Elaborate usage of PlatformTransactionManager

[quaff]

There are many places to configure transactionManager, it's unclear whether it's mandatory or not, from my understanding, it's should be optional since dataSource is mandatory, Spring Batch could create DataSourceTransactionManager() as default, correct me if I'm wrong.

And it's unclear whether it's used for batch metadata operations or JDBC reader/writer of step, if Spring Boot's @BatchDataSource and @BatchTransactionManager are used for separated DataSource, which transactionManager should be used for StepBuilder::chunk?

https://github.com/spring-projects/spring-batch/blob/e1b0f156e4db9ae2c3b60b83ec372dac8bddad68/spring-batch-core/src/main/java/org/springframework/batch/core/step/builder/StepBuilder.java#L118