Comment by belter

Comment by belter a day ago

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No. The real world is full of eventual consistency, and we simply operate around it. :-)

Think about a supermarket: If the store is open 24/7, prices change constantly, and some items still have the old price tag until shelves get refreshed. The system converges over time.

Or airlines: They must overbook, because if they wait for perfect certainty, planes fly half empty. They accept inconsistency and correct later with compensation.

Even banking works this way. All database books have the usual “you can’t debit twice, so you need transactions”…bullshit. But think of a money transfer across banks and possibly across countries? Not globally atomic...

What if you transfer money to an account that was closed an hour ago in another system? The transfer doesn’t instantly fail everywhere. It’s posted as credit/debit, then reconciliation runs later, and you eventually get a reversal.

Same with stock markets: Trades happen continuously, but final clearing and settlement occur after the fact.

And technically DNS is eventual consistency by design. You update a record, but the world sees it gradually as caches expire. Yet the internet works.

Distributed systems aren’t broken when they’re eventually consistent. They’re mirroring how real systems work: commit locally, reconcile globally, compensate when needed.

sethev 19 hours ago

These analogies (except for DNS, perhaps) aren't very illuminating on the difference between a CP system and an AP system in the CAP sense, though. In banking, there are multiple parties involved. Each of those parties is likely running a CP system for their transactions (almost guaranteed). Same with stock exchanges - you can look up Martin Thompson's work for a public glimpse of how these systems work (LMAX and Aeron are systems related to this).

These examples are closer to control loops, where a decision is made and then carried out or finalized later. This kind of "eventual consistency" is pervasive but also significantly easier to reason about than what people usually mean by that term when talking about a distributed database, for example.

To expand on the 24/7 grocery store example: if the database with prices is consistent, you will always know what the current price is supposed to be. If the database is eventually consistent, you may get inconsistent answers about the current price that have to be resolved in the code somehow. That's way harder to reason about then "the price changed, but the tag hasn't been hung yet". The first case, professional software engineers struggle to deal with correctly. The second case, anyone can understand.

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hobs a day ago

None of the systems you describe are the 90% of businesses - grocery, airlines, banking, stock markets, dns, they are all modeling huge systems with very active logistics compared to most businesses, I still don't agree with you at all.

Banks across countries - not again a problem most businesses ever have to deal with.

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awesome_dude a day ago

> Even banking works this way. All database books have the usual “you can’t debit twice, so you need transactions”…bullshit. But think of a money transfer across banks and possibly across countries? Not globally atomic..

Banking is my "go to" anology when it comes to eventual consistency because 1: We use banking almost universally the same ways, and 2: we understand fully the eventual consistency employed (even though we don't think about it)

Allow me to elaborate.

When I was younger we had "cheque books" which meant that I could write a cheque (or check if you're American) and give it to someone in lieu of cash, they would take the cheque to the bank, and, after a period of time their bank would deposit funds into their account, and my bank would debit funds from mine - that delay is eventual consistency.

That /style/ of banking might have gone for some people, but the principle remains the same, this very second my bank account is showing me two "balances", the "current" balance and the "available" balance. Those two numbers are not equal, but they will /eventually/ be consistent.

The reason that they are not consistent is because I have used my debit card, which is really a credit arrangement that my bank has negotiated with Visa or Mastercard, etc. Whereby I have paid for some goods/services with my debit card, Visa has guaranteed the merchant that they will be paid (with some exceptions) and Visa have placed a hold on the balance of my account for the amount.

At some point - it might be overnight, it might be in a few days, there will be a reconciliation where actual money will be paid by my bank to Visa to settle the account, and Visa will pay the merchant's bank some money to settle the debt.

Once that reconciliation takes place to everyone's satisfaction, my account balances will be consistent.

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  • kukkeliskuu a day ago

    I have been working on payment systems and it seems that in almost all discussions about transactions, people talk about toy versions of bank transactions that have very little to do with what actually happens.

