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Nimbus uses ehcache for persistence of several things related to the EC2 interfaces and spot instances. By design, ehcache favors safety over persistence and we have seen several examples of caches being flushed in the wild. This can be a minor annoyance (losing SSH keys stored in the service) or a serious error (elastic instance IDs changing for running instances).
This has been fixed for the elastic interface persistence, merged in c7f75a9. Ehcache use has been replaced with sqlite.
The fix for spot instances is pending, being worked on by @oldpatricka.
This was fixed in commit 100f503