Redis is used for plenty of things, not just memory caches.

For example if you use it for session storage, you can't have your application read from a random instance that may or may not contain the session.

▲

tossandthrow an hour ago | parent [-]

This case is exactly what he talks about. To get HA just setup more than one redis cache - or rebuild the session if it was lost in the redis cache.

▲

9dev 41 minutes ago | parent [-]

It’s not. Imagine a web app that stores your user information in a session store, mapped by your cookie-provided session ID. Your web app searches redis 1 for the session id, but since that key is on redis 2, the lookup fails and the application thinks there is no such session, and rejects the request.

Now you could solve this specific case by sharding by prefix, or by querying all instances, but then you still do not have high availability: if the instance a specific session is on is down, these users cannot authenticate. At that point you’re better off with a single instance.

▲

olavgg 19 minutes ago | parent [-]

But that is his point. If you cannot find the session id in redis, you login again. If your Redis server crash, you start a new one and everyone just login again. No data is lost.

	▲	9dev 13 minutes ago \| parent [-]
		Sure the data is lost. A session commonly holds arbitrary state, and even if it’s just the login information. This is ridiculous.