Use the Print connector to write data for debugging - Realtime Compute for Apache Flink

The Print connector writes each row to the standard output or standard error stream of the TaskManager. Use it during development to inspect intermediate results or verify final output without configuring a production sink.

Print connector output appears in TaskManager logs, not in the Flink Web UI console. Set the TaskManager log level to INFO to see the output.

Supported capabilities

Item	Description
Table type	Result table and data ingestion sink
Running mode	Batch mode and streaming mode
Data format	N/A
Metric	N/A
API type	SQL API and data ingestion YAML API
Data update or deletion in a sink table	Supported

Use cases

Development testing: Add a Print result table alongside your existing sink to verify that the correct rows are flowing through before writing to a production system.

SQL

Syntax

CREATE TABLE print_table (
  a INT,
  b VARCHAR
) WITH (
  'connector' = 'print',
  'logger' = 'true'
);

To mirror an existing table schema without redefining columns, use the LIKE clause:

CREATE TABLE print_table WITH ('connector' = 'print')
LIKE table_source (EXCLUDING ALL)

Parameters

Parameter	Description	Data type	Required	Default value
`connector`	The connector type. Set to `print`.	String	Yes	—
`logger`	Whether to display output in the console.	Boolean	No	`false`
`print-identifier`	A label prepended to each output line. Use this to distinguish results when multiple Print sinks are active.	String	No	—
`sink.parallelism`	The parallelism of the Print sink.	Int	No	Same as upstream

Data ingestion

For data ingestion YAML jobs, use the values connector with print.enabled: true to route output to logs or out files.

Syntax

source:
  type: xxx

sink:
  type: values
  name: Values Sink
  print.enabled: true

Parameters

Parameter	Description	Data type	Required	Default value
`type`	The sink connector type. Set to `values`.	STRING	Yes	—
`name`	A display name for the sink.	STRING	No	—
`print.enabled`	Enables print behavior. Set to `true`.	BOOLEAN	Yes	—
`sink.print.standard-error`	Whether to write output to stderr instead of stdout.	BOOLEAN	No	`false`
`sink.print.logger`	Whether to display output in the console.	BOOLEAN	No	`false`
`sink.print.limit`	The maximum number of records to print.	LONG	No	`2000`
`materialized.in.memory`	Whether to persist binary log events to memory.	BOOLEAN	No	`false`
`error.on.schema.change`	Whether to raise an error when a schema change occurs.	BOOLEAN	No	`false`

Examples

SQL result table

The following example reads from a source table and writes all rows to a Print sink. Check TaskManager logs to see the output.

CREATE TEMPORARY TABLE table_source (
  name VARCHAR,
  score BIGINT
) WITH (
  ...
);

CREATE TEMPORARY TABLE print_sink (
  name VARCHAR,
  score BIGINT
) WITH (
  'connector' = 'print'
);

INSERT INTO print_sink SELECT * FROM table_source;

Data ingestion sink

The following example ingests data from MySQL and prints all rows to the out file.

source:
  type: mysql
  name: MySQL Source
  hostname: ${mysql.hostname}
  port: ${mysql.port}
  username: ${mysql.username}
  password: ${mysql.password}
  tables: ${mysql.source.table}
  server-id: 7601-7604

sink:
  type: values
  name: Values Sink
  print.enabled: true

Limitations

Log level: Set the TaskManager log level to INFO to see Print connector output. Output is written to TaskManager logs, not the Flink Web UI console.
Record limit: TaskManager.out displays a maximum of 2,000 log entries. To inspect specific rows when the data volume is large, add a WHERE clause to filter rows before they reach the Print sink.