Common variables and values

video_id vs. youtube_id

Conceptually...

video_id is an abstract ID that corresponds to a video lesson; youtube_id corresponds to an actual video (from YouTube!). Usually video_id corresponds to many youtube_ids (one per language), and a youtube_id corresponds to a single video_id. However, this is not strictly so--one youtube_id can be used for multiple video_ids (if a single video encapsulates multiple video lessons), and a video_id could have multiple youtube_ids for a single language (if the first was not very good, and was re-recorded)

In our code....

• video_id should be used for all reporting and for completion.
• Youtube_id is used for metadata (exactly which video was watched that led to this video_id being completed?) and for streaming videos (including showing subtitles), downloading, and deleting videos.

Therefore, youtube_id is used mainly in the i18n and updates apps, while video_id is used largely in the coachreports and caching code, while both are used in main, khanload, and elsewhere.