Implement OWA Downloading algorithm

[ozten]

We should build a server side version of the algorithm that is in Fennec for deciding if an OWA Version has changed.

Assigning to @mykmelez to document via comments in this bug (or wherever).

[mykmelez]

The algorithm lives in the very large function checkForUpdate. It looks something like this:

• retrieve the manifest, specifying the etag of the old manifest in the If-None-Match request header
• if the response status is not 200, the app is unchanged
• otherwise, ensure the new manifest is valid (valid JSON, passes checkManifest, etc.)
• save the etag of the new manifest
• if the hashes of the old and new manifests differ, the app has been updated
• otherwise, if the app is a hosted app, and it has an _appcachepath property, check for an appcache update
• if the appcache has been updated, the app has been updated

Up until the appcache check, the algorithm seems fairly straightforward. But the appcache check is implemented in a different part of the codebase and seems complex. So I'm leery of duplicating its algorithm on the server, although I'm unsure what the alternatives are. This part needs more research.

[ozten]

Awesome, thanks!

[ozten]

otherwise, if the app is a hosted app, and it has an appcache_path property, check for an appcache update

also...

otherwise, if the app is a packaged app, resolve package_path and check to see if the hash of the package has changed.

Save hash of package with the has of the manifest.

(right?)

[mykmelez]

otherwise, if the app is a packaged app, resolve package_path and check to see if the hash of the package has changed.

I thought we were doing this, but then I didn't see it when I read the algorithm. But I might have missed it.

[ozten]

Design: Issue #13 creates the APK Metadata. Issue#18 integrates with S3. Each controller deamon will make requests with the OWA Downloader and then check it's results (using the above algorithm) against the APK Metadata. If it is a cache hit, it will get the APK out of S3.

The OWA Downloader will use either the modified module or a Squid reverse proxy to optimize the requests.

[mykmelez]

After thinking about it more, I think we shouldn't try to support appcache-based updates. At least not in the initial implementation (nor, for that matter, should we try to pre-cache the appcache and include it in the APK). It's too likely to be a rat hole, given the complexity of the API. And would only help a small portion of apps in any case. Let's focus on the majority cases.

[ozten]

Filed Issue #29. Assuming we deploy with Squid, everything else is this bug is finished. Closing.