user29947: 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?

user85183: 
How does something like ethereum nodes actually store data?

Taking a quick look at the Go-Ethereum GitHub repository, it seems that Ethereum uses LevelDb--as does Bitcoin Core--.
On another note, to address a few points that were brought up by the OP in an older thread, the benefits of using a blockchain are mitigated once they are implemented in a centralized manner. One of the main allures of the blockchain is that by its very nature, it does not require trust; it is permissionless. To have a central party or authority means you have to trust that central party--thus removing one of the main benefits of a decentralized blockchain--. 
TL;DR: There's no real noticeable advantage to using a centralized blockchain over just a simple MySQL database.

user2306: 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.

29947: <p>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?</p>

<p>how does something like ethereum nodes actually store data? In some custom database? Why not use existing tech?</p>
 85183: <blockquote>
  <p>How does something like ethereum nodes actually store data?</p>
</blockquote>

<p>Taking a quick look at the <a href="https://github.com/ethereum/go-ethereum/search?q=leveldb&amp;unscoped_q=leveldb" rel="nofollow noreferrer">Go-Ethereum GitHub repository</a>, it seems that Ethereum uses LevelDb--as does Bitcoin Core--.</p>

<p>On another note, to address a few points that were brought up by the OP in an older thread, the benefits of using a blockchain are mitigated once they are implemented in a centralized manner. One of the main allures of the blockchain is that by its very nature, it does not require trust; it is permissionless. To have a central party or authority means you have to trust that central party--thus removing one of the main benefits of a decentralized blockchain--. </p>

<p>TL;DR: There's no real noticeable advantage to using a centralized blockchain over just a simple MySQL database.</p>
 2306: <p>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.</p>

<blockquote>
  <p>How does something like ethereum nodes actually store data?</p>
</blockquote>

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

<p>I don't know what backend Ethereum uses.</p>
