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The Two Generals’ Problem

Tom Scott
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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: www.dashlane.com/tomscott
MORE BASICS: usvid.net/group/PL96C35uN7xGLLeET0dOWaKHkAlPsrkcha
Written by Sean M Elliott and Tom Scott
Directed by Tomek
Graphics by Mooviemakers www.mooviemakers.co.uk/
Audio mix by Haerther Productions haerther.net/
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 dashlane.com/tomscott
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Aug 12, 2019

tom scotttomscottthe basicscomputer sciencetwo generals problemtwo generals' problemthe two generals problemidempotencyidempotency keyidempotency tokenidempotence

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Comments 2 107
Tom Scott
Tom Scott 27 days ago
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
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 😊👌
Masked Death
Masked Death Day ago
@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.
Sweet's Channel
@Masked Death oh... now I understand. your talking about the sponsor of the video. I'm sorry 😔
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
Masked Death
Masked Death Day ago
@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
Ahmad Pujianto
Ahmad Pujianto 10 hours ago
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.
فاء بن ألف'
فاء بن ألف' 10 hours ago
Can’t just 1 red messenger send the message then come back?
فاء بن ألف'
فاء بن ألف' 10 hours ago
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
Souvik Aswad
Souvik Aswad 14 hours ago
Great video!
J Espinola
J Espinola 16 hours ago
How did this not happen sooner?
Yasir Arshad
Yasir Arshad 18 hours ago
So like free food?
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.
Brian Bacc
Brian Bacc Day ago
you're awesome subbed and liked
gepisar
gepisar Day ago
!! 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?
Autotrope
Autotrope Day ago
By your own description of the two generals problem, an idempotency token does not solve the problem.
alex chamber
alex chamber Day ago
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.
jammin023
jammin023 Day ago
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!
Orion Red
Orion Red Day ago
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.
ruwiki
ruwiki Day ago
"it's unsolveable" me at 2 am: well, there must be a way ...
Oz El Coskuner
Just giving thumbs down for showing 2 long (unskippable) ads. I hope creators puts some pressure on USvid. Because USvid literally don't give a damn about feelings of customers who are not paying. WHY WE STARTED WATCHING USvid? The biggest argument was "so that you don't have to watch long commercials of TV and get brainwashed". Now average ad on USvid is surpassing TV. Anyway, since I got Amazon prime, I reduced USvid time to 2 hours a week. Much happier. And I will never pay for USvid, just because their approach. You cannot shove your will down to people's throat and expect success. When I see USvid logo, I see a RABID DOG with dollar symbols on his red eyes. Disgusting.
John Dean
John Dean Day ago
There IS answer to the Two Generals' Problem: go to your local chippie.
Airpolygon
Airpolygon Day ago
Very interesting!
Rens Breur
Rens Breur Day ago
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 2 days ago
Can you find a different shirt?
iammaxhailme
iammaxhailme 2 days ago
"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
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 2 days ago
The main problem is that they built a castle in a valley.
ruwiki
ruwiki Day ago
@Nubby Tope as well
Nubby Tope
Nubby Tope Day ago
ruwiki - fortifications like castles are built on high ground.
ruwiki
ruwiki Day ago
why? to protect an important river crossing, for example.
Zen Lucas-Divers
Zen Lucas-Divers 2 days ago
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 2 days ago
And then that "idempotency key" resets when you hit "try again" :D
less kiss
less kiss 2 days ago
The fact you spent £1,795 inc vat per minute of this videos graphics is beyond me
Techno_mage_21
Techno_mage_21 2 days ago
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 2 days ago
Are you in the Centre for Computing History in Cambridge?
less kiss
less kiss 2 days ago
very close to this idea. Her paper was entitled "Network layer protocols with Byzantine robustness"
Caleb_ Artzs
Caleb_ Artzs 2 days ago
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 2 days ago
to prevent jamming of wireless signals.
soap's alt
soap's alt 2 days ago
The messengers could meet in the valley and then return with an agreement.
Jose Rojas
Jose Rojas 2 days ago
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
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 2 days ago
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 2 days ago
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 2 days ago
Castles aren't built in valleys, generally, generally.
ruwiki
ruwiki Day ago
well, if they protect a river crossing
Lou
Lou 2 days ago
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
Bitcoin solved the Byzantine generals' problem
ruwiki
ruwiki Day ago
blockchain you mean
Emily An
Emily An 2 days ago
I earn like 20 quid an hour for deliveroo it’s not bad 😂
ruwiki
ruwiki Day ago
with tip
=NolePtr
=NolePtr 2 days ago
I've never heard it referred to as an idempotency key. I've always heard it called a "nonce"
jammin023
jammin023 Day ago
The problem is that word means something else in British English...
Giovanni Joe
Giovanni Joe 2 days ago
Super informative! Thanks very much for the great content!
Emily An
Emily An 2 days ago
story short I got 50 quids worth of Wagamamas for free.
flutty bitch
flutty bitch 2 days ago
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 2 days ago
"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.
Orion Red
Orion Red Day ago
Well, you're dead on.
Dan Scherck
Dan Scherck 2 days ago
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 3 days ago
The fact you spent £1,795 inc vat per minute of this videos graphics is beyond me
Bluestripe
Bluestripe 3 days ago
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 3 days ago
I feel so smart now😂
Real Sky Luke
Real Sky Luke 3 days ago
*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 3 days ago
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
Ptao Tom
Ptao Tom 3 days ago
Bitcoin
alnoso
alnoso 3 days ago
i dont get food delivery apps is it so hard to call a human and say "hey i want a pizza"
