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by Animats 3254 days ago
Oh, they loaded the Etherium blockchain into an SQL database. Easy. From the title, it appeared that someone had figured out how to represent an SQL database in the blockchain using the virtual machine for contracts. That would be hard.

As I've pointed out before, smart contracts need atomic transactions. Either everything commits, or nothing commits. This is a basic property needed for accounting systems.
3 comments
DennisP 3254 days ago
Transactions on Ethereum are atomic. If something throws, everything rolls back.

There's one well-known exception, which is that if your contract sends ETH to another contract, invoking its fallback function, then a throw in the callee just means the call returns false. So in that particular case you have to check the return value and rethrow to make it atomic; this sounds crazy but in some circumstances you don't actually want to throw. The compiler gives you a warning if you don't check.

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Animats 3254 days ago
Didn't the DAO hack happen because someone found a way to make an "atomic" transaction fail without full rollback?
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DennisP 3254 days ago
No, it was a reentrant attack. The contract was doing a state change after sending ETH, and since the recipient called back, it was able to get repeated ETH sends before the state updated.
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Animats 3254 days ago
That's an atomicity failure. That class of bug, incidentally, is a classic source of trouble in window/widget GUI systems.
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DennisP 3254 days ago
Hah, I used to run into a lot of those GUI issues, and hadn't made the connection until now.
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always_good 3254 days ago
It's a logic error. It's actually still atomic.
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always_good 3254 days ago
To clarify, reentrant errors are not atomicity errors. Fully serializable transactions can have reentrant errors and they often do, but that class of error is a case of the code not doing what you expect rather than an atomic violation.

What I would instead wager is that it's too easy to introduce reentrant errors in Solidity.

I've noticed since the DAO exploit, the ecosystem has been better about this though. For example, Solidity's docs has a section of reentrancy, and even the in-browser editor can warn you about reentrancy in some cases. Seems to be improving, though unfortunately after big expense.

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xiaoyao 3254 days ago
It's not loaded into a SQL database. You can think of Presto as simply as a SQL query engine. It sources data from Ethereum and run queries on it.
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simondedalus 3254 days ago
not only would it be hard, but using a blockchain (a type of database) to represent a functional SQL database using smart contracts is a mind-numbingly bad idea.

(insert "should i use a blockchain" infographic, which is amusingly impossible to google image search for, because this question evidently does not often occur to people...)

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misterdata 3254 days ago
That might just depend on what parts of a "SQL database" you'd implement using a blockchain, and on the application type. You could e.g. segregate the data and only use the blockchain to authorize or timestamp transactions, for instance. In the latter case, Blockchain is just another consensus layer, and all distributed databases need one. See also https://github.com/pixelspark/catena, which uses the blockchain like a replication journal, logging only mutating queries and providing authorization based on public key crypto (disclosure: I am the author).
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simondedalus 3253 days ago
not to put too fine a point of it, but there's a world of difference between creating a ledger that creates a "distributed" SQL database whose integrity is guaranteed among non-trusted nodes through a blockchain, and using smart contracts to "put a functioning SQL database on the blockchain."

"get used to blockchain when all you know is SQL" - cool.

"guarantee all us non-trusted nodes have the same SQL database" - ...sure, but for any project such that i want this, i'm pretty sure i don't need a blockchain. (but of course, your project doubtless has uses i haven't thought of)

"i want a trustless SQL database so i'm going to use EVM and solidity and put it on The Blockchain because i'm awesome" - very bad.

"i'm starting a new project, so naturally i need to use a blockchain" - extremely bad.

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ah- 3254 days ago
https://github.com/pixelspark/catena
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