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can you tell me how you would design WhatsApp?

Hey everyone, welcome back to another Exponent System Design mock interview.

My name is Kevin Wei and on today's show we have Roshan.

Before we get started, Roshan, do you mind telling the audience just a little bit about yourself and then we'll get into the mock interview?

Of course, Kevin.

Thank you.

Thank you so much for inviting me to this interview.

My name is like roshan i work as a sd2 at amazon

and yeah i spend most of my time building highly scalable systems

and you know uh creating impact in the lives of billions of people excellent thank you roshan

so let's do this question today can you tell me how you would design whatsapp yeah of course

so maybe like usually whenever we start the system designs first we start with collecting all the requirements

so Kevin what I'm gonna do is first you know try to get all the requirements

that the WhatsApp application could have because today like you know everybody's aware of WhatsApp

and it has like a bunch of features

but in the interest of time maybe we will be developing only some you know essential features is that fine Kevin?

Yeah that sounds good maybe you can tell me what these essential features what the priorities might be sure

so yeah maybe i'll start writing it down

so first thing like maybe we can start with very basic

so today whatsapp is used uh you know to send

and receive messages uh you know um between um two peers

so yeah this is one of the most basic feature where

two person like one person one person can you know send

and receive message to another person

so this is like a you know the minimum feature

that any typical chat application should have the second uh one could be you know a group messaging where a member

would create a group they will add some x number of people in the group

and whenever the person sends the message all the other parties in the group will receive it

and everybody will know that who has sent that message

so that's the second feature that we may have

and we can also have like once the message has been

sent you know um the other person would be really interested

to know whether it has been delivered second one would be

like rather it should there should be three things three receipts basically the sent receipt the delivered receipt

and the read receipt so maybe in case

if we want we can add that as well sent delivered and read receipt

so yeah this could be another feature um so kevin anything uh

so far like on these three features do you want to add something yeah this makes sense sending

and receiving messages this seems like p0

group messaging is interesting and sending delivering read receipts just to let the user know

that the app is working and that everything that they're doing is working as planned

i think we can stick with this for now and then

if we want to add more features we can

if we have time okay so now maybe i'll just ask some more clarifying questions on this uh the first feature

so when we say messages

so are we talking only about the text messages are uh

you know the the person can send documents images videos um is that feature available in the first version

yeah great question let's assume that most of the messages are going to be text or images okay okay

okay got it so i think uh yeah so maybe i let me add some uh title in here

whatsapp system requirements okay

so now uh maybe i'll proceed with the you know the

design of like what what is going through my mind so um let me let me draw some icons in here

nice i love it some visuals for us yeah so let's say like this is the user a

and we also have another user in here B.

Okay.

So now we need some kind of a protocol where A can communicate to B

but in a more secured way so that we don't get, you know, like some third party should not be able to, you know, decode the message and,

you know, read it because that would be a pretty awful experience for the user B.

like to the to both the users for that matter.

So we need some kind of a protocol where we can you know establish

that communication and I think we have something called web sockets.

So web sockets was specially designed for peer-to-peer communication.

So maybe I can quickly write that down one second.

um web sockets however uh just using web sockets won't work uh how would like how would my system know

that you know this uh this user b is present in

some network like b should b would be connected to some network right

so we need some kind of a you know database where we know

to which box is a connected to and

which box is b connected to

so let me just change this a bit let me put this here

and let me remove this line over here and yeah

so let's say maybe i can here okay so now let me see

okay so we need api gateway

okay

so basically what would happen is this you know this phone

this this user a is connected to api gateway one

and there's another user b which is connected to ap api gateway two Now, maybe we can have a microservice.

Let me put that here.

And we can call it as a session service.

One second.

Session microservice.

Basically, the responsibility of session microservice is to connect with a DB.

And it will tell us which user is connected to which box or which API gateway.

So, maybe I think, yeah.

So say suppose when the message

when A wants to send a message to B it would first go to API Gateway

and then connect to this microservice

and this microservice will fetch you know the box number of the user B

and then the session microservice will connect back to API Gateway number 2

and from there it will go to API Gateway to be

and the protocol that we will be using here is web sockets maybe i can write that down real quick

so this is how i think i would be able to you know send

and receive messages and i think this solution is also pretty scalable how are we going to ensure

that is maybe the type of database that we will maintain here is maybe NoSQL database.

