[Xerte-dev] Re: Lockfiles

[David Goodwin]

Yes - the flock thing should work fine (at least it has for me in the past) - but the lock is only held for the duration of the process - which wouldn't necessarily be sufficient for you - as a http request wouldn't persist while someone was editing the stuff. I think I didn't quite understand what you were trying to achieve.



Perhaps something like the following would work for Xerte: ?

1 On going to edit a LO, you could use the flock stuff to ensure you (the user) are the only ones to have gained an EDIT status on an LO. Once you get EDIT status, you mark it as being edited within the db (e.g. learning_object.edited_by = 'dg', learning_object.edited_time = '2011-02-29 10:00').


2. Your edit page has a background AJAXy HTTP request which fires every 30 seconds to update learning_object.edited_time 

3. If someone else goes to edit the LO, if there is a learning_object.edited_time within 45 (?) seconds of now then you don't allow them to edit it - and say it's "in use by dg"


This will only fall down if there are multiple people using the same login within Xerte; so perhaps you'd need to make it slightly more distinct - e.g. using the user's login and a uniqid() token - which would be stored in their $_SESSION? Or use their IP address?

David.


On 29 Feb 2012, at 09:58, Pat Lockley wrote:

> That's the issue. I have a lock system already, and it suffers because they become stale.
> 
> On 29 Feb 2012, at 00:53, David Goodwin <david at palepurple.co.uk> wrote:
> 
>> Oh, it also looks like you need to explicitly call flock($fp, LOCK_UN) afterwards.
>> 
>> The alternative is, I guess, having a field in the DB to store who is currently editing a file - presumably storing a session_id in a field.
>> The only problem with that is it would be possible on either a slow/busy/whatever webserver for two processes to think they had a "write" lock when in fact only one does, and you'd need some way of removing/detecting stale locks - so an LO isn't marked as being edited forever.
>> 
>> David
>> 
>> 
>> On 29 Feb 2012, at 00:34, David Goodwin wrote:
>> 
>>> 
>>> On 28 Feb 2012, at 23:50, Pat Lockley wrote:
>>> 
>>>> Stop concurrent access
>>>> 
>>> 
>>> $fp = fopen('/path/to/something.xml.lock', 'w');
>>> if(flock($fp, LOCK_EX, FALSE)) {
>>>   // we have an exclusive lock on someting.xml.lock; within the scope of this webserver.
>>> }
>>> else {
>>>  // someone else has access.
>>> }
>>> 
>>> See also http://php.net/flock  - note it's behaviour is slightly different on Windows, and it wouldn't scale past more than one web server.
>>> 
>>> 
>>> David.
>>> 
>>>> On 28 Feb 2012, at 23:45, David Goodwin <david at palepurple.co.uk> wrote:
>>>> 
>>>>> What do you mean by "lock an XML file down"
>>>>> 
>>>>> Are you trying to stop concurrent access to it, or something else?
>>>>> 
>>>>> David.
>>>>> 
>>>>> On 28 Feb 2012, at 22:26, Pat Lockley wrote:
>>>>> 
>>>>>> Hi all,
>>>>>> 
>>>>>> Thoughts on how to lock an XML file down?
>>>>>> 
>>>>>> I think it might be the session expiring, or the js might not fire properly. I am pretty sure the lockfile remove code doesn't use the session anymore.
>>>>>> 
>>>>>> 
>>>>>> 
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