[MapProxy] mapproxy-seed: why re-request tiles that are in cache?

[Anne Blankert]

Hello,

If I remember correctly, mapproxy-seed should only request new data from
the remote source if the tiles in the cache are out of date or not
available?

According to documentation, https://mapproxy.org/docs/nightly/seed.html,
there are mapproxy-seed options '--continue' and '--progress-file'. Why do
these options exist? mapproxy-seed could find existing tiles in the cache
and quickly skip tiles until it finds out-of-date or non-existing tiles?

I am suspecting there is something wrong in my configuration. File
permissions in the cache seem to be correct and I see the cache is being
filled by mapproxy-seed. However, if I restart mapproxy-seed, the seeding
starts requesting data from the remote source also for tiles that are
already in the cache.

Is this re-requesting by design or is something wrong in my configuration?

My seed.yaml:
seeds:
  nextzenelevation:
    caches: [nextzenelevation_cache]
    grids: [webmercator]
    levels:
      from: 0
      to: 3

My cache definition:
caches:
  nextzenelevation_cache:
    sources: [nextzenelevation_tiles]
    format: image/png
    grids: [webmercator]

sources:
   nextzenelevation_tiles:
     type: tile
     url:
https://tile.nextzen.org/tilezen/terrain/v1/256/terrarium/%(tms_path)s.%(format)s?api_key=xxxx
     grid: webmercator

grids:
  webmercator:
    srs: EPSG:900913
    origin: nw
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