UploadWizard/Operations assessment

I was asked about the potential effects of Extension:UploadWizard on Wikimedia operations. Here's what I (User:NeilK) know, which amounts to a lot of educated guesses. It's easy to tell you the differences between one uploaded file the old way versus the new way. It's harder to guess how this is going to affect uploading patterns overall.


  1. We do not face any new kinds of threats. This tool just uses the existing API. We are not any more vulnerable now than we were before.
  2. Per uploaded file, Special:UploadWizard uses less resources than the old upload page on Commons.
  3. Due to increased usability, it probably will greatly increase the amount of uploads per day.
  4. Ops must pay a little more attention to keeping temporary storage "clean".

Changes in serving the upload page edit

Table of typical bandwidth usage of Commons' enhanced Special:Upload versus UploadWizard

When it comes to serving the actual page itself, Special:UploadWizard is actually somewhat more efficient than the old Special:Upload. This is because more functionality is concentrated in jQuery and better delivered, using ResourceLoader. So, to simply load the application, you won't see any major differences.

Both pages also make lots of API calls to validate various fields, but it turns out to be about the same in terms of efficiency.

The major difference is that Special:UploadWizard displays thumbnails from stashed files. When the browser can't create the thumbnail locally, it does an API call. This does not add very much to the page in terms of downloaded bytes (maybe 100K at most) but it will require more processing power to make these thumbnails.

Increased volume/burstiness in thumbnailing edit

When UploadWizard uploads a file, it may immediately request a thumbnail, and may request a larger one if the user clicks on the small thumbnail. Previously, it was unlikely that any thumbnail size other than the "standard" one would be requested.

Often the thumbnail isn't there on the first try, so the UploadWizard now does exponential backoff (will retry to fetch the thumbnail in 2, 4, 8, 16, 32 seconds).

This applies more to less advanced browsers (FFox before 3.0, any version of IE before 10). Modern browsers such as Chrome and Firefox >=3.5 may not need to use the thumbnail server at all, since we have ways of creating the thumbnail in-browser.

Recommendation for ops: monitor CPU, cache, network utilized by thumbnailing servers; expect increased usage

Potential for some kinds of resource exhaustion edit

This is not a new problem, but UploadWizard may make it easier for users to upload zillions of small files, and leave them in the stash (the FileRepo temporary zone). See bugzilla:26063. Note: this potential problem exists in the API, and has since late fall 2010; it is not actually UploadWizard-specific.

The proposed solution, to be implemented by UploadWizard and/or MediaWiki developers, is to have a maximum on the number of files a user can have in the stash at any given time (say, 100 or so). When any more are uploaded, some are removed, even if incomplete.

Recommendation for ops: the per-user check mentioned above can be implemented in MediaWiki software, so no ops action is needed there. But for extra security against this class of problem, cronjobs should be employed to sweep that area clean of files older than 6 hours or so, and to also maintain the total number of files stashed below some threshold.

Increased volume of uploads edit

We expect that the increased ease of use will, over time, accelerate the number of uploads and the need for permanent storage. We have no idea how much.

Recommendation: monitor growth of number of files and average size, see if increased volume will accelerate storage growth needs

Increased 'burstiness' of uploads from a single user edit

UploadWizard allows users to upload multiple files relatively easily. This will cause numerous files to be uploaded within a few seconds of each other.

The tool is currently limited to only allow ten uploads in total per invocation. However, this configuration is on the client and thus can be changed by the client. Or, the user can open the tool in multiple browser tabs.

We can block abuse server-side with the measures discussed above under "Potential for some kinds of resource exhaustion".

Recommendation: monitor daily/hourly/minute-by-minute variation in number of uploaded files, see if increased burstiness needs extra capacity

More simultaneous uploads from the same user edit

The tool is designed to allow simultaneous uploads.

When simultaneous uploads are turned on, the tool will allow some configured number of simultaneous transactions. For example, if the user is trying to upload seven items, it will at first start three of them. When one finishes, the tool will start another upload, and so on, until the full list of seven has been uploaded.

This can be circumvented client side by hacking the configuration (it's just Javascript) or opening multiple browser windows.

We can block abuse server-side with the measures discussed above under "Potential for some kinds of resource exhaustion". However, in general, we are not aware of any design limitations in MediaWiki or its backend file store that cause problems when the same user makes simultaneous accesses.

Recommendation for ops: none, other than cronjobs already mentioned

Increased number of files going to temporary zone (and/or being abandoned there) edit

Currently, the temporary stash area is only used when a file has a problem that can be corrected by some user action. For UploadWizard, it is the first step on the journey to publishing the file. It is possible for the user to abandon the file in the stash if they don't complete the process.

Recommendation for ops: none, other than cronjobs already mentioned

Leftover records from abandoned uploads edit

Following Raindrift's changes to UploadWizard, temporary files will have a database record. This will need to be periodically cleaned.

There is a maintenance script that does this, $IP/maintenance/cleanupUploadStash.php. It should be run every once in a while, and is currently set to remove files more than 6 hours old.

Recommendation for ops: none, other than cronjobs already mentioned