Amazon Aurora

As we know it can have up to 15 read-only database

I think that these replicas are cache! as i understand they do not keep information about locks.. etc. They just keep pages without lock information. So it looks like complex cache. i mean complex because we ca do some complex logic here: join, group by etc. I guess you may think of read replicas as read-only entity, but that is not true. don't forget about WAL How does WAL impact in case of replica? In Aurora replica does not do any writes? For instance - update. you have to lock an entity before an update, right? Yes. I have to set lock byte in particular row. But only master will do it. Replica does not do any writes to a storage. How a data appears in read replica if so? In case if raft/paxos replicas do writes to log(wal). But in case if aurora replica only do reads. By writes I mean synchronization (in a wide sense) with primary. How aurora does that? Aurora replica just know that its time to reconstract page from storage. as i understand master will send just an event that replica should apply new changes from distributed storage. The main idea that replica does not do any writes ! It looks like cache! I am not sure about sharding. But aurora support multi-master.

Can you send an article for that? I kind of agree that read replica mostly required for 2 things:

failover
read queries that can tolerate eventual consistency

but I didn't get the way how the data is transferred from primary to that replicas

Its also clea why they propose only 15 read-only replicas. It because all replica have the same data. They do not propose sharding on case of single master. Please correct me if i am wrong. If we want to have sharding we need to have multi-master