    You don't even need to talk about credit cards to have multiple kinds of accounts (internal bank accounts for payment settlement etc.), multiple involved systems, batch processes, reconciliation etc. Having a single atomic database transaction is not realistic at all.

    On the other hand, the toy transaction example might be useful for people to understand basic concepts of transactions.

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    • mrkeen a day ago

      And then they take that toy transaction model and think that they're on ACID when they're not.

      Are you stepping out of SQL to write application logic? You probably broke ACID. Begin a transaction, read a value (n), do a calculation (n+1), write it back and commit: The DB cannot see that you did (+1). All it knows is that you're trying to write a 6. If someone else wrote a 6 or a 7 in the meantime, then your transaction may have 'meant' (+0) or (-1).

      Same problem when running on reduced isolation level (which you probably are). If you do two reads in your 'transaction', the first read can be at state 1, and the second read can be at state 2.

      I think more conversations about the single "fully consistent" db approach should start with it not being fit-for-purpose - even without considering that it can't address soft-modification (which you should recognise as a need immediately whenever someone brings up soft-delete) or two-generals (i.e. consistency with a partner - you and VISA don't live in the same MySql instance, do you? Or to put it in moron words - partitions between your DB and VISA's DB "don't happen often" (they happen always!))

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      • fishstamp82 a day ago

        RE: "All it knows is that you're trying to write a 6. If someone else wrote a 6 or a 7 in the meantime, then your transaction may have 'meant' (+0) or (-1)."

        This is not how it works at all. This is called dirty writes and is by default prevented by ACID compliant databases, no matter the isolation level. The second transaction commit will be rejected by the transaction manager.

        Even if you start a transaction from your application, it does not change this still.

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        • mrkeen 18 hours ago

          I have no problem with ACID the concept. It's a great ideal to strive towards. I'm sure your favourite RDBMS does a fine job of it. If you send it a single SQL string, it will probably behave well no matter how many other callers are sending it SQL strings (as long as the statements are grouped appropriately with BEGIN/COMMIT).

          I'm just pointing out two ways in which you can make your system non-ACID.

          1) Leave it on the default isolation level (READ_COMMITTED):

          You have ten accounts, which sum to $100. You know your code cannot create or destroy money, only move it around. If no other thread is currently moving money, you will always see it sum to $100. However, if another thread moves money (e.g. from account 9 to account 1) while your summation is in progress, you will undercount the money. Perfectly legal in READ_COMMITTED. You made a clean read of account 1, kept going, and by the time you reach account 9, you READ_ what the other thread _COMMITTED. Nothing dirty about it, you under-reported money for no other reason than your transactions being less-than-Isolated. You can then take that SUM and cleanly write it elsewhere. Not dirty, just wrong.

          2) Use an ORM like LINQ. (Assume FULL ISOLATION - even though you probably don't have it)

          If you were to withdraw money from the largest account, split it into two parts, and deposit it into two random accounts, you could do it ACID-compliantly with this SQL snippet:

              SELECT @bigBalance = Max(Balance) FROM MyAccounts
    SELECT @part1 = @bigBalance / 2;
    SELECT @part2 = @bigBalance - @part1;
    ..
    -- Only showing one of the deposits for brevity
    UPDATE MyAccounts
    SET Balance = Balance + @part1
    WHERE Id IN (
        SELECT TOP 1 Id
        FROM MyAccounts
        ORDER BY NewId()
    );
          Under a single thread it will preserve money. Under multiple threads it will preserve money (as long as BEGIN and COMMIT are included ofc.). Perfectly ACID. But who wants to write SQL? Here's a snippet from the equivalent C#/EF/LINQ program:

              // Split the balance in two
    var onePart = maxAccount.Balance / 2;
    var otherPart = maxAccount.Balance - onePart;

    // Move one half
    maxAccount.Balance -= onePart;
    recipient1.Balance += onePart;

