Fix Connection to Redis Failed Error in Magento 2.2.6

You might try checking for network connection limits. We've seen that happen before where something opens a TON of connections to Redis, and stops further connections, then goes away on its own. The error messages are deceptive because they won't flat out say that and it takes some poking around to actually figure that out.

I.e. Redis can only have so many open connections, as may a given service on your particular setup, so if you hit that it won't appear in any Magento/Redis logs directly.

For #2, Magento creates a session for each visitor; to get an estimate of how much space you need, just multiply the size of one session storage by the highest number of visitors (and multiply by.. let's say 4-5 or maybe even 10, to be safe :D). You can also setup alters in AWS, for when you reach like 80% memory usage. Also, you can check if garbage collection is working and play with the expires.

For #3, see this response on SO https://stackoverflow.com/a/8654367/64944