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.
Ptao Tom
Ptao Tom 3 days ago
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 3 days ago
youtube recommended, wtf? Why? I neither understand computer logics or having problems with my food arriving.
Alexander Cavrich
I
less kiss
less kiss 3 days ago
err, Works fine for me... sounds like user error....
Drake H.
Drake H. 3 days ago
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 3 days ago
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 3 days ago
What if someone wanted to do the same order for multiple people. A certain restaurant verifies double orders
Mike Pratt
Mike Pratt 3 days ago
Suggestion for dinner next time - Salmon in a dishwasher..?
Alexi Hickin
Alexi Hickin 3 days ago
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 3 days ago
I earn like 20 quid an hour for deliveroo it’s not bad 😂
Raymon Crane
Raymon Crane 3 days ago
Those look like HP 7933/7935 disk drives in back. Used to fix those back in the day.
roasted pancakes
roasted pancakes 3 days ago
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 3 days ago
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 3 days ago
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 3 days ago
It's not "unsolvable", the word is "insuperable - Pedant
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
invisiblebears
invisiblebears 3 days ago
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
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 3 days ago
why did this video get uploaded 3 times??
Robert South
Robert South 3 days ago
Seems like once both generals have received at least one acknowledgement they can just attack since the time hasn't changed.
007one
007one 3 days ago
Bitcoin
TheTech9
TheTech9 4 days ago
This channel is awsome, and this guy is pleasant voice to listen to. No homo
Carl
Carl 4 days ago
Hey dude do you still see dead people? oh nevermind.
Jake Wish
Jake Wish 4 days ago
What if someone wanted to do the same order for multiple people. A certain restaurant verifies double orders
Stuart Wilson
Stuart Wilson 4 days ago
err, Works fine for me... sounds like user error....
Tinka Like AngelOK
Justeat 😂
hmmm
hmmm 4 days ago
No plussed how this solves the two generals actual problem. Poorly explained.
xirsamoht x
xirsamoht x 4 days ago
Speckled Jim & his friends!
Imelda Chandra
Imelda Chandra 4 days ago
I wonder what if like this : Gen A sent message (8 pm) to Gen B Gen B reply (ok. 8 pm) to Gen A is it possible?
Silverwolf0080
No because now B. would need a reply to ack that A got his Ack.
Imelda Chandra
Imelda Chandra 4 days ago
I wonder what if like this : Gen A sent message (8 pm) to Gen B Gen B reply (ok. 8 pm) to Gen A is it possible? or there is another hindrance?
Poldovico
Poldovico 3 days ago
Gen B doesn't know that A got the message
Ailsa Ni
Ailsa Ni 4 days ago
ally with the other general, and then assume the message got through.
Verne Jules
Verne Jules 4 days ago
Tom, I understand that the idempotency token/key can solve the problem of double orders/messages. But how does that solve the Two Generals Problem? Forgive my slow brain.
Ailsa Ni
Ailsa Ni 4 days ago
So the solution to the two generals problem is to send a third of your army into the dangerous valley, all of them with a message to attack at a certain time with the survivors to
Cullenak47
Cullenak47 4 days ago
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. B 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
Daniel Newton
Daniel Newton 4 days ago
Have each army send a messenger when they're ready. When the two messengers meet in the middle (taking precautions to avoid getting spotted), each one goes back to their army and says "we're good to go". If the messengers don't make it back, they will assume that they died so can send another, until the two meet and send a return message.
Andy B-G
Andy B-G 2 days ago
facts.
Kingsly9802
Kingsly9802 4 days ago
>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". But they're fun!
soSAMuk's UK slot channel
Why do kids toys adverts have to say “batteries not included?”
TheDeathMongrel
TheDeathMongrel 4 days ago
2 Generals is an interesting conundrum in and of itself but I think it's a poor paradigm for avoiding the processing of repeated messages. In 2 Generals, no one proceeds without an acknowledgment. It's a Schrodinger's Cat problem. You don't know the answer until 8 p.m. when either everyone is at the castle or is still waiting for another acknowledgment. Applying idempotency (which would be against the "strict" rules) to 2 Generals, still wouldn't solve the problem. It would only affirm that additional messages, if any, are the same (attack at 8 p.m.), and only if the messages actually make it. 2 Generals doesn't guarantee message delivery in either direction, so even with idempotency tokens, Army B may still take no action if they don't get their acknowledgment. With the delivery issue, if the additional messages don't make it, it doesn't matter, the other side will respond appropriately to the original message. In delivery, the return acknowledgment goes far beyond the 2 Generals' (receipt of message), as you can get an erroneous message, a success or failure, and is likely not to have an effect on processing if no acknowledgment is sent. In any case, I think the delivery problem and applications of idempotency tokens as you've described, is its own best explanation of what's going on, not 2 Generals, which idempotency can't be used to solve (again strict rules).
Max Mouse
Max Mouse 4 days ago
General A sends "8pm" General B sends "8pm ack" - General a doesn't attack unless it receives the ack...
Aaron
Aaron 4 days ago
General A sends 2 Messengers. Both go to general B to confirm the message was received. Both go to General A but when passing the castle at the exact halfway point one messenger goes back to general b the other goes on to a. They both know the message was received by general b and the acknowledgement got through back to a.
Mike Nerone
Mike Nerone 2 days ago
This assumes that a messenger can only be killed at the exact halfway point. You can't make that assumption. Remember that this is a dramatization of a real communications problem in computer networking. A data packet could be lost *anywhere* along the path. Further, even if you did send two packets (messengers) in parallel, there's no way to get one of them to turn around at the halfway point (indeed, there's not even a way to determine that a packet has *reached* the halfway point).
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