    // Move the other half
    maxAccount.Balance -= otherPart;
    recipient2.Balance += otherPart;
          Now the RDBMS couldn't manage this transactionally even if it wanted to. By the final lines, 'otherPart' is no longer "half of the balance of the biggest account", it's a number like 1144 or 1845. The RDBMS thinks it's just writing a constant and can't connect it back to its READ site:

              info: 1/31/2026 17:30:57.906 RelationalEventId.CommandExecuted[20101] (Microsoft.EntityFrameworkCore.Database.Command) 
        Executed DbCommand (7ms) [Parameters=[@p1='a49f1b75-4510-4375-35f5-08de60e61cdd', @p0='1845'], CommandType='Text', CommandTimeout='30']
        SET NOCOUNT ON;
        UPDATE [MyAccounts] SET [Balance] = @p0
        WHERE [Id] = @p1;
        SELECT @@ROWCOUNT;
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    • arter45 a day ago

      I don't have a lot of payment experience, but AFAIK actual payment systems work in an append-only fashion, which makes concurrency management easier since you're just adding a new row with (timestamp, from, to, value, currency, status) or something similar. However, how can you efficiently check for overdrafts in this model? You'd have to periodically sum up transactions to find the sender's balance and compare it to a known threshold.

      Is this how things are usually done in your business domain?

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      • mrkeen 18 hours ago

        > how can you efficiently check for overdrafts in this model?

        You already laid the groundwork for this to be done efficiently: "actual payment systems work in an append-only fashion"

        If you can't alter the past, it's trivial to maintain your rolling sums to compare against. Each new transaction through the system only needs to mutate the source and destination balances of that individual transaction.

        If you know everyone's balance as of 10 seconds ago, you don't need to consider any of the 5 million transactions that happened before 10 seconds ago.

        (If your system allowed you to alter the past and edit arbitrary transactions in the past, you could never trust your rolling sums, and you'd be back to summing up everything for every operation.)

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    • awesome_dude a day ago

      The point is to give people who don't realise that they have been dealing with eventual consistency all along, that it's right there, in their lives, and they already understand it.

      You're right I go into too much detail (maybe I got carried away with the HN audience :-) and you are right that multiple accounts is something else that people generally already understand and demonstrate further eventual consistency principles.

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      • kukkeliskuu a day ago

        I wasn't criticizing you, just making the point that when people talk about toy example bank transactions, they usually want to just introduce the basic understanding. And I think it ok, but I would prefer that they would also mention that REALLY the operations are complex.

        I modified my comment above that by multiple types of accounts I meant that banks have various accounts for settlements with the other banks etc. even in the common payment case.

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        • awesome_dude 14 hours ago

          My bad, I didn't mean to sound too upset, but I do get a bit "trigger happy" from time to time

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  • da_chicken a day ago

    No, this is confusing how the financial institutions operate as a business with how the data store that backs those institutions operates as a technology.

    You can certainly operate your financial system with a double entry register and delayed reconciliation due to the use of credit and the nature of various forms of promissory notes, but you're going to want the data store behind the scenes to be fully consistent with recording those transactions regardless of how long they might take to reconcile. If you don't know that your register is consistent, what are you even reconciling against?

    What you're arguing is akin to arguing that because computers store data in volatile RAM and that data will often differ from what is on disk, that you shouldn't have to worry about file system consistency or the integrity of private address spaces. After all, they aren't going to match anyways.

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    • awesome_dude a day ago

      No.

      I clearly state

      > analogy (sorry about the initial misspell) when it comes to eventual consistency because 1: We use banking almost universally the same ways, and 2: we understand fully the eventual consistency employed (even though we don't think about it)

      The point is, you understand that your bank account is eventually consistent, and I have given an explanation of instances of eventual consistency that you already usually know and understand.

      You make the mistake of thinking about something else (the back end storage, the double entry bookeeping).

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