ImageResizer DiskCache+AzureReader strange behaviour

Question

I'm using ImageResizer + Diskcache plugin and I'm finding issues to get cache working properly. Either the images are cached forever (regardless how many times I upload a new image) or changing some settings I get the old image in some browsers/computers and the new one in others.

This is what I have now in my web.config:

<add name="AzureReader2" connectionString="blahblahblah" endpoint="http://blahblahblah.blob.core.windows.net/" prefix="~/" redirectToBlobIfUnmodified="false" requireImageExtension="false" checkForModifiedFiles="true" cacheMetadata="true"/>

and:

    <diskcache dir="~/imagecache" autoclean="true" hashModifiedDate="true" subfolders="8192" asyncWrites="true" asyncBufferSize="10485760" cacheAccessTimeout="15000" logging="true" />

Not sure if is something I can achieve using the existing parameters. My goal is to invalidate the cache preferably when the new image has been uploaded without having to change the query string serving the image to get the new one.

I was thinking:

• Maybe having a blob storage trigger that when a replacement image has been uploaded, fires a webhook that deletes the cache for that image?
• Or a web request to my imageresizer app to preload the new image in cache so it replaces the old cached image???

I've seen some posts about using IVirtualFileWithModifiedDate but from what I understand that would have a big performance impact? Is probably 5% of our images request that will have someone uploading an image and expecting it to see it right away since most of the images barely change but it's really frustrating if the image doesn't show the new one not even a day after they have uploaded it!

If I can use IVirtualFileWithModifiedDate to invalidate the cache when the image has changed and not in every image request? Would that be possible?

Answer 1

I get the old image in some browsers/computers and the new one in others.

That different browsers are displaying different versions indicates that either browser caching or proxy/CDN caching is at fault.

ImageResizer's DiskCache hashes the modified date, so it is always as correct as the storage provider.

Regarding your expectations around server-side invalidation:

You're using checkForModifiedFiles="true" cacheMetadata="true", which means that Azure is queried for the latest modified date, but that metadata is cached with a sliding expiration window of 1 hour. I.e, if a URL hasn't been accessed in 1 hour, the next request will cause the modified date to be checked. See StandardMetadataCache.

You can change this behavior by implementing IMetadataCache yourself and assigning that cache to the .MetadataCache member of the storage provider you're using.