Create a KSQL Stream

In KSQL, you create streams from Apache Kafka® topics, and you create streams of query results from other streams.

  • Use the CREATE STREAM statement to create a stream from a Kafka topic.
  • Use the CREATE STREAM AS SELECT statement to create a query stream from an existing stream.

Note

Creating tables is similar to creating streams. For more information, see Create a KSQL Table.

Create a Stream from a Kafka topic

Use the CREATE STREAM statement to create a stream from an underlying Kafka topic. The Kafka topic must exist already in your Kafka cluster.

The following examples show how to create streams from a Kafka topic, named pageviews. To see these examples in action, create the pageviews topic by following the procedure in Writing Streaming Queries Against Apache Kafka® Using KSQL (Docker).

Create a Stream with Selected Columns

The following example creates a stream that has three columns from the pageviews topic: viewtime, userid, and pageid.

KSQL can't infer the topic's data format, so you must provide the format of the values that are stored in the topic. In this example, the data format is DELIMITED. Other options are Avro and JSON.

In the KSQL CLI, paste the following CREATE STREAM statement:

CREATE STREAM pageviews \
  (viewtime BIGINT, \
   userid VARCHAR, \
   pageid VARCHAR) \
  WITH (KAFKA_TOPIC='pageviews', \
        VALUE_FORMAT='DELIMITED');

Your output should resemble:

 Message
----------------
 Stream created
----------------

Inspect the stream by using the SHOW STREAMS and DESCRIBE statements:

SHOW STREAMS;

Your output should resemble:

Stream Name | Kafka Topic | Format
---------------------------------------
PAGEVIEWS   | pageviews   | DELIMITED
---------------------------------------

Get the schema for the stream:

DESCRIBE PAGEVIEWS;

Your output should resemble:

Name                 : PAGEVIEWS
 Field    | Type
--------------------------------------
 ROWTIME  | BIGINT           (system)
 ROWKEY   | VARCHAR(STRING)  (system)
 VIEWTIME | BIGINT
 USERID   | VARCHAR(STRING)
 PAGEID   | VARCHAR(STRING)
--------------------------------------
For runtime statistics and query details run: DESCRIBE EXTENDED <Stream,Table>;

Create a Stream with a Specified Key

The previous KSQL statement makes no assumptions about the Kafka message key in the underlying Kafka topic. If the value of the message key in the topic is the same as one of the columns defined in the stream, you can specify the key in the WITH clause of the CREATE STREAM statement.

For example, if the Kafka message key has the same value as the pageid column, you can write the CREATE STREAM statement like this:

CREATE STREAM pageviews_withkey \
  (viewtime BIGINT, \
   userid VARCHAR, \
   pageid VARCHAR) \
 WITH (KAFKA_TOPIC='pageviews', \
       VALUE_FORMAT='DELIMITED', \
       KEY='pageid');

Confirm that the KEY field in the new stream is pageid by using the DESCRIBE EXTENDED statement:

DESCRIBE EXTENDED pageviews_withkey;

Your output should resemble:

Name                 : PAGEVIEWS_WITHKEY
Type                 : STREAM
Key field            : PAGEID
Key format           : STRING
Timestamp field      : Not set - using <ROWTIME>
Value format         : DELIMITED
Kafka topic          : pageviews (partitions: 1, replication: 1)
[...]

Create a Stream with Timestamps

In KSQL, message timestamps are used for window-based operations, like windowed aggregations, and to support event-time processing.

If you want to use the value of one of the topic's columns as the Kafka message timestamp, set the TIMESTAMP property in the WITH clause.

For example, if you want to use the value of the viewtime column as the message timestamp, you can rewrite the previous CREATE STREAM AS SELECT statement like this:

CREATE STREAM pageviews_timestamped \
  (viewtime BIGINT, \
   userid VARCHAR, \
   pageid VARCHAR) \
  WITH (KAFKA_TOPIC='pageviews', \
        VALUE_FORMAT='DELIMITED', \
        KEY='pageid', \
        TIMESTAMP='viewtime');

Confirm that the TIMESTAMP field is viewtime by using the DESCRIBE EXTENDED statement:

DESCRIBE EXTENDED pageviews_timestamped;

Your output should resemble:

Name                 : PAGEVIEWS_TIMESTAMPED
Type                 : STREAM
Key field            : PAGEID
Key format           : STRING
Timestamp field      : VIEWTIME
Value format         : DELIMITED
Kafka topic          : pageviews (partitions: 1, replication: 1)
[...]

