Transform
This documentation is for an unreleased version of Apache Flink CDC. We recommend you use the latest stable version.

Definition #

Transform module helps users delete and expand data columns based on the data columns in the table.
What’s more, it also helps users filter some unnecessary data during the synchronization process.

Parameters #

To describe a transform rule, the following parameters can be used:

Parameter Meaning Optional/Required
source-table Source table id, supports regular expressions required
projection Projection rule, supports syntax similar to the select clause in SQL optional
filter Filter rule, supports syntax similar to the where clause in SQL optional
primary-keys Sink table primary keys, separated by commas optional
partition-keys Sink table partition keys, separated by commas optional
table-options used to the configure table creation statement when automatically creating tables optional
description Transform rule description optional

Multiple rules can be declared in one single pipeline YAML file.

Metadata Fields #

Fields definition #

There are some hidden columns used to access metadata information. They will only take effect when explicitly referenced in the transform rules.

Field Data Type Description
namespace_name String Name of the namespace that contains the row.
schema_name String Name of the schema that contains the row.
table_name String Name of the table that contains the row.
data_event_type String Operation type of data change event.

Metadata relationship #

Type Namespace SchemaName Table
JDBC Catalog Schema Table
Debezium Catalog Schema Table
MySQL Database - Table
Postgres Database Schema Table
Oracle - Schema Table
Microsoft SQL Server Database Schema Table
StarRocks Database - Table
Doris Database - Table

Functions #

Flink CDC uses Calcite to parse expressions and Janino script to evaluate expressions with function call.

Comparison Functions #

Function Janino Code Description
value1 = value2 valueEquals(value1, value2) Returns TRUE if value1 is equal to value2; returns FALSE if value1 or value2 is NULL.
value1 <> value2 !valueEquals(value1, value2) Returns TRUE if value1 is not equal to value2; returns FALSE if value1 or value2 is NULL.
value1 > value2 value1 > value2 Returns TRUE if value1 is greater than value2; returns FALSE if value1 or value2 is NULL.
value1 >= value2 value1 >= value2 Returns TRUE if value1 is greater than or equal to value2; returns FALSE if value1 or value2 is NULL.
value1 < value2 value1 < value2 Returns TRUE if value1 is less than value2; returns FALSE if value1 or value2 is NULL.
value1 <= value2 value1 <= value2 Returns TRUE if value1 is less than or equal to value2; returns FALSE if value1 or value2 is NULL.
value IS NULL null == value Returns TRUE if value is NULL.
value IS NOT NULL null != value Returns TRUE if value is not NULL.
value1 BETWEEN value2 AND value3 betweenAsymmetric(value1, value2, value3) Returns TRUE if value1 is greater than or equal to value2 and less than or equal to value3.
value1 NOT BETWEEN value2 AND value3 notBetweenAsymmetric(value1, value2, value3) Returns TRUE if value1 is less than value2 or greater than value3.
string1 LIKE string2 like(string1, string2) Returns TRUE if string1 matches pattern string2.
string1 NOT LIKE string2 notLike(string1, string2) Returns TRUE if string1 does not match pattern string2.
value1 IN (value2 [, value3]* ) in(value1, value2 [, value3]*) Returns TRUE if value1 exists in the given list (value2, value3, …).
value1 NOT IN (value2 [, value3]* ) notIn(value1, value2 [, value3]*) Returns TRUE if value1 does not exist in the given list (value2, value3, …).

Logical Functions #

Function Janino Code Description
boolean1 OR boolean2 boolean1 || boolean2 Returns TRUE if BOOLEAN1 is TRUE or BOOLEAN2 is TRUE.
boolean1 AND boolean2 boolean1 && boolean2 Returns TRUE if BOOLEAN1 and BOOLEAN2 are both TRUE.
NOT boolean !boolean Returns TRUE if boolean is FALSE; returns FALSE if boolean is TRUE.
boolean IS FALSE false == boolean Returns TRUE if boolean is FALSE; returns FALSE if boolean is TRUE.
boolean IS NOT FALSE true == boolean Returns TRUE if BOOLEAN is TRUE; returns FALSE if BOOLEAN is FALSE.
boolean IS TRUE true == boolean Returns TRUE if BOOLEAN is TRUE; returns FALSE if BOOLEAN is FALSE.
boolean IS NOT TRUE false == boolean Returns TRUE if boolean is FALSE; returns FALSE if boolean is TRUE.

