Seems this is related to this one?

#26954

Thank you.

Yes. I think so. I’ll test and confirm. I’ll make sure to update the docs if fixed.

Hm.. There is definitely some weird behaviour here:

This is my (semi) working example

framework:
    cache:
        pools:
            cache.array:
                adapter: cache.adapter.array

            cache.apcu:
                adapter: cache.adapter.apcu

services:
    cache.app:
        class: Symfony\Component\Cache\Adapter\ChainAdapter
        arguments:
            - [ '@cache.array', '@cache.apcu']
            - 2419200 # One month

But if I try to add the cache.pool tag to my cache.app service then I'll get the same error.

Argument 1 passed to Symfony\Component\Cache\Adapter\ChainAdapter::__construct() must be of the type array, string given,

This is indeed because of CachePoolPass line 109

This would be fine (but strange).. however, other services are using cache.app as their parent. And because of that I get an error again.

So with some help from Symfony Slack, I now know the following:

• Not all adapters support being tagged with cache.pool
• ChainAdapter is one of those pools.
• You can decorate the pool with ProxyAdapter to make sure it can be tagged.

So, I can do something like this:

framework:
    cache:
        app: cache.chain
        pools:
             cache.chain:
                adapter: cache.adapter.psr6 # ProxyAdapter
                provider: my_chain_provider

            cache.array:
                adapter: cache.adapter.array

            cache.apcu:
                adapter: cache.adapter.apcu

services:
    my_chain_provider:
        class: Symfony\Component\Cache\Adapter\ChainAdapter
        arguments:
            - [ '@cache.array', '@cache.apcu']
            - 2419200 # One month

I've updated the PR to the docs: symfony/symfony-docs#11813

Thank you for the help.

I created the cache chain with cache.adapter.redis as my first choice and cache.adapter.array the second. I expected this chain setting will auto switch to cache.adapter.array if redis server is not available. But symfony throws error when I shut down the redis server. What I'm doing here. （symfony 5.1）

framework:
    cache:
        pools:
            custom.app.cache:
                adapter: cache.adapter.psr6
                provider: app.my_chain_provider
                tags: true
            my.cache.redis:
                adapter: cache.adapter.redis
                tags: true
                provider: app.my_custom_redis_provider
            my.cache.array:
                adapter: cache.adapter.array

services:
    app.my_chain_provider:
        class: Symfony\Component\Cache\Adapter\ChainAdapter
        arguments:
            - ['@my.cache.redis', '@my.cache.array']
            - 2419200
    app.my_custom_redis_provider:
        class: \Redis
        factory: ['Symfony\Component\Cache\Adapter\RedisAdapter', 'createConnection']
        arguments:
            - 'redis://localhost'
            - { retry_interval: 2, timeout: 31536000 }

That's not what a chain is for. Please use support channels if you need help.