@Override
        public String toString() {
            return "Comment{" +
                "key=" + key +
                ", timestamp=" + Instant.ofEpochMilli(timestamp) +
                ", text='" + text + '\'' +
                ", articleId=" + articleId +
                '}';
        }
This test builds on a basic relational data caricature:
a set of articles, each with a set of comments on them.
<p>
The particular relation in this data is that each Comment
has an articleId foreign-key reference to an Article.
<p>
We traverse this relation in both directions to verify
correct operation of Kafka Streams:
- Embed on each Comment a prefix of the text in the Article it references
aka SELECT * FROM Comment JOIN Article ON Comment.ArticleID = Article.ID
- Embed on each Article a count of all the Comments it has
streams/src/test/java/org/apache/kafka/streams/tests/RelationalSmokeTest.java