[mapserver-users] Mapcache - best way to find broken or white tiles in a service?

[Mark Volz]

Travis,

Thank you for your ideas.  The tiles themselves existed in Mapcache as white images so Mapcache returned a “valid” image instead of a 404.  I was able to find the white tiles by just deleting all the <1K files in the area.  However, if I ever notice the issue again I will definitely consider using the mapcache_seed command line with a shapefile to force the tiles to rebuild...  That would have been quicker than searching for small files!

Thank You!

Sincerely,
Mark Volz, GISP
Lyon County GIS Coordinator

From: Travis Kirstine <traviskirstine at gmail.com>
Sent: Wednesday, February 20, 2019 12:45 PM
To: Mark Volz <MarkVolz at co.lyon.mn.us>
Cc: mapserver-users at lists.osgeo.org
Subject: Re: [mapserver-users] Mapcache - best way to find broken or white tiles in a service?

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You shouldn't have any cached "broken" tiles unless your source WMS is generating the white images (metatiles).  I believe MapCache will generate a 404 and return a empty tile if configured.  This may be the white tiles you are seeing.

   <!-- errors
        configure how error will be reported back to a client:
          - log : no error is reported back, except an http error code.
          - report : return the error message to the client in textual format
          - empty_img : return an empty image to the client. the actual error code is in the X-Mapcache-Error http header
          - report_img : return an image with the error text included inside. not implemented yet.
        the default setting is to report the error message back to the user. In production, you might want to set this to "log"
        if you're paranoid, or to "empty_img" if you want to play nice with non-conforming clients.
   -->
   <errors>report</errors>

It can be very difficult to locate and clean up tiles from a cache, the easiest method and in some cases the fastest is simply to create a new cache and delete the old one.  One alternative is to use the mapcache_seed command line tool to either -f force the creation of new tiles or use the -mode delete option to delete the tiles from the cache.  BTW you can use a polygon (shapefile) with mapcache_seed to only update / delete the desired area using the -d flag.

Also if you are using a disk cache you might be using symlinks which will allow mapcache to "reuse" blank tiles.  If this is the case you might need to find / delete the symlinks rather than a "file"

<cache name="disk" type="disk">
   <!-- base
        Absolute filesystem path where the tile structure will be stored.
        This directory needs to be readable and writable by the user running
        Apache.
   -->
   <base>/tmp</base>
   <!-- symlink_blank
        Enable blank (i.e. uniform color) tile detection. Blank tiles will be
        detected at creation time and linked to a single blank tile on disk
        to preserve disk space.
   -->
   <symlink_blank/>
</cache>



On Tue, 19 Feb 2019 at 16:47, Mark Volz <MarkVolz at co.lyon.mn.us<mailto:MarkVolz at co.lyon.mn.us>> wrote:
Hello,

On my webmap I noticed a few broken (white) tiles in a single area from my mapcache server.  I would like to know the best way to find broken tiles in my mapcache service.

•         Initially I was concerned that I would need to the tile number and folder location by using the BBOX, WIDTH, and TILE RESOLUTION from the request.  However, I figured out using the mapcache demo is easier to work with /mapcache/demo/

•         Would it be better just to delete any tile that is 1K or smaller?  These problibly are tiles that are broken, or intended to be blank in the first place.

•         What do others do to validate that the mapcache tiles have been built appropriately?
Thanks!

Sincerely,
Mark Volz, GISP
Lyon County GIS Coordinator

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