@Override
    public String toString() {
        return metaId + ":"
                + partIndex + ":"
                + getRecordIndex().orElse(0L) + " - "
                + childStreamType;
    }
@return The part index, zero based. 0 for single-part segmented streams.
stroom-core-client/src/main/java/stroom/data/client/SourceKey.java