The Two Generals’ Problem

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  • Опубліковано 24 гру 2024

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  • @TomScottGo
    @TomScottGo  5 років тому +11253

    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 www.dashlane.com/tomscott

    • @johandaun874
      @johandaun874 5 років тому +59

      2 weeks ago?!?

    • @azarus8806
      @azarus8806 5 років тому +8

      whattt

    • @AritroVlogs
      @AritroVlogs 5 років тому +7

      2 weeks ago... Hmm...

    • @PepsiMan42069
      @PepsiMan42069 5 років тому +18

      Tom Scott time travelling tom

    • @davidfordom1216
      @davidfordom1216 5 років тому +67

      for those asking this was uploaded 2 weeks ago but just scheduled to be released today

  • @Statsy10
    @Statsy10 5 років тому +9618

    I also put in a food order that night... and 30 minutes later a messenger showed up with orders about attacking some castle. Crazy night!

    • @StayAwayFromMyCat
      @StayAwayFromMyCat 5 років тому +245

      So did the other army show up at the same time as you?

    • @painauchocolate2395
      @painauchocolate2395 5 років тому +210

      What a crazy knight

    • @michaelfranciotti3900
      @michaelfranciotti3900 5 років тому +177

      @glyn hodges we were waiting on your acknowledgement of our acknowledgment.

    • @theapexsurvivor9538
      @theapexsurvivor9538 4 роки тому +42

      @@michaelfranciotti3900 I told you this is why we needed an idempotency key.

    • @thiccchungo1041
      @thiccchungo1041 4 роки тому +20

      @glyn hodges your attack was pitiful from my view in the castle....

  • @wolfelkan8183
    @wolfelkan8183 3 роки тому +10731

    "A single human error is never the root cause"
    Because if a single human error _is_ the root cause, then the blame also lies with everyone who allowed the project to be susceptible to a single human error.

    • @iiiiitsmagreta1240
      @iiiiitsmagreta1240 3 роки тому +99

      This is funnier if you just came from Tom's fireworks video

    • @IsaBella-ir4rf
      @IsaBella-ir4rf 3 роки тому +37

      content content content

    • @KnakuanaRka
      @KnakuanaRka 3 роки тому +98

      Remember the episode where Tom accidentally deleted an entire database?

    • @dionysusnu3881
      @dionysusnu3881 3 роки тому +220

      @@KnakuanaRka where the supervisor that tasked Tom with converting the database should've maybe backed it up in some way? Yes.

    • @json_bourne3812
      @json_bourne3812 3 роки тому +75

      *Writes entire application on my own for my own personal solo venture*
      *Issues arise with the application*
      "At least it's not ALL my fault!"

  • @moomoo2214
    @moomoo2214 4 роки тому +24167

    Tom Scott: A single person can never be the root problem.
    Also Tom Scott: ONCE I DELETED AN ENTIRE DATABASE

    • @christopherstokes9393
      @christopherstokes9393 4 роки тому +3008

      Even then, there were multiple poor decisions leading up to that problem (e.g. not taking a backup, working on the live system, not starting a transaction) - so, that case is another example in favour of his point.

    • @eddy3038
      @eddy3038 4 роки тому +687

      @@christopherstokes9393 good point actually! Thanks for the insight, I was thinking the same way as the guy above you.

    • @mannyokafor4609
      @mannyokafor4609 4 роки тому +57

      Like @Christopher Stokes said there were multiple errors that lead up to that.

    • @mr.animation5259
      @mr.animation5259 4 роки тому +248

      @@christopherstokes9393 weren't they still all his fault? Multiple mistakes by one Tom

    • @akkere4052
      @akkere4052 4 роки тому +177

      @@mr.animation5259 A system should ideally have its own backups put in place as a collective decision (I don't know the context of the database deletion, so I don't know if this was just a personal project or one done by a company). Everything else is on the single Tom though, even if it is multiple poor decisions being made compounded to one outcome.

  • @alfy01
    @alfy01 3 роки тому +3519

    "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." had me dying omg

    • @horsermchead2504
      @horsermchead2504 3 роки тому +22

      Me too. I laughed out loud.

    • @AjayKumar-fd9mv
      @AjayKumar-fd9mv 2 роки тому +12

      What. is this series ? i searched for the series I got a comedy show related series.

    • @stoobidthing
      @stoobidthing 2 роки тому +96

      @@AjayKumar-fd9mv This video is part of a series called The Basics, in witch Tom explains The Basics of computer science

    • @csurname
      @csurname 2 роки тому +14

      You lost your life

    • @SuprSBG
      @SuprSBG Рік тому

      Same

  • @ameliagryffon7097
    @ameliagryffon7097 3 роки тому +4316

    The real problem is when the castle intercepts the message and sends back a guy in the messengers clothes changing the time of the attack so they both attack at the wrong time.

    • @berksterb8475
      @berksterb8475 3 роки тому +183

      Then, when someone comes up with asymmetric crypto, you can introduce the integrity problem, where I don’t care what false time I sent, only that it is different :) it’s a fun little conceit!

    • @ameliagryffon7097
      @ameliagryffon7097 3 роки тому +55

      @@berksterb8475 I don't know what asymmetric crypto and integrity problem are XD

    • @irrelevant_noob
      @irrelevant_noob 3 роки тому +82

      @@ameliagryffon7097 asymmetric crypto is when you don't use a single password for both encryption and decryption... Like how the "public key + private key' system works. And integrity is just a hash (checksum) that you can check in order to determine whether the message was changed in transit or not.

    • @ameliagryffon7097
      @ameliagryffon7097 3 роки тому +10

      @@irrelevant_noob Oh okay I think I understand.

    • @Predator20357
      @Predator20357 3 роки тому +13

      I sort of like these little hypothetical scenarios, like you have to take it at face value and no other way or it’ll quickly fall apart.

  • @mathieukransfeld3183
    @mathieukransfeld3183 5 років тому +26358

    the real question: why do you put a castle in a valley??

    • @chexyt3504
      @chexyt3504 5 років тому +1797

      Ur right they were always on hills

    • @ExaltedPhoenix
      @ExaltedPhoenix 5 років тому +4697

      Mathieu Kransfeld the problem actually makes more sense if the castle was in a hill, then you wouldn’t be able to see the other army

    • @mathieukransfeld3183
      @mathieukransfeld3183 5 років тому +207

      @@ExaltedPhoenix someone didn't get the joke

    • @techwizpc4484
      @techwizpc4484 5 років тому +779

      MACHICOLATIONS!!!
      Cheers if you get the reference.

    • @wibaswibas92
      @wibaswibas92 5 років тому +23

      ha ha ha

  • @robertkeddie
    @robertkeddie 3 роки тому +5352

    When I worked at the warehouse of a well-known internet retailer, I found myself picking 25 copies of Adele's new CD. I began to wonder if someone had accidentally typed the album title into the quantity box.

    • @tactcom7
      @tactcom7 3 роки тому +461

      Maybe it was Adele bumping her numbers up.

    • @DespicableDishwasher
      @DespicableDishwasher 3 роки тому +385

      @@tactcom7 actually it was a very devoted Latin American dictator who was a fan of hers

    • @jonmendelson1104
      @jonmendelson1104 3 роки тому +115

      @@DespicableDishwasher Nah, that only happens to Cherlene.

    • @slapslicks7531
      @slapslicks7531 3 роки тому +55

      100 BUCKS says he's at amazon

    • @MrGreghome
      @MrGreghome 3 роки тому +42

      @@jonmendelson1104 but that only happens with outlaw country music

  • @albertbatfinder5240
    @albertbatfinder5240 2 роки тому +795

    Finally I understand the Two Generals Problem. It’s dressed up as something fancy, but all it is saying is that if you don’t have a reliable means of communication, you don’t have a reliable means of communication.

