API4KP11 — Avoid conflicts in case of concurrent updates

When more than one users are updating a KA concurrently, only the last update prevails. Some sort of locking is necessary.

An 'optimistic locking' scheme is proposed:

• Upon reading an artifact, it MUST deliver its versionTag.
• When updating an artifact, the caller MUST specify the versionTag upon which the change was made. If another change has meanwhile created a new version, the update referencing a previous version must be refused.

The OpenAPI definitions and the schemata are affected.