Feature Ideas/Reprocessing

Rationale

Sometimes, when we try to process new media, we'll fail. This could be for lots of reasons: maybe a transcoding process was killed by a crazed sysadmin, or the file is corrupted, or there might even be a bug in MediaGoblin (crazy, I know!). Right now, when this happens, the unprocessed media lives in the database forever, a zombie. Instead of that, we should periodically retry processing the media, when it makes sense. Maybe we'll have better luck next time; if we do, it'll make the user happy.

Refactoring

Before I get started writing new code, I'd like to refactor the existing processing code. All of the media type processing.py files have chunks of code that look like this:

prepare an output file
with output file:
    process the media
    saved the processed version to the file

After all this is done, we save changes to the MediaEntry. The processing bit is unique, but the code for actually creating the files stays more or less the same, and I think it could be factored out to common functions in the main processing module. As a side benefit, I think separating the file handling code will give us the opportunity to make it more robust, which should help us fix issues like #419.

Core reprocessing code

When should we try to reprocess?

Reprocessing can be more or less helpful depending on why previous processing attempts failed. If we failed because the machine was low on memory or disk at the time, reprocessing stands a good chance of succeeding. If we failed because the media is corrupt, reprocessing will never work unless some code has changed in the meantime.

TODO: We should collect known cases of when processing failed, what it looked like. That will help us write code to determine why processing failed, and whether or not it's worthwhile to retry.

When should we start reprocessing?

There are two forces pushing us in different directions on this. On the one hand, the more often we retry, the sooner the user's media will appear on the site, which makes them happy. On the other hand, if we retry so often that not much can change between different attempts, we're just wasting computing resources to little end. This could hurt our performance on deployments without resources to spare, like SheevaPlugs or Raspberry Pi systems.

Like scheduling code in Linux, there are a million different ways we could do this, no one system is going to be perfect for every site, and we're going to need feedback from lots of users telling us what's good and what's bad before we can make effective adjustments. With all that said, here are some ideas to consider for version 1:

• Note when we last tried to process some media, and don't bother retrying more often than every X minutes. Alternatively, schedule the next time it's okay to retry the media, with some kind of algorithm like exponential backoff.

• Whenever the processing queue is empty, if there's something worth retrying (based on the scheduling above), go ahead and retry it. In order to make sure that reprocessing happens on busy sites, we should probably also give entries a time where we will force the item onto the queue if it hasn't already been retried.

TODO: Discuss (at a meeting?) general priorities about how we want to balance "users see their media ASAP" vs. general site performance. Get feedback about the version 1 scheme, and maybe get alternative proposals.

How does this actually happen?

TODO: Brett needs to investigate the code to figure out which part is responsible for scheduling tasks like this, and start writing in more detail where these different pieces get implemented. (Feel free to give him hints here!)

cwebber's vague thoughts

09:09 < paroneayea> I've been thinking vaguely about a few things related to 
                    that like
09:10 < paroneayea> "what if you don't have the original anymore?  Does it 
                    reprocess it into something more lossy?"
09:11 < paroneayea> "Should we set it up so that things can determine 
                    conditionally if they should be reprocessed?  Ie, if 
                    resolutions have changed, but this one was smaller than the 
                    new lowest resolution anyway?"
09:11 < paroneayea> I'm not sure what the answer to those are but I've only 
                    thought vaguely about them.

User visibility

After we have reprocessing code, logged in users should be able to see information about where their entries stand in the queue: it's going to be processed, it's going to be reprocessed by such-and-such time, it failed completely. There are already some bugs about this (TODO: collect them here).