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Autoscan rewrites

These rewrites seem to cause a lot of consternation.

This article will attempt to clear them up.

Why are rewrites needed in autoscan?

Basically, autoscan rewrites are a way to convert a path as one thing [typically an app like Sonarr] sees it to a path where another thing [typically an app like Plex] sees it.

The source of the scan [Sonarr, for example] sees a thing at one path, and the target of scan [Plex, for example] may see the same file at a different path. Autoscan uses "rewrites" to convert the source path to the target path.

Do I always need rewrites?

No.

If Sonarr and Plex both see an episode at /mnt/unionfs/Media/TV/SomeShow/Season 01/SomeShow S01E01.mkv, then no rewrite is needed.

If you are setting up saltbox from scratch and use our recommended paths, your autoscan config needs no rewrites for Sonarr/Radarr/Lidarr [a-train or inotify triggers probably still need rewrites].

Rewrites are needed when: 

/the/path/where/the/trigger/sees/files
and 
/the/path/where/the/target/sees/files
are different.

Sonarr has a root directory:

This directory might be internal to the container, as this one is.

Shows are stored in that root:

As are episodes, of course:

This episode path is what Sonarr is going to send to Autoscan.

Similarly, a Plex library has a root path:

And things in that library are relative to that root:

Those two paths are different, so Autoscan needs a rewrite to change this path: 

/tv/#killerpost (2016) (tvdb-307250)/Season 1/#killerpost - S01E01 (tvdb-307250) Payne Potter [HDTV-720p x264 AC3 5.1]-W4F.mkv
to this path: 
/data/TV/#killerpost (2016) (tvdb-307250)/Season 1/#killerpost - S01E01 (tvdb-307250) Payne Potter [HDTV-720p x264 AC3 5.1]-W4F.mkv
so Plex can see it.

What happens during a scan:

Sonarr tells Autoscan:

"Here's a new thing at /tv/#killerpost (2016)/Season 1/#killerpost - S01E01.mkv, tell your friends."

Autoscan first needs to figure out which library to tell Plex about, since it's going to say:

"Please scan this folder: /tv/#killerpost (2016)/Season 1/; it's in library something-or-other."

So Autoscan looks for a Plex library that has /tv/ as one of its root directories. If it finds that to be library ID 12, it tells Plex:

"Please scan this folder: /tv/#killerpost (2016)/Season 1/; it's in library 12."

Howver, in our example above, the Plex library is pointed at /data/TV, so Autoscan won't be able to find the target library [there isn't one with /tv/ as one of its root dirs], so Autoscan will report:

No target libraries found error="/tv/#killerpost (2016)/Season 1/: failed determining libraries" target=plex url=https://plex.mydomain.tld/

This problem is what rewrites are used to solve.

Example rewrite process:

One way to do this is shown in Autoscan's README:

  sonarr:
    - name: sonarr
      priority: 2

      rewrite:
        - from: /tv/
          to: /mnt/unionfs/Media/TV/

targets:
  plex:
    - url: https://plex.domain.tld
      token: XXXX
      rewrite:
        - from: /mnt/unionfs/Media/
          to: /data/

Sonarr sends a path to Autoscan, which uses this rewrite: 

      rewrite:
        - from: /tv/
          to: /mnt/unionfs/Media/TV/
to make this change: 
                  /tv/#killerpost (2016) (tvdb-307250)/Season 1/#killerpost - S01E01 (tvdb-307250) Payne Potter [HDTV-720p x264 AC3 5.1]-W4F.mkv
|||||||||||||||||||||
/mnt/unionfs/Media/TV/#killerpost (2016) (tvdb-307250)/Season 1/#killerpost - S01E01 (tvdb-307250) Payne Potter [HDTV-720p x264 AC3 5.1]-W4F.mkv
then queues the scan.

Later, when Autoscan goes to send the scan to Plex, it uses this rewrite: 

      rewrite:
        - from: /mnt/unionfs/Media/
          to: /data/
to make this change: 
/mnt/unionfs/Media/TV/#killerpost (2016) (tvdb-307250)/Season 1/#killerpost - S01E01 (tvdb-307250) Payne Potter [HDTV-720p x264 AC3 5.1]-W4F.mkv
||||||||||||||||||
             /data/TV/#killerpost (2016) (tvdb-307250)/Season 1/#killerpost - S01E01 (tvdb-307250) Payne Potter [HDTV-720p x264 AC3 5.1]-W4F.mkv
so it matches what Plex expects.

Using this method might be useful if you have multiple targets: 

targets:
  plex:
    - url: https://plex.domain.tld
      token: XXXX
      rewrite:
        - from: /mnt/unionfs/Media/
          to: /plex/sees/files/here/
  emby:
    - url: https://emby.domain.tld
      token: XXXX
      rewrite:
        - from: /mnt/unionfs/Media/
          to: /emby/sees/files/here/
so that the local /mnt/unionfs/Media/ path is used as a common interchange.

You could also do this with a single rewrite as: 

  sonarr:
    - name: sonarr
      priority: 2

      rewrite:
        - from: /tv/
          to: /data/TV/

targets:
  plex:
    - url: https://plex.domain.tld
      token: XXXX

The Sonarr rewrite goes straight to the final path required by Plex: 

     /tv/#killerpost (2016) (tvdb-307250)/Season 1/#killerpost - S01E01 (tvdb-307250) Payne Potter [HDTV-720p x264 AC3 5.1]-W4F.mkv
||||||||
/data/TV/#killerpost (2016) (tvdb-307250)/Season 1/#killerpost - S01E01 (tvdb-307250) Payne Potter [HDTV-720p x264 AC3 5.1]-W4F.mkv
and Plex accepts the path as is.

Of course, that works best when there's only a single target.