wstd-aws-example: using wstd support in the AWS Rust SDK

The AWS Rust SDK has support for using the wstd crate on the wasm32-wasip2 target to use the wasi-http interface. This example shows how to use it.

TL;DR

• depend on aws-* crates released recently enough to have MSRV of 1.91.1 (on or after March 4, 2026). Use default-features = false so tokio doesn't get sucked in.
• depend on aws-smithy-wasm and setup your Config with:

  config
      .sleep_impl(aws_smithy_wasm::wasi::WasiSleep)
      .http_client(aws_smithy_wasm::wasi::WasiHttpClientBuilder::new().build())
  

Explanation

In many wasi settings, its necessary or desirable to use the wasi-http interface to make http requests. Wasi-http interfaces provide an http implementation, including the sockets layer and TLS, outside of the user's component. wstd provides user-friendly async Rust interfaces to all of the standardized wasi interfaces, including wasi-http.

The AWS Rust SDK, by default, depends on tokio, hyper, and either rustls or s2n_tls, and makes http requests over sockets (which can be provided as wasi-sockets). Those dependencies may not work correctly under wasm32-wasip2, and if they do, they will not use the wasi-http interfaces. To avoid using http over sockets, make sure to set the default-features = false setting when depending on any aws-* crates in your project.

To configure the AWS Rust SDK to use wstd's wasi-http client, use the aws_smithy_crate at version 0.10.0 or later. Provide aws_smithy_wasm::wasi::WasiSleep and aws_smithy_wasm::wasi::WasiHttpClientBuilder::new().build() to your aws_config::ConfigLoader:

    let config = aws_config::defaults(BehaviorVersion::latest())
        .sleep_impl(aws_smithy_wasm::wasi::WasiSleep)
        .http_client(aws_smithy_wasm::wasi::WasiHttpClientBuilder::new().build())
        ...;

Example

An example s3 client is provided as a wasi cli command. It accepts command line arguments with the subcommand list to list a bucket's contents, and get <key> to get an object from a bucket and write it to the filesystem.

This example must be compiled in release mode - in debug mode, the aws sdk's generated code will overflow the maximum permitted wasm locals in a single function.

Compile it with:

cargo build -p wstd-aws-example --target wasm32-wasip2 --release

When running this example, you will need AWS credentials provided in environment variables, and you should substitute in a region and bucket where your credentials have permissions to list the bucket and read items.

Run it with:

wasmtime run -Shttp \
    --env AWS_ACCESS_KEY_ID \
    --env AWS_SECRET_ACCESS_KEY \
    --env AWS_SESSION_TOKEN \
    --dir .::. \
    target/wasm32-wasip2/release/s3.wasm \
    --region us-west-2 \
    --bucket wstd-example-bucket

or alternatively run it with:

cargo run --target wasm32-wasip2 -p wstd-aws-example --example s3 -- \
    --region us-west-2 --bucket wstd-example-bucket

which uses the wasmtime cli, as above, via configiration found in this workspace's .cargo/config.toml.

By default, the subcommand list will be run, listing the contents of the bucket. To get an item from the bucket, use the subcommand get <key> [-o <output>]. Use --help when in doubt.