I need to store (for the each user action in the system) a pair of strings: TransId, DetailId and mainly I need to read this value, sometimes update and finally delete.
Data should be persistent available after system restart or other maintenance.
The easiest way is to use CloudSQL, but I am thinking about Memorystore with failover (standard, not basic).
Memorystore is suitable for key value storage, but rather for cache... (and probably more expensive in compare to CloudSQL)
So maybe ideal option is CloudSQL, and in case that better performance is needed (additional cache by Memorystore).
What do you think?
Price aside, your use case fits more neatly with Memorystore. From a technical implementation standpoint, you are likely to have a much easier time developing with it. (Which IMO is well worth the increased cost.)
An SQL solution will likely cost you less, but will add some headaches around management.
If you're looking for a cheap solution, and you're running your code from a managed VM, I'd suggest storing your key/values in the machine's metadata. This is a hack / workaround and should not be used in enterprise settings.