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Transactions spanning several microservices violate their autonomy - one service should not lock resources of another. Jeppe Cramon explains it pretty well in

• PeterGriffin 12y ago
  That's often not practical. Say you're a shop front. If you have a goods purchase service and a goods delivery service, you want to reserve the good (limited quantity) and reserve a time slot for transport (limited number of deliveries a day) before you charge the customer, and make the "commit". And if you'd merge two separate companies into one uber-service, you've just created more problems than you've solved.
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  • superuser2 12y ago
    Atomic transactions are one of several strategies for handling this problem, and not always the right answer. Starbucks is a good analogy (http://www.eaipatterns.com/ramblings/18_starbucks.html http://www.eaipatterns.com/ramblings/18_starbucks.html). If Starbucks were transactional, you would have to stand there at with the money on the counter and wait until your drink was finished, then exchange at the exact same time. Even the mental image is ridiculous. In practice, it makes much more sense to allow the system to enter certain inconsistent states and then remediate them. Out of an ingredient? Refund the money. Can't pay? Pull the drink out of the queue, or just write it off. This can involve some cost, but less cost than than the throughput you'd lose by enforcing consistency. For your specific example of a goods delivery service, most take-out restaurants fly in the face of these claims: you can pay the driver in cash when they arrive. They are willing to accept the risk that you made your order fraudulently, won't pay, etc. because it's still better for them to have your business. If you reserved a time-slot or inventory for someone whose credit card is declined, so what? Just un-reserve it. It's not hard to UPDATE a row in a database. The system was in an inconsistent state for 10 seconds. Big whoop. (You might have turned down a legitimate customer, but only if they were consuming the last resource, and if demand is that high, another customer will buy the item.) Notice that Ticketmater and most airlines will hold your seats (i.e. leave the system in an inconsistent state) for up to 15 minutes while you enter your card details. Hell, Ticketmaster can easily reverse a transaction at any point up until the moment your ticket is scanned at the door. So could an airline, and they often do. The classic example of account balances isn't necessarily the case either - banks can INSERT records of individual transactions. Since addition is commutative, race conditions are irrelevant. Balances are then calculated nightly offline by playing back the transaction records. I guess that's a form of locking, but even if it weren't, the balance isn't the source of truth - the transactions are, and the balance can be recomputed. Banks also don't hide the complexities of transaction processing. Your "current balance" is different from your "available balance" and your most recent transactions show as pending anyway. You don't need transactions as often as you think you do.
    • PeterGriffin 12y ago
      > "In practice, it makes much more sense to allow the system to enter certain inconsistent states and then remediate them. Out of an ingredient? Refund the money. Can't pay? Pull the drink out of the queue, or just write it off. This can involve some cost, but less cost than than the throughput you'd lose by enforcing consistency." I don't know what practice you're referring to, but unlike commodity coffee drinks paid for in cash 1) CC refunds are not free, and they're not cheap in volume at all if you intend to do it casually during normal operation 2) not all goods are standardized and available in large quantities. Your examples are all over the place. Ticketmaster is an example of a reservation system similar to one I was trying to give an example of (two step commit). A resource is locked, the lock is held for a short period while collecting answers from the other subsystems (in this case, payment gateway), and then a final commit is issued (or a rollback is issued). Airline overselling isn't done because it was some microservice design dogma about how great inconsistent state is, but because every seat costs the airline a fortune if left empty, and a certain % of passengers cancel or reschedule their tickets, and the airline is trying to arrive at an airplane with as few empty seats as possible. Having your tickets canceled is certainly not something that happens "often", thank god, but it does happen as a result of that tradeoff. But if I reserve and buy my seats online for a cinema movie, and then I go with company and get handed my money back because "it's practical", I'll make a scene. And so no one implements cinema ticket reservation this way. For bank overdrafting, it's a very special case - your money is a number in a computer, and the bank owns that computer. They make the rules... so they did. It's easy to mess around with numbers like that. Bank account overdrafting is probably the biggest exception of them all as no physical products and services are involved. No one's going to have their lawn un-mowed because the bank allowed your account to overdraft. The only common thing between your examples is that they're driven by business concerns, not some ivory tower concern about service design. And this is why they're so different, and reserving resources is and will remain a common practice for many, as long as the business logic calls for it. There's nothing wrong about it.
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      • superuser2 12y ago