Alpakka S3 - not giving feedback (document not stored)

Hello everyone

I am facing a weird issue with the Alpakka S3 sink/flow: I am not getting any feedback (errors) and no document is stored, but the stream continues.

I should note that I am running against a local Minio and not AWS. The bucket has been created in advance and a read-write policy has been set in Minio. I can use the API via Postman.

    private Sink<MyClass, NotUsed> createS3Flow() {
        final S3Settings s3Settings =
                        .withS3RegionProvider(() -> Region.EU_CENTRAL_1)
                                AwsBasicCredentials.create("FOO", "BAR")))

        return Flow.of(MyClass.class)
                .log("x", MyClass::getX).addAttributes(Attributes.createLogLevels(Attributes.logLevelInfo()))
                .map(x -> ByteString.fromString(x.getKundennummerArbeitgeber()))
                .to(S3.multipartUpload("testbucket", "x",

What strikes me is, that even when I configure a wrong password, the stream continues without complaining.

Another thing to note: we are running Quarkus and hence our source is actually provided by Mutiny (shipped with Quarkus), but since I prefer the akka-streams api I plugged the two together

var source = Source.fromPublisher(mutinySource);;

The source is configured to pull an item from the DB every 2 seconds and I can see the element in the log-operator, but the S3-sink isnt doing anything (despite pulling new items)

After chaning my flow to using S3.putObject it is now working.

return Flow.of(MyClass.class)
                .flatMapConcat(x -> {
                    var bytes = ByteString.fromString(x.getSome());
                    return S3.putObject("testbucket",
                }).log("x", ObjectMetadata::headers).addAttributes(

Could it be that the multiplartUpload did not confirm because I was always overwriting the same value?

advance and a read-write policy has been set in Minio. I can use the API via Postman.