Placeholder Image

字幕表 動画を再生する

  • September 2018.

  • I was tired, I was exhausted,

  • I'd just got back home from something,

  • and I didn't have the energy to cook.

  • So I went to my phone,

  • opened up a certain food-delivery app that's popular in the UK, and I ordered pizza.

  • Now, I know: that food-delivery company's employment practices are questionable,

  • there are more ethical ways to get dinner delivered.

  • But I was tired, and I was hungry.

  • As are a lot of their drivers.

  • But that was the Night of the Multiple Orders,

  • when a bug in that app meant that some people around Britain ended up with

  • identical food orders being delivered two or three times,

  • and others got nothing at all.

  • And I nearly got caught up in the chaos.

  • To explain what happened,

  • I need to tell you a story about two generals.

  • The Two Generals' Problem is a classic of computer science,

  • and it goes like this: picture a valley.

  • In the middle of the valley is a heavily fortified castle.

  • On the edges of the valley are two armies.

  • The generals of these armies know that the only way they can win a battle and overwhelm

  • the castle is if they both attack at the same time.

  • A single army isn't going to make it.

  • They need the combined strength from both sides of the valley to win.

  • And the only way they can communicate

  • is by sending messengers on a risky path through the valley.

  • And General A won't know what the right time is until everyone's already in position.

  • How can those two generals coordinate

  • to make sure they attack at the same time?

  • This is a magical computer-science-land problem, by the way,

  • so reasonable suggestions likesemaphoreortelescopesdon't apply.

  • On the surface the problem seems trivial.

  • General A could just send a message to General B with a proposed time.

  • Say, 8 o'clock.

  • But the messenger has to pass through the valley,

  • and if they're spotted, they're, um,

  • not going to make it to the other side to deliver the message.

  • So how does General A know that General B received the message?

  • The messenger might not have made it.

  • And if that happens,

  • A will attack, B won't, and they'll lose.

  • So maybe they arrange it so General B has to send an acknowledgment back,

  • and General A will only attack if that acknowledgement arrives.

  • But that now runs into the same problem:

  • how does B know that A has received the acknowledgement?

  • If it doesn't get through,

  • A won't attack, B will, and they'll lose.

  • So, General A could send another acknowledgement for the acknowledgement.

  • But how do they know that message has gotten through? Well,

  • General B could send an acknowledgment for the acknowledgement for the acknowledgement

  • and so on, and so on, and so on.

  • This problem is unsolvable.

  • I know, it feels like there should be some hacky workaround

  • like sending 200 messengers, and sure,

  • that would probably work in the real world.

  • But this is magical information-theory computer-science land.

  • Under these strict rules,

  • there is never a guarantee, there is no certainty,

  • there is no arrangement that can be made, there is no way,

  • that the two generals, the two computers sending data,

  • can agree that the message has definitely been received and acknowledged.

  • Now, with computers you're not usually dealing with such high stakes.

  • If you are in computer science and working on a problem that involves potential loss of life,

  • I really hope you aren't watching a series called "The Basics".

  • Anyway. I was ordering food.

  • And I put my order together, I tapped 'pay',

  • I put my fingerprint on my phone's reader.

  • I got the little Apple Pay progress bar, and the little tick.

  • And then I got a message from the app saying that there had been a problem,

  • and my order had failed to go through. Would I like to try again?

  • And I was about to.

  • I was about to hit 'pay' again.

  • And then something, just in the back of my head, said: hang on.

  • There was that little tick saying payment had worked.

  • And I'm enough of a computer nerd to go "I'm not sure I believe that failed".

  • So I checked the 'order history' page.

  • It took a few tries to load, but when it finally did, there was my order.

  • Processing. It had gone through, but the acknowledgement hadn't come back.

  • Or, rather, something had gone wrong on the app's servers,

  • and the logic they'd written thought it had failed when it hadn't.

  • So I sat tight, I hoped that my food would arrive,

  • and I figured that the engineers were probably having a very bad day.

  • They really were.

  • Because I wasn't the only one.

  • People all over the UK ordering via the app were going to the payment screen,

  • hitting the button and getting "try again".

  • And a lot of them did. Again, and again, and again.

  • They were General A, and the app's server was General B,

  • and they were part of a real-life, complicated version of the Two Generals' problem.