Arithmetic Functions #

Function Janino Code Description
numeric1 + numeric2 numeric1 + numeric2 Returns NUMERIC1 plus NUMERIC2.
numeric1 - numeric2 numeric1 - numeric2 Returns NUMERIC1 minus NUMERIC2.
numeric1 * numeric2 numeric1 * numeric2 Returns NUMERIC1 multiplied by NUMERIC2.
numeric1 / numeric2 numeric1 / numeric2 Returns NUMERIC1 divided by NUMERIC2.
numeric1 % numeric2 numeric1 % numeric2 Returns the remainder (modulus) of numeric1 divided by numeric2.
ABS(numeric) abs(numeric) Returns the absolute value of numeric.
CEIL(numeric) ceil(numeric) Rounds numeric up, and returns the smallest number that is greater than or equal to numeric.
FLOOR(numeric) floor(numeric) Rounds numeric down, and returns the largest number that is less than or equal to numeric.
ROUND(numeric, int) round(numeric) Returns a number rounded to INT decimal places for NUMERIC.
UUID() uuid() Returns an UUID (Universally Unique Identifier) string (e.g., “3d3c68f7-f608-473f-b60c-b0c44ad4cc4e”) according to RFC 4122 type 4 (pseudo randomly generated) UUID.

String Functions #

Function Janino Code Description
string1 || string2 concat(string1, string2) Returns the concatenation of STRING1 and STRING2.
CHAR_LENGTH(string) charLength(string) Returns the number of characters in STRING.
UPPER(string) upper(string) Returns string in uppercase.
LOWER(string) lower(string) Returns string in lowercase.
TRIM(string1) trim(‘BOTH’,string1) Returns a string that removes whitespaces at both sides.
REGEXP_REPLACE(string1, string2, string3) regexpReplace(string1, string2, string3) Returns a string from STRING1 with all the substrings that match a regular expression STRING2 consecutively being replaced with STRING3. E.g., ‘foobar’.regexpReplace(‘oo|ar’, ‘’) returns “fb”.
SUBSTRING(string FROM integer1 [ FOR integer2 ]) substring(string,integer1,integer2) Returns a substring of STRING starting from position INT1 with length INT2 (to the end by default).
CONCAT(string1, string2,…) concat(string1, string2,…) Returns a string that concatenates string1, string2, …. E.g., CONCAT(‘AA’, ‘BB’, ‘CC’) returns ‘AABBCC’.

Temporal Functions #

Function Janino Code Description
LOCALTIME localtime() Returns the current SQL time in the local time zone, the return type is TIME(0).
LOCALTIMESTAMP localtimestamp() Returns the current SQL timestamp in local time zone, the return type is TIMESTAMP(3).
CURRENT_TIME currentTime() Returns the current SQL time in the local time zone, this is a synonym of LOCAL_TIME.
CURRENT_DATE currentDate() Returns the current SQL date in the local time zone.
CURRENT_TIMESTAMP currentTimestamp() Returns the current SQL timestamp in the local time zone, the return type is TIMESTAMP_LTZ(3).
NOW() now() Returns the current SQL timestamp in the local time zone, this is a synonym of CURRENT_TIMESTAMP.
DATE_FORMAT(timestamp, string) dateFormat(timestamp, string) Converts timestamp to a value of string in the format specified by the date format string. The format string is compatible with Java’s SimpleDateFormat.
TIMESTAMPDIFF(timepointunit, timepoint1, timepoint2) timestampDiff(timepointunit, timepoint1, timepoint2) Returns the (signed) number of timepointunit between timepoint1 and timepoint2. The unit for the interval is given by the first argument, which should be one of the following values: SECOND, MINUTE, HOUR, DAY, MONTH, or YEAR.
TO_DATE(string1[, string2]) toDate(string1[, string2]) Converts a date string string1 with format string2 (by default ‘yyyy-MM-dd’) to a date.
TO_TIMESTAMP(string1[, string2]) toTimestamp(string1[, string2]) Converts date time string string1 with format string2 (by default: ‘yyyy-MM-dd HH:mm:ss’) to a timestamp, without time zone.

Conditional Functions #

Function Janino Code Description
CASE value WHEN value1_1 [, value1_2]* THEN RESULT1 (WHEN value2_1 [, value2_2 ]* THEN result_2)* (ELSE result_z) END Nested ternary expression Returns resultX when the first time value is contained in (valueX_1, valueX_2, …). When no value matches, returns result_z if it is provided and returns NULL otherwise.
CASE WHEN condition1 THEN result1 (WHEN condition2 THEN result2)* (ELSE result_z) END Nested ternary expression Returns resultX when the first conditionX is met. When no condition is met, returns result_z if it is provided and returns NULL otherwise.
COALESCE(value1 [, value2]*) coalesce(Object… objects) Returns the first argument that is not NULL.If all arguments are NULL, it returns NULL as well. The return type is the least restrictive, common type of all of its arguments. The return type is nullable if all arguments are nullable as well.
IF(condition, true_value, false_value) condition ? true_value : false_value Returns the true_value if condition is met, otherwise false_value. E.g., IF(5 > 3, 5, 3) returns 5.

