in today's video actually i have asked a friend of  mine who is an ex microsoft and ex airbnb employee  

to actually take a mock interview and he will  be asking a system guy interview questions which  

he hasn't told me yet so i haven't prepared for  what he will be asking me and so i will actually  

try to design this whole system at one time so  please consider this youtube video as an example  

for how you have to take a systems interview  especially when the interviewer asks you something  

which you haven't prepared in advance and you will  see now in this video doing the mock interview how  

i am thinking out loud how i am going through all  the different things how i'm actually expressing  

my opinion with confidence how i'm discussing  different design approaches why one is better than  

the other or not and you will also see that even  in the interview there are something which i don't  

personally like but i did still even i did  that for example the back of the envelope  

calculation i i don't personally prefer them  but since the interviewer asked me to actually  

perform the back of the envelope calculation so  instead of arguing with the interviewer actually  

i did perform uh some back of the envelope in  a calculation very quickly so i hope that you  

will find this video useful so let me know in the  comments below if you find this video useful and  

if you find it useful and valuable and you learn  something about how to tackle a system designed  

by going through this mock interview then please  also like this video and if you haven't subscribed  

to my channel yet then please subscribe to this  channel and click the bell icon so that you get  

notified when i am uploading some new videos  also please note that i have not edited this  

mock interview video so that you will have the  experience of real mock interview so let's go and  

see the mock interview uh hi let's do uh design  mock design interview and i want you to design  

youtube okay so before designing youtube i  think we should first discuss what would be the  

uh we should try to scope down the the whole  big system and see what are the different  

functional and non-functional requirements uh that  youtube have and that we would like to actually  

uh tackle in this uh interview so let's see  whether we have a text box or something yes

okay so let's see what are the  different uh functional requirements  

that we can think of about the youtube so first  of all i think uh we would like user to actually  

have an account of course user need  a gmail account in order to upload  

videos to youtube so user account the second  i would say the functional requirement is  

basically we would like user to upload a video  so upload video functionality and in the same  

way we would like once the user has uploaded the  video he should be able to provide some metadata  

information about the video for example the  the title the description the tags etc so

metadata information about the video so this  step this functionality is only related to the  

to the user who is who is actually a creator  or who is a youtube video creator but let's see  

what let's now discuss the functionality and the  requirements from the perspective of a user who's  

who's watching different youtube videos so the  very first i would say functionality would be  

that user should be able to  search some youtube video

and of course when the once the user find the  

the video then the user should be able to  play the video play the video and of course  

the other all the different uh actions  that the user can perform like fast like  

forwarding the video move to a particular  time in the video etc uh apart from that

we would like also youtube to  actually recommend videos to the user

recommend videos to the user  

so in a user home page the the user could see  different videos recommended by the by the youtube  

at the same time users should be able to  subscribe to different channels in youtube

and of course this is all you functionality  to use a watching videos and this is the  

functionality which is a user creating  or publishing videos and of course here

we could also say that user  should be able to create  

a youtube channel and in the channel he  can share different different youtubes  

so i think this week what do you think  i think we can actually uh keep all the  

functional non-fictional requirements for this  interview uh i think we can just keep these  

uh functional requirements  they look good yeah yeah

so now let's see what are the different  non-functional requirements that youtube could  

have so i think the very first requirement is  always we want our system to be highly available  

we don't want youtube to go down  and we don't want user to actually  

see the message that the users is down and  we would like also our system to fault all  

then so even it's totally possible that some  servers in youtube may go down but it will not  

actually affect the overall functionality  or overall availability of the service  

we also would like our service to be highly  scalable as the load on the users as a number  

of users increase the number of videos in case we  also would like our system to be highly scalable

we would like our youtube service to be durable so  once a video has been uploaded to youtube we don't  

want the the video to be deleted and we would  like to keep that video in the in the system  

then as far as the consistency is concerned  

consistency consistency requirements  has i would say these are these will be  

there will be different consistency requirements  for different parts of youtube for example when  

a user is uploading a video then after  uploading a video he should be able to  

see that video that he has uploaded that video  so we need us somewhat strong consistency there  

but as far as when a user is searching a video and  let's say another user actually upload a video at  

that time and that video does not appear in the in  the search it's totally fine because at that level  

we can have some sort of eventual consistency  similarly when a user actually make a video public  

at that time it's totally possible that some users  will see that the video is public and some may not  

see but i think here also it would be okay to  have eventual consistency where uh some users  

would see that the video is public a bit later  it's fine i think from the consistency requirement  

yeah so i think these would be uh some  functional and non-functional requirements  

i would also say that of course when we are  designing this youtube service we will have  

we will have some monitoring service analytics  because as part of service development  

and also operations of of service we always  need some monitoring and analytics as part of  

the service operations as well so which will tell  us the help of the service the load on the service  

etc so which i'm just discussing right now but of  course we are not going to uh if you want we can  

go into that detail as well but i think right  now i will keep uh only i will keep focus on  

these uh these functionality non-functional  and social requirements what do you think i  

think that's the right approach we can discuss  uh monitoring later if we have time okay cool so  