    • @legendgames128
      @legendgames128 Рік тому +27

      As far as I can tell, I have a half-solution to the problem: General A sends a messenger to General B proposing a messenger meeting in the valley, say through 8:00PM to 8:10PM, General A doesn't care if anyone replies, but replies acknowledging that General B has heard the message are welcome. At 8:00PM, send a second messenger to the valley. If no one comes back by 8:10PM, or a messenger died during that meeting, assume failure, repeat the above. If a messenger comes back saying the other messenger made it out alive with a proposed attacking time, that must have been a success. Both armies are ready to attack.
      This ensures that neither army attacks individually, only that no army attacks or both armies attack.

    • @JordanYee
      @JordanYee Рік тому +93

      ​@@legendgames128this doesn't ensure that though. What if the two messengers meet and agree, but one of them dies before making it back to report? The original problem still exists.

    • @legendgames128
      @legendgames128 Рік тому +2

      @@JordanYee Would the other messenger not realize their death and thus report that death?

    • @JordanYee
      @JordanYee Рік тому +61

      @@legendgames128 only if they know about it, which they can't. The messengers meet and agree, but you cannot trust that both messengers make it back to their generals with the information. If you could trust in that, you wouldn't have this problem to begin with.

    • @josephthomas4900
      @josephthomas4900 Рік тому +40

      @@legendgames128 Keep in mind that, as this is a parallel to sending data, any messenger can die at any point, and the messengers wont be able to see each other or check each others status whilst moving. You could have them 'meet in the valley' on a third server, but either messenger could 'die' on the way back from that meeting without the other messenger or other general knowing.

  • @itaybron
    @itaybron 5 років тому +3127

    As astronaut John Glenn put it: "As I hurtled through space, one thought kept crossing my mind - every part of this rocket was supplied by the lowest bidder."

    • @Roxolan
      @Roxolan 5 років тому +229

      Including the astronaut! Even if the labour market has a little more friction.

    • @Graknorke
      @Graknorke 5 років тому +54

      @@Roxolan
      Labour power has a set price generally rather than going to auction. I actually can't think of any time where workers have gone to auction that I know of.

    • @proliferatingprofligate7032
      @proliferatingprofligate7032 5 років тому +157

      @@Graknorke slavery, graknorke. It was during slavery.

    • @Graknorke
      @Graknorke 5 років тому +8

      Are slaves auctioned? I would've assumed it was just a set price.

    • @itaybron
      @itaybron 5 років тому +77

      @@Graknorke I think it was both

  • @Tamhvm
    @Tamhvm 5 років тому +5487

    "If you're in computer science and work on a problem which involves potential loss of life, I really hope you're not watching a series called The Basics."
    And here I am. Because Tom Scott.

    • @tummasmohr7160
      @tummasmohr7160 5 років тому +124

      What are you working on? - if I may ask :)

    • @kakarot9919
      @kakarot9919 5 років тому +23

      *USE MIRROR FLASH*

    • @casey6556
      @casey6556 5 років тому +14

      Tamashii M. H. What do you work on?

    • @davidflores909
      @davidflores909 5 років тому +33

      @@tummasmohr7160 getting sum likes... I hope...

    • @WaffleAbuser
      @WaffleAbuser 5 років тому +168

      He's working on killer robots, obviously.

  • @WMTeWu
    @WMTeWu 5 років тому +759

    There are only two hard problems in distributed systems: 2. Exactly-once delivery 1.Guaranteed order of messages 2. Exactly-once delivery
    ~ Mathias Verraes

    • @ElectricityTaster
      @ElectricityTaster 5 років тому +62

      I'll thumb you up to let you know I received your message.

    • @douggodsoe
      @douggodsoe 5 років тому +14

      Don’t forget, messages of order guaranteed.

    • @Shepard_AU
      @Shepard_AU 5 років тому +3

      WMTeWu I received message regarding the 10 million dollars you’re giving me.

    • @kagi95
      @kagi95 5 років тому +1

      I laughed oud loud! :D

  • @TacComControl
    @TacComControl 2 роки тому +121

    This issue is so old, it's delightful, anyone who's ever run a print server in a school can tell you just exactly how common it is, especially when a printer suddenly starts handling spooled jobs that have been spooling up for an entire day, that it for some reason didn't start before, and it won't stop running them until well on into the night. And most of them are duplicates. Thankfully there are mitigation methods for that these days, but there didn't used to be

    • @furdiburd
      @furdiburd Рік тому +5

      once at a school i worked as it supoort the printer got stuck due to trash wifi (thanks hp for making it internet based) and when it got resolved it started printing a math test 7 times 32 copies. and when you think it finaly stops, it just start another stack of test. i think i would be good to have a quick erase print queve button and not need to nail that 5 sec between print jobs

  • @romi3996
    @romi3996 5 років тому +10071

    Oi nothing wrong with ordering the exact same things 5 times in a row I have my reasons

    • @CleverCrumbish
      @CleverCrumbish 5 років тому +378

      Order five copies of the same thing in the same order you're not fooling anyone at the restaurant

    • @elevown
      @elevown 5 років тому +247

      Lots of us do this. But there's normally a week or so between each of the orders not 1 minute :)

    • @richardhee
      @richardhee 5 років тому +142

      In some (rare) cases you are right, because when I buy one it's cheaper than buying multiple. I know it sounds totaly wrong and in my opinion it is wrong, however, some shops make an error, like, buy 3 get one free, buy 5 get 20% discount. (yep, buy 3 get 1 free is getting 4 items and translates in 25% discount)

    • @markstafford1410
      @markstafford1410 5 років тому +16

      This "Two Generals Prob." has been quite frequently happened to us but in are minds eye we generally calculatingly cop it. Scott is just bring it out to the forefront in layman's terms, i.e the snag. That's is why most apps., have a cart order for the one order.

    • @Abdega
      @Abdega 5 років тому +85

      “Haha… I ordered this all for a bunch of friends!”
      **cries eating 5 pizzas alone**

  • @Tyguy161
    @Tyguy161 4 роки тому +4789

    Have you tried negotiating with the lord of the castle? Was peace even an option?

    • @anuh4308
      @anuh4308 4 роки тому +814

      *P E A C E W A S N E V E R A N O P T I O N*

    • @addisonchan3053
      @addisonchan3053 4 роки тому +159

      I can think of a solution. Move A to B (or vice versa). Two armies!

    • @haroldfisher7528
      @haroldfisher7528 4 роки тому +24

      @@addisonchan3053 I was looking for that comment.

    • @NeibasThe5th
      @NeibasThe5th 4 роки тому +104

      Addison Chan but then again the entire army could be destroyed just like how any number of messengers could be destroyed and then you only have one army

    • @KPIBM
      @KPIBM 4 роки тому +134

      This is magical computer science land, so *peace was never an option.*

  • @TLGame
    @TLGame 3 роки тому +1834

    Imagine being the line cooks on that day. "I swear I made this order two times already."

    • @JohnRay1969
      @JohnRay1969 2 роки тому +62

      That was my thoughts exactly. I would immediately notice duplicate orders on my line. Similar is one thing, happens all the time. But duplicates, exactly the same is rare and noticable.

    • @gwen6622
      @gwen6622 2 роки тому +89

      i mean if it was something like a fast food place, theyre making a billion copies of the same order already anyway. a big mac is a big mac is a big mac. or in this case, a pizza

    • @joshyoung1440
      @joshyoung1440 2 роки тому +33

      @@gwen6622 small, simple orders sure. But most people order a combination of things, especially when they're already paying to get it delivered. I'm a delivery driver. Unique orders are DEFINITELY very rare and noticeable.

    • @nappman1999
      @nappman1999 Рік тому +12

      I worked at a pizza place where our system failed one year due to time change, so we'd get orders when they arrived, and 1 hour after they arrived, we ended up with like 60 extra pizzas that night, it was chaos XD.

    • @natchu96
      @natchu96 Рік тому +5

      @@joshyoung1440 And in this case, since the system is bugging out at the payment acknowledgement step and giving a retry prompt on the spot, the duplicate orders would be coming in at rapid succession. Which always looks dodgy.

  • @RvB_Fan_since_8
    @RvB_Fan_since_8 3 роки тому +227

    I’ve been saved so many times by this video. I remember to always double check when it says “order failed” after I get that check mark. That check mark, that Tom explained the significance of, saves me every time.

