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course: ksqlDB 101

Push Queries and Pull Queries

2 min
Allison

Allison Walther

Integration Architect (Presenter)

Robin Moffatt

Robin Moffatt

Principal Developer Advocate (Author)

Push Queries and Pull Queries

There are two ways of querying data in ksqlDB: push queries and pull queries.

  • Push queries are identified by the EMIT CHANGES clause. By running a push query, the client will receive a message for every change that occurs on the stream (that is, every new message).

  • Pull queries return the current state to the client, and then terminate. In that sense, they are much more akin to a SELECT statement executed on a regular RDBMS. They can only be used against ksqlDB tables with materialized state, that is, a table in which there is an aggregate function. Currently, tables declared against an existing Apache Kafka topic cannot be queried with a pull query (as of ksqlDB v0.15 / February 2021).

Applying this to the aggregate ORDERS_PER_HOUR_BY_MAKE table from Stateful Aggregations (Materialized Views), we can query it both ways. A pull query returns the current state of the aggregate:

pull-query-returns

New events will update the aggregate, but we’d need to execute the pull query again in order to see the new value:

aggregate-two

Compare this to a push query against the same table. Here, we receive a continuous stream of all changes to the aggregate:

KSQLDB PUSH QUERY

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Total 28

  1. Introduction to ksqlDB
    2 min
  2. Interacting with ksqlDB
    2 min
  3. Hands On: Interacting with ksqlDB
    5 min
  4. Creating, Exporting, and Importing Data
    2 min
  5. Filtering with ksqlDB
    1 min
  6. Hands On: Filtering with ksqlDB
    1 min
  7. Lookups and Joins with ksqlDB
    2 min
  8. Hands On: Lookups and Joins with ksqlDB
    2 min
  9. Transforming Data with ksqlDB
    2 min
  10. Hands On: Transforming Data with ksqlDB
    2 min
  11. Flatten Nested Records with ksqlDB
    1 min
  12. Hands On: Flatten Nested Records with ksqlDB
    2 min
  13. Converting Data Formats with ksqlDB
    2 min
  14. Hands On: Converting Data Formats with ksqlDB
    2 min
  15. Merging Two Streams with ksqlDB
    1 min
  16. Hands On: Merging Two Streams with ksqlDB
    2 min
  17. Splitting Two Streams with ksqlDB
    2 min
  18. Hands On: Splitting Two Streams with ksqlDB
    2 min
  19. Streams and Tables
    2 min
  20. Stateful Aggregations (Materialized Views)
    2 min
  21. Hands On: Stateful Aggregations (Materialized Views)
    2 min
  22. Push Queries and Pull Queries
    2 min
  23. Hands On: Push Queries and Pull Queries
    2 min
  24. Under the Covers of ksqlDB
    2 min
  25. ksqlDB Code Lifecycle
    2 min
  26. Apply Lambda Functions to Arrays and Maps
    2 min
  27. Hands On: Apply Lambda Functions to Arrays and Maps
    2 min
  28. Monitoring ksqlDB
    1 min

Related Resources

Presentation

Inside ksqlDB

Docs

Queries

Docs

SELECT (Push Query)

Docs

SELECT (Pull Query)

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