The Two Generals’ Problem

Tom Scott

Tom Scott

1 157 410 Views754

    Time to tell a story about idempotency, computer science, and the Night of the Multiple Orders. • Sponsored by Dashlane -try 30 days for free at:
    Written by Sean M Elliott and Tom Scott
    Directed by Tomek
    Graphics by Mooviemakers
    Audio mix by Haerther Productions
    Thanks to Dashlane for sponsoring the video! If you're techie enough to watch this video, you should be using a password manager. Get a 30-day free trial at
    I'm at
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    and on Instagram as tomscottgo

    Published on 10 days ago


    1. Tom Scott

      Yes, I had help with the graphics for this series. There's no way I'd have animated that myself! On that note, thanks to Dashlane for sponsoring and helping me hire an animator: their free trial link is

      1. Sweet's Channel

        @Masked Death True... I admit that the reply was a bit aggressive. I Should have been more polite from the start. Have a nice day 😊👌

      2. Masked Death

        @Sweet's Channel no problem, mistakes happen, sorry for lashing out - your reply was a bit aggressive and it caught me off guard. My bad.

      3. Sweet's Channel

        @Masked Death oh... now I understand. your talking about the sponsor of the video. I'm sorry 😔

      4. Sweet's Channel

        @Masked Death No it doesn't. the issue talked about in the video... starting at 3:00 it has to do with the implementation of Fetch/Ajax/Json the coder should have made sure that communication works as it should with some sort of REST API Tester example with Restlet Client or postman I'm not aggressive but frustrated because the app you are talking about is app for end user and not a coders broblem

      5. Masked Death

        @Sweet's Channel yes it does, it's an alternative to Dashlane except you don't have to pay for it. Not sure why your response is so aggressive unless you're shilling for Dashlane tho

    2. Burning Wang

      I use just eat, never used Deliveroo so I’m reet

    3. Bernd Knauer

      Send smoke signals, PrObLeM SoLvEd

    4. Christian Rasmussen

      Neat. I'll float this to pupils who might appreciate this kind of problem. :)

    5. Stephen Cooper

      There is a fix though, General A just sends the messenger over and over again until they receive a reply?! General B knows that as long as the message keeps arriving then his own messenger hasn't gotten through?!

    6. Lysergesaure1

      7:40 you can definitely bribe the company to make the app send the master pasword to their servers, and none would be the wiser. Use something open source, and audited.

    7. Guinness

      What you're saying is use the order, user and date but not the time add the key?

    8. Guinness

      Easy way to run a scam

    9. Dominic Quick

      Make sure the same soldier gets back?

    10. Ahmad Pujianto

      This is quiet a basic thing for payment gateway.. System should consider multiple order as a sign of failure, either in user device or on any infrastructure within the system itself.

    11. فاء بن ألف'

      Can’t just 1 red messenger send the message then come back?

      1. فاء بن ألف'

        And if they want confirmation the blue’s would send one and verify it then go back, it doesn’t have to arrive even; both of them would attack anyway and they’ll win

    12. Souvik Aswad

      Great video!

    13. J Espinola

      How did this not happen sooner?

    14. Yasir Arshad

      So like free food?

    15. Silverwolf0080

      So what would be the equivalent of the idempotency for the two generals. Damn it, now I'm gonna be thinking about that all day.

    16. Brian Bacc

      you're awesome subbed and liked

    17. gepisar

      !! I just came from another one of your videos that warns us not say "Tired and Emotional"... i think i know what you mean by "Tired and Exhausted" AND "Tired and Hungry"... is this self-libel?

    18. Autotrope

      By your own description of the two generals problem, an idempotency token does not solve the problem.

    19. alex chamber

      Why don't you keep acknowledging until everyone has more than 3 ack-ah... Why don't you send 20 messengers at once? One will get through.... Ah, that too. Right time to watch the solution.

    20. jammin023

      I wish these videos had been around back when I was doing Soft Eng at uni... your explanations are so much clearer than my lecturers could manage!

    21. Orion Red

      This is exactly how I ended up with two gallons of milk in my fridge and two loaves of bread in my cupboard last week. It's also how I ended up with a gallon of spoiled milk and a loaf of stale bread this week.

    22. ruwiki

      "it's unsolveable" me at 2 am: well, there must be a way ...

      1. Stephen Cooper

        General A just sends the messenger over and over again until they receive a reply?! General B knows that as long as the message keeps arriving then his own messenger hasn't gotten through?

