Rqlite: The lightweight, distributed relational database built on SQLite
It looks like this layers Raft on top of SQLite. I don't like when systems replicate high-level changes like "update users set salary = salary + 1000 where ...;" Instead, I prefer they replicate low-level changes like "replace key/block X, which should have contents C_x, with C_x'".<p>Why? Imagine you're doing a rolling update. Some of your replica are running the newer version of SQLite and some are running the older version. They may not execute the high-level query in exactly the same way. For example, in the absence of an "order by" clause, select results' order is unstated. So imagine someone makes a mutation that depends on this: "insert ... select ... limit". (Maybe a dumb example but it can happen anyway.) Now the databases start to diverge, not only in underlying bytes and implementation-defined ordering but in actual row data.<p>I worked on a major distributed system that originally replicated high-level changes and switched to replicating low-level changes for this reason. We had a system for detecting when replicas didn't match, and replication of high-level changes was the biggest reason for diffs. (Hardware error was the second biggest reason; we added a lot of checksumming because of that.)