Why are existing distributed database technologies not applicable to cryptocurrencies Why are existing distribued database technologies such as mongodb not useful for hosting the blockchains of large cryptoapplications, after all they are just distributed key-value stores?

how does something like ethereum nodes actually store data? In some custom database? Why not use existing tech?
The security model of MongoDB assumes that all MongoDB nodes are run by the same entity, and trusts the servers in the cluster to not tamper with the data they're given. There are checksums and such, but those are intended to detect and correct accidental corruption of data, not intentional changes.


  How does something like ethereum nodes actually store data?


Bitcoin uses LevelDB to store blockchain data, though this is not sharded for the reasons mentioned above.

I don't know what backend Ethereum uses.