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WARCIT Media Conversion Workflow

With the 0.4.0, warcit introduces a new workflow for converting video/audio files into web-friendly formats and then placing them into WARCs along with transclusion metadata to enable access from within a containing page.

To allow for maximum flexibility, this process is split into two phases: conversion and transclusion WARC creation.

Media Conversion

warcit includes a standalone conversion utility, warcit-converter which can be used to batch convert files in a directory structure based on extension/regex matching.

The conversion process can be run separately from WARC creation and outputs converted files into a separate directory, recreating the same directory structure.

For example, given a directory structure:

- data/
    - videos/
         - video_file.flv
         - an_audio_rec.ra

Running:

warcit-converter http://www.example.com/ ./data/

with the default rules will result in converted files written into ./conversions directory (by default).

The full url of each file, as with warcit, is created by prepending the prefix to the path in the directory.

The input media in this example would have a full url of http://example.com/media/video_file.flv and http://example.com/media/an_audio_file.ra. The converted files simply have additional extensions added for the full url, such as: http://example.com/media/video_file.flv.mp4, http://example.com/media/an_audio_file.ra.webm, etc...

- data/
    - media/
         - video_file.flv
         - an_audio_rec.ra
- conversions/
    - warcit-conversion-results.yaml
    - media/
         - video_file.mp4
         - video_file.webm
         - video_file.mkv
         - an_audio_rec.mp3
         - an_audio_rec.ogg
         - an_audio_rec.opus

The results of each conversions are written into warcit-conversion-results.yaml. This file can then be used to analyze the results of the conversion, and to inform the transclusion metadata workflow.

Conversion Rules

The default rule set currently specifies conversions for .flv, .mp4, and RealMedia formats into several standardized formats, using ffmpeg.

The current output formats are two web-focused formats and a preservation format:

  • .webm -- vpx9 + opus encoded video + audio, an open format for the web
  • .mp4 -- H.264 + AAC encoded video + audio, primarily for Safari and Apple based platforms.
  • .mkv -- FFV1 codec in a Matroska container.

(For audio only content, .webm, .mp3 and .flac are used instead)

The first two formats are designed to be used for the web in <video> (or <audio>) tags.

The FFV1 format is a recommended preservation format and not designed to be shown in the browser.

It is also possible to specify a custom rules YAML file via the warcit-converter --rules custom-rules.yaml ...

WARC Conversion Record Creation

warcit includes the capability to write converted files as WARC conversion records with a reference to the original file that was the source of the conversion.

To include conversion record creation along with simply include the conversion results output as a parameter to warcit:

warcit --conversions ./conversion/warcit-conversion-results.yaml http://example.com/ ./data/ -o output.warc.gz

The resulting WARC will contain the original urls, eg. http://example.com/media/video_file.flv and http://example.com/media/an_audio_file.ra as resource records, as well as all of the converted files, eg. http://example.com/media/video_file.flv.mp4 and http://example.com/media/an_audio_file.ra.mp3 as conversion records. The conversion records will refer to the record ids and urls + timestamps of the original resource records.

Transclusion Manifest and Metadata

The above procedure allows for converting files in batch and adding them as WARC conversion records. However, it is often useful to reference "transcluded" video and audio from another page, which embeds/transcludes the content.

The information on which resources are transcluded from which pages is not possible to deduce from the media itself, and so must be provided as an additional input to warcit.

warcit supports a transclusion manifest YAML file, which can map resources to their containing/transcluding pages. A manifest might look as follows:

transclusions.yaml:

transclusions:
  http://example.com/media/video_file.flv:
    url: http://example.com/watch_video.html
    timestamp: 20160102
    selector: 'object, embed'

  http://example.com/media/an_audio_file.ra
    url: http://example.com/sample_audio.html
    timestamp: 20170102
    selector: 'a[id="#play"]'

Given this input, running warcit --transclusions transclusions.yaml will generate a reverse index, a metadata record for the containing pages, which point to the media files transcluded from that page. For the above example, two metadata records, with target uris metadata://example.com/watch_video.html and metadata//example.com/sample_audio.html will be created.

The metadata records will simply point to the transclusions, http://example.com/media/video_file.flv and http://example.com/media/an_audio_file.ra respectively.

However, when combining the transclusion manifest with conversion results, all of the conversion records will also be added as metadata.

warcit might then be run as follows:

warcit --transclusions transclusions.yaml --conversions ./conversion/warcit-conversion-results.yaml http://example.com/ ./data/ -o output.warc.gz

Note that the transclusion manifest should only contain the urls of the original, not the converted records, as they will be added automatically.

