@Override
    protected void validateSpecificSettings() {
        if (feed == null) {
            if (feedRef != null) {
                feed = docRefInfoService.name(feedRef).orElse(null);
                if (Strings.isNullOrEmpty(feed)) {
                    fatal("Feed not found");
                }

            } else {
                final Meta parentMeta = metaHolder.getMeta();
                if (parentMeta == null) {
                    fatal("Unable to determine feed as no parent set");
                } else if (Strings.isNullOrEmpty(parentMeta.getFeedName())) {
                    fatal("Parent has no feed name");
                } else {
                    // Use current feed if none other has been specified.
                    feed = parentMeta.getFeedName();
                }
            }
        }

        if (Strings.isNullOrEmpty(streamType)) {
            throw ProcessException.create("Stream type not specified");
        }
    }
Joins text instances into a single text instance.
stroom-pipeline/src/main/java/stroom/pipeline/writer/RollingStreamAppender.java