Genuinely interested why we need HA in redis, just not read round robin from multiple non-HA instances? Redis (and memcache) are memory caches and should be treated like that, not like highly consistent distributed session store.

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compumike an hour ago | parent | next [-]

> Redis (and memcache) are memory caches and should be treated like that

If you haven't come across Kvrocks yet, it may be worth a look: https://github.com/apache/kvrocks https://kvrocks.apache.org/ . It's a database with a Redis-compatible wire protocol, but the database is stored on disk. This means your working set is not limited by RAM and can be a few orders of magnitude larger! On modern SSDs this is still very fast. I think it improves the durability story as well. But the big win is the orders of magnitude larger database space.

As I've been improving my side project https://totalrealreturns.com/ recently I've ended up using both Redis and Kvrocks together. Redis is great for small global state that needs to be super fast. Kvrocks is great for larger bulk data storage (large precomputed datasets), but also supports a lot of the Redis data structures as well as Lua scripts.

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n_e 2 hours ago | parent | prev | next [-]

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.

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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.

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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.

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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.

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9dev 2 hours ago | parent | prev | next [-]

Redis doesn't necessarily have to be used as a cache. Streams, for example, make it a great message queue; but a single-node message queue is a single point of failure and thus not viable for many setups.

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acejam an hour ago | parent [-]

That's why you run Redis Sentinel in production

	▲	9dev 38 minutes ago \| parent \| next [-]
		That you do. Until you realise that there is only a single writer in that scenario, it doesn’t address any sharding concerns, you need to use compatible clients that opt into the sentinel protocol, during failover you’ll see client errors… there’s lots of room for improvement on redis HA.
	▲	lukaslalinsky 34 minutes ago \| parent \| prev [-]
		With the amount of problems I had using Redis Sentinel, I really wish there was another way. On multiple occasions, with completely different deployments, it got itself into a non-repairable state where the only option was to drop it and setup the replicas manually. I was hoping someone would do a Patroni-like project for Redis, but I've not found it yet. I've moved all persistent data to PostgreSQL and use a number of Valkeys behind Envoy proxy as a cache.

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__s 2 hours ago | parent | prev [-]

Years ago I enabled durability on redis & used it as database for an online card game