A Carbon data source is a RDBMS or a custom data source created using the Data Sources sub menu of the Configuration menu in the management console. You can simply use that as the data source of a data service. A Carbon data source is persisted, and can be used whenever required. To create a Carbon data source, see Configuring Data Sources.
After creating a Carbon data source, be sure to copy the JDBC driver to <PRODUCT_HOME>/repository/components/lib directory. For example, cp mysql-connector-java-5.XX-bin.jar /home/user/wso2ds/wso2-dataservices-XX/repository/components/lib.
Next, simply use that data source at the time a data service is created. You can get the Add New Datasource screen shown below on the management console by executing steps 1 to 3 in creating a data service. Then, select Carbon Data Source as the Datasource Type. It opens the Carbon-datasource-specific options as shown below:
To write a query to this data source, see Writing Data Service Queries.