Create a Persistent Streaming Query from a Stream

Use the CREATE STREAM AS SELECT statement to create a persistent query stream from an existing stream.

CREATE STREAM AS SELECT creates a stream that contains the results from a SELECT query. KSQL persists the SELECT query results into a corresponding new topic. A stream created this way represents a persistent, continuous, streaming query, which means that it runs until you stop it explicitly.

Note

A SELECT statement by itself is a non-persistent continuous query. The result of a SELECT statement isn't persisted in a Kafka topic and is only printed in the KSQL console. Don't confuse persistent queries created by CREATE STREAM AS SELECT with the streaming query result from a SELECT statement.

Use the SHOW QUERIES statement to list the persistent queries that are running currently.

Use the PRINT statement to view the results of a persistent query in the KSQL CLI. Press CTRL+C to stop printing records. When you stop printing, the query continues to run.

Use the TERMINATE statement to stop a persistent query. Exiting the KSQL CLI does not stop persistent queries. Your KSQL servers continue to process the queries, and queries run continuously until you terminate them explicitly.

To stream the result of a SELECT query into an existing stream and its underlying topic, use the INSERT INTO statement.

Note

The CREATE STREAM AS SELECT statement doesn't support the KEY property. To specify a KEY field, use the PARTITION BY clause. For more information, see Partition Data to Enable Joins.

The following KSQL statement creates a pageviews_intro stream that contains results from a persistent query that matches "introductory" pages that have a pageid value that's less than Page_20:

CREATE STREAM pageviews_intro AS \
      SELECT * FROM pageviews \
      WHERE pageid < 'Page_20';

Your output should resemble:

 Message
----------------------------
 Stream created and running
----------------------------

To confirm that the pageviews_intro query is running continuously as a stream, run the PRINT statement:

PRINT pageviews_intro;

Your output should resemble:

Format:STRING
10/30/18 10:15:51 PM UTC , 294851 , 1540937751186,User_8,Page_12
10/30/18 10:15:55 PM UTC , 295051 , 1540937755255,User_1,Page_15
10/30/18 10:15:57 PM UTC , 295111 , 1540937757265,User_8,Page_10
10/30/18 10:15:59 PM UTC , 295221 , 1540937759330,User_4,Page_15
10/30/18 10:15:59 PM UTC , 295231 , 1540937759699,User_1,Page_12
10/30/18 10:15:59 PM UTC , 295241 , 1540937759990,User_6,Page_15
^CTopic printing ceased

Press CTRL+C to stop printing the stream.

Note

The query continues to run after you stop printing the stream.

Use the SHOW QUERIES statement to view the query that KSQL created for the pageviews_intro stream:

SHOW QUERIES;

Your output should resemble:

 Query ID               | Kafka Topic     | Query String
--------------------------------------------------------------------------------------------------------------------------------------------
 CSAS_PAGEVIEWS_INTRO_0 | PAGEVIEWS_INTRO | CREATE STREAM pageviews_intro AS       SELECT * FROM pageviews       WHERE pageid < 'Page_20';
--------------------------------------------------------------------------------------------------------------------------------------------
For detailed information on a Query run: EXPLAIN <Query ID>;

A persistent query that's created by the CREATE STREAM AS SELECT statement has the string CSAS in its ID, for example, CSAS_PAGEVIEWS_INTRO_0.

Delete a KSQL Stream

Use the DROP STREAM statement to delete a stream. If you created the stream by using CREATE STREAM AS SELECT, you must first terminate the corresponding persistent query.

Use the TERMINATE statement to stop the CSAS_PAGEVIEWS_INTRO_0 query:

TERMINATE CSAS_PAGEVIEWS_INTRO_0;

Your output should resemble:

 Message
-------------------
 Query terminated.
-------------------

Use the DROP STREAM statement to delete a persistent query stream. You must TERMINATE the query before you can drop the corresponding stream.

DROP STREAM pageviews_intro;

Your output should resemble:

 Message
-------------------
 Source PAGEVIEWS_INTRO was dropped.
-------------------