locking mechanism for shared documents

[GVogeler]

If one user has loaded the shared document in an editor it should be locked for other users: e.g.:

• When loading the document check if xrx:lock exists in document; if exists: check how old the timestamp is (expire time span to be defined, could have same time as the session expire).
• if not: write xrx:lock with sessionID, user and timestamp into the document in the database, load document
• on autosave: update xrx:lock/timestamp
• events.onunload(): save document and delete xrx:lock from the document in the database

[StephanMa]

What should happen if the Document is still locked? Should an i18n-Message appear and inform the user about it or should the input-fields are be marked as read-only?

This Issue isn't easy to solve because of the following points:

1) Before the Document is opened, it must be checked if the Charter is still locked. But at this point, the Charter as a document is already loaded! So it is not possible to perform a Statuscheck on document.load etc.... 2) It should be checked seperately before the editor is shown by the Form. If the Charter is still locked within a session timespan (?), the user should get an Information about that. 3) If the Charter is not locked, write the Lock-Node into the Charter-document. 4) Release Charter after leaving the Editor.

[GVogeler]

The user should receive a "Document currently in use by xy"-i18n message instead of the editmom3. we certainly have to do the check as early as possible:If we store the lock in the xml we could even create a service which issue tested before the document is loaded,couldn't we? A major issue is releasing the document with a document.onUnload event can be interrupted by loosing connectivity: possible Evolution might be to consider documents "in use" automatically as release after a defined time span (e.g. a working day of 8 h) 

[gioele]

Locking is hard and there are plenty of pitfalls:

• https://en.wikipedia.org/wiki/Mutual_exclusion#Types_of_mutual_exclusion_devices
• https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock
• https://en.wikipedia.org/wiki/Two-phase_locking

If a lock has an expiration time, it is no longer a normal lock, but a lease: https://books.google.de/books?id=L-LXO9Rkj54C&lpg=PA3&ots=tBus_-U6Qb&dq=lock%20time%20process%20lease&pg=PA3#v=onepage&q=lock%20time%20process%20lease&f=false

All the aforementioned concepts require the presence of a kernel/supervisor (a process that runs in background, never crashes, knows about every running process). If there is no such process, then you are in the domain of distributed consensus ("who holds the lock?") https://en.wikipedia.org/wiki/Paxos_%28computer_science%29

[StephanMa]

Scheduled for a later release and some further investigations