The Use or register a shared DB Connection option in Talend Studio is an option that allows you to share a single database connection between multiple components in a job or between parent and child jobs. This option can improve the performance and efficiency of your job by avoiding opening and closing multiple connections to the same database. The Use or register a shared DB Connection option is compatible with the following scenarios:

Sharing a connection between parent and child jobs. You can use this option to pass a database connection from a parent job to a child job that is called by a tRunJob component. This way, you do not need to create a new connection in the child job, but reuse the existing one from the parent job.

Sharing a connection between components in the same subjob. You can use this option to register a database connection in one component (such as tMysqlConnection) and use it in other components (such as tMysqlInput or tMysqlOutput) in the same subjob. This way, you do not need to configure each component with the same connection properties, but use a shared one.

The Use or register a shared DB Connection option is incompatible with the following scenarios:

Using an independent process to run subjob option of tRunJob component. This option allows you to run a child job in a separate JVM process from the parent job. This option is not compatible with sharing a database connection, because each JVM process has its own memory space and cannot access the objects of another process.

Using dynamic job option of tRunJob component. This option allows you to call a child job dynamically based on a context variable that contains the name of the child job. This option is not compatible with sharing a database connection, because dynamic jobs are loaded at runtime and cannot access the metadata of other jobs. References: Talend Open Studio: Open-source ETL and Free Data Integration | Talend, [Use or register a shared DB Connection - 7.3], [tRunJob properties - 7.3]