{ dist sys, formal verification, event streaming }
In the last post I described the necessary protocol changes to ensure that all entries in closed ledgers reached Write Quorum (WQ) and all entries in all but the last fragment in open ledgers reach write quorum.
In this post we’re going to look at another tweak to the protocol to allow ledgers to be unbounded and allow writes from multiple clients over their lifetime.
Recently I wrote a blog post on my team blog about the differences between Raft and the Apache BookKeeper replication protocol. In it I covered one difference that surprises people which is that a ledger can have multiple blocks of entries that only ever reach Ack Quorum and not Write Quorum due to how ensemble changes work. A Raft log on the other hand has the property that the replication factor (RF) reached by any given entry matches the following:
Prefix RF >= Entry RF >= Suffix RFThat is to say, if a given entry has reached RF of 3, then the entire log prefix must also be at 3 or above (depending on the desired RF configured). But with BookKeeper that is not the case. For example, with WQ=3/AQ=2, a given entry that has reached RF of 3 may have entries before it that only reached RF of 2