  • @blu0065
    @blu0065 5 років тому +1971

    "If you're in computer science and working on a problem that could cause a potential loss of life, I really hope you aren't watching a series titled 'the basics.'"
    Oh. If it's any comfort, I'm watching because it's my Lunch break and I love your videos.

    • @derek96720
      @derek96720 5 років тому +188

      Sometimes intelligent people get too inside their own heads, and going back to basics can greatly help eliminate unnecessary considerations, leaving you with a clear answer.

    • @RKBock
      @RKBock 4 роки тому +63

      @@derek96720 sometimes, especially in programming, it leads to a programmer stopping his trust in certain packages/libraries and them reinventing the wheel.

    • @sniperbloom1305
      @sniperbloom1305 4 роки тому +1

      @@marclurr "explodes when you try to burn it"... doesnt that defeat the purpose 🤔

    • @Dolthra
      @Dolthra 4 роки тому +8

      @@marclurr Depends. Sometimes it produces a worse solution, sometime it produces a better solution... that only works for a certain specific type of cart.

    • @user-iu3ii8sq6t
      @user-iu3ii8sq6t 4 роки тому

      same here, and I'd never heart of 2 generals

  • @muh1h1
    @muh1h1 5 років тому +1611

    "Next time i'll just cook for myself"
    I was expecting:
    "This series of the Basics is sponosred by Blue Arpon"

    • @diamondflaw
      @diamondflaw 5 років тому +18

      Same thought, but with Hello Fresh since I listen to NDQ and Hello Internet podcasts.

    • @tab160
      @tab160 5 років тому +5

      @@diamondflaw Fellow Tim I see

    • @yoshilovesyoshi
      @yoshilovesyoshi 5 років тому +4

      @LueLou WOAH THAT'S AWESOME! Step up California

    • @WilliamAndrea
      @WilliamAndrea 5 років тому +2

      something something The Basics with Babish

    • @BrokenMonocle
      @BrokenMonocle 5 років тому +1

      I thought "That's a lie, and you know it."

  • @MrTrees
    @MrTrees 5 років тому +2759

    4:49
    "A single human error is never the root cause"
    Me: *looking at line 456 of my code that has a missing semi-colon* "SEE?"

    • @ZippyDan
      @ZippyDan 5 років тому +20

      Never

    • @da4127
      @da4127 5 років тому +15

      Well that’s not your only life error making you miss that semi colon

    • @johnbyrne8948
      @johnbyrne8948 5 років тому +1

      😅😅😅😅

    • @ChuckNorrizHIM
      @ChuckNorrizHIM 5 років тому +6

      @@da4127 My exact excuse of a thought

    • @Kalahee
      @Kalahee 5 років тому +34

      Missing a semi-colon, app would just not work and not result in such catastrophe.

  • @Reczack
    @Reczack 3 роки тому +188

    A very similar thing happened on Uber Eats a few months ago. I'm a driver, and I repeated a route from a specific restaurant to a specific home at least six times.

  • @SjondeJongen
    @SjondeJongen 4 роки тому +4865

    For me it would be more logical if the castle was on a hill. In that case the generals couldnt just see eachother

    • @cowboymooman8776
      @cowboymooman8776 4 роки тому +350

      you make a really valid point

    • @dylandarnell3657
      @dylandarnell3657 4 роки тому +526

      Or even more logically, a long ridge, so they can't just go around the side.
      It's also a more logical place to build a castle.

    • @woutertesselaar5725
      @woutertesselaar5725 4 роки тому +183

      most castles also are on hills

    • @xirenzhang9126
      @xirenzhang9126 4 роки тому +25

      Can’t they just go around the hill then?

    • @Alittlefruitgoesalongway
      @Alittlefruitgoesalongway 4 роки тому +108

      No, the generals need a way of knowing one another is there while also not being able to coordinate. For example if you were across a real valley you could see an army encampment on the other side if there was one, but you wouldn't be able to see well enough to make out specific people or to coordinate an attack properly. If it was the inverse then you'd have no idea another general is on the opposite base of the mountain also trying to attack the castle.

  • @MuhsinFatih
    @MuhsinFatih 4 роки тому +4581

    "I was lucky" it's the opposite. You were cursed with the computer science knowledge. Had you made multiple orders you would have gotten a refund and also eat 5 meals :D

  • @DerAykac
    @DerAykac 5 років тому +620

    "But I was tired, and i was hungry. As are a lot of their drivers"
    sheeeeeesh, that was a harsh one xD

  • @Hybzy
    @Hybzy Рік тому +13

    Another solution is what we would do in the electronics industry. Sometimes when you press an analog button, due to the speed of electricity, if the button doesn't make a clean connection, it registers multiple presses. To combat this, we would use a 'debounce' circuit, where a single press of the button temporarily disconnects the button through a semiconductor. This then means that the button will no longer work until a time-delay (usually discharge of a capacitor) has taken place. This would be akin to the solution mentioned here where an order to a restaurant could not be repeated for a specified duration (An hour, for example).

  • @lancejordan9638
    @lancejordan9638 4 роки тому +1488

    Tesla just had this issue with their website. Someone ordered 28 cars by clicking the buy button again and again after it said there was an error.

    • @eve_the_eevee_rh
      @eve_the_eevee_rh 4 роки тому +30

      XD

    • @absolutelybagel2218
      @absolutelybagel2218 4 роки тому +15

      @imahumanandimagamer tesla is full online

    • @huisbaasbob1923
      @huisbaasbob1923 4 роки тому +172

      @imahumanandimagamer can you imagine spending 50k and when you get an error, you don't immediately call the seller in panic but you just try and try again

    • @frozenheartedgiant8330
      @frozenheartedgiant8330 4 роки тому +58

      @@huisbaasbob1923 When you have money for a Tesla, you probably don't worry about getting charged twice.

    • @TheMajorpickle01
      @TheMajorpickle01 4 роки тому +19

      @imahumanandimagamer Not having to go to a Dealer and just instantly getting what you want with no faff sounds amazing to me. You would only really need to see the car if it was used

  • @SGTSkylor
    @SGTSkylor 3 роки тому +1667

    “When something goes this drastically wrong… A single human error is never a root cause.” Challenge accepted

    • @mrocto329
      @mrocto329 3 роки тому +210

      but your birth required 2 people

    • @TheIdiotPlays
      @TheIdiotPlays 3 роки тому +11

      @@mrocto329 :D

    • @lordsiomai
      @lordsiomai 3 роки тому +14

      @@mrocto329 yooooooooooooo

    • @한승원-p5m
      @한승원-p5m 3 роки тому +14

      @@mrocto329 yooooooooo duuuuuuuuuuude
      uncool

    • @list9016
      @list9016 2 роки тому +5

      @@mrocto329 OHHHHHHHHHHHHHHHHHH dude....

  • @IkisDragonFist
    @IkisDragonFist 5 років тому +2326

    -"So what are we having? Chinese? Mexican?"
    -"Byzantine".

    • @Rolando_Cueva
      @Rolando_Cueva 5 років тому +153

      Constantinople.exe stopped working.

    • @asrnyigit4040
      @asrnyigit4040 5 років тому +112

      Do you want to execute Istanbul.exe?

    • @alexwang982
      @alexwang982 5 років тому +16

      Asrın Yiğit Execute Constantine

    • @nikolatasic9452
      @nikolatasic9452 5 років тому +30

      We will take Jerusalem.

    • @kevinboros7427
      @kevinboros7427 5 років тому +10

      Then the winged hussars arrived!

  • @UltimateAzumanger
    @UltimateAzumanger 2 роки тому +51

    0:07 "A certain food-delivery app"
    5 seconds later:
    Deliveroo

  • @lonagen9340
    @lonagen9340 4 роки тому +1070

    2:53: “if you’re dealing with a problem that involves potential loss of life, I really hope your not watching a series called the basics.”
    The military drone programmer watching this: *sweats profusely*

  • @Kinetik07
    @Kinetik07 5 років тому +5662

    So it's like double posting by accident?

  • @fluffy9050
    @fluffy9050 2 роки тому +17

    As someone working in IT without any formal training, I hate how much of this I understood.
    I really like how easily you explain this complex idea in a way normal people... could... could, understand.

