The Two Generals’ Problem


Comments • 2 068

  • Tom Scott
    Tom Scott   4 weeks back

    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

    • Liberals Get the Bullet Too
      Liberals Get the Bullet Too  2 days back

      Why don't the two generals have the messengers meet in the middle, and each return with confirmation of having met? This should immediately double the survivability of any particular messenger - they're only travelling half the hostile distance.
      Further, you could have a second set of messengers in an intermediate space to act as a witness or a redundancy.

    • steve d
      steve d  2 days back

      Doesn"t mention food delivery company by name, shows headline with delivery company's name...

    • Andrew F
      Andrew F  4 days back

      How is it unsolvable? Wouldn't you just need a maximum of two confirmations sent in order after the original message? If you get at least one confirmation in the correct order after the original message then you know both parties received the original, right? What am I not understanding here?

    • john smith
      john smith  4 days back

      who would put the castle in the valley but a computer tech

      ERMAN ATES  4 days back

      It was very well put. Animations were only fabulous. Thanks

  • John Dean
    John Dean  2 hours back

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

    • Airpolygon
      Airpolygon  4 hours back

      Very interesting!

      • Rens Breur
        Rens Breur  4 hours back

        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.

        • Joseph Thomas
          Joseph Thomas  6 hours back

          Can you find a different shirt?

          • iammaxhailme
            iammaxhailme  8 hours back

            "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

            • Jonathan Tikhonoff
              Jonathan Tikhonoff  8 hours back

              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.

              • Nubby Tope
                Nubby Tope  11 hours back

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

                • Zen Lucas-Divers
                  Zen Lucas-Divers  14 hours back

                  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.

                  • M
                    M  15 hours back

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

                    • less kiss
                      less kiss  11 hours back

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

                  • Techno_mage_21
                    Techno_mage_21  15 hours back

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

                    • Zachary Wilson
                      Zachary Wilson  17 hours back

                      Are you in the Centre for Computing History in Cambridge?

                      • less kiss
                        less kiss  11 hours back

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

                    • Caleb_ Artzs
                      Caleb_ Artzs  17 hours back

                      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

                      • Deep hug
                        Deep hug  18 hours back

                        to prevent jamming of wireless signals.

                        • soap's alt
                          soap's alt  19 hours back

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

                          • Jose Rojas
                            Jose Rojas  14 hours back

                            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'.

                        • BleedingRaindrops
                          BleedingRaindrops  20 hours back

                          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.

                          • Deep hug
                            Deep hug  18 hours back

                            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

                        • XerO
                          XerO  20 hours back

                          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

                          • George Hugh
                            George Hugh  21 hours back

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

                            • Lou
                              Lou  22 hours back

                              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?

                              • CS:GO сФинщини
                                CS:GO сФинщини  23 hours back

                                Bitcoin solved the Byzantine generals' problem

                                • Emily An
                                  Emily An  1 days back

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

                                  • =NolePtr
                                    =NolePtr  1 days back

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

                                    • Giovanni Joe
                                      Giovanni Joe  1 days back

                                      Super informative! Thanks very much for the great content!

                                      • Emily An
                                        Emily An  1 days back

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

                                    • flutty bitch
                                      flutty bitch  1 days back

                                      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.

                                      • Michael Thomas
                                        Michael Thomas  1 days back

                                        "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.

                                        • Dan Scherck
                                          Dan Scherck  1 days back

                                          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"

                                          • Magic Morgan
                                            Magic Morgan  1 days back

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

                                            • Bluestripe
                                              Bluestripe  1 days back

                                              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.

                                              • NikoHD203
                                                NikoHD203  1 days back

                                                I feel so smart now😂

                                                • Real Sky Luke
                                                  Real Sky Luke  1 days back

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

                                                  • Reagan Epps
                                                    Reagan Epps  1 days back

                                                    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

                                                  • alnoso
                                                    alnoso  2 days back

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

                                                    • Sebastian Nielsen
                                                      Sebastian Nielsen  2 days back

                                                      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.

                                                      • Ptao Tom
                                                        Ptao Tom  1 days back

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

                                                    • Black Light
                                                      Black Light  2 days back

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

                                                      • Alexander Cavrich
                                                        Alexander Cavrich  2 days back


                                                        • less kiss
                                                          less kiss  2 days back

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

                                                      • Drake H.
                                                        Drake H.  2 days back

                                                        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.

                                                        • [TRCZ] NoidEXE
                                                          [TRCZ] NoidEXE  2 days back

                                                          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.

                                                          • less kiss
                                                            less kiss  2 days back

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

                                                        • Mike Pratt
                                                          Mike Pratt  2 days back

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

                                                          • Alexi Hickin
                                                            Alexi Hickin  2 days back

                                                            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.

                                                            • Alexi Hickin
                                                              Alexi Hickin  2 days back

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

                                                              • Raymon Crane
                                                                Raymon Crane  2 days back

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

                                                                • roasted pancakes
                                                                  roasted pancakes  2 days back

                                                                  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.

                                                                  • TelFiRE
                                                                    TelFiRE  2 days back

                                                                    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

                                                                    • TelFiRE
                                                                      TelFiRE  2 days back

                                                                      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

                                                                  • Simon Johnson
                                                                    Simon Johnson  2 days back

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

                                                                    • Ask to seduce Miss
                                                                      Ask to seduce Miss  2 days back

                                                                      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

                                                                  • invisiblebears
                                                                    invisiblebears  2 days back

                                                                    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?

                                                                    • Ask to seduce Miss
                                                                      Ask to seduce Miss  2 days back

                                                                      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

                                                                  • lampree
                                                                    lampree  2 days back

                                                                    why did this video get uploaded 3 times??

                                                                    • Robert South
                                                                      Robert South  2 days back

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

                                                                      • 007one
                                                                        007one  2 days back