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KSQL and ksqlDB

Note

For the purposes of this topic, "ksqlDB" refers to ksqlDB 0.6.0 and beyond, and "KSQL" refers to all previous releases of KSQL (5.3 and lower).

ksqlDB is not backward compatible with previous versions of KSQL. This means that, ksqlDB doesn't run over an existing KSQL deployment. This topic provides an overview of each incompatibility, as well as guidance around migrating a KSQL deployment to ksqlDB.

Syntax

With the introduction of a new query form, pull queries, ksqlDB's syntax had to evolve such that pull queries and push (streaming) queries can be expressed unambiguously. As a result, one of these two fundamental query forms required new syntax in order to be identified. Although push queries have existed since KSQL's inception, we ultimately decided to add new syntax to express them in ksqlDB so that pull query syntax would remain as standard as possible, because pull queries are much more likely to be used by external SQL-based systems that may be integrated with ksqlDB. For more information, see KLIP-8: Queryable State Stores.

This syntax addition is incompatible with previous versions of KSQL. This means that a streaming query in KSQL will not be interpreted correctly as a streaming query in ksqlDB. Streaming queries in ksqlDB require the EMIT CHANGES modifier, which informs ksqlDB that the caller wants to receive incremental changes to the query result as it runs perpetually.

The following statements show a comparison between a simple streaming query in KSQL versus an equivalent streaming query in ksqlDB:

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-- KSQL:
SELECT * FROM my_stream;

-- ksqlDB:
SELECT * FROM my_stream EMIT CHANGES;

While streaming KSQL queries will not be interpreted as streaming queries in ksqlDB, we have taken measures to mitigate the impact of this incompatibility to the greatest extent possible. In particular, persistent streaming queries don't require the EMIT CHANGES modifier. All persistent queries are interpreted implicitly as if they include an EMIT CHANGES modifier. This implicit behavior means that persistent queries created by using KSQL will run properly, as is, in ksqlDB.

Note

EMIT CHANGES will be required for persistent queries in the future. We recommend migrating all of your persistent streaming queries to include the EMIT CHANGES modifier.

Storage

ksqlDB introduces changes to the underlying materialization storage that are incompatible with the storage format of previous KSQL versions. ksqlDB will therefore not be able to run over an existing KSQL deployment.

HTTP Response Format

In previous KSQL versions, push query responses could return multiple JSON objects that weren't wrapped in an array, so a collection of objects returned by the server wasn't a valid JSON document. In ksqlDB, the HTTP response format for push queries has changed such that all responses are now valid JSON documents. Push query results returned by a ksqlDB server are wrapped in a JSON array.

Also, push query HTTP responses in ksqlDB now include a query result schema as well as the ID assigned to the query.

Finally, null fields are no longer included in push query result rows.

SHOW TOPICS

In ksqlDB, the output of the SHOW TOPICS command no longer includes the "Consumers" and "ConsumerGroups" columns. To obtain this information, use the SHOW TOPICS EXTENDED command.

Last update: 2022-08-19