Use the Blackhole connector to debug deployments - Realtime Compute for Apache Flink

The Blackhole connector works like /dev/null on Unix: it accepts all input records and discards them silently. Use it to isolate whether a problem in your Flink SQL job comes from the sink configuration or from upstream processing logic.

The Blackhole connector supports both batch mode and streaming mode, and supports data updates and deletions in the sink table.

Use cases

Isolate sink configuration errors: If a sink table using another connector type throws an error and you're not sure whether the cause is a system issue or an invalid WITH clause parameter, change connector to blackhole and click Validate in the upper-right corner of the Drafts tab. If no error appears, the system is healthy—check your original WITH clause parameters.
Measure performance overhead: Run a deployment in the RUNNING state with a Blackhole sink to check whether performance consumption occurs when inserting data into a table, without the overhead of a real external system.
Validate UDF output: Route user-defined function (UDF) output to a Blackhole sink table to inspect the data without writing to a physical table.

Capabilities

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

Limitations

Realtime Compute for Apache Flink using Ververica Runtime (VVR) 2.0.0 or later is required.

Create a Blackhole sink table

Define the sink table with 'connector' = 'blackhole':

CREATE TABLE blackhole_sink(
  name VARCHAR,
  score BIGINT
) WITH (
  'connector' = 'blackhole'
);

To create a Blackhole sink table from an existing source table definition, use the LIKE clause:

CREATE TABLE blackhole_sink WITH ('connector' = 'blackhole')
LIKE table_source (EXCLUDING ALL);

Connector options

Option	Required	Default	Type	Description
`connector`	Yes	None	STRING	The connector type. Set to `blackhole`.

Example

The following example reads from a source table and writes all records to a Blackhole sink:

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

CREATE TEMPORARY TABLE blackhole_sink(
  name VARCHAR,
  score BIGINT
) WITH (
  'connector' = 'blackhole'
);

INSERT INTO blackhole_sink SELECT * FROM table_source;