Implement redis redirects (cluster), switch to enable/disable MGET

[hofrob]

multi get in a cluster with read replicas only works when the keys to be
retrieved are allocated in the same hash slot
https://redis.io/topics/cluster-spec#keys-hash-tags

Q	A
Is bugfix?	yes
New feature?	no
Breaks BC?	no
Tests pass?	yes
Fixed issues	comma-separated list of tickets # fixed by the PR, if any

[hofrob]

When I have time I may take a look at how to solve this with using hash tags in multiSet as proposed in the redis documentation.

Avoiding multiGet would solve this, but you would also need to avoid yii\caching\TagDependency.

[hofrob]

Turns out, this still doesn't solve the issues we have with yii2-redis and AWS ElasticCache. Mainly they are:

• redis redirects are not implemented in yii2-redis
• multiGet (MGET) only works if cache keys are built a specific way

I thought adding read replicas to the config would prevent redirects. Then I just would have to avoid multiGet.

I'm currently trying to implement the MOVED redis redirect response. This way you won't need to configure read replicas in the config (just use the main endpoint of your AWS cluster) and disable multiGet.

[hofrob]

@samdark @cebe

Anything else I should be doing before you can pull this change?

The only thing I can think of right now, is that there should be an implementation of setting multiple keys in the same hash slot. Then the enableMultiGet would be obsolete.

If there's time, I will take a look at it. But right now, this change works well for us (we're using it on AWS EB with ElasticCache Redis cluster mode enabled).

[cebe]

I don't think it's a good idea to open and close the connection each time a redirect occurs. It would be better to have a pool of connections to choose from and call the method on the correct connection object for the other host.
We have a cluster implementation in Yii core for SQL DB Connection. Maybe it is possible to make a generic multi-connection implementation that works for both SQL clusters and redis clusters.

[hofrob]

That's a good point. Will take a look at it when I have time.

[hofrob]

Meanwhile we discovered that even multiSet (MSET) throws an error when:

Redis error: CROSSSLOT Keys in request don't hash to the same slot
Redis command was: MSET (...)

Which I will fix first, before looking at the connection pool.

[hofrob]

#178