What was the most interesting thing that you learned while implementing the WAL? Have you thought about how WAL is going to work in the multi-master setup?
We write to WAL and then register the transaction in the transaction sequence registry.
If a concurrent transaction registered between the start and the end of the transaction, we update the current uncommitted transaction data with concurrent transactions and re-try registering it in the sequencer again.
To scale to multi-master we will move the transaction sequence registry to a service with a consensus algorithm.