  • @blooddragonnetwork9545
    @blooddragonnetwork9545 5 років тому +3976

    This guy is something between 20 and 60 years old i cant tell

    • @Flint_Inferno
      @Flint_Inferno 5 років тому +168

      I literally have no idea what his age is

    • @algorithm1193
      @algorithm1193 5 років тому +190

      I... I think you just discovered a new paradox

    • @colbyd7618
      @colbyd7618 5 років тому +163

      He’s 34

    • @isaacbruner65
      @isaacbruner65 5 років тому +248

      @@colbyd7618 so OP was spot on!

    • @teddywolfes1042
      @teddywolfes1042 5 років тому +34

      Welcome to computer science...

  • @nemahs
    @nemahs 5 років тому +784

    Tom, I am a safety critical programmer and I am watching a series called the basics, but more cause I like your stories and less for use at work :)

    • @JoeBleasdaleReal
      @JoeBleasdaleReal 5 років тому +12

      Robby Maura is potential loss of life involved? 😂

    • @TecSanento
      @TecSanento 5 років тому +1

      But how do you solve this problem at work?

    • @ratdude747
      @ratdude747 5 років тому +7

      Same here... not my main job, but I do design and tweak safety systems at work when needed (robotic welding cells, etc.). Rule #1: "Why is this wrong?"

    • @NatoBoram
      @NatoBoram 5 років тому

      @@ratdude747
      Rule #2 : Why is this right?

    • @57thorns
      @57thorns 5 років тому +1

      It is also a good idea to get a refresher now and then, especially when it is entertaining.

  • @Dark_Ronius
    @Dark_Ronius 3 роки тому +89

    This really opens your eyes as to how the financial system works when you pay for something. Like, the reality there is no solution to the two general problem. So anything, no matter how advanced and seemingly secure with how it works everyday, is limited by this. Anything is a work around, which just happens to work 99.9999% of the time

    • @stargazer7644
      @stargazer7644 Рік тому +7

      Except that there is a solution. And he showed it to you.

    • @codinghub3759
      @codinghub3759 Рік тому +1

      ​@@stargazer7644 There is still a fault in that system. Just rewatch the part at 5:50

    • @LineOfThy
      @LineOfThy Рік тому +3

      @@stargazer7644 that isn't a foolproof solution, because the return data is still impossible to confirm

    • @unsolicitedditkapics9722
      @unsolicitedditkapics9722 Рік тому +2

      There a multiple solutions to the two generals problems. Smoke signals have been used throughout history to solve problems just like this. I hate when metaphors completely miss the point in life and EVERYONE eats it up thinking it's some higher wisdom

    • @unixtreme
      @unixtreme Рік тому

      @@unsolicitedditkapics9722the metaphor is to try to visualize a computer problem. Which tbh isn’t a real problem, the way we solve this is by having an authoritative source in the middle, that say stores the order with a timeout, then repeat orders cannot be placed until acknowledged, then the server can set it a sack low edged and the client can see it’s been acknowledged. If the timeout is exceeded both now to cancel that order and it has to be booked again by the client.

  • @marbleswan6664
    @marbleswan6664 5 років тому +990

    I find the idea of two armies going one by one through a valley sending acknowledgments back and forth until everyone is dead hilarious
    Just imagine the army in the castle being confused.

    • @goldfishy
      @goldfishy 5 років тому +137

      “Why do they keep attacking one at a time?!?”

    • @Nonsense010688
      @Nonsense010688 5 років тому +103

      I'm more confused on who builds the castle IN the valley and not on a Hill...

    • @TheUltraSonicKid
      @TheUltraSonicKid 5 років тому +17

      Si Wi it’s over anakin! I have the high ground!

    • @10babiscar
      @10babiscar 5 років тому +32

      @@Nonsense010688 That would work better for the problem actually. If the castle is in a valley the two armies would be able to see each other but if it was on a hill there wouldn't be a direct line of sight between the armies.

    • @dekaaizer2550
      @dekaaizer2550 5 років тому +4

      What about them both sending a messenger at 8pm so they will meet in the middle to sync there attacks.

  • @Putindidnothingwrong
    @Putindidnothingwrong 5 років тому +677

    See you all in 2023 when this video gets recommended again

  • @alexihickin230
    @alexihickin230 5 років тому +192

    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.

    • @luckymouse1988
      @luckymouse1988 4 роки тому +4

      That's rather _selfish_

    • @natebit7167
      @natebit7167 4 роки тому +4

      wtf is a wagamama

    • @brentcouncil1489
      @brentcouncil1489 4 роки тому

      Sorry but do you speak English? I have no idea what what you are talking about :(

    • @AlfiRizkyR
      @AlfiRizkyR 4 роки тому +1

      @@luckymouse1988 underrated pun right there.

    • @natebit7167
      @natebit7167 4 роки тому +1

      No actually, wtf is a wagamama someone pls

  • @MeltyMel
    @MeltyMel Рік тому +2

    the fact that it's 2023 and this exact thing has happened to me TWICE in the past month. I'm just tired at this point. HOW is this still happening. thank you for describing what's been happening to me i thought it was a me problem for the longest time

  • @jlinkpro
    @jlinkpro 4 роки тому +348

    "I was tired and hungry... as are a lot of their drivers."
    Never stop being savage.

    • @petarus625
      @petarus625 4 роки тому +3

      @@DvH_2 ikr? I had no clue the word savage had anything to do with the Pauls.

  • @erraticonteuse
    @erraticonteuse 3 роки тому +3934

    Tom: The Two Generals Problem is a classic of computer science.
    Me: Given how much the development of computer science was tied up with World War II and then the Cold War, I wonder if there's any bonus military history insights.
    Tom: So there's a castle in a valley...
    Me: The insight is that computer scientists don't know military history.

    • @GeneralAceTheAwesome
      @GeneralAceTheAwesome 3 роки тому +337

      The thing is though, the valley has nothing to do with the problem. If it was a flat plane, or a castle on a hill, you could still have the same problem. I wonder if the valley was in the original version of the problem?

    • @zeroyuki92
      @zeroyuki92 3 роки тому +160

      Possible if the castle is protecting something like a holy site or is an important crossroad between mountains, for example.

    • @noahjordan6761
      @noahjordan6761 3 роки тому +64

      Like there would be military history when a point needed to be made. Peoples attention spans are at stake here!

    • @unrelatedK
      @unrelatedK 3 роки тому +22

      @@noahjordan6761 and so are the two generals

    • @NoriMori1992
      @NoriMori1992 3 роки тому +63

      The insight is that it's not supposed to resemble a real military situation.

  • @Carter-dv4hz
    @Carter-dv4hz 4 роки тому +1880

    General A could just move his whole army to general B.
    I guess that involves driving to the pizza place though.

    • @jyothia3958
      @jyothia3958 4 роки тому +14

      Haha

    • @SuperSox97
      @SuperSox97 4 роки тому +58

      @ivan ivanovitch ivanovsky What if all the sent messengers are killed on their way back?

    • @SuperSox97
      @SuperSox97 4 роки тому +60

      @ivan ivanovitch ivanovsky If all my sent messengers are killed on their way back, how do I know the messengers I sent weren't killed on the way there instead of on the way back? If they were killed on the way there, the other army wouldn't know when they're supposed to attack, so our attack would fail.

    • @Vousie
      @Vousie 4 роки тому +37

      Or general A could just move himself to general B... I guess that means going to the pizza place & leaving your computer at home.
      Now I have the mental image of a guy driving his car to a pizza place, but with his entire massive desktop computer along with all of the peripherals in his car with him - "I brought my army".

    • @Goombalove3000
      @Goombalove3000 4 роки тому +20

      @@Vousie if general a moves to general b's position, the army will get intercepted on the way and wiped out, since you need both armies to win a battle

  • @GodfreyMann
    @GodfreyMann Рік тому +2

    Data transactions solved this problem decades ago where the database only writes the transaction if it’s successful end-to-end. If any part of the transaction fails, then the database rolls back the entire transaction.
    Therefore, if the system was telling users that the transaction was successful when bits of it failed, then that’s either a transaction design or programming mistake.

