To provide high reliability, we need to replicate the event list across multiple nodes. During the replication process, we have to guarantee the following properties.

No data loss.

The relative order of data within a log file remains the same across nodes.

To achieve those guarantees, consensus-based replication is a good fit. The consensus algorithm makes sure that multiple nodes reach a consensus on what the event list is. Let’s use the Raft [17] consensus algorithm as an example.

The Raft algorithm guarantees that as long as more than half of the nodes are online, the append-only lists on them have the same data. For example, if we have 5 nodes and use the Raft algorithm to synchronize their data, as long as at least 3 (more than half) of the nodes are up as Figure 23 shows, the system can still work properly as a whole: