Kafka Streams as table Patch log not full Post - apache-kafka

Question

Desired functionality: For a given key, key123, numerous services are running in parallel and reporting their results to a single location, once all results are gathered for key123 they are passed to a new downstream consumer.
Original idea: Using AWS DynamoDB to hold all results for a given entry. Every time a result is ready a micro-service does a PATCH operation to the database on key123. An output stream checks each UPDATE to see if the entry is complete, if so, it is forwarded downstream.
New Idea: Use Kafka Streams and KSQL to reach the same goal. All services write their output to the results topic, the topic forms a change log Kstream that we KSQL query for completed entries. Something like:
CREATE STREAM competed_results FROM results_stream SELECT * WHERE (all results != NULL).
The part I'm not sure how to do is the PATCH operation on the stream. To have the output stream show the accumulation of all messages for key123 instead of just the most recent one?
KSQL users, does this even make sense? Am I close to a solution that someone has done before?

Answer 1

If you can produce all your events to the same topic, with the key set, then you can collect all of the events for a specific key using an aggregation in ksqlDB such as:
CREATE STREAM source (
KEY INT KEY, -- example key to group by
EVENT STRING -- example event to collect
) WITH (
kafka_topic='source', -- or whatever your source topic is called.
value_format='json' -- or whatever value format you need.
);
CREATE TABLE agg AS
SELECT
key,
COLLECT_LIST(event) as events
FROM source
GROUP BY key;
This will create a changelog topic called AGG by default. As new events are received for a specific key on the source topic, ksqlDB will produce messages to the AGG topic, with the key set to key and the value containing the list of all the events seen for that key.
You can then import this changelog as a stream:
CREATE STREAM agg_stream (
KEY INT KEY,
EVENTS ARRAY<STRING>
) WITH (
kafka_topic='AGG',
value_format='json'
);
And you can then apply some criteria to filter the stream to only include your final results:
STREAM competed_results AS
SELECT
*
FROM agg_stream
WHERE ARRAY_LEN(EVENTS) = 5; -- example 'complete' criteria.
You may even want to use a user-defined function to define your complete criteria:
STREAM competed_results AS
SELECT
*
FROM agg_stream
WHERE IS_COMPLETE(EVENTS);