    23. Oz El Coskuner

      Just giving thumbs down for showing 2 long (unskippable) ads. I hope creators puts some pressure on US-new. Because US-new literally don't give a damn about feelings of customers who are not paying. WHY WE STARTED WATCHING US-new? The biggest argument was "so that you don't have to watch long commercials of TV and get brainwashed". Now average ad on US-new is surpassing TV. Anyway, since I got Amazon prime, I reduced US-new time to 2 hours a week. Much happier. And I will never pay for US-new, just because their approach. You cannot shove your will down to people's throat and expect success. When I see US-new logo, I see a RABID DOG with dollar symbols on his red eyes. Disgusting.

    24. John Dean

      There IS answer to the Two Generals' Problem: go to your local chippie.

    25. Airpolygon

      Very interesting!

    26. Rens Breur

      But this anecdote isn't REALLY an example of the two generals problem right? The only 'attacker' is the server processing the order request. The two generals problem can't be solved by using a unique key.

    27. Joseph Thomas

      Can you find a different shirt?

    28. iammaxhailme

      "Next time, I'll just cook for myself" Or, you can just call the restaurant and place your order verbally. Magic! I don't know how it is in the UK, but in NYC you sometimes get it a bit cheaper this way becuase apps take a cut

    29. Jonathan Tikhonoff

      Actually the two general story is already solved through encryption system. I know of one encryption system that can confirm to both users at same time that each other got the message. I assume that Deliveroo didn't use such encryption because it might be thought of as a bit too complicated and too costly for such simple service. Source: I took cryptography class few years ago so my memory are a bit shaky on details but I clearly remembered Bob, Alice, and Oscar model system about this problem.

    30. Nubby Tope

      The main problem is that they built a castle in a valley.

      1. ruwiki

        @Nubby Tope as well

      2. Nubby Tope

        ruwiki - fortifications like castles are built on high ground.

      3. ruwiki

        why? to protect an important river crossing, for example.

    31. Zen Lucas-Divers

      wouldn't the best option to just meet somewhere else, send a message to one side and keep doing so until a messenger gets through and tell them to meet a one point so that you can attack together.

    32. M

      And then that "idempotency key" resets when you hit "try again" :D

      1. less kiss

        The fact you spent £1,795 inc vat per minute of this videos graphics is beyond me

    33. Techno_mage_21

      Maybe a joke comment cause excuse my incompetence but why isn’t the problem set in 3D? Cause then they could just go around?

    34. Zachary Wilson

      Are you in the Centre for Computing History in Cambridge?

      1. less kiss

        very close to this idea. Her paper was entitled "Network layer protocols with Byzantine robustness"

    35. Caleb_ Artzs

      Why not A send a messager at same time as B? So both can meet at the middle and both can go back and inform there general's

    36. Deep hug

      to prevent jamming of wireless signals.

    37. soap's alt

      The messengers could meet in the valley and then return with an agreement.

      1. Jose Rojas

        without some form of instantaneous communication, neither general will never know with 100% confidence that the other general's messenger made it back safely to relay the agreement. The point is there's always some assumptions made at some point by one general that the last message went through and by the other general that the previously sent messages were sufficient to proceed with 'enough confidence'.

    38. BleedingRaindrops

      this is a really cool problem. and a really great explanation. Not being a software engineer myself, a lot of why I'm here is through sheer curiosity, but your explanations are always entertaining and easy to understand, so I keep them on a backlog for the day they become relevant to me. Thanks for keeping this channel awesome.

      1. Deep hug

        attack. If you stop receiving messages, count 1 for each missed message, Reset counter if you receive a message. If count exceeds 6, attack. If the number "I last saw mess

    39. XerO

      couldnt you just send one messenger with a message to have the a messenger from each side to meet at some kind of halfway point and then return to their general

    40. George Hugh

      Castles aren't built in valleys, generally, generally.

      1. ruwiki

        well, if they protect a river crossing

    41. Lou

      You could just get past the castle with your whole force, telling the other force to attack with you - so you become one big army. Problem solved?

    42. CS:GO сФинщини

      Bitcoin solved the Byzantine generals' problem

      1. ruwiki

        blockchain you mean

    43. Emily An

      I earn like 20 quid an hour for deliveroo it’s not bad 😂

      1. ruwiki

        with tip

    44. =NolePtr

      I've never heard it referred to as an idempotency key. I've always heard it called a "nonce"

      1. jammin023

        The problem is that word means something else in British English...

    45. Giovanni Joe

      Super informative! Thanks very much for the great content!

      1. Emily An

        story short I got 50 quids worth of Wagamamas for free.

    46. flutty bitch

      You are my new favorite channel. Subscribed. BTW, I had the same problem when purchasing Dragon Age: Inquisition. Took a month for EA to return my money.

    47. Michael Thomas

      "A single human error is never the root cause" Tell that to my development manager who came in to the position with nothing more than a background in marketing & graphic design and understands absolutely nothing about coding anything more complicated than changing a few things on an HTML template. edit: I don't even work in tech currently, so I'm just meme-ing not speaking from experience.

