Glossary
The video, streaming and infrastructure terms used across these docs.
Video and streaming
| Term | Meaning |
|---|---|
| VOD | Video on demand: pre-recorded content a viewer can start at any time, as opposed to live streaming. |
| Transcoding | Decoding a video and re-encoding it, usually to a different codec, resolution or bitrate. |
| Rendition | One encoded version of a video at a specific resolution and bitrate (for example 720p at 2,500 kbps). VOD produces up to seven. |
| Bitrate ladder | The full set of renditions offered for one video, from highest to lowest quality. |
| ABR (adaptive bitrate streaming) | The player measures bandwidth and switches between renditions mid-playback to avoid buffering. |
| HLS | HTTP Live Streaming, Apple's ABR protocol. Video is cut into short segment files listed in .m3u8 playlists, served over plain HTTP. |
| Segment | A short chunk of a rendition. VOD targets 6 seconds and uses MPEG-TS (.ts) segments. |
| Media playlist | An .m3u8 file listing one rendition's segments in order. |
| Master (multivariant) playlist | The top-level .m3u8 that lists every rendition with its bandwidth and resolution; the player loads it first. |
| Keyframe (I-frame) | A frame that can be decoded on its own. Segments can only start on a keyframe. |
| GOP | Group of pictures: a keyframe and the frames that depend on it until the next keyframe. |
| H.264 / AVC | The video codec VOD outputs (encoder: libx264), chosen for universal browser support. |
| AAC | The audio codec VOD outputs (128 kbps). |
| CRF | Constant Rate Factor: an x264 quality setting (VOD uses 23) that targets constant visual quality rather than a fixed bitrate. |
| Storyboard / scrubbing preview | Thumbnails shown when hovering the player timeline. VOD stores them as sprite sheets (many thumbnails tiled in one image) indexed by a WebVTT file. |
| ffprobe | FFmpeg's inspection tool; VOD uses it to read duration, resolution, codecs and bitrate. |
Uploads
| Term | Meaning |
|---|---|
| tus | An open HTTP protocol for resumable uploads: the client sends the file in chunks and can resume from the last acknowledged offset. |
| tusd | The reference tus server. In VOD it writes chunks into MinIO and calls the API through hooks (pre-create, post-finish, post-terminate). |
| Uppy | The browser upload library VOD uses, with its tus plugin. |
| Admission control | Limiting how many uploads can run at once (VOD: 8 by default), rejecting extra ones with HTTP 503 and Retry-After. |
Backend and infrastructure
| Term | Meaning |
|---|---|
| Celery task | A Python function run by a background worker instead of inside the web request. |
| Chain / group / chord | Celery workflow primitives: a chain runs tasks in order; a group runs tasks in parallel; a chord is a group plus a callback that runs once all of them finish. |
| Broker | Where queued task messages wait (Redis in VOD). |
| Processing lock | The Redis key video_processing:active_lock that allows only one video's pipeline at a time. |
| Object storage | Storage for files ("objects") addressed by bucket and key over an HTTP API, like Amazon S3. VOD uses MinIO. |
| Bucket | A top-level container in object storage. VOD uses three: raw uploads, thumbnails, processed HLS. |
| Presigned URL | A time-limited URL that grants access to one private object without credentials. VOD uses one for admin downloads of originals. |
| Reverse proxy | A server that receives public requests and forwards them to internal services. VOD's is Caddy. |
| JWT | JSON Web Token: a signed token carrying claims. VOD issues a short-lived access token and a longer-lived refresh token. |
| Soft delete | Marking a row deleted (deleted_at timestamp) instead of removing it. |
| ADR | Architecture Decision Record: a short document explaining one design decision and its alternatives. |