@Override
    public void startProcessing() {
        if (s3ConfigRef == null) {
            fatal("No S3 config has been provided for S3 appender.");
        }

        final Optional<S3ClientConfig> optional = s3ClientConfigCache.get(s3ConfigRef);
        if (optional.isEmpty()) {
            fatal("Unable to load S3 client config from " + s3ConfigRef);
        } else {
            s3ClientConfig = optional.get();
        }

        super.startProcessing();
    }
Joins text instances into a single text instance.
stroom-aws/stroom-aws-s3-impl/src/main/java/stroom/aws/s3/impl/S3Appender.java