Fixing MySQL CDC Snapshot Backfill Error In RisingWave

When working with RisingWave and MySQL CDC (Change Data Capture), you might encounter a frustrating error during the snapshot backfill process. This error often manifests as: Could not retrieve 'bool': Couldn't convert the value 'Null' to a desired type. This article dives deep into the causes of this error, provides a step-by-step guide to reproduce it, and offers a potential solution.

Understanding the Root Cause

At the heart of the issue lies in how RisingWave's connector parses MySQL data, specifically boolean values. The error message indicates that the connector is attempting to convert a Null value from MySQL into a boolean (bool) type within RisingWave. This conversion fails because a Null value cannot be directly interpreted as either true or false. This problem often arises when dealing with tables where boolean columns allow Null values.

The specific location in the RisingWave code where this error occurs is highlighted in the provided bug report:

https://github.com/risingwavelabs/risingwave/blob/main/src/connector/src/parser/mysql.rs#L83
There's a redundant take here, we can remove this line.

While the bug report mentions a redundant take operation, the core issue remains the handling of Null values during boolean conversion. Let's investigate a scenario to replicate this error and then propose a workaround.

Keywords: MySQL CDC, snapshot backfill, RisingWave, boolean conversion, Null values, data integration.

Step-by-Step Guide to Reproduce the Error

To effectively troubleshoot, reproducing the error in a controlled environment is crucial. Follow these steps to replicate the Could not retrieve 'bool' error:

1. Set up a MySQL Database: You'll need a MySQL database instance. Ensure you have the necessary credentials (host, port, username, password) and a database named test.

2. Create a Table with a Nullable Boolean Column: Execute the following SQL statements in your MySQL database to create a table named t1 with an id (BIGINT, PRIMARY KEY) and a value (TINYINT(1), NULLABLE) column:

   drop table if exists t1;
   create table t1 (
       id bigint primary key,
       value tinyint(1) null default null
   );
   insert into t1 (id, value) values (1, null);
   commit;
   

   Note that tinyint(1) is commonly used in MySQL to represent boolean values, and we're explicitly allowing Null values in the value column.

3. Configure RisingWave: Connect to your RisingWave instance using a PostgreSQL client (e.g., psycopg2).

4. Create a MySQL CDC Source: Define a source in RisingWave that connects to your MySQL database. Replace the placeholders with your actual MySQL credentials:

   create source mysql with (
       connector = 'mysql-cdc',
       hostname = '127.0.0.1',
       port = '3306',
       username = 'root',
       password = 'hehehe',
       database.name = 'test',
       debezium.poll.interval.ms = '1'
   );
   

5. Create a Materialized View: Define a materialized view in RisingWave that ingests data from the MySQL source. Importantly, map the value column to a bool type and enable snapshotting:

   create table t1 (
       id bigint primary key,
       value bool
   ) with (
       snapshot = 'true'
   ) from mysql table 'test.t1';
   

6. Observe the Error: Upon executing the CREATE TABLE statement with snapshot = 'true', RisingWave will attempt to perform a snapshot backfill of the data from MySQL. This is when the Could not retrieve 'bool' error is likely to occur.

Keywords: reproduce error, MySQL setup, nullable boolean, RisingWave source, materialized view, snapshotting.

Analyzing the Error Message and Log

The provided error message and log offer valuable clues about the error's origin. Let's break down the key components:

• Panic Location: The log points to mysql_common-0.32.1/src/value/convert/mod.rs, indicating that the error occurs during the conversion of a MySQL value.
• Error Message: Could not retrieve 'bool': Couldn't convert the value 'Null' to a desired type clearly states the conversion failure.
• Stack Trace: The stack trace provides a call stack that shows the sequence of function calls leading to the error. Notably, it includes risingwave_connector::parser::mysql::mysql_datum_to_rw_datum, which is responsible for converting MySQL data to RisingWave's internal data representation.
• Await Tree Context: The await tree context shows that the error occurs during the backfill_snapshot_read stage, confirming that it's related to the snapshot backfill process.

By examining these details, we can pinpoint the issue to the combination of a Null value in a MySQL boolean column and the snapshot backfill mechanism in RisingWave.

Keywords: error message analysis, log analysis, stack trace, await tree, snapshot backfill.

Solution and Workarounds

To resolve this error, you have several options:

1. Adjust the data type in RisingWave

One of the simplest solutions is to change the data type of the value column in the RisingWave table to BOOLEAN. This allows the column to store NULL values and should prevent the error.

create table t1 (
    id bigint primary key,
    value BOOLEAN
) with (
    snapshot = 'true'
) from mysql table 'test.t1';

2. Data Transformation during Ingestion

Another approach is to transform the data during ingestion to replace Null values with a default boolean value (e.g., false). This can be achieved using a CASE statement in the CREATE TABLE statement:

CREATE TABLE t1 (
    id BIGINT PRIMARY KEY,
    value BOOLEAN
) WITH (
    snapshot = 'true'
) AS
SELECT
    id,
    CASE
        WHEN value IS NULL THEN false
        ELSE value
    END
FROM mysql.test.t1;

This statement selects the id and transforms the value column. If value is Null, it's replaced with false; otherwise, the original value is used. This ensures that the value column in RisingWave never contains Null values.

3. Handle Nulls in the Application Layer

If modifying the RisingWave table definition or data transformation during ingestion isn't feasible, you can handle Null values in your application layer. This involves writing code to check for Null values and handle them appropriately before processing the data.

Addressing the Redundant Take

The bug report mentions a redundant take operation in the mysql.rs file. While removing this redundant operation might not directly fix the Null value conversion error, it can improve the code's efficiency. If you're comfortable modifying the RisingWave source code, you can remove the line and rebuild the connector. However, ensure you thoroughly test the changes to avoid introducing new issues.

Conclusion

The Could not retrieve 'bool' error during MySQL CDC snapshot backfill in RisingWave arises from the improper handling of Null values in boolean columns. By understanding the error's cause, reproducing it with a step-by-step guide, and applying the suggested solutions, you can overcome this challenge and ensure seamless data integration between MySQL and RisingWave.

Keywords: error resolution, data type adjustment, data transformation, Null handling, RisingWave best practices.

For further information on data integration and CDC, refer to reliable sources such as the Debezium documentation.