usually uh i know that there are people who  actually uh prefer some back of the envelope uh  

estimation but usually i don't want to waste time  usually in estimating the back of the envelope  

by in doing this back of the envelope calculation  because i think that we should try to actually  

first try to design the service based on the  functional non-functional requirements and try to  

keep the design as flexible enough so that if it  needs to be highly scalable and available we can  

make so and as far as the estimation is concerned  we can actually of course we can go and like this  

five ten minutes doing that uh but eventually  i think at the end i think what we would need  

is basically uh just to know okay what are if  let's say if you're uploading videos what would  

be the size of the video how much uh storage  we would require and of course uh this would be  

estimations and we will have an electronic service  to actually uh keep an eye on all those metrics  

and that's why we can start with some initial  storage and we can always increase that that's  

why i don't want to waste time on that but if  you would like i can go into backup the envelope  

estimation as well otherwise we can go directly to  the high level design and then the detail design  

so is the estimation is useless like doing  the cost estimation is really never helpful or  

it is going to be useful at some point later i  mean it is usually useful when you are designing  

your service in real life it would be a bit useful  because we would like to know what would be at the  

beginning what would be the scale could have or  there would be a of course it will be just an  

estimation and the real numbers might be different  but it will actually make you decide okay  

do i need to allocate let's say three app  servers or six app servers or 10 app servers  

for my service based on the load that i'm  predicting that that will happen from day one  

because as far as the adding news servers  are concerned we can always add new servers  

uh if we see that the load is increasing and we  can also start with some bare minimum number of  

servers that would that we really need in order to  design the service okay let's do some five minutes  

uh cost estimation uh uh some some estimation of  uh numbers so that we have some data available

okay let's do them some cost estimation

so let's just assume that right now  uh i don't know right now what would  

be the right now the current i think how  many users uh youtube has maybe almost uh  

i would say almost uh a billion users but as  far as the users who are uploading the videos  

uh i'm not sure how many of them are right  now let's consider them maybe uh i would say  

100 million let's see if there are 100 million  users who are actually publishing videos okay  

and let's suppose if they are actually publishing  video uh on average i would say every week  

like one one user is uploading video every once  a week for example once a week video upload

and let's say in that case uh a video on average  usually well nowadays you will find videos from  

like under a minute to more than like like  more than like hours like almost six seven  

eight hours of videos you will find on youtube but  let's just assume right now that on average there  

will be video for around 15 to 20 minutes that's  a 20 minute video on average there will be videos  

which will be of course less than 20 minutes and  there will be videos which will be more than 20  

minutes but let's just assume these are the on  average there will be a video of size 20 minute  

and of course it will be using let's say 1080p  and usually when you youtube uploads when you  

upload a video on youtube youtube actually save  those videos in multiple different bit layers in  

different formats so for example if you upload  our video at 1080p usually uh youtube usually  

stores those videos not just in s 1080p but also  at 720p and 480p and 320p etc because we we will  

discuss later also that youtube actually uses  adaptive uh bit streaming to actually uh make the  

the the experience of use of user to be like  help to make the user experience good actually  

this is also i think one was the non-social  department that we we didn't discuss that we  

would like the experience of user to be like  fast we would like video playback to be faster

so it means that the the server would be  actually able to recognize the the client  

so the client will inform the server about  its bit weight that it has or and also the  

screen size etc and the server can actually  send the right size video uh to the client

so apart from that i think so let's say if i  if we are doing a 20 minute average 1080p video  

so usually as i said uh there are  videos like usually 20 minute video  

depending on the bit where it  could take around uh on average  

10 to 20 gb and if it's a very high quality  video and so let's say on average we say it's a  

10 gb video and of course uh use youtube actually  also store uh this video and other different  

formats as well like if it's if you're uploading a  1080p uh you youtube actually also store uh one of  

the other formats of the video like which have  reduced resolution like 7080p etc so usually i  

would say on average 10 10 gb for that around  5 to 4 gb for 720p around 1gb for i would say  

let's say 480p videos so usually i would say on  average let's just say that there will be around  

15 to 20 gb a video of storage that we  will have for a video and now depending on  

let's say 100 million videos per week so you  can say 100 multiplied by almost let's say 20  

gb which is almost 2 terabytes of video per week  

that would be uploading and in that and based  on that we can see how many videos we would see  

let's say in a month or in a year for example  do you want to go further into that or should we  

go to the high level yeah i think this is good  enough we can move to the next section okay  

so let's let me draw the high level uh diagram  i think is there a way to make it bigger

okay so i am actually going to draw the high level  diagram so there are multiple parts if you see

from the user's perspective the youtube  service will be actually comprised of multiple  

microservices there will be an upload service this  upload service will be responsible for uploading  

a youtube video and it will we will go further  into it and see how those upload service what  

is the functionality of the cloud service how it  works etc apart from that there will be of course

let me copy

there will be

a user service

there will be of course

i would say a channel service

a user can have multiple channels so the  channel service which actually show all the  

information about the about  the channel the user has uh  

apart from user service and these channels there  will be also other service which is like let's say

search service

there will be

recommendation service