access_mode - Access mode

The data source access mode defines whether data might be updated or not:

• Read - read access allowed, only
• Update - instances might be updated. Asynchronous updates by different users are supported, but conflicts are signaled and solved. This option is suggested for modifying data,
• Write - instances might be updated, but not asynchronously. Instances are locked after reading.
• undefined - the data source inherits the access mode from its parent (default).

Defining undefined for global data sources, which usually do not have got a parent, will be changed to Read.