Example #

Add computed columns #

Evaluation expressions can be used to generate new columns. For example, if we want to append two computed columns based on the table web_order in the database mydb, we may define a transform rule as follows:

transform:
  - source-table: mydb.web_order
    projection: id, order_id, UPPER(product_name) as product_name, localtimestamp as new_timestamp
    description: append calculated columns based on source table

Reference metadata columns #

We may reference metadata column in projection expressions. For example, given a table web_order in the database mydb, we may define a transform rule as follows:

transform:
  - source-table: mydb.web_order
    projection: id, order_id, __namespace_name__ || '.' || __schema_name__ || '.' || __table_name__ identifier_name
    description: access metadata columns from source table

Use wildcard character to project all fields #

A wildcard character (*) can be used to reference all fields in a table. For example, given two tables web_order and app_order in the database mydb, we may define a transform rule as follows:

transform:
  - source-table: mydb.web_order
    projection: \*, UPPER(product_name) as product_name
    description: project fields with wildcard character from source table
  - source-table: mydb.app_order
    projection: UPPER(product_name) as product_name, *
    description: project fields with wildcard character from source table

Notice: When * character presents at the beginning of expressions, an escaping backslash is required.

Add filter rule #

Use reference columns when adding filtering rules to the table web_order in the database mydb, we may define a transform rule as follows:

transform:
  - source-table: mydb.web_order
    filter: id > 10 AND order_id > 100
    description: filtering rows from source table

Computed columns can be used in filtering conditions, too. For example, given a table web_order in the database mydb, we may define a transform rule as follows:

transform:
  - source-table: mydb.web_order
    projection: id, order_id, UPPER(province) as new_province 
    filter: new_province = 'SHANGHAI'
    description: filtering rows based on computed columns

Reassign primary key #

We can reassign the primary key in transform rules. For example, given a table web_order in the database mydb, we may define a transform rule as follows:

transform:
  - source-table: mydb.web_order
    projection: id, order_id
    primary-keys: order_id
    description: reassign primary key example

Composite primary keys are also supported:

transform:
  - source-table: mydb.web_order
    projection: id, order_id, UPPER(product_name) as product_name
    primary-keys: order_id, product_name
    description: reassign composite primary keys example

Reassign partition key #

We can reassign the partition key in transform rules. For example, given a table web_order in the database mydb, we may define a transform rule as follows:

transform:
  - source-table: mydb.web_order
    projection: id, order_id, UPPER(product_name) as product_name
    partition-keys: product_name
    description: reassign partition key example

Specify table creation configuration #

Extra options can be defined in a transform rule, and will be applied when creating downstream tables. Given a table web_order in the database mydb, we may define a transform rule as follows:

transform:
  - source-table: mydb.web_order
    projection: id, order_id, UPPER(product_name) as product_name
    table-options: comment=web order
    description: auto creating table options example

Tips: The format of table-options is key1=value1,key2=value2.

Classification mapping #

Multiple transform rules can be defined to classify input data rows and apply different processing. Only the first matched transform rule will apply. For example, we may define a transform rule as follows:

transform:
  - source-table: mydb.web_order
    projection: id, order_id
    filter: UPPER(province) = 'SHANGHAI'
    description: classification mapping example
  - source-table: mydb.web_order
    projection: order_id as id, id as order_id
    filter: UPPER(province) = 'BEIJING'
    description: classification mapping example

User-defined Functions #

User-defined functions (UDFs) can be used in transform rules.

Classes could be used as a UDF if:

  • implements org.apache.flink.cdc.common.udf.UserDefinedFunction interface
  • has a public constructor with no parameters
  • has at least one public method named eval

It may also:

  • overrides getReturnType method to indicate its return CDC type
  • overrides open and close method to do some initialization and cleanup work

For example, this is a valid UDF class:

public class AddOneFunctionClass implements UserDefinedFunction {
    
    public Object eval(Integer num) {
        return num + 1;
    }
    
    @Override
    public DataType getReturnType() {
        return DataTypes.INT();
    }
    
    @Override
    public void open() throws Exception {
        // ...
    }

    @Override
    public void close() throws Exception {
        // ...
    }
}

To ease the migration from Flink SQL to Flink CDC, a Flink ScalarFunction could also be used as a transform UDF, with some limitations:

  • ScalarFunction which has a constructor with parameters is not supported.
  • Flink-style type hint in ScalarFunction will be ignored.
  • open / close lifecycle hooks will not be invoked.

UDF classes could be registered by adding a user-defined-function block:

pipeline:
  user-defined-function:
    - name: addone
      classpath: org.apache.flink.cdc.udf.examples.java.AddOneFunctionClass
    - name: format
      classpath: org.apache.flink.cdc.udf.examples.java.FormatFunctionClass

Notice that given classpath must be fully-qualified, and corresponding jar files must be included in Flink /lib folder, or be passed with flink-cdc.sh --jar option.

After being correctly registered, UDFs could be used in both projection and filter expressions, just like built-in functions:

transform:
  - source-table: db.\.*
    projection: "*, inc(inc(inc(id))) as inc_id, format(id, 'id -> %d') as formatted_id"
    filter: inc(id) < 100

Known limitations #

  • Currently, transform doesn’t work with route rules. It will be supported in future versions.
  • Computed columns cannot reference trimmed columns that do not present in final projection results. This will be fixed in future versions.
  • Regular matching of tables with different schemas is not supported. If necessary, multiple rules need to be written.