Shaka Offline Download - Text segments are downloaded before audio&video

[ilFuma]

Have you read the FAQ and checked for duplicate open issues?
Yes

What version of Shaka Player are you using?
4.1.2

Can you reproduce the issue with our latest release version?
Yes

Are you using the demo app or your own custom app?
Custom

If custom app, can you reproduce the issue using our demo app?
Yes

What browser and OS are you using?
Chrome 108 - Windows 11

What configuration are you using? What is the output of player.getConfiguration()?
{ "drm": { "retryParameters": { "maxAttempts": 2, "baseDelay": 1000, "backoffFactor": 2, "fuzzFactor": 0.5, "timeout": 30000, "stallTimeout": 5000, "connectionTimeout": 10000 }, "servers": { "com.widevine.alpha": "https://rng.stage.ott.irdeto.com/licenseServer/widevine/v1/........" }, "clearKeys": {}, "advanced": { "com.widevine.alpha": { "distinctiveIdentifierRequired": false, "persistentStateRequired": false, "videoRobustness": "SW_SECURE_CRYPTO", "audioRobustness": "SW_SECURE_CRYPTO", "sessionType": "", "serverCertificate": {}, "serverCertificateUri": "", "individualizationServer": "" } }, "delayLicenseRequestUntilPlayed": false, "logLicenseExchange": false, "updateExpirationTime": 1, "preferredKeySystems": [], "keySystemsMapping": {} }, "manifest": { "retryParameters": { "maxAttempts": 2, "baseDelay": 1000, "backoffFactor": 2, "fuzzFactor": 0.5, "timeout": 30000, "stallTimeout": 5000, "connectionTimeout": 10000 }, "availabilityWindowOverride": null, "disableAudio": false, "disableVideo": false, "disableText": false, "disableThumbnails": false, "defaultPresentationDelay": 0, "segmentRelativeVttTiming": false, "dash": { "clockSyncUri": "", "ignoreDrmInfo": false, "disableXlinkProcessing": false, "xlinkFailGracefully": false, "ignoreMinBufferTime": false, "autoCorrectDrift": true, "initialSegmentLimit": 1000, "ignoreSuggestedPresentationDelay": false, "ignoreEmptyAdaptationSet": false, "ignoreMaxSegmentDuration": false, "keySystemsByURI": { "urn:uuid:1077efec-c0b2-4d02-ace3-3c1e52e2fb4b": "org.w3.clearkey", "urn:uuid:e2719d58-a985-b3c9-781a-b030af78d30e": "org.w3.clearkey", "urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed": "com.widevine.alpha", "urn:uuid:9a04f079-9840-4286-ab92-e65be0885f95": "com.microsoft.playready", "urn:uuid:79f0049a-4098-8642-ab92-e65be0885f95": "com.microsoft.playready", "urn:uuid:f239e769-efa3-4850-9c16-a903c6932efb": "com.adobe.primetime" } }, "hls": { "ignoreTextStreamFailures": false, "ignoreImageStreamFailures": false, "defaultAudioCodec": "mp4a.40.2", "defaultVideoCodec": "avc1.42E01E", "ignoreManifestProgramDateTime": false, "mediaPlaylistFullMimeType": "video/mp2t; codecs=\"avc1.42E01E, mp4a.40.2\"" } }, "streaming": { "retryParameters": { "maxAttempts": 2, "baseDelay": 1000, "backoffFactor": 2, "fuzzFactor": 0.5, "timeout": 30000, "stallTimeout": 5000, "connectionTimeout": 10000 }, "rebufferingGoal": 2, "bufferingGoal": 10, "bufferBehind": 30, "ignoreTextStreamFailures": false, "alwaysStreamText": false, "startAtSegmentBoundary": false, "gapDetectionThreshold": 0.1, "durationBackoff": 1, "forceTransmuxTS": false, "safeSeekOffset": 5, "stallEnabled": true, "stallThreshold": 1, "stallSkip": 0.1, "useNativeHlsOnSafari": true, "inaccurateManifestTolerance": 2, "lowLatencyMode": false, "autoLowLatencyMode": false, "forceHTTPS": false, "preferNativeHls": false, "updateIntervalSeconds": 1, "dispatchAllEmsgBoxes": false, "observeQualityChanges": false, "maxDisabledTime": 30 }, "offline": { "usePersistentLicense": true, "numberOfParallelDownloads": 8 }, "abr": { "enabled": true, "useNetworkInformation": true, "defaultBandwidthEstimate": 1000000, "switchInterval": 8, "bandwidthUpgradeTarget": 0.85, "bandwidthDowngradeTarget": 0.95, "restrictions": { "minWidth": 0, "maxWidth": null, "minHeight": 0, "maxHeight": null, "minPixels": 0, "maxPixels": null, "minFrameRate": 0, "maxFrameRate": null, "minBandwidth": 0, "maxBandwidth": null }, "advanced": { "minTotalBytes": 128000, "minBytes": 16000, "fastHalfLife": 2, "slowHalfLife": 5 } }, "preferredAudioLanguage": "en", "preferredTextLanguage": "", "preferredVariantRole": "", "preferredTextRole": "", "preferredAudioChannelCount": 2, "preferredVideoCodecs": [], "preferredAudioCodecs": [], "preferForcedSubs": false, "preferredDecodingAttributes": [], "restrictions": { "minWidth": 0, "maxWidth": null, "minHeight": 0, "maxHeight": null, "minPixels": 0, "maxPixels": null, "minFrameRate": 0, "maxFrameRate": null, "minBandwidth": 0, "maxBandwidth": null }, "playRangeStart": 0, "playRangeEnd": null, "cmcd": { "enabled": false, "sessionId": "", "contentId": "", "useHeaders": false } }

What did you do?
Download a DASH (Widevine) content with

• Multiple subtitles languages
• Multiple audio languages
• Multiple video resolutions

What did you expect to happen?
Shaka should download video/audio/text segments in parallel (accordingly to the numberOfParallelDownloads parameter) and in sequential order (same as playback).

What actually happened?
Shaka downloads all the Text segments at the beginning, then Video, than Audio.

Since text segments are many and very small, the browser is very slow to retreive them and (based on the actual number of sub languages) it can take up to 5/10 minutes to download just all of them for a standard-duration movie.

I tried increasing the number of parallel downloads, but the performance differences are negligible.

Please not that this behaviour started with the version 4.x, oldest version (I tried with Shaka 3.2) downloads the segments in the expected order and are way more fast to complete a download (same manifest, same pc, same network).

[theodab]

The download order was changed in preparation for adding support for background fetching in service workers. Background fetch can't fetch text tracks, if I recall, so the download order was changed to download text first. Otherwise text might not be downloaded at all. See #879.

Unfortunately, when I tried to implement the next step, there ended up being severe bugs in Chrome's implementation of that feature that made going forward impossible. They've fixed those bugs since, but in the meantime our priorities have shifted and background fetch is no longer as important to us.

Since we don't have plans to add background fetch anytime soon, I suppose we could partially/fully revert those changes.

Also, so GitHub links these, this is related to #4625

[ilFuma]

This would be great!

BTW, if in the future the "background fetch" will become a priority again, would it be possible to download the text tracks in advance only if the "background fetch" is requested?
(Keeping the mixed -and faster- download mode in case of foreground fetch)

[theodab]

Probably, yeah.
To be honest, when I made that change, I simply hadn't thought of the case of an asset with segmented text. Most of our sample content has all one file per text language, so I hadn't expected the time overhead for the change to be too bad.