#async #client #http


Asynchronous, zero-allocation HTTP client that is agnostic to choice of executor

1 unstable release

0.1.0 Oct 25, 2020

#12 in #async-client




Agnostic Aysynchronous HTTP Client (aahc) is a lightweight, pure Rust asynchronous HTTP/1.1 client that does no dynamic memory allocation, is not tied to any specific asynchronous executor or executors, and does not spawn additional threads.

Design goals

Agnostic to choice of executor

Most other asynchronous HTTP client crates depend on a specific asynchronous runtime environment (usually tokio or async-std, or less frequently fibers). Most of the rest have feature flags that let them work with two or three specific choices of executor. aahc does not depend on any particular asynchronous executor or runtime environment and should work with any executor that provides socket capabilities. This is achieved by requiring the user to provide the transport socket over which the HTTP request should be sent, which must be appropriate to the asynchronous runtime environment used by the application—an application running in tokio would pass in tokio sockets, an application running in async-std would pass in async-std sockets, an application running in GTK would pass in GLib sockets, and so on.


The only other asynchronous HTTP client crate I am aware of that is truly executor-agnostic is Isahc. Isahc accomplishes this goal by spawning an auxiliary thread which performs I/O. aahc does not spawn any threads.


aahc is #![forbid(unsafe_code)]. Its dependencies are httparse and futures-io, which both have relatively little unsafe code.

Zero allocations

aahc does not perform any heap allocations during normal operation. Neither does its underlying parser, httparse. Note that if the detailed-errors feature is enabled (which is the default) then some errors are returned with heap-allocated detailed messages; if the feature is disabled, then these errors are returned with less informative data but with no heap allocation.



aahc is an HTTP/1.x client. It does not currently support HTTP/2+.


aahc does not currently support HTTP compression in the Transfer-Encoding header.

Compression via the Content-Encoding header can be used; as an end-to-end header (rather than a hop-by-hop header), aahc takes the perspective that Content-Encoding is the application’s responsibility.

HTTP proxies

SOCKS proxies work entirely at the transport layer, below the HTTP layer. Thus, an application can connect to a SOCKS proxy, perform the initial SOCKS negotiation, and pass the resulting socket into aahc.

HTTP proxies using the CONNECT method work similarly to SOCKS proxies. Thus, an application can similarly connect to an HTTP proxy, perform the initial CONNECT request, and pass the resulting socket into aahc. However, aahc does not support making CONNECT requests itself, and thus cannot be used to provide tunnels through CONNECT-style proxies for other consumers (or other instances of itself).

HTTP proxies not using CONNECT are supported by aahc. As always, the application is responsible for creating and connecting the transport socket, in this case to the proxy rather than to the origin server. The application can then provide a request target in absolute form rather than origin form, as is appropriate when communicating with a proxy server.

The Upgrade header

aahc does not support the Upgrade header. It therefore cannot speak WebSocket.

1xx status codes

aahc currently ignores 1xx (informational) status codes. Only the final ≥2xx response is returned to the application.


aahc does not currently support the use of trailers following a response body using chunked transfer encoding. Applications must not set TE: trailers in their request headers.



If this feature is enabled, error returns of kind InvalidData contain a nested error object with more details about the exact error; however, because of the implementation of std::io::Error, such additional details must be allocated on the heap. If this feature is disabled, error returns of kind InvalidData do not contain a nested error object.