private String resolveRegion(RegionFromEndpointResolver resolver, String endpointPrefix, String serviceSigningName) {
        if (resolver == null) {
            resolver = new DefaultRegionFromEndpointResolver();
        }
        String host = request.getEndpoint().getHost();
        String region = resolver.guessRegionFromEndpoint(host, endpointPrefix != null ? endpointPrefix : serviceSigningName);
        return region != null ? region : "us-east-1";
    }
Generates an instance of AWS4signerRequestParams that holds the
parameters used for computing a AWS 4 signature for a request
aws-java-sdk-core/src/main/java/com/amazonaws/auth/internal/AWS4SignerRequestParams.java