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Is metadata also replicated in S3?

Else I don't understand how the metadata can be persistent after reboot as AFAIK redis cannot dump and reload its state.

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Redis can be persisted with RDB and AOF, can also be replicated to another machine. In the cloud, you don't need to worry about that, hosted Redis are ready to use.

The is an ongoing effort [1] to improve the persistency and availability in general, which is expected to be GA in 2021.

[1] https://github.com/RedisLabs/redisraft

given a lost write to Redis would translate to corruption or missing data at the filesystem level, the only "safe" way to run this ATM is using Redis' extremely inefficient "always fsync" setting for its AOF log [1].

Keep in mind that Elasticache does not support it (in general, it doesn't really support running Redis in a durable way).

[1] https://redis.io/topics/persistence#:~:text=The%20suggested%....

Thanks! I really have trouble to trust in memory databases that require 100% uptime. What happens with a power outage?

As mentioned in other place, the perfect solution would be adding Raft to redis [1].

[1] https://github.com/RedisLabs/redisraft

Right now, the metadata is not replicated to S3, you can replicated it to another Redis, or backup the persisted RDB and AOF to S3.

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