The fetchers

Overview

The fetchers is the part of the code interacting with the transport protocol, defined in transports, to download and parse:

• the Manifest
• media Segments

Each of those task is performed by a discrete component of the fetchers:

• The Manifest fetcher is used to download and parse the manifest file.

• The SegmentFetcherCreator is used to create Segment fetchers, allowing to download and parse media segments.

The Manifest fetcher

The Manifest fetcher allows to download and parse the Manifest/Playlist of the current transport protocol to return an unified Manifest object.

This is the part of the code that interacts with transports to perform the request and parsing of the Manifest file.

The SegmentFetcherCreator

The SegmentFetcherCreator allows to easily perform segment downloads for the rest of the code. This is the part of the code that interacts with the transport protocols - defined in transports - to load and parse media segments.

To do so, the SegmentFetcherCreator creates “segment fetchers” of different types (example: a video or audio segment fetcher) when you ask for it. Through those fetchers, you can then schedule various segment requests with a given priority.

The priority of this request is then corroborated with the priority of all requests currently pending in the SegmentFetcherCreator (and not only with those on the current segment fetcher) to know when the request should effectively be done - more prioritary requests will be done first.

During the lifecycle of the request, the segment fetcher will communicate about data and metrics through several means - documented in the code.

Priorization

Each Segment request can be linked to a priorization number. Such number will indicate which segment is needed more immediately than other (the lower it is, the higher the priority of the segment is).

This is for example used to indicate that a very close video segment has a higher priority than some distant audio segment (both might be scheduled at the same time depending on the situation).

If the request has no priorization number, the lowest priorization number (the highest priority) will be set on it: 0

Basically, any new request will have their priorization number compared to the one of the current request(s) done by the SegmentFetcherCreator:

• if no request is already pending, we perform the request immediately

• if (and only if) this priorization number is higher (so lower priority) than all current requests, this new request will be postponed to let the higher-priority ones finish.

• If a new request has a priorization number lower or equal than all current downloads, we perform the request immediately without interrupting the current, lower-priority ones.

The priority of a download can be updated at any time, until this request either has finished, was canceled or failed. The same rules apply when the priorization number is updated (if this request is already pending, we keep it going in any case).