Transclusion Metadata Format(s)

The generated metadata JSON is originally modeled on the youtube-dl metadata (and has Content-Type application/vnd.youtube-dl_formats+json)

The JSON metadata record for metadata://example.com/watch_video.html might look as follows:

{

  "formats": [
    {
      "command": "ffmpeg -y -i {input} -c:v vp9 -c:a libopus -speed 4 {output}",
      "ext": "webm",
      "mime": "video/webm",
      "name": "webm",
      "original_url": "http://example.com/media/video_file.flv",
      "url": "http://example.com/media/video_file.flv.webm"
    },
    {
      "command": "ffmpeg -y -i {input} -strict -2 {output}",
      "ext": "mp4",
      "mime": "video/mp4",
      "name": "mp4",
      "original_url": "http://example.com/media/video_file.flv",
      "url": "http://example.com/media/video_file.flv.mp4"
    },
    {
      "command": "ffmpeg -y -i {input} -c:v ffv1 -c:a flac {output}",
      "ext": "mkv",
      "mime": "video/x-matroska",
      "name": "ffv1_flac",
      "original_url": "http://example.com/media/video_file.flv",
      "skip_as_source": true,
      "url": "http://example.com/media/video_file.flv.mkv"
    },
    {
      "ext": "flv",
      "mime": "video/x-flv",
      "original": true,
      "url": "http://example.com/media/video_file.flv"
    }
  ],
  "selector": "object, embed",
  "webpage_timestamp": "20160102",
  "webpage_url": "http://www.example.com/watch_video.html"
}

This metadata includes all of the converted records and URLs to load for each format. Based on this metadata, the pywb client side rewriting system can replace an <object> or <embed> tag based on the selector object,embed for example:

<video>
  <source src="http://example.com/media/video_file.webm" type="video/webm"/>
  <source src="http://example.com/media/video_file.flv.mp4" type="video/mp4"/>
  <source src="http://example.com/media/video_file.flv" type="video/x-flv"/>
</video>

(The .mkv format is not included as its marked as skip_as_source while the .flv is included for completeness as the original)

Proposal: Multiple Transcluded Objects Per Page

This is a proposal and has not yet been implemented.

The current format is designed for a single transcluded object. However, it is likely that there will be pages with multiple transcluded objects. Further, it is possible that additional transclusions + conversions may be added later. And, different versions of a page may have different numbers of videos.

For example:

  • A page has multiple videos but only one was initially available. Later, additional content with two more videos was discovered.
  • A page has one video that needed to be converted and one that played natively in the current browser. Later, the other video also needed to be converted.
  • An initial capture of a page has two video that were converted. A later capture has only one video (the other was removed, or shifted to a new page, etc...)

To handle all of these cases, a more flexible metadata format is proposed.

The top-level JSON object in the previous example is instead placed into its own dictionary.

This format, given content-type application/vnd.transclusions+json might look as follows. For a given entry, metadata://example.com/watch_page.html, 2 videos may be listed:

{
  "transclusions":
    {"http://example.com/media/video_file.flv": {..., "formats": {...}},
    {"http://example.com/media/another_video_file.flv": {..., "formats": {...}},

  "webpage_timestamp": "20160102",
  "webpage_url": "http://www.example.com/watch_video.html",
  "creation_timestamp": "20190301",
}

Multiple Transclusion Records

However, at a later time, another transclusion is discovered for the same page and added with a new metadata record:

{
  "transclusions":
    {"http://example.com/media/yet_another_video.flv": {..., "formats": {...}},

  "webpage_timestamp": "20160102",
  "webpage_url": "http://www.example.com/watch_video.html",
  "creation_timestamp": "20191001",
}

When loading the page 20160102/http://www.example.com/watch_video.html, both transclusion metadata records will be loaded, and all 3 videos will be readded to the page, if possible.

- "http://example.com/media/video_file.flv"
- "http://example.com/media/another_video_file.flv"
- "http://example.com/media/yet_another_video.flv"

The transclusion metadata WARC-Date is set to the date of the containing page, while the actual creation date is set in the WARC-Creation-Date header.

Transclusions for Different Timestamps of Same Page

However, if a later version of the same page contains different transclusions, only those transclusions should be loaded. For example, the 20170102 version of the page may have only one video:

{
  "transclusions":
    {"http://example.com/media/video_file.flv": {..., "formats": {...}},

  "webpage_timestamp": "20170102",
  "webpage_url": "http://www.example.com/watch_video.html",
  "creation_timestamp": "20190105",
}

Replay Lookup Behavior

When replaying a particular page, all of the exact match transclusions will be used:

  • When replaying 20160203/http://www.example.com/watch_video.htm, the closest transclusion metadata are:
20160102/http://www.example.com/watch_video.html -- 2 videos
20160102/http://www.example.com/watch_video.html -- 1 video

Since there two records at the exact same timestamp, they will be combined and 3 videos will be added.

  • When replaying 20170203//http://www.example.com/watch_video.htm the closest transclusion metadata record is:
20170102/http://www.example.com/watch_video.html -- 1 video

Since there is only one match, the 1 video from this record is used. Additional transclusion records farther away are not searched.

If additional captures require custom sets of transclusions, additional records can be added at the exact capture time.