Should we document "cache.adapter.system" ?

[javiereguiluz]

In the main Cache article, you can see this example at the beginning:

# config/packages/cache.yaml
framework:
    cache:
        app: cache.adapter.filesystem
        system: cache.adapter.system

However, the cache.adapter.system is not documented anywhere (we document all the other adapters). So, is this something to be documented for end users or not? Is this something "special" or "internal" and should not be documented? Ping @nicolas-grekas Thanks!

[nicolas-grekas]

the default are good for anyone so not something we need to tell ppl about imho

[plashenkov]

Hi @nicolas-grekas

It is not clear from the docs: whether it possible to use other adapters for system cache or not. I've tried to use redis for system cache but I found that files inside var/cache folder are still created. And I cannot find anything in the documentation about that.

[nicolas-grekas]

Cache warmers may generate files yes, but they won't be used if you configure the system cache for something else IIRC.

We could tell in the doc that while it is possible to reconfigure the system cache, we don't recommend doing so, because the default is really the best possible configuration.

[plashenkov]

We could tell in the doc that while it is possible to reconfigure the system cache, we don't recommend doing so, because the default is really the best possible configuration.

This would be a valuable addition.

[carsonbot]

Thank you for this issue.
There has not been a lot of activity here for a while. Has this been resolved?

[sustmi]

The documentation (https://symfony.com/doc/current/cache.html#configuring-cache-with-frameworkbundle) contains the following example:

# config/packages/cache.yaml
framework:
    cache:
        app: cache.adapter.filesystem
        system: cache.adapter.system

but the succeeding list:

The Cache component comes with a series of adapters pre-configured:

does not contain this adapter.

What I miss are details about this adapter:

• What storage does it it use internally?
• When should I use this adapter over the other adapters?

When I dig in the code I can see that the adapter is created by \Symfony\Component\Cache\Adapter\AbstractAdapter::createSystemCache() factory method.
This method chooses between PhpFilesAdapter and ApcuAdapter based on some logic.

---

I will clarify my situation even though I know it is a bit off topic but I hope that it helps you to understand why I miss more information about cache.adapter.system.

I am currently deciding which cache adapter to use for Doctrine's metadata cache and query cache.

When using these caches in configuration with multiple load-balanced application servers that are updated one-by-one when deploying new version, these caches must not be shared between the old and new versions of the application because each version has a different idea about what value should be stored under certain cache key (eg. SELECT u FROM Users u DQL query gets translated to different SQL queries based on current ORM mapping in the deployed application).

In this case, a cache storage on filesystem of the application instance is a pretty neat way to solve this problem because each deployed version has it's own cache directory. Even Doctrine's documentation itself recommends PhpFileCache.
However PhpFileCache is deprecated and no longer available in doctrine/cache:2.x. I still think that OPCache-backed filesystem cache is a good choice for Doctrine's metadata and query caches so I am looking for some similar cache adapter. PhpFilesAdapter looks like a good replacement.

Can I use cache.adapter.system for Doctrine's metadata and query caches? What are the documented properties of cache.adapter.system?
Can I rely on the fact that it uses filesystem storage that is naturally not shared between multiple server instances?
Can I rely on the fact that it is OPCache-backed so when using PHP-FPM, it stays in RAM between multiple requests and does not cause unnecessary IO load?

If the documentation stated something like this:

The storage provider for cache.adapter.system was chosen to be PhpFilesAdapter which is suitable for things like annotations or metadata that does not change often and is not guaranteed to not change in the future.

it will be much better than now when I basically do not know what to expect from cache.adapter.system so I try to keep away from it (even thought from the name "system" and the fact that it is created inside AbstractAdapter class it seems that it is some kind of main cache storage).

Am I supposed to keep away from cache.adapter.system and system cache pool as an application developer or am I supposed to use it?

[sustmi]

I just noticed that cache.adapter.system is used for Doctrine's metadata and query cache in this example: https://symfony.com/doc/current/reference/configuration/doctrine.html#caching-drivers