  • Imagine all the customers as General A,

  • sending message after message to General B. B received the messages,

  • dutifully took the money from the credit card every time --

  • they attacked --

  • but something had happened that stopped the confirmation message getting through.

  • According to the flood of angry reports on Twitter,

  • sometimes the restaurant would realise the problem and just send one order.

  • Sometimes the restaurant wouldn't realise,

  • and three different drivers would arrive with three identical orders.

  • Sometimes no food would arrive at all.

  • The app's customer service line was swamped.

  • To be clear: this was not the sort of thing that is one engineer's fault.

  • When something goes this drastically wrong,

  • there have been many poor decisions made over a long period of time.

  • A single human error is never a root cause.

  • So what else could the app team have done?

  • How can you solve the Two Generals' problem in the real world?

  • Well, first, maybe no-one should have been able to place two identical orders

  • on the same credit card, for the same restaurant,

  • within a few minutes of each other.

  • That seems like the sort of thing there should have been a check for?

  • But the real solution is anidempotency token”, or anidempotency key”.

  • This is a unique value generated on the app, or on the web site:

  • and it's a shopping cart ID, basically,

  • and it's sent along with the order.

  • it's not just for shopping carts, though:

  • the idempotency token could be attached to instructions to delete the oldest log file,

  • or send a text message,

  • or anything that you only want to happen once.

  • The server stores the idempotency key to keep track of the request.

  • And if another request arrives with the same key attached,

  • then the server knows it's already dealt with that request.

  • So it doesn't fulfill it again; instead it knows that the reply didn't get through,

  • so it just sends back a copy of that first acknowledgement again.

  • Now, that still won't help if none of the messengers get through,

  • if the connection completely fails,

  • but for real-world problems, humans will notice that.

  • Idempotence means that you can request the same thing multiple times

  • and it'll only ever happen once.

  • That's the way to fix the Two Generals' Problem.

  • I was lucky.

  • I placed one order, I was charged for one order,

  • and one order of food arrived half an hour later.

  • Next time, I'll just cook for myself.

  • This series of The Basics is sponsored by Dashlane, the password manager.

  • I mentioned in the previous sponsored section that they sync all your passwords

  • and payment details between all your devices

  • without ever knowing those passwords.

  • Which sounds a bit like magic.

  • When you sign up to Dashlane, you choose a Master Password.

  • And incidentally,

  • you can do that by going to dashlane.com/tomscott for a free 30-day trial.

  • Anyway, when you sign up,

  • you pick a single Master Password,

  • and that is never transmitted over the internet.

  • Not even to Dashlane, not to their servers, nowhere.

  • If you don't know that password,

  • all that private data just looks like random noise.

  • So: when you sign up to a new website and Dashlane generates

  • a long, complicated password for you,

  • it is bundled up and encrypted using your master password, that only you know.

  • That encryption takes just long enough, a few fractions of a second,

  • that there's no way to brute-force it back open.

  • That encrypted bundle gets sent to Dashlane:

  • they just see random noise with a label saying 'please synchronise this'.

  • So they pass the bundle on to your other devices,

  • and those devices, and only those devices, can decrypt it because, at some point,

  • you're going to open up the app and type in your Master Password.

  • In truth, it's actually a little more secure than that,

  • because behind-the-scenes they also generate a different key for each device you log in to,

  • but that is a whole other level of security

  • that I have actually found it impossible to explain in a script.

  • But, in the massively unlikely event that someone did compromise Dashlane's servers,

  • or bribe some employee, it wouldn't work.

  • All they could do is watch those packets of random noise get shuffled around.

  • All your data stays on your own devices.

  • Which means, if you lose your Master Password,

  • Dashlane can't help you.

  • But that's fine,

  • because now you've just got a single password to remember.

  • That is massively more secure than reusing the same password

  • or variations on a password everywhere online.

  • Like I said last time: you should use a password manager.

  • So: dashlane.com/tomscott for a free 30-day trial of Dashlane Premium,

  • and if you like it you can use the codetomscottfor 10% off at purchase.

September 2018.

字幕と単語

動画の操作 ここで「動画」の調整と「字幕」の表示を設定することができます

B1 中級

二大将問題 (The Two Generals’ Problem)

  • 0 0
    林宜悉 に公開 2021 年 01 月 14 日
動画の中の単語