  • @ruwiki
    @ruwiki 5 років тому +576

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

    • @stephencooper5155
      @stephencooper5155 5 років тому +68

      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?

    • @abeltutor9034
      @abeltutor9034 5 років тому +46

      @@stephencooper5155 This. If both A and B sends messengers and calls them back with the confirmation of an 8pm attack, they will know 100% that the other side got the message. 2-3 messengers does the trick. This "problem" is like elementary school level, I don't understand how can't people figure it out.

    • @Lamawalrus
      @Lamawalrus 5 років тому +24

      @@stephencooper5155 How does general B know that "as long as the message keeps arriving then his own messenger hasn't gotten through"? You send a pre-message to tell what the strategy is? Human intuition? This is fantasy computer land, these things do not apply

    • @Lamawalrus
      @Lamawalrus 5 років тому +147

      @@abeltutor9034 If a famously unsolvable problem appears to be an elementary school level problem, then it's more likely you misunderstood the problem

    • @dLzzzgaming
      @dLzzzgaming 5 років тому +28

      @@Lamawalrus Yea they are missing the point, that this is computers.
      Say general (computer) A spams general B, B sends message to A saying let's go at 8. A receives the message, yes, but B can't know for sure that A received the message just because A stopped sending messages to B.
      If B doesn't get to A, and A stops because it eventually has to stop, how does B know it got through? If A updates the message it was spamming to say "We got your message, attack at 8 am doubly confirmyed!" then A can't know for sure that B is receiving THAT message.

  • @turdanc
    @turdanc 5 років тому +1685

    "A single human error is never the root cause"
    A single human error: I'm about to end this man's whole career

    • @KaiserTom
      @KaiserTom 5 років тому +62

      Ah, but why does that single human have so much power to completely ruin such a system by themselves? In which case the root cause is actually poor change management or lack of adherence to principle of least privilege or lack of protections for certain key equipment. The key word is "root" cause which dives down much further than the fact that a single human may have caused the problem, but isn't themselves the "root" cause. And a malicious actor bringing down something is not really an "error", and even still may have root causes in poor security measures.

    • @ghosty918
      @ghosty918 5 років тому +15

      It's not a SINGLE error. Tom mentions a couple things that should be there (the token, the inability to order the same thing in a set period, that sort of thing).

    • @Voron_Aggrav
      @Voron_Aggrav 5 років тому +7

      Usually most failures of systems are a culmination of various factors that either together or independently causes something to behave in unintended ways

    • @tcoren1
      @tcoren1 5 років тому +4

      EpicKaiserTom sometimes a single programming error can bring down an entire operation (I think once an unmanned spacecraft crashed because a typo).
      There really isn’t any think that could’ve been done to prevent it, it’s not a result of poor choices or management, it’s just the way things are in this world

    • @SendarSlayer
      @SendarSlayer 5 років тому +1

      @@KaiserTom Ahem Ahem Y2K was caused by one dude saying "Lets do dates DD/MM/YY and everyone copying him. Single human could've destroyed almost all computers if no one had picked up on it.

  • @tiyenin
    @tiyenin 3 роки тому +581

    4:48 Old Tom: A single human error is *never* the root cause.
    18 or 19 y.o. Tom: *ONOSECOND*

    • @j.hawkins8779
      @j.hawkins8779 3 роки тому +24

      Or you know, *accidentally deleting an entire data base* do get the joke tho if there is one

    • @SSM24_
      @SSM24_ 3 роки тому +23

      @@j.hawkins8779 "onosecond" is a reference to that exact video you're talking about

    • @j.hawkins8779
      @j.hawkins8779 3 роки тому +1

      @@SSM24_ yes, I know

    • @crimsonstrykr
      @crimsonstrykr 3 роки тому +9

      @@j.hawkins8779 You know, but how do I know that you know? And if I know then how do you know that I know that you know? AND IF YOU KN-.....

    • @j.hawkins8779
      @j.hawkins8779 3 роки тому +3

      @@crimsonstrykr hm, yes. *infinite loop*

  • @RonaldSimkin
    @RonaldSimkin 2 роки тому +15

    2:45 What if each side sends a guy to the middle, and they communicate at the middle, and they each return and each report the success of an agreement.

    • @pritamdavis
      @pritamdavis 3 місяці тому +5

      What if one person doesn't return

    • @nikhilweerakoon1793
      @nikhilweerakoon1793 2 місяці тому +7

      the problem is that even if their messenger returns from such an arrangement in the middle neither general will be able to know with one hundred percent certainty that the other sides messenger has made it home safely since they would only know if their respective messengers arrived safely .

  • @sosirisos
    @sosirisos 4 роки тому +1239

    imagine if the sponsor at the end was deliveroo

    • @Nat-jf2ge
      @Nat-jf2ge 4 роки тому +3

      😅 That would be fun!

    • @mohammedhussain6749
      @mohammedhussain6749 4 роки тому +10

      I got a pre video ad for deliveroo.

    • @MrJoeDone
      @MrJoeDone 3 роки тому +4

      @@mohammedhussain6749 genius, that was exactly what i wanted to comment about, the algorythim just knows that this video is talking about food deliveries but not if good or bad :D

    • @seanmurphy4465
      @seanmurphy4465 3 роки тому +1

      Who Deliveroo? 😬

    • @Nat-jf2ge
      @Nat-jf2ge 3 роки тому

      @@seanmurphy4465 A chinese owned version of uber eats

  • @uniqueusername_
    @uniqueusername_ 3 роки тому +2542

    “I know.”
    “I know you know.”
    “I know you know I know.”

    When I was little, I was so confused by this. Good to know it’s actually an unsolvable problem!

    • @RGC_animation
      @RGC_animation 3 роки тому +54

      It's unsolvable, I know.

    • @jebbsredemption
      @jebbsredemption 3 роки тому +71

      @@RGC_animation I know you know it's unsolvable.

    • @JohnFiala
      @JohnFiala 3 роки тому +43

      @@jebbsredemption I know you know I know it’s unsolvable.

    • @scriptshowcasechannel4515
      @scriptshowcasechannel4515 3 роки тому +34

      It's solvable - if you send a person to deliver a message tell them to come back once they have delivered it. If they don't come back send another message until one person comes back. Then you know they got the message and therefore will attack at the same time.

    • @scriptshowcasechannel4515
      @scriptshowcasechannel4515 3 роки тому +12

      It's solvable - if you send a person to deliver a message tell them to come back once they have delivered it. If they don't come back send another message until one person comes back. Then you know they got the message and therefore will attack at the same time.

  • @vulpeeze
    @vulpeeze 3 роки тому +167

    Watched this months ago and now I'm being told to watch it again by my university professor. Tom Scott never fails to impress.

  • @MeditationalState
    @MeditationalState 2 роки тому +2

    *Am I the only one who realises that this still doesn't solve the 'classic problem' involving the 2 armies?*

  • @nebulamage8394
    @nebulamage8394 4 роки тому +824

    Is this problem the thing where you send a text message and it says “message not sent” so you spam it and then your friend says you sent 10 of the same message?

    • @ryanmunn4134
      @ryanmunn4134 4 роки тому +68

      yep

    • @airkami
      @airkami 4 роки тому +55

      And the solution is anti-spam bots in twitch chat. You try to say the exact same thing twice in a row and it will let you know your duplicate message won't go through and you need to wait X amount of time before posting the same message again.

    • @MrJoeDone
      @MrJoeDone 3 роки тому +2

      @@airkami well in twitch its just identical messages not the same message (different massage id if you would create one)

    • @airkami
      @airkami 3 роки тому +7

      @@MrJoeDone Depends on how they implement the bot. Since the bot will have a cache of the original messages, it could update the id of the message to be the same as the id of the old message thus making it effectively the same exact message as far as the system is concerned.

    • @unliving_ball_of_gas
      @unliving_ball_of_gas 3 роки тому +4

      That sometimes happens on yt too

  • @nickolas474
    @nickolas474 5 років тому +238

    If I was an engineer working on a computer science problem with human lives on the line, I might still watch a series titled "the basics" because Tom Scott is worth watching.

