request to a Response.
  • ServiceExtutil
    An extension trait for Services that provides a variety of convenient adapters

Functions

Type Aliases

  • BoxError
    Alias for a type-erased error type.