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Bug: Problem with merging meta data in Dakar release

On the Dakar merge model, two revisions are attempted to be merged, when edited concurrently. With respect to meta data, merging is defined only for two types of data: file attachments and form fields.

There are several problems:

Nr Problem description Resolution
1 Attribute and title values are not merged for type FIELD  
2 Type FORM is not merged  
3 No attempt is made to merge any other type of meta data  
4 When the stored topic and the later topic have a different form, it takes the stored version as primary while the text takes the later version as primary. (This also creates problems with mandatory fields if the stored version has a mandatory field, which cannot be filled in on the later form.) 1  
5 Form fields are merged by textually merging their values. The undesirable consequences of this are
1. fields that are merged do no longer match the definitions in the form and are not displayed in edit mode, and
2. fields that are unordered as to their values (e.g., checkbox fields) may confuse the merging algorithm.
 
6 The result of merging for forms is only visible for text and textarea fields  

Notes:

1 This scenario arises as follows: User A has previously edited this topic. User B begins to edit the topic. User A goes back in the browser to the topic in edit mode, changes the form, and saves. Then user B saves the topic.

In particular, item (3) makes it pretty much impossible to leverage meta data storage for user application. This is not appropriate.

Test case

Environment

TWiki version: Daklar
TWiki plugins: DefaultPlugin, EmptyPlugin, InterwikiPlugin
Server OS:  
Web server:  
Perl version:  
Client OS:  
Web Browser:  

-- ThomasWeigert - 08 May 2005

Impact and Available Solutions

It is impossible to leverage meta data for user TWikiApplications.

Follow up

OK, I accept all your observations; but what algorithms do you propose instead? I tried to avoid too many assuptions in the meta-data merging code about the type of the data being merged - perhaps that is not going to work, and we need to load the form definition after all. That would allow us to skip merges for all field types except text and textarea, which I think addresses your concerns above? I don't think it's meaningful to merge field titles, is it? And what do you mean by "attribute values"?

If the form is changed, I wanted to merge across as many same-named form fields as possible. Is that wrong? BTW, be careful of the terms "stored topic" and "later topic". The model is as follows:

---------> time
User1 ---A---------------------------------------E-------
         ^edits topic                            ^saves topic, triggering merge
User2 ---------B------------C--------------D--------------
               ^edits topic ^changes form  ^saves topic
When User1 edits the topic it has form type "old form type". User 2 changes that to "new form type", and saves to create what you refer to as the "saved topic". Now User 1 tries to save the topic, but it still has the "old form type". User 1's chosen form type is used because unless User 2 has also changed the form type, it is more likely to reflect the changes. It's a bit of a lottery, I confess. It's tempting to put in META entries for all fields from both forms, which would work if we followed up on what we discussed before and retain fields not in the form in the meta-data.

I don't think any other type of meta-data is meaningful to merge, is it?

-- CrawfordCurrie - 08 May 2005

Fix record

Discussion

Crawford, thanks for the detail. I am still trying to get my head around the scheme, so above was more questions than statements. I'll play around with this and will come back with feedback/proposals.

-- ThomasWeigert - 08 May 2005

But in the meantime, I think we have a real problem here. Please see the discussion in DakarMergeModel.

-- ThomasWeigert - 08 May 2005

A number of strategies have been employed as described in DakarMergeModel. This is a duplicate of that report.

-- CrawfordCurrie - 05 Jun 2005

DakarMergeModel has been merged to core, so I'm marking this one as well.

-- TWikiJanitor - 17 Sep 2008

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Topic revision: r6 - 2008-09-17 - TWikiJanitor
 
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