This is really useful - thanks. I have used the 'lock file' method, but there is always the need to cater for people leaving the office for a week's holiday and the lock files are not released. There need to be other housekeeping functions in this scenario.
I see that some NXD such as eXist-db have some utility functions to help with getting an exclusive lock on a file and I was wondering if anyone had real working systems that use these.
Regards
Peter
-----Original Message-----
From: [email protected]
Sent: Mon, 17 Feb 2014 06:11:11 -0600
To: [email protected]
Subject: Re: [Xsltforms-support] getting exclusive lock when editingHello Peter,I think you may be referring to the problem that often comes up is that two people are trying to update the same record at the same time with an XForms application. This is in general known as "Concurrency Control" and locking is really only one option.The problem is that the second person to save their data will overwrite the updates made by the first person. This is often called the "Missing Updates" problem and is also common in version controls systems.There are, in general, there are a few alternative approaches.1) Pessimistic methods aka locking- add a "lock" file to the server when a user opens the file with an "Edit"- only allow read access for locked files- this is fine as long as the server never goes down and users complete their edits- if users open a file for edit and "go to lunch" this blocks all future users unless you have tools that automatically unlock records- forms with high-traffic often have to employ full-time staff to manage locks and timeout settings2) Optimistic methods aka checksum change detection- when a user opens a record, also send the client a checksum of the record before it was used using and algorithm such as an MD5 hash- many web forms application use the HTTP ETAG element to store this information- if a difference is detected then you must warn the second user that did the "Update save" that the underlying data has changed and guide them through a merge processThere are also some nice features like "automatic merge" that we would like to have in our tools. For example when a user saves, detect the change, do a diff and merge each of the changes. This is what GIT and other revision control systems do. There are some XQuery "merge" tools around but I don't know any that are robust.Some useful references:I have wanted to put together an article on this for a while so your question was a good prompt. Please let us know if this helps and which option you take.- DanOn Mon, Feb 17, 2014 at 2:26 AM, peter winstanley <[email protected]> wrote:
Hi List
I know it's marginally off-topic, but I was wondering if anyone had a good illustration or design pattern of getting an exclusive lock on a native XML DB file from an XSLTForm to prevent write collisions?
Kind regards
Peter
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