The nice thing about the RPis is that you can easily pull the plug to test failover. Ok, that is only one possible failure scenario but for sure the most obvious and more impressive than "kill -9".
That demo application is constantly using the database for storing new lines, removing old lines and reading all line data for the graphical view. There is no caching. It uses JDBC directly.
To document the setup here is the config.ini file for MySQL Cluster:
[ndbd]I made the cluster diskless so it will not write any logs and table spaces to disk. The SD card performance was not great and it does not affect failover behavior.
I also reduced the HeatbeatIntervallDbDb so that nodes detect immediately (well, 10ms) if a heartbeat is missed. After a few missed heartbeats cluster reconfigures and the remaining node takes responsibility and service continues.
BTW: Pulling the plug is nice but every now and then I had to manually fsck the root-fs during reboot.