    • @maxximumb
      @maxximumb 5 років тому +5

      Everyone can always learn something new. The secret is keeping your mind open to different ways of doing things.

    • @huttj509
      @huttj509 5 років тому +16

      @@maxximumb There's also the aspect of "That's a really good way of putting what I wasted an hour trying to explain to that one manager!"

    • @icemysta30
      @icemysta30 5 років тому

      ishmael

    • @jirm18
      @jirm18 5 років тому

      Exactly!

  • @mattpinkerton6562
    @mattpinkerton6562 4 роки тому +22

    Because of this video I avoided a similar problem with uber eats, it said there was a problem ordering and I remembered this video, made sure to check my bank account. Sure enough while Uber said the order wasn't placed, the money was out of my account. 20 minutes later my order arrives, thanks Tom!

  • @kyriwalkthrough
    @kyriwalkthrough 2 роки тому +3

    The CS solution I learned in my network analysis class:
    1. "A" sends a messenger (MSG) to "B". It waits a specified amount of time. If it has not heard back an acknowledgement (MSG-ACK) from "B" within this time, it resends another messenger.
    2. When "B" receives a MSG from "A", it sends back a MSG-ACK to "A". Every time that "B" receives another MSG from "A" or some specified amount of time passes, it will send that same MSG-ACK in response.
    3. When "A" finally receives one of these MSG-ACKs from "B" , it will send back its own acknowledgement (ACK) to "B". Again, it will send back one of these ACKs for every MSG-ACK it receives, but NOT after some specified time period like it did previously.
    4. Once "B" receives this ACK from "A", it knows it can stop sending MSG-ACKs, since an ACK from "A" means it has received B's MSG-ACK.
    At this point, both A and B have authenticated the other's messages.
    If I missed anything or was wrong about something let me know!

    • @ammarratnani6209
      @ammarratnani6209 6 місяців тому

      Question: How does A know that B has received its ACK. It seems you're relying on the absence of MSG-ACKs to signal that. But, the same absence of MSG-ACKs could be produced if:
      * the ACK from A gets dropped, and
      * every retransmitted MSG-ACK from B also gets dropped.
      In that case, A will have sent its final ACK and decide to attack, but B will still be waiting for A's ACK and decide not to.
      There's also the question of how long A should wait for retransmitted MSG-ACKs before comitting to an attack time. The goal is for A and B to come to a consensus about when to attack within a bounded amount of time. But in your approach, A might have to service MSG-ACKs indefinitely. You'd have to tell A to assume B received the ACK if it doesn't see MSG-ACKs for some time. It's possible that B was sending MSG-ACKs for that entire time, but none of them were getting through. In that case, A will decide to attack but B won't.
      It's worth noting that the approach you describe is what's used by TCP when terminating a connection. Because of the Two Generals Problem, the last person to send an ACK can't be sure the ACK has gone through. So, we tell them to keep the connection open for some TIME_WAIT period - 120s by default. If they see a retransmission within that time, they resend their ACK and (I think) reset their timer. If the TIME_WAIT period expires and we haven't seen any retransmissions, we assume the network will never fail so badly as to deliver no retransmissions for that long, and we conclude that B has seen our ACK and we can terminate the connection.

  • @megiant
    @megiant 4 роки тому +83

    Total War AI is so advanced it doesn’t need a two generals problem to just rush towards a castle without waiting for your half of the army.

  • @seanobrien5350
    @seanobrien5350 5 років тому +4532

    Nobody would put a castle in a valley between two perfectly good hills, therefore the entire video is invalid.

    • @mrnice4434
      @mrnice4434 5 років тому +305

      Maybe the castle was on a hill but then the two generals order there army to pill up to hills on the two sides. ;)

    • @Jehty_
      @Jehty_ 5 років тому +169

      That is not true. There were a bunch of castles in valleys.
      For example one that comes to mind is Castle Eltz in Germany.

    • @itchykami
      @itchykami 5 років тому +457

      Maybe the castle should be on the hill, and the armies on two valleys. It'd eliminate the need to say 'don't use flags' too, since the armies wouldn't have line of sight.

    • @Arthur0000100
      @Arthur0000100 5 років тому +155

      @@mrnice4434 Actually, that would make more sense in two ways, as the hill would also create a (irl) barrier for transmitting messages

    • @badmeme486
      @badmeme486 5 років тому +18

      @@Jehty_ was about to say that, there are pros and cons to building a castle in a dip

  • @riveraluciano
    @riveraluciano 5 років тому +324

    "If you're in computer science and work on a problem which involves potential loss of life, I really hope you're not watching a series called The Basics."
    Meanwhile, everyone at StackOverflow...

    • @Hyperus
      @Hyperus 5 років тому +5

      Not a lot to go wrong when all you have is a "Ctrl", C and a V Button to use :>

    • @kalebbruwer
      @kalebbruwer 5 років тому +27

      "Hey so I got this error, it says 'Segmentation fault (Core Dumped)'. You know what I did wrong?

    • @SubjectMRF37
      @SubjectMRF37 5 років тому +10

      Kaleb Bruwer oh that’s nothing, just turn ur computer off and on again and it should be fine.. computers just tired

    • @TheAlison1456
      @TheAlison1456 5 років тому +2

      what the hell is the basics

    • @LukaMiljak
      @LukaMiljak 5 років тому +2

      Unknow0059 One of his video series. This episode belongs to it.

  • @catcoder7812
    @catcoder7812 Рік тому +1

    My solution: Send the entire army as messengers and all attack together from 1 side
    (I know that this only works in the 2 Generals Problem and not in the real world but I was still pleased to find a work-around)

  • @P7777-u7r
    @P7777-u7r 3 роки тому +244

    Option C: Both sides just sit there and let the forces in the castle who are cut off from the outside sit and run out of supplies

    • @goekhanbag
      @goekhanbag 2 роки тому +20

      The attacking sides would run out of supplies before the castle would.

    • @alex2005z
      @alex2005z 2 роки тому +10

      Option D: smoke signals

    • @baron2062
      @baron2062 2 роки тому +35

      @@goekhanbag seems unlikely if they can figure out how a supply wagon works

    • @chanclink2383
      @chanclink2383 2 роки тому +1

      Well that was a strategy used during the Civil War in the USA; Check the Fort Sumter

    • @alex2005z
      @alex2005z 2 роки тому +29

      @@chanclink2383 sieging has been a thing since forever

  • @findingteemo6833
    @findingteemo6833 5 років тому +602

    "That's how to solve the two generals problem", but that doesn't really solve it right? It just is a good way to deal with it.

    • @AndrewJJ-0114
      @AndrewJJ-0114 5 років тому +78

      Correct, it's just a way to work around it in practice.

    • @airRozi
      @airRozi 4 роки тому +6

      bitcoin solves it.

    • @HTFFanOfFlaky
      @HTFFanOfFlaky 4 роки тому

      @@hairyairey it already did tho

    • @chantolove
      @chantolove 4 роки тому +6

      ...otherwise known as a solution?

    • @PiezasdeAjedrez
      @PiezasdeAjedrez 4 роки тому

      Only in that very specific orders repetition case :v

  • @somethinggreatishappening8536
    @somethinggreatishappening8536 3 роки тому +155

    lesson learned: if you want to attack a castle, just cook for yourself

  • @silentwhisper868
    @silentwhisper868 9 місяців тому +1

    Just set up a protocol:
    General A sends a messanger. If more than twice the time it takes a messanger to get to the other side and no messanger got back this means one of the messangers failed. So general A sends another one.

    • @null3081
      @null3081 2 місяці тому

      what if the messenger gets lots on the way back from general B? then general B received the message but general A didnt receive the confirmation. now general B might attack while general A is still sending another messenger

  • @Voodoomancer
    @Voodoomancer 5 років тому +150

    Moral of the story: If you want to be a programmer, learn to cook.
    ...wait.

  • @Marinealver
    @Marinealver 5 років тому +195

    "Would you like to pay again?"
    Sounds like everything is working as intended to me.

