SQL: Incorrect by Construction (chreke.com)

Wonder what the [check-constraints] part meant or if it's a placeholder.

SQL is intended as a declarative query language. That it is not the correct tool for imperative processing of updates feels expected? And mostly fine?

Fair that things often grow beyond their original intent.

I've encountered this dozens of times. It's not intuitive, but this implicitly locks the row from concurrent reads, where as SELECTing first won't:

  UPDATE accounts  
  SET balance = balance - 10  
  WHERE owner = 'alice' AND balance >= 10;

Another possible surprise, say two xacts do this at the same time:

  INSERT INTO foo(num) (
    SELECT 1 WHERE NOT EXISTS (
      SELECT * FROM foo WHERE num = 1
    )
  );

Without a UNIQUE on num, you get num=1 twice. Of course adding UNIQUE would prevent this, but what you might not expect is UNIQUE implicitly adds a lock too. So not only do you only get num=1 once, but also both xacts are guaranteed to succeed, which in some situations is an important distinction.

Schools teach that databases are ACID, but in most cases they aren't by default, and enabling full ACID comes with other caveats and also a large performance hit.

This assumes you don't do any sort of caching or use distributed systems that can cache the data and choose to hold off to write it all to the DB. The cached system can show both users the in-process transactions as well.

If you need concurrency design your system for concurrency.

Have a transactions table with the payer and receiver and calculate the current balance using the transactions.

Each transaction must have a unique Id (pk)

This document relies strongly upon Transact-SQL:

https://en.wikipedia.org/wiki/Transact-SQL

A more universal industry standard is SQL/PSM, which originated from Oracle PL/SQL:

https://en.wikipedia.org/wiki/SQL/PSM

Demonstrating the flaws in question in the PSM standard would be more useful.

[dead]

You don't update balances, you enter transactions. Then you derive balances from transactions.

You can even insert them in an unvalidated state, then validate them later. That way if you have two transactions that come one after another, it doesn't matter because you can process them sequentially anyway.

This is just like... SQL 101 for transactions and locking. These are basic, elementary concepts in databases.

There's nothing "incorrect by construction".

The author claims the original snippet "looks completely reasonable". It absolutely does not, if you know anything about client-server databases.

> I want an alternative to SQL

I don't think it's SQL itself - it's the DB vendors ship weak isolation so people aren't hit by deadlocks, isn't it?

> Make transactions atomic by default

Not the issue, right? It's the weak isolation.

Generally I would recommend an append only data structure here, not a bunch of updates

  BEGIN TRANSACTION;

  IF EXISTS (
    SELECT 1
    FROM account_balances WITH (UPDLOCK, HOLDLOCK)
    WHERE owner = 'alice'
      AND balance >= 10
  )
  BEGIN
    INSERT INTO account_ledger (owner, amount, memo)
    VALUES
        ('alice', -10, 'transfer to bob'),
        ('bob',    10, 'transfer from alice');
  END

  COMMIT TRANSACTION;

or if I wanted to use the update pattern since I am taking the lock anyway

  BEGIN TRANSACTION;

  UPDATE accounts WITH (UPDLOCK)
  SET balance = balance - 10
  WHERE owner = 'alice'
  AND balance >= 10;

  IF @@ROWCOUNT = 1
  BEGIN
    UPDATE accounts
    SET balance = balance + 10
    WHERE owner = 'bob';
  END

  COMMIT TRANSACTION;

> Let the user manage locks themselves, and make sure the correct locks are acquired before mutating a database object.

??? This doesn't make sense. It's like saying "just implement it properly".

what about distributed clients? what about _different_ clients?

I don't understand how, after the preceding sections, this makes it into the conclusion:

> Let the user manage locks themselves, and make sure the correct locks are acquired before mutating a database object.

As demonstrated by the extended (corrected) version of the transaction, the user is controlling which locks get used. So how does this make it into the conclusion as a want, when it already is how it works?

Post shows beginner T-SQL under SQL Server lock-based READ COMMITTED defaults, then generalizes that to "SQL is incorrect by construction."

The conclusion should be much narrower. For the same row double debit, true SNAPSHOT isolation, enabled and actually used, not just RCSI, turns the loser into an update-conflict/retry. The idiomatic SQL fix is a conditional debit UPDATE, row-count check, constraints, and one transaction for the transfer.

The deadlock example is also overstated. SQL Server detects the cycle, rolls back a victim, returns error 1205 [1], and the application is expected to retry. Microsoft has shitty defaults and the author turned it into a much broader claim ..

[1] - "Your transaction (process ID #...) was deadlocked on {lock | communication buffer | thread} resources with another process and has been chosen as the deadlock victim. Rerun your transaction"

"Deadlocks guide" - https://learn.microsoft.com/en-us/sql/relational-databases/s...

It's interesting reading this.

Preventing these kinds of concurrency issues is exactly why I built https://socketcluster.io years ago. Though it solves the problem at the app layer rather than the storage layer.

But not many developers care about these race conditions it seems.

It's not just an issue with SQL but a more general issue with many programming languages and approaches.

This is a great example because it shows how concurrent executions can lead to significant issues.

Yes, that's incorrectly constructed SQL for the selected isolation level.

> What if Alice and Bob both try to transfer money to each other at the same time?

> Let’s map out the transactions again:

> T1: Acquire a lock on Alice’s account

> T2: Acquire a lock on Bob’s account......

::loud sigh::

In MySQL, if you acquire a lock on Alice, then attempt acquire on Bob... then another session acquire Bob, then acquires Alice: The engine notices. Nothing bad happens. One of the threads gets marked as a deadlock and is rolled back, the other succeeds. So... yeah, not really a problem. MySql chooses at "random" which one lives.

Actually... hang on, pause, let me remind you what you learned in your undergrad CS 400 class: Acquire Locks in consistent order. You could sort by account id, or even their first names. Hell sort by their cats name. It doesn't matter, it just has to be deterministic, so: Alice comes before Bob. So doing this properly:

T1: Acquire a lock on Alice’s account with SELECT FOR UPDATE

T2: Acquire a lock on Alice’s account with SELECT FOR UPDATE... ::waits::

Annnnnnd, yeah, magic. Problem solved.

I don't find any of this too difficult and I really the only thing SQL engines do poorly is reporting on what locks are being held. Adding in XA Transactions can make things even more intuitive and gives you a nice building block if you need to coordinate sending messages to a broker or something else.

Oh and lastly, you should be using a ledger table for this; not storing balances as columns. Lock on both accounts, insert parallel rows into the ledger. And don't forget to use fixed point decimal datatypes.

I'll show myself to the door now. Thank you.

yes wrap everything in transactions

  set @balance = (....

heh assigning value to @balance is that really part of the transaction? because @balance isn't even part of the database lol

still, anytime you use an isolation mode besides serializable you have to know the details and even with serializable if you aren't catching on failure in your app then you're sending your deadlock or timeout message to your end user.

I used to debug procedures with "sleep" tsql interspersed through code, this way I could deliberately overlap procedure executions and see how that went.

In my last job I had to support app developers who were happy with the "nolock" keyword even when marshalling info for updates. Shortly before I left that job in disgust I was at least able to have the developers quit using "nolock" everywhere, unfortunately their remediation to prevent use of the "nolock" keyword was to use "SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED" at the top of their code haha