QGIS/Windows setting cache size to zero fails on certain WMS services

[rduivenvoorde]

Thanks Andrea for letting us know.
See also this thread on the mailinglist:
https://lists.osgeo.org/pipermail/qgis-user/2019-August/043569.html

On certain WMS services, there is an issue when the users has set the cache size of QGIS to 0 (zero).
I can only reproduce it on Windows though, both in 3.8 and master/3.9

To reproduce:

• start QGIS 3.8 or 3.9
• make sure you normal a valid cache setting
• add a WMS-service with this public url:
  https://geodata.nationaalgeoregister.nl/bestuurlijkegrenzen/ows
• now just add the layer 'provincies', you will see NL in all it's glory :-)
• now open the network settings of QGIS and set the cache size to 0
  
• close the project, create a new one
• go to your wms connections, connect to service above and try to load the same provincies layer
• NOW you see an error (red message in the wms connection dialog)
  the logs tell you:
  2019-08-02T16:42:58 CRITICAL Invalid Layer : Raster layer Provider is not valid (provider: wms, URI: contextualWMSLegend=0&crs=EPSG:4326&dpiMode=7&featureCount=10&format=image/png&layers=provincies&styles&url=https://geodata.nationaalgeoregister.nl/bestuurlijkegrenzen/ows (file: ......srccorerasterqgsrasterlayer.cpp row: 614function QgsRasterLayer::setDataProvider:)

All this ONLY on windows, same service and QGIS versions on Linux work fine.

Making the cache size 1 already gives a valid result.

[gioman]

@rduivenvoorde FYI, this problem (or a very similar one) is bugging us since... long. For example this was also an issue in 2.*

[rduivenvoorde]

@gioman : a hacky way to 'fix' this is to make the size dropdown have a minimum >= 1
What do you mean by 'a similar one' ?
I think there has been some problems with the creation of the GetCapabilities url (if it contained or NOT contained something in the Query params) some versions ago. Which made QGIS give you about the same error. But I'm not aware of a real issue anymore besides this reproducable one.

@jef-n this is probably hard to debug isn't it? One because it occurs only on Windows, and Two because if I'm correct the caching mechanism used is deep-Qt specific?

I think the REAL problem is that people want the cache to be zero because they think/have problems which they think is because of the caching.
So another option would be to have a tickbox to just NOT have any network caching... (probably with some warning that this will really have performance results with wmts/tiled layers...)

[gioman]

What do you mean by 'a similar one' ?

well I remember clearly the following:

suppose you are creating a WMS service, and you are constantly fine tuning your advertised extent, layer list, etc. Then if you use QGIS as client you may want to have that cache set to "0", because you don't want it to make a (cached) request to a service that has changed in the meantime. Problem is (was?) that "0" means (meant?) "no limit" rather than "no cache", so the only solution is (was?) to constantly clear the cache with the "trash" button.

[jmckenna]

Can you add a tooltip in the "size" field saying "warning: changing the default cache size value may prevent parsing of some WMS/WFS/OGC services" ?

Personally I never change it, and instead use the "Clear Cache" button when needed.

[chau-intl]

I'm testing WMTS performance and just ran into this issue on latest master. In my case I don't want QGIS to cache anything and by setting the cache size to 0 I was expecting to accomplish exactly this.

[gioman]

I'm testing WMTS performance and just ran into this issue on latest master. In my case I don't want QGIS to cache anything and by setting the cache size to 0 I was expecting to accomplish exactly this.

long standing issue, that bit me several times. As far as I have understand in the past 0 means "no limit" which is counter-intuitive/plain wrong.

[chau-intl]

Maybe the entire caching configuration could use a little remake. It seems like people are expecting three options for caching:

• None
• All-you-can
• A specific size.

I do however would like to see a more granular approach to caching. I would like to be able to adjust caching more individually. Especially I would like to be able to avoid clearing the cache for databases, since I have 30-50 schemas in a database and each schema has a lot of tables. Reloading this takes about half an hour - which is a very long time.

[Pedro-Murteira]

@rduivenvoorde Hello, is this issue still valid on more recent versions?

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