Flink guarantees exactly-once application state access/updates. There is no mention of exactly-once delivery anywhere and it is pretty much impossible to offer this for just any general sink. That does not exclude the possibility of offering a special-purpose transactional sink implementation in the future though (e.g. via external distributed locking/state reconciliation), just saying...
Exactly-once delivery really depends where and how you actually "deliver". Even if your writes are idempontent you need to know if they have been properly committed on the other side, not trivial. If the system you are committing your output offers version control and/or proper transactional support then delivery can be eventually re-conciliated, otherwise with no such assumptions it is pretty much impossible.
Apache Flink's snapshotting algorithm solely guarantees exactly-once application state access, plain and simple.
I happened to work on some of the decentralised computing and networking projects in Barcelona. Guifi.net is the largest community network at the moment that among others lets users share and publish their content including internet access [1]. Last year I worked in Clommunity [2], an effort to build the first large scale community cloud. Guifi.net users can just install the Clommunity distro [3] on top of and use its service discovery and decentralised cloud management tools to contribute and benefit from marginal resources. I think it's a cool idea that we will keep revisiting in the long future...