  • @twinklestar3556
    @twinklestar3556 5 років тому +64

    I almost ordered pizza multiple times recently when they said payment wasn't recieved, but I was too tired to care about reordering and gave up.
    Imagine my surprise when it showed up an hour later, lmao

  • @resentfulscarf4170
    @resentfulscarf4170 2 роки тому +2

    All I’m thinking is why the hell would you build a castle in a valley

  • @RedesCat
    @RedesCat 5 років тому +76

    "next time I'll just cook for myself"
    Tom, we both know next time you'll order out again

  • @iamtheusualguy2611
    @iamtheusualguy2611 5 років тому +7

    As a computer science student, it's really cool to see Tom introducing problems like these to the general public!
    This is one of the many things people learn in their network communication courses in undergrad! It's crazy how many seemingly simple problems there are with communicating in distributed systems

  • @eggyrepublic
    @eggyrepublic 5 років тому +83

    "A single human error is never the root cause"
    Me: *observe*

  • @sweatcityheatwave
    @sweatcityheatwave Рік тому +1

    Message 1: We attack at 8.
    Message 2: Okay, 8 it is.
    Message 3: Great. See you then.
    Message 4: Great.
    Don't both parties know what's up at this point? This is hurting my head.

    • @noraduong8755
      @noraduong8755 Рік тому

      what if message 4 doesnt arrive? that could mean message 3 didnt arrive to the other person and they would still be waiting, and they wont attack.

  • @markusr3259
    @markusr3259 5 років тому +322

    "I was tired and I was hungry, as are a lot of their drivers"
    Jim Ross: Tom Scott Slam! Tom Scott Slam! GOOD GOD ALMIGHTY he's destroyed them! 21 seconds, TWENTY ONE SECONDS and its all over.

    • @appleslover
      @appleslover 5 років тому +5

      ?

    • @bogomilmitev7904
      @bogomilmitev7904 5 років тому +11

      apple's lover It's a WWE reference. Jim Ross is a commentator and this is his legendary quote.

    • @tacitus5665
      @tacitus5665 5 років тому +4

      Much better qoute would be: "Wow, what'd you say after that one? Brutal, savage, rekt? Absolutley taken down..."

    • @Priest92
      @Priest92 5 років тому +1

      MUUUHHH GAWWD

  • @z-beeblebrox
    @z-beeblebrox 5 років тому +346

    idempotency: when you're able to have children, but just the one time.

    • @satibel
      @satibel 5 років тому +24

      Remember : killing your children is ok in IT.

    • @CogniVision
      @CogniVision 5 років тому +4

      Sounds like China needed this.

    • @eidolor
      @eidolor 5 років тому +1

      Twins have to fight for vaccination

    • @jimifloydrix
      @jimifloydrix 5 років тому +17

      I mean, for women it's kind of like that. No matter how many other soldiers you send, once she's preparing your order you'll have to wait for it to be delivered to order a new one.

    • @andersenzheng
      @andersenzheng 5 років тому

      @@satibel Me: child.kill();
      processor: thats reasonable, let me do it for ya.

  • @fabianlauterbach2845
    @fabianlauterbach2845 5 років тому +42

    And that's how you do a sponsorship. Don't just be a talking billboard but actually do something with it. Great work (as always), Tom!

  • @rosellastar468
    @rosellastar468 2 роки тому +1

    I don’t know what it is about this specific video of toms but I always find myself coming back to rewatch it

  • @dalek2538
    @dalek2538 5 років тому +26

    For once, this is a very helpful sponsorship. I thought it was concerning that all of someone's passwords are stored in one location, so thanks for clearing that up!

  • @sator_project
    @sator_project 3 роки тому +60

    "next time I'll just cook for myself"
    and ever since then, Tom Scott has cooked very meal he's eaten

  • @YaDerpyyy
    @YaDerpyyy 5 років тому +33

    One evening in late 2018, our neighbours came over and gave us free pizza saying they received more than they ordered, but we never knew why or what happened. Funny that this video seems to have solved that mystery haha

    • @aryanpatel2924
      @aryanpatel2924 5 років тому +2

      Damn, you've got some good neighbours

  • @jemangerrit1747
    @jemangerrit1747 2 місяці тому +1

    This is how you solve the two generals problem. You move your entire army to the other side, that way there will be no one left to not know whether the message came through. And then you just attack together.

  • @alexanderreichard9594
    @alexanderreichard9594 3 роки тому +623

    The REAL problem is the general who built a castle in a valley…

    • @jeremiahmuth9732
      @jeremiahmuth9732 3 роки тому +105

      I mean, you say that, but it seems to be working out for him so far

    • @schwarzerritter5724
      @schwarzerritter5724 3 роки тому +22

      Since the messengers can not go around, the place where the castle stands is the only connection between the sides. That makes it a strategically important point.

    • @gustandberg7553
      @gustandberg7553 3 роки тому +61

      @@schwarzerritter5724 But building a castle in a low point means besieging parties can set up siege engine on the hills and bombard with a height advantage. If the valley can project force in the surrounding area, the hill can do it better.

    • @dariusluca5469
      @dariusluca5469 3 роки тому +12

      It is debatable, if the valley is really that important and the local garrison is big enough said fortress may be always able to hold out until reinforcements arrive

    • @MagicSpamDragon
      @MagicSpamDragon 3 роки тому +3

      The real issue is linear pathing who decided the rules of point a to point b
      If this was real internet with reverse dns you can go from point A to point C to point B cause a+b failed connection

  • @minimanofiron2501
    @minimanofiron2501 5 років тому +423

    do you want a new tom sc-
    me: YES

    • @joestefanik6197
      @joestefanik6197 5 років тому +14

      I acknowledge I have seen your comment.

    • @jeromej1234
      @jeromej1234 5 років тому +10

      Joshua Stockin i acknowledge your acknowledgement of his reply of acknowledgement of the comment

    • @diamondflaw
      @diamondflaw 5 років тому +3

      So we're cloning Tom now? Cool! Do we have enough red shirts?

    • @takeonjapan7992
      @takeonjapan7992 5 років тому +1

      confirmed, loading new tom scat video

    • @GentlemanlyOtter
      @GentlemanlyOtter 5 років тому +5

      Mormon Freegan, i acknowledge your acknowledgement of the acknowledgement of his reply of acknowledgement of the comment.

  • @spaceninjasteve3356
    @spaceninjasteve3356 5 років тому +101

    After about the third acknowledgement I reckon the generals would know they were on the same page

    • @coltonbates629
      @coltonbates629 4 роки тому

      If one starts to attack, wouldn't the other see this and attack too?

    • @GentlemanlyOtter
      @GentlemanlyOtter 4 роки тому +1

      The Athiest Messiah, again, this is “magic computer science land”.

    • @Stormagaming
      @Stormagaming 4 роки тому

      @@coltonbates629 And I think it would make more sense if the castle was on a hill so the army's couldn't see each other

  • @8-bitsarda747
    @8-bitsarda747 Рік тому +1

    I'm gonna be honest, as a military history nut, and someone who's convinced that computers are magic boxes, I completely missed the part at the beginning where you mentioned that this was a metaphor for computer related witchcraft, so I was just like, "send up a smoke signal or something when the message is received"

  • @JoostMehrtens
    @JoostMehrtens 5 років тому +146

    Me: no you can't have pizza for dinner.
    7 year old Son:Why not?
    Me: because you had pizza for lunch.
    Son: why can't I have pizza for lunch and dinner.
    Me: because of idempotency.

    • @Clickum
      @Clickum 4 роки тому +4

      @Jay Arre You must be eating terrible pizza.

    • @tuyiren781
      @tuyiren781 4 роки тому

      @@Clickum eating "good" pizza is what kills you, veggie pizza won't

    • @jlammetje
      @jlammetje 4 роки тому +2

      but lunch != dinner, so idempotency has nothing to do with it ;-)

    • @JoostMehrtens
      @JoostMehrtens 4 роки тому +1

      @@jlammetje not exactly, dinner can be lunch, but lunch is not necessarily dinner.

