PRINT¶
Synopsis¶
PRINT topicName [FROM BEGINNING] [INTERVAL interval] [LIMIT limit]
Description¶
Print the contents of Kafka topics to the ksqlDB CLI.
The topicName is case sensitive. Quote the name if it contains invalid characters. See Valid Identifiers for more information.
The PRINT statement supports the following properties:
Property | Description |
---|---|
FROM BEGINNING | Print starting with the first message in the topic. If not specified, PRINT starts with the most recent message. |
INTERVAL interval | Print every interval th message. The default is 1, meaning that every message is printed. |
LIMIT limit | Stop printing after limit messages. The default value is unlimited, requiring Ctrl+C to terminate the query. |
Example¶
The following statement shows how to print all of the records in a topic named
_confluent-ksql-default__command_topic
, (the default name for the topic ksqlDB stores your submitted command in).
Note, the topic name has been quoted as it contains the invalid dash character.
PRINT '_confluent-ksql-default__command_topic' FROM BEGINNING;
ksqlDB attempts to determine the format of the data in the topic and outputs what it thinks are the key and value formats at the top of the output.
Note
Attempting to determine a data format from only the serialized bytes is not an exact science!
For example, it is not possible to distinguish between serialized BIGINT
and DOUBLE
values,
because they both occupy eight bytes. Short strings can also be mistaken for serialized numbers.
Where it appears different records are using different formats ksqlDB will state the format is MIXED
.
Your output should resemble:
Key format: JSON or SESSION(KAFKA_STRING) or HOPPING(KAFKA_STRING) or TUMBLING(KAFKA_STRING) or KAFKA_STRING
Value format: JSON or KAFKA_STRING
rowtime: 12/21/18 23:58:42 PM PSD, key: stream/CLICKSTREAM/create, value: {statement":"CREATE STREAM clickstream (_time bigint,time varchar, ip varchar, request varchar, status int, userid int, bytes bigint, agent varchar) with (kafka_topic = 'clickstream', value_format = 'json');","streamsProperties":{}}
rowtime: 12/21/18 23:58:42 PM PSD, key: table/EVENTS_PER_MIN/create, value: {"statement":"create table events_per_min as select userid, count(*) as events from clickstream window TUMBLING (size 10 second) group by userid EMIT CHANGES;","streamsProperties":{}}
^CTopic printing ceased
The key format for this topic is KAFKA_STRING
. However, the PRINT
command does not know this and
has attempted to determine the format of the key by inspecting the data. It has determined that the
format may be KAFKA_STRING
, but it could also be JSON
or a windowed KAFKA_STRING
.
The value format for this topic is JSON
. However, the PRINT
command has also determined it could
be KAFKA_STRING
. This is because JSON
is serialized as text. Hence you could choose to deserialize
this value data as a KAFKA_STRING
if you wanted to. However, JSON
is likely the better option.