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Add XSLT podcast post
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v2/content/posts/2023/xslt-podcast/index.md
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v2/content/posts/2023/xslt-podcast/index.md
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title = "Creating a Podcast From a Mastodon Account With XSLT"
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date = 2023-03-01T18:33:33+10:00
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#[extra]
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#updated = 2023-01-11T21:11:28+10:00
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{% aside(title="Just want the feed?", float="right") %}
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Here you go:<br>
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[ATPrewind podcast feed](https://files.wezm.net/aptrewind.rss)
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{% end %}
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I recently discovered the [ATPrewind account on Mastodon][ATPrewind]. It's an account
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sharing "gems discovered while re-listening to [@atpfm] from the very first
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episode. By [@joshua]". ATP is a tech Podcast that's been running for about 10
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years. Each post (so far) from ATPrewind includes a short clip from the show in the
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form of a little video.
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This post describes how I was nerd sniped into creating a podcast from the ATPrewind posts.
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<!-- more -->
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It all started when [I posted the following][my-post] on Mastodon:
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> Ahh this ATP Rewind account is gold https://social.tupo.space/@ATPrewind
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>
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> Keep up the great work @joshua
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[Kashyap replied][Kashyap]:
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> Indeed! This should also be a podcast 🙃
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This was a great idea and it got me thinking about how to do it with the least
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amount of effort.
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In this day and age I imagine many programmers would reach for
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their favourite programming language and code up something to generate a
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podcast feed (real podcasts are just RSS). Perhaps using the Mastodon API or
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similar and then work out a way to host their program.
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With [my recent experience with Deno Deploy][deno-deploy] fresh in my mind I
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considered using it. However I opted for a decidedly late 90s solution: [XSLT].
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According to Wikipedia "XSLT is a language originally designed for transforming
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XML documents into other XML documents". The 'originally' refers to fact that
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you can now generate any text with it, not just XML.
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Since it was created in the era of "XML ALL THE THINGS", XSL templates are
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themselves XML documents. It also makes extensive use of [XPath] expressions to
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select nodes and extract their content.
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### Creating a Podcast Feed
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Every Mastodon account has an RSS feed so I created an XSL template to process
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the ATPrewind RSS feed and add the missing elements required to turn it into a
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valid podcast feed. With some help from [this Dr. Drang][drdrang] post this is
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what I came up with:
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```xml
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<?xml version="1.0" encoding="utf-8" ?>
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<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:media="http://search.yahoo.com/mrss/">
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<xsl:output method="xml" encoding="utf-8" indent="yes"/>
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<!-- First, get everything. -->
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<xsl:template match="node() | @*">
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<xsl:copy>
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<xsl:apply-templates select="node() | @*"/>
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</xsl:copy>
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</xsl:template>
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<!-- Update items to include title and enclosure elements. -->
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<xsl:template match="/rss/channel/item">
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<item>
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<xsl:apply-templates select="node()" />
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<title>Post on <xsl:value-of select="pubDate"/></title>
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<enclosure url="{media:content/@url}" length="{media:content/@fileSize}" type="audio/mp4; codecs="mp4a.40.2"" />
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</item>
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</xsl:template>
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</xsl:stylesheet>
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```
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XML noise aside this document should be fairly self explanatory. It copies all
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nodes from the source RSS feed, then processes each `item` element. To each one
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it adds a `title` element derived from the text "Post on" and the publication
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date of the post, and an `enclosure` element derived from the `media:content`
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element.
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I lie a bit by saying that the enclosure MIME type is `audio/mp4;
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codecs="mp4a.40.2"`. I.e. AAC-LC in MP4 container for the video in each post.
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Running `curl -L https://social.tupo.space/@ATPrewind.rss | xsltproc
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podcast.xsl` produces the podcast feed.
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It took a few tries to get it to work but eventually I was able to convince
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[Overcast] that it was a real podcast. One trick that I'm exploiting here is
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that the MP4 container for video and audio is the same, the audio only version
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is just lacking the video stream. I figured that podcast players might still be
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able to play the video and at least for Overcast it works:
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{{ figure(image="posts/2023/xslt-podcast/overcast-screenshot.png", link="posts/2023/xslt-podcast/overcast-screenshot.png", alt="Screenshot of Overcast showing the podcast 'episodes'.", caption="Screenshot of Overcast showing the 'episodes'.", width=393) }}
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### Deployment
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To deploy this contraption I created a Docker image with `curl` and `libxslt`
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installed:
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```dockerfile
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FROM wezm-alpine:3.17.2
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# UID needs to match owner of /home/rss/feeds volume
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ARG PUID=1000
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ARG PGID=1000
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ARG USER=rss
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RUN addgroup -g ${PGID} ${USER} && \
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adduser -D -u ${PUID} -G ${USER} -h /home/${USER} -D ${USER}
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RUN apk --update add curl libxslt
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COPY ./entrypoint.sh /home/${USER}/entrypoint.sh
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COPY ./podcast.xsl /home/${USER}/podcast.xsl
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WORKDIR /home/${USER}
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USER ${USER}
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VOLUME ["/home/rss/feeds"]
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ENTRYPOINT ["./entrypoint.sh"]
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```
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I made a small script as the entrypoint to the container:
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```sh
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#!/bin/sh
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set -e
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trap 'exit' TERM INT
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while true; do
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curl -L https://social.tupo.space/@ATPrewind.rss | xsltproc podcast.xsl - > /home/rss/feeds/atprewind.rss
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sleep 3600 # 1 hour
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done
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```
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It regenerates the podcast feed each hour. I bind mount the directory for
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files.wezm.net into the container in my Docker Compose config, which takes
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advantage of the fact that nginx is already serving that directory.
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Some may argue that the Docker part of this is not in the spirit of, "least
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amount of effort", but I already have all this set up on my server so adding
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one more container is very little effort. I've written previously about [my
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Alpine Linux server][server] if you'd like to read more.
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Once I pushed the Docker image the podcast was live. The URL is:<br>
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<https://files.wezm.net/aptrewind.rss> if you'd like to subscribe.
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### Future Work
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I whipped all this up before work today and made an assumption that might not
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always hold: there is a single video media attachment on each post. So far
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that's true but I should update the XSL template to only try to generate an
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`enclosure` element if the attachment is present. It would also be a good idea
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to filter for audio and video only in case a post appears with images.
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For now I shall eagerly look forward to the next post appearing in Overcast.
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[@atpfm]: https://mastodon.social/@atpfm
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[@joshua]: https://social.tupo.space/@joshua
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[ATPrewind]: https://social.tupo.space/@ATPrewind
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[Overcast]: https://overcast.fm/
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[XPath]: https://www.w3.org/TR/xpath-31/
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[XSLT]: https://www.w3.org/TR/xslt-30/
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[deno-deploy]: https://www.youtube.com/watch?v=d-tsfUVg4II
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[drdrang]: https://leancrew.com/all-this/2022/08/filtering-my-rss-reading/
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[server]: https://www.wezm.net/technical/2019/02/alpine-linux-docker-infrastructure/
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[my-post]: https://mastodon.decentralised.social/@wezm/109940341596949214
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[Kashyap]: https://mastodon.social/@kgrz/109942702497796855
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