SourceCacheIdentity

The host's own name for one version of one source, keying the keyframe sidecar (keyframeIndexCacheDirectory) instead of the address the engine was handed.

The URL-based identity cannot survive an address that changes between launches while the bytes stay put — a host's localhost range proxy on a fresh port with a fresh token is exactly that, so every app restart was a cache miss. A host that knows which item it is playing, and holds a strong validator for it from its own server's metadata, says so here, and the sidecar then binds to that: the address, the transport and its ETag stop mattering, so it works over FFmpeg's own HTTP and a host input as well as the coordinated reader.

• namespace: whose item it is — a server ID, so two servers' item numbers never meet.
• item: the item (or media part) on that server.
• etag: a strong validator of the item's bytes, as the server reports it. A weak one (W/…) or an empty one is no proof of the same bytes, so it turns the sidecar off for the source — never back to the URL, which is the key the host is replacing.

The host vouches for it, so it must change whenever the bytes do: a map trusted on a stale tag plans segment cuts on keyframes the new file does not have. The byte size and container duration of the open still go into the key beside it.