How can we measure/confirm that a shard has "enough" nodes to maintain a healthy level of security?



The more nodes that have been in a shard, the more secure a shard history is due to the number of redundant copies of the shard information being maintained.

It is also possible for nodes to leave a shard, come back later and re-affirm history. This makes any attack vector to not just attack all the current nodes on the shard, but all the nodes that have ever witnessed the event you are trying to change.

There is also a high degree of incentive for nodes to maintain under-served shards as the more shards low node count shards they maintain (serve) the higher the probability of getting fees.

For shard selection incentives, see:​


I don’t understand this sentence.


Updated it. It means any attacker using any attack vector against a decentralized system will have to alter history on all the nodes that have ever witnessed the transaction in question. Some of those nodes might be online and some might be offline. It is hard to attack temporarily offline nodes. And when they do come back online they can detect the anomaly and announce it to the entire network.