  • @philipmelchert4093
    @philipmelchert4093 5 років тому +877

    Everyone else: Goes to bed a reasonable time
    Me: Stays up all night trying to solve the unsolvable general problem with 0 knowledge of computer science

    • @goldfishy
      @goldfishy 5 років тому +4

      Philip Melchert bitcoin solves it.

    • @seanwebb605
      @seanwebb605 5 років тому +54

      How hard could it possibly be? I mean I figured out which side to park my car to fill it with gas. Only took three tries.

    • @emeraldspiders4046
      @emeraldspiders4046 5 років тому +13

      I dunno, I don't think it's that hard, just have army A keep sending messengers until one of them returns

    • @goldfishy
      @goldfishy 5 років тому +17

      Sean Webb - same number of tries it takes me to plug in a USB cable

    • @Mo16030
      @Mo16030 5 років тому

      Like LABOUR and the Tories

  • @DanksterPaws
    @DanksterPaws 3 роки тому +545

    Solution: Send the entire army as “messengers” combine with the blue, attack together.

    • @dipperjc
      @dipperjc 3 роки тому +71

      You'd only be attacking from one side. Which in this metaphor would translate to the two computers being somehow physically conjoined and not connected to the internet at all.

    • @DanksterPaws
      @DanksterPaws 3 роки тому +18

      @@dipperjc then just send the army back as “messengers” again. Atleast now you had communication with the other side. Ofcourse, this doesn’t really apply since it’s a computer science question as mentioned in the video. But yk just having some fun

    • @dipperjc
      @dipperjc 3 роки тому +8

      @@DanksterPaws Suppose none in the army make it back?

    • @youdontknowmegigna
      @youdontknowmegigna 3 роки тому +34

      Problem: No guarantee the messengers make the trip, remember.

    • @tba2204
      @tba2204 3 роки тому +4

      army A could also send a messenger every "x" time interval until they get a reply

  • @LRM12o8
    @LRM12o8 2 роки тому +4

    Reminds me of my Christmas shopping experience last year:
    I ordered some gifts from a store that I hadn't ordered from in a while and didn't realize that the credit card still linked to that account has expired in the meantime. So I ordered and used the stored credit card info to pay and the website didn't notice either. It just displayed the usual "thanks for your order" page and I went on to different things.
    A week later I wondered why I still haven't received the stuff and realized that I hadn't gotten an order confirmation per e-mail either. So I ordered again and the website got me through checkout again, saying everything's fine. This time I monitored my inbox and after two hours of nothing, checked my order history in my account, found no order and ordered again, but got no mail again.
    It took two days of back and forth with the customer support for them to find out why my orders weren't processed. I wonder why they were confirmed on the website though... 🤔

    • @eaaeeeea
      @eaaeeeea 2 роки тому +1

      I would imagine a check and prompt for a card expiration would be one of the must-have features in any webshop. Crazy to think that on this day and age someone didn't implement it.

  • @TheRichardHonor
    @TheRichardHonor 4 роки тому +75

    "The root cause is that our processes aren't robust enough to prevent a person making this mistake" - a former colleague set this as a slack autoresponse any time any dare write the letters "RCA"

  • @lukeystuff
    @lukeystuff 4 роки тому +248

    "Next time, I'll cook for myself"
    **After 4 math tests in one week**
    Me: Yes, I would like a pizza
    Edit: Changed exams to tests

    • @ME0WMERE
      @ME0WMERE 3 роки тому

      By ‘exams’ do you mean tests or exams?

  • @henriquelegoinha3806
    @henriquelegoinha3806 4 роки тому +32

    Hey, for those interested in the physical background of this problem, the two generals problem is an anology related to the limitations of synchronizing two clocks (due to the speed of transfering information).
    Since information can't travel (up to our best knowledge) faster than the speed of light (which means that there must be a finite value for it's speed so that there can't be instantaneous change of information) both clocks will always be differing by some amount of time. Very very tiny small difference of course (unless they are super spaced but that's not the case of the valley here). However, that tiny difference is enough to say the atack is not simultaneous as general A and B will advance with a slight time diference (like clock A and B that can't be synchronized).

    • @grimreefer4366
      @grimreefer4366 4 роки тому +1

      That's a great way to look at the problem. But that means that the answer to this question is telephones. That large difference is a small one with our current technology.

    • @cara-seyun
      @cara-seyun 2 роки тому +1

      But can’t you use that knowledge to ‘sync ahead’ the other clock so they are both equal? Like move Clock B so it appears 1 femtosecond ahead, but in reality is the same, just lagging.

  • @sipos0
    @sipos0 2 роки тому +1

    "Next time I'll just cook for myself" - bit drastic! It's not that bad.

  • @AndrewBlucher
    @AndrewBlucher 5 років тому +55

    "A single human error is never the root cause"
    This applies to almost everything.

    • @sillyking1991
      @sillyking1991 5 років тому +15

      @Aaron you missed the point. in that situation the root cause isn't the single influential person, its the system, which has been built up to allow such a thing by multiple errors compounding over time.
      the point being that there is a difference between the *instigating event* and the *root cause*

    • @FullMetalChains
      @FullMetalChains 4 роки тому

      Then again a rocket launched by NASA crashed due to a single error in coding.

  • @over00lordunknown12
    @over00lordunknown12 5 років тому +12

    6:11 “Next time I’ll just cook for myself”
    I was REALLY a hoping that the sponsor for this video would’ve been Hello Fresh.

  • @CreepyUncleIdjit
    @CreepyUncleIdjit 3 роки тому +44

    So in either case, the 5P rule applies: Prior Planning Prevents Poor Performance.

  • @Lampe2020
    @Lampe2020 Рік тому +3

    That was about what I expected to be the solution - just send several messages if you don't get a reply and find a way for the server to identify duplicate requests.

  • @stephenholland6328
    @stephenholland6328 3 роки тому +65

    I think it would be instructive to add that TCP/IP recognizes this problem and includes a packet ID. The two generals problem explanation increases my appreciation for TCP/IP. Thanks!

    • @waldolemmer
      @waldolemmer Рік тому +7

      TCP, to be specific. TCP/IP is a protocol stack consisting of many different protocols

  • @EEVblog
    @EEVblog 5 років тому +108

    I'm surprised there is no mention about Byzantine Faults (a.k.a the Byzantine generals problem), and there are solutions that claim to be "Byzantine compliant" or "Byzantine fault tolerant", and this is a big thing in the cryptocurrency space right now.

    • @shawnleary
      @shawnleary 5 років тому +1

      Nakamoto consensus using proof of work for the win!

    • @pikekeke
      @pikekeke 3 роки тому

      It's the same problem and still only solved by at-least-once delivery + idempotency.

  • @moosemaimer
    @moosemaimer 5 років тому +133

    *ERROR*
    The operation completed successfully

    • @josugambee3701
      @josugambee3701 5 років тому +9

      ERROR:
      "Uh.. nothing. Nevermind."

    • @OrigamiMarie
      @OrigamiMarie 5 років тому +17

      Error! AutoCAD has closed due to an unexpected situation!
      Uh, I quit the program intentionally a couple seconds ago?

    • @MatthewStinar
      @MatthewStinar 5 років тому +3

      @@OrigamiMarie Google podcasts does something similar when I delete a podcast after I've finished listening but there's still a few seconds of audio left.

    • @derecwilsom4546
      @derecwilsom4546 5 років тому +5

      "ERROR: Keyboard not found, press F1 to continue" (actually a real error message!)

    •  5 років тому +4

      The operation failed successfully.

  • @Objectified
    @Objectified 2 роки тому

    It's always striking to me how on one hand you seek to dispel memes, stereotypes, tropes, and misconceptions yet intersperse your videos with interjections of sociopolitical memes, stereotypes, tropes, and misconceptions.

  • @SimulationEvolve
    @SimulationEvolve 4 роки тому +58

    "A single human error is never the root cause"
    I would dare to disagree.

    • @thewanderinggamer1369
      @thewanderinggamer1369 4 роки тому +6

      *o n o s e c o n d i n t e n s i f i e s*

    • @GamerLeFay
      @GamerLeFay 3 роки тому

      The root problem in that situation was the company that allowed an inexperienced teenager to work on a live database without backups. Not one person.