8 ms·

As someone who very briefly poked around Silk Road at the time (just created an account), and as someone who has used PHP with the LAMP stack occasionally over

As someone who very briefly poked around Silk Road at the time (just created an account), and as someone who has used PHP with the LAMP stack occasionally over the years, I am completely unsurprised. Race conditions can be pretty tricky to prevent in PHP, since the state is contained within different threads that can’t trivially communicate with each other. You need to be proficient with global state mutation such as via Redis or SQL.

pmontra 4y ago
Well, the database is there to manage and defend the data if only properly instructed to do so (CHECK constraints, locking, serialization.)
- xyzzy4747 4y ago
  True, but it’s still not trivial, unless you’ve thought of the race condition problems in advance. My guess is that instead of keeping all the withdrawal logic in a comprehensive DB transaction with good constraint checking, the code was broken up into multiple sequential PHP statements with transactions of simpler SQL (which is likely easier from a coding perspective) and with insufficient locking. This is the easier way to code it if you haven’t thought of or heard about race condition issues.
  - pmontra 4y ago
    Yes, I'm sure that's what happened. The money withdrawal scenario is the classic example for locking in any database programming classroom but not everybody went through that or remembers it.
  - ilyt 4y ago
    Even if you're not 100% sure in your DB code you can do defense in depth approach and have the code that does the money operations do some checking too. Even executing queued operations with few seconds delay allow you to notice say a series of withdrawals putting account in negative
    - Scoundreller 4y ago
      Yeah, and it’s Bitcoin withdrawals, nobody expects it to be anywhere near instant. I doubt the market place set its transaction fees very high either. But then you have to create a queue that only runs 1 transaction per second. I guess another imperfect but simpler way is to create a batch file of transactions, execute hourly, but randomize or sort the sequence in a way to make sure related transactions aren’t executed together.
amalcon 4y ago
At the same time, this is like... data integrity 101. For future reference if you want to build a system that manipulates valuable information (such as monetary accounts), and you've never done anything like that before: Please pick a database (relational or otherwise) with ACID transactions and replication, buy at least two different textbooks on that specific database, read both, do any exercises they suggest, and then decide whether you still want to build that system. If yes, then cool: don't hold me responsible if anything goes wrong, but you need to learn somehow. Yes, it's possible to build systems like that on non-ACID-compliant databases, or without using the ACID functionality. It's much harder, though, and you really must understand what you're giving up in order to have a reasonable chance of success.
- xyzzy4747 4y ago
  Probably the code had logic such as the following, which from an amateur perspective, seems to work. After all, the first check prevents the withdrawal if the account balance is too low, so what could go wrong? $balance = sql_query(“SELECT balance FROM accounts WHERE account_id = “ . $account_id); if ($withdrawal_amount < $balance) { withdraw_funds($account_id, $withdrawal_amount); } The correct solution of course requires putting the checks in an ACID transaction along with the account balance updating. This requires more work upfront (SQL is harder to write than PHP) and an impatient amateur coder probably didn’t think of it. Also I’m guessing they didn’t properly index all the DB columns and this made transactions take longer which exacerbates race conditions.
- lmm 4y ago
  IMO this is backwards advice. Traditional relational databases with ACID go to great lengths to create the illusion that there's a single, global state, and that's what leads to people doing things like check-and-set that seem to work. You're far better off using a non-ACID database, simply because incorrect approaches will fail more visibly and more often. Ultimately you do the same amount of work either way, but using a non-ACID database forces you to do it upfront rather than letting you silently mostly succeed with a fundamentally broken data model.
  - 4y ago

pmontra 4y ago

Well, the database is there to manage and defend the data if only properly instructed to do so (CHECK constraints, locking, serialization.)

xyzzy4747 4y ago
True, but it’s still not trivial, unless you’ve thought of the race condition problems in advance. My guess is that instead of keeping all the withdrawal logic in a comprehensive DB transaction with good constraint checking, the code was broken up into multiple sequential PHP statements with transactions of simpler SQL (which is likely easier from a coding perspective) and with insufficient locking. This is the easier way to code it if you haven’t thought of or heard about race condition issues.
- pmontra 4y ago
  Yes, I'm sure that's what happened. The money withdrawal scenario is the classic example for locking in any database programming classroom but not everybody went through that or remembers it.
- ilyt 4y ago
  Even if you're not 100% sure in your DB code you can do defense in depth approach and have the code that does the money operations do some checking too. Even executing queued operations with few seconds delay allow you to notice say a series of withdrawals putting account in negative
  - Scoundreller 4y ago
    Yeah, and it’s Bitcoin withdrawals, nobody expects it to be anywhere near instant. I doubt the market place set its transaction fees very high either. But then you have to create a queue that only runs 1 transaction per second. I guess another imperfect but simpler way is to create a batch file of transactions, execute hourly, but randomize or sort the sequence in a way to make sure related transactions aren’t executed together.

amalcon 4y ago

At the same time, this is like... data integrity 101. For future reference if you want to build a system that manipulates valuable information (such as monetary accounts), and you've never done anything like that before: Please pick a database (relational or otherwise) with ACID transactions and replication, buy at least two different textbooks on that specific database, read both, do any exercises they suggest, and then decide whether you still want to build that system. If yes, then cool: don't hold me responsible if anything goes wrong, but you need to learn somehow. Yes, it's possible to build systems like that on non-ACID-compliant databases, or without using the ACID functionality. It's much harder, though, and you really must understand what you're giving up in order to have a reasonable chance of success.

xyzzy4747 4y ago
Probably the code had logic such as the following, which from an amateur perspective, seems to work. After all, the first check prevents the withdrawal if the account balance is too low, so what could go wrong? $balance = sql_query(“SELECT balance FROM accounts WHERE account_id = “ . $account_id); if ($withdrawal_amount < $balance) { withdraw_funds($account_id, $withdrawal_amount); } The correct solution of course requires putting the checks in an ACID transaction along with the account balance updating. This requires more work upfront (SQL is harder to write than PHP) and an impatient amateur coder probably didn’t think of it. Also I’m guessing they didn’t properly index all the DB columns and this made transactions take longer which exacerbates race conditions.
lmm 4y ago
IMO this is backwards advice. Traditional relational databases with ACID go to great lengths to create the illusion that there's a single, global state, and that's what leads to people doing things like check-and-set that seem to work. You're far better off using a non-ACID database, simply because incorrect approaches will fail more visibly and more often. Ultimately you do the same amount of work either way, but using a non-ACID database forces you to do it upfront rather than letting you silently mostly succeed with a fundamentally broken data model.
- 4y ago