When to use - redis or memory in mysql?

Question

MySQL

1) When would you use redis to store cache data (or not cache), and when would storage engine = MEMORY (in MySQL) ?
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2) there is another curious crutch - InnoDB on RAMdisk , who thinks about this?

Answer 1

In which cases would you use redis to store cache data (or not cache), and in which storage engine = MEMORY

Personally, I - probably in all cases in which Redis is suitable for this. Partly due to the fact that projects that require caching of this level are mainly (* specifically in my practice, in fact - this does not mean anything) are completed with Postgres.
Among other advantages - radish can save data and disk, this may be important / necessary.
In other cases, the main thing is to do it without fanaticism. There is no "one-size-fits-all" solution that would give a 100% result. You have certain requirements for the task, there is a tool. If MEMORY_ENGINE works fine (i.e. you are satisfied with the indicators), and you already have MySQL installed and it works - why drag more Redis? This will complicate the system, although not much.
If you do not have MySQL, or MEMORY_ENGINE for some reason did not suit you, for example, you need to save data to disk or [reason-N] - take Redis.
Also, it is worth noting that the functionality of MEMORY_ENGINE is a little wider, in view of the presence of a full-fledged SQL engine, and in general is logically different from a radish.
And then there's Memcached...

2) there is another curious crutch - InnoDB on RAMdisk , who thinks about this?

In my opinion, the decision level "so-so". I don't like this idiom.
Yes, and in general - Redis is exactly a cache storage, with a cache lifetime out of the box (as an example), MEMORY_ENGINE is a full-fledged table, with all the ensuing facts. The functionality, as I wrote above, is a bit different...
PS In MySQL, I remember, they threatened to introduce some new engine, just in order to "outshine" all Redis with Memcached together, but something then I do not find information about this ... but I remember exactly that I read this!

Answer 2

redis is probably faster.
memory mysql - you can work not only as a key-value, but also make different selections / sorting / grouping, etc.
radish also has some data structures besides key-value, but others, far from those that relational databases provide (they still need to be studied and used to).
so the choice is either simplicity and speed or sql functionality.