Concurrency – Handling or Preventing Conflicts in a Multi-User System

concurrencylocks

I have a web application that is accessed by multiple users from different locations worldwide. Let's say I have an "edit" form for a specific document and two (or more) users are editing the document at the same time. I know that both of the users can edit the document and submit it, but user1 will not see that user2 made changes to the document and might submit an older version of it.

I was thinking about adding some kind of a lock, but then, if user1 only opened the document for edit, but never changed anything (and kept it open) user2 will never be able to edit it. Therefore, I was thinking to add a Timeout for the editor, but then user1 might time out before he finished doing his changes to the document (let's say, went out for launch).

The question is, how would one prevent the document from being edited from one user while the other changes it?

Note: if this question does not belong here, please explain why and where could I ask such a question.

Best Answer

The basics of implementing optimistic concurrency aren't that hard.

First, you'll need a way to identify a row (or series of rows) in your database (think version number).

Second, when your application loads the data for your user's to review or edit, load the version of the data as well (see above).

Third, when your user goes to save their changes, you'll want to ensure that the version of the data they're changing matches the version of data that exists in the database. If the version numbers don't match then you'll raise and handle some kind of stale data exception.

Related Solutions

PHP file_put_contents File Locking

I know this is ages old, but in case someone runs into this. IMHO the way to go about it is like this:

1) Open the original file (e.g. original.txt) using file_get_contents('original.txt').

2) Make your changes/edits.

3) Use file_put_contents('original.txt.tmp') and write it to a temp file original.txt.tmp.

4) Then move the tmp file to the original file, replacing the original file. For this you use rename('original.txt.tmp', 'original.txt').

Advantages: While the file is being processed and written to the file is not locked and others can still read the old content. At least on Linux/Unix boxes rename is an atomic operation. Any interruptions during the file writing don't touch the original file. Only once the file has been fully written to disk is it moved. More interesting read on this in the comments to http://php.net/manual/en/function.rename.php

Edit to address commments(too for comment):

https://stackoverflow.com/questions/7054844/is-rename-atomic has further references to what you might need to do if you are operating across filesystems.

On the shared lock for the reading I am not sure why that would be needed as in this implementation there is no writing to the file directly. PHP's flock (which is used to get the lock) is a little but unreliable and can be ignored by other processes. Thats why I am suggesting using the rename.

The rename file should ideally be named uniquely to the process doing the renaming so as to make sure not 2 processes do the same thing. But this of course does not prevent editing of the same file by more than one person at the same time. But at least the file will be left intact (last edit wins).

Step 3) & 4) would then become this:

$tempfile = uniqid(microtime(true)); // make sure we have a unique name
file_put_contents($tempFile); // write temp file
rename($tempfile, 'original.txt'); // ideally on the same filesystem

API Concurrency – How to Prevent User from Requesting API Method Multiple Times in Parallel

I think your locking strategy will work, although I'd suggest looking at a distributed lock manager (e.g., Apache's Helix project) instead of going to the database just to get a lock. This would ensure that users didn't get locked out if your service gets restarted mid-request.

Another possibility would be to bucket the user IDs to queue them to specific request handlers; this would serialize the parallel requests, but is probably more complicated and could lead to processing bottlenecks (at least until you got your configuration tuned).

Best Answer

Related Solutions

PHP file_put_contents File Locking

API Concurrency – How to Prevent User from Requesting API Method Multiple Times in Parallel

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