      1. Orion Red

        Well, you're dead on.

    48. Dan Scherck

      Interesting factoid: The Spanning Tree Protocol, used in Networking to prevent network loops, was invented by Radia Perlman, whose doctoral thesis at MIT was on something very close to this idea. Her paper was entitled "Network layer protocols with Byzantine robustness"

    49. Magic Morgan

      The fact you spent £1,795 inc vat per minute of this videos graphics is beyond me

    50. Bluestripe

      I had this with steam with a gift and sent 3 skyrims to my friend, by the time we'd realised the refund was no longer possible.

    51. NikoHD203

      I feel so smart now😂

    52. Real Sky Luke

      *I was a sub before you hit 200k, WTF WHEN DID YOU BLOW UP???!!!!! 1.8 Million!!! I never even noticed!!!!*

    53. Reagan Epps

      I know this probably wouldn’t work with computer science stuff but if they sent a messenger to meet the other and they both go back that might work

      1. Ptao Tom


    54. alnoso

      i dont get food delivery apps is it so hard to call a human and say "hey i want a pizza"

    55. Sebastian Nielsen

      There is actually a way to "solve" this problem, and that is continually send messages. They could contain a number, my message number, and the last message number I saw from you. Of course they must be encrypted and end2end verified. Continuially send these messages, lets say with 10 minutes apart. Stop sending messages 1 hour before the attack. If you stop receiving messages, count 1 for each missed message, Reset counter if you receive a message. If count exceeds 6, attack. If the number "I last saw message number X" is more than 6 - (count), attack. Regardless on how the adversiary capture the messages, it will result in a successful attack. HINT: This is how alarm system works to prevent jamming of wireless signals.

      1. Ptao Tom

        Seems like once both generals have received at least one acknowledgement they can just attack since the time hasn't changed.

    56. Black Light

      youtube recommended, wtf? Why? I neither understand computer logics or having problems with my food arriving.

      1. less kiss

        err, Works fine for me... sounds like user error....

    57. Drake H.

      Wouldn't you be able to include a signal response request in the initial message (talking about the two generals problem) - like general a gives a time and requests that general b light a large fire in a visible area to confirm receipt and agreement of the message.

    58. [TRCZ] NoidEXE

      4:55 but how can you make the same order three times and the server not notice it's the same order? I mean can't you use a pseudo random number generator or something? You only get the next random number if you receive a confirmation or cancel the order. That way if the server gets the same random number for the same client it can tell it's the same order. I'm just a game designer so maybe it's harder than this but I can't think of why it wouldn't work.

      1. less kiss

        What if someone wanted to do the same order for multiple people. A certain restaurant verifies double orders

    59. Mike Pratt

      Suggestion for dinner next time - Salmon in a dishwasher..?

    60. Alexi Hickin

      I work for deliveroo and I remember this evening, i delivered Wagamamas to a house who had already received their order. I had to wait with them while they got a refund, long story short I got 50 quids worth of Wagamamas for free.

    61. Alexi Hickin

      I earn like 20 quid an hour for deliveroo it’s not bad 😂

    62. Raymon Crane

      Those look like HP 7933/7935 disk drives in back. Used to fix those back in the day.

    63. roasted pancakes

      Just send two million messengers with a paper each towards the blue army. The paper should have a time to attack and a check mark at the bottom of the page. Make the blue army check every single paper and send all of the messengers back. Then both armies will have a reasonable guarantee that each army got the message. Alternatively, reroute one of the armies to meet the other army and have a combined attack with both armies.

    64. TelFiRE

      There’s nothing unethical about ordering food. I work for Grubhub. Yes there are things I would rather have different, but wouldn’t at their job? By not ordering food all you’re doing is denying me money and making my job harder

      1. TelFiRE

        And frankly the only unethical think you’re is to frame this is a question of ethics. Businesses are allowed to have different policies and people are allowed to agree or disagree on those policies without it being a matter of ethics. If you don’t want to work there you don’t have to

    65. Simon Johnson

      It's not "unsolvable", the word is "insuperable - Pedant

      1. Ask to seduce Miss

        knows if A made it and A knows B made it. Both army’s attack. Solved. I know this wouldn’t work in computers but it is a solution if it were just a puzzle

    66. invisiblebears

      Why couldn't they just continually send back messengers with confirmation until the proposed time and if one of the messengers didn't show up Abort or in the case of a computer disconnect or and endhandshake?

      1. Ask to seduce Miss

        A sends a messenger to b. If it makes it , b sends both that messenger and a new messenger from b to the castle. Messenger B turns back and messenger A continues to a

    67. lampree

      why did this video get uploaded 3 times??

    68. Robert South

      Seems like once both generals have received at least one acknowledgement they can just attack since the time hasn't changed.

    69. 007one