There are three kinds of queries in ksqlDB: persistent, push, and pull. Each gives you different ways to work with rows of events.
Persistent queries are server-side queries that run indefinitely processing rows of events. You issue persistent queries by deriving new streams and new tables from existing streams or tables.
A push query is a form of query issued by a client that subscribes to a result as it changes in real-time. A good example of a push query is subscribing to a particular user's geographic location. The query requests the map coordinates, and because it's a push query, any change to the location is "pushed" over a long-lived connection to the client as soon as it occurs. This is useful for building programmatically controlled microservices, real-time apps, or any sort of asynchronous control flow.
Push queries are expressed using a SQL-like language. They can be used to query either streams or tables for a particular key. Also, push queries aren't limited to key look-ups. They support a full set of SQL, including filters, selects, group bys, partition bys, and joins.
Push queries enable you to query a stream or materialized table with a subscription to the results. You can subscribe to the output of any query, including one that returns a stream. A push query emits refinements to a stream or materialized table, which enables reacting to new information in real-time. They’re a good fit for asynchronous application flows. For request/response flows, see pull Query.
Execute a push query by sending an HTTP request to the ksqlDB REST API, and the API sends back a chunked response of indefinite length.
The result of a push query isn't persisted to a backing Kafka topic. If you need to persist the result of a query to a Kafka topic, use a CREATE TABLE AS SELECT or CREATE STREAM AS SELECT statement.
A pull query is a form of query issued by a client that retrieves a result as of "now", like a query against a traditional RDBMS.
As a dual to the push query example, a pull query for a geographic location would ask for the current map coordinates of a particular user. Because it's a pull query, it returns immediately with a finite result and closes its connection. This is ideal for rendering a user interface once, at page load time. It's generally a good fit for any sort of synchronous control flow.
Pull queries enable you to fetch the current state of a materialized view. Because materialized views are incrementally updated as new events arrive, pull queries run with predictably low latency. They're a great match for request/response flows. For asynchronous application flows, see Push Query. Pull queries are expressed using a strict subset of ANSI SQL.
Execute a pull query by sending an HTTP request to the ksqlDB REST API, and the API responds with a single response.
If you want to learn more about Pull Queries, see Pull Queries.