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I can't find any docs on this so I assume it's not possible, but...

If you run Sitecore in Azure PaaS and have scale-out enabled (e.g. you're running 2x S3 instances) then can you have a shared/centralized cache, much like the Redis Session State cache?

I'm talking about the built-in item, data, prefetch caches that you can clear from /sitecore/admin/cache.aspx. Do these have to be stored in-memory on each instance?

If so, it seems a massive performance problem that each time a new instance spins up it will have to re-create all of those caches from scratch when all the other instances have already done the heavy lifting.

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The answer of your question “Do these have to be stored in-memory on each instance”? is “YES

By default, All the Sitecore cache (Item, data, Prefetch, html) are stored in the in-memory/In-Proc of the each server(CM,CD).

But technically it’s possible to implement Redis as output cache and stored the in-memory/In-Proc HTML cache into the output/Out-Proc cache(Redis), Sitecore 8.2 has introduced the abstraction for the cache which makes it possible.

But this implementation has its own limitation which could impact the website performance as well; Martin Ivan explained this limitation in detail, please read this thread for more information:

https://community.sitecore.net/developers/f/8/t/8512#pi214filter=all&pi214scroll=false

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  • To clarify here - you can only do this for the HtmlCache - the Item, Data & Prefech caches are baked into the Database object and there is no abstraction for them AFAIK.
    – Richard Seal
    Commented Jun 19, 2018 at 19:33
  • Thanks @RichardSeal for the clarification..I updated the same. Commented Jun 19, 2018 at 19:36
  • Thanks both, I guess this answers my question pretty neatly.
    – theyetiman
    Commented Jun 20, 2018 at 8:18
  • For performance tuning if patch config created to increase cache size for both master and web db then it's need to be deployed on both CM and CD app services in Azure PaaS?
    – Amit Kumar
    Commented Mar 6, 2019 at 17:12

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