dynamic-config-s3

Read dynamic-config configuration from an S3 object — on AWS, or on anything that speaks its API.

[dependencies]
dynamic-config = { version = "0.5.0", features = ["async"] }
dynamic-config-s3 = "0.5.0"

use dynamic_config_s3::S3;

DbConfig::set_remote_async(S3::new("myapp-config", "prod/db.json").await?);

DbConfig::refresh_remote_async().await?;

The AWS SDK is async throughout, so this implements the async AsyncRemoteSource trait rather than the blocking one.

What it reads

One object, whose body is a whole configuration document — the same bytes that would be in a config file. The format comes from the key's extension, or from with_format.

Credentials

Through aws-config, which is the chain every AWS tool uses: AWS_ACCESS_KEY_ID, the shared profile, the EC2 instance role, the ECS task role, and IRSA on EKS. That is deliberately not re-implemented here — a second credential chain in a program that already has one is a bug waiting for a rotation.

Not only AWS

with_config takes an SdkConfig the program already built, which is how a non-AWS endpoint is reached. Path-style addressing is forced, because http://bucket.host/key needs DNS entries only AWS has:

let config = aws_config::from_env()
    .endpoint_url("http://minio.internal:9000")
    .load()
    .await;

let s3 = S3::with_config(&config, "myapp-config", "prod/db.json");

MinIO, Ceph, Cloudflare R2 and Backblaze B2 all work this way — and the test suite runs against MinIO, so that is checked rather than claimed.

Watching

S3 cannot say when an object changes without a notification pipeline — SNS, SQS, EventBridge — and that is a deployment's decision rather than a library's. So watch polls, and says so.

What it does not do is download the object every tick. HEAD returns the ETag, which changes when the body does, so an unchanged configuration costs one small request and no transfer — which matters on a bucket that charges per gigabyte.

s3.watch(&watching, Duration::from_secs(30), move |document| sink.apply(document)).await

The current object is not delivered at startup.
The ETag is taken from the read, not from the check that preceded it, so a write landing between the two is not delivered twice.
A failed check does not end the watch. Stopping is noticed within a quarter second whatever the interval is.

Builders

Method	Default
`with_format(..)`	from the key's extension
`with_config(..)`	credentials from the environment
`from_client(..)`	builds its own client

Testing

The test suite drives MinIO in a container — the same API, offline and free. That the crate works against MinIO at all is itself the assertion that matters for everyone on Ceph, R2 or B2.

cargo test -p dynamic-config-s3    # needs a working Docker daemon

MSRV

1.88 — higher than dynamic-config's own 1.71, because the AWS SDK moves faster than that crate wants to.

License

MIT

dynamic-config-s3 0.5.0