We can use that for scalability purposes as well.

Yeah, NoSQL like maybe DynamoDB.

So why would you use NoSQL here rather than a relational or SQL database?

As in when the number of users grows, we want the systems to scale, right?

And NoSQL are one of the most of the time why we use no sequels

100

because it helps us during scaling and

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we don't have to stick to one schema the schema could be pretty dynamic

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so yeah okay for scalability yeah it makes sense yeah

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and maybe i think i can quickly write that format as well

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so for now i'm just thinking of a key value pair so it could have something like um the key

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and yeah so the key could be the you know um user

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and the value could be the the box uh or the gateway

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so by that by that way we'll be able to identify

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which user is in which box so yeah so

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that would only this this problem this particular design will solve the first requirement

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but we still have not addressed the images thing

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so usually for static content we have s3

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that is the most preferred feature like you know most preferred option

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so maybe i i think i would stick to that

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so maybe i think when

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when user wants to send a image right the only difference between text

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and image here is um by the way we are not planning to store this uh text anyway right

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because as soon as uh as soon as this um text

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reaches the session microservice we are only going to get the user name

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and their box number

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and then send the text message as it is we don't

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need to store this text anyway however in case of images i'm thinking would it make sense to store it um

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we we can say that we don't want to store it just

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because we don't store text

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so we also don't want to store images for privacy yeah privacy purposes okay

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so yeah in

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that case then i think we are good with first requirement

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maybe before we go to second requirement let's complete the third one So send, delivered and released.

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Now, as soon as, you know, this user A has sent a message,

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like as soon as a message is received by the session microservice, what we can also do is immediately send a response back,

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like an acknowledgement to user a

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that your message has been received by us this session microservice can do

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that so maybe what we can also do is like let me put

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that real quick okay and the text here would be a read um message um send acknowledgement

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okay and maybe let me put some numbers as well

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so basically the the number one here indicates uh the first part of it

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because the message has been sent now once the session microservice has got the box number of the user b

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and once it has sent that message to b

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what we can do is it can send back an acknowledgement to session microservice

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so maybe let me put it this way um api gateway

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and from here to session microservice and let me add some numbers here

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so this is more of a message received acknowledgement and

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And what I can also do here is I can add another text in here.

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It says message delivered acknowledgement.

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Okay.

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So just to summarize, user A has sent a message to APA Gateway.

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APA Gateway would have communicated to session microservice.

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And as soon as the session microservice has got a message from A, it will send a message sent acknowledgement.

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That is point number one.

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and session microservice would interact with DB.

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It would get the box number and it knows exactly where user B is.

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So it would go to API gateway 2 and API like you know this message will get redirected to B.

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Now as soon as the message is you know received by B, B can again send a acknowledgement.

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So that is point number 2 and as soon as session microservice gets that acknowledgement

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it will send a message delivered acknowledgement to A.

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So, you know, sent has been covered, delivered has been covered.

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Now read receipt.

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So the thing is, as soon as the user B opens the message, we can do exactly similar thing what we did for, you know, point number two.

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So maybe I think I can draw another box in here.

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So what happens here when user B reads the message?

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yeah message read acknowledgement yeah

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so now once the message has been read we need to you know somehow indicate

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that to user a right

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so what we can do is again we can use the

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typical feature of what whatsapp is already averaging like maybe changing the color of the double ticks

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so as soon as session microservice communicates that to a the the double ticks might be changed to double blue ticks.

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So that would be point number four, I believe.

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This is point number four.

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Yeah, that makes sense.

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Cool, yeah, that makes sense.

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So this makes sense for peer-to-peer texts and images.

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The texts and images are not stored.

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They're just transmitted through the network.

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We have sent, delivered, and read receipts.

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Can we cover the group messages here?

175

So now let us talk about the group messaging.

176

So what essentially are we trying to achieve here is step number one, some user will create a group.

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Step number two, the user will add some X number of people.

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By the way, I forgot to ask this Kevin, how many, what is the maximum number of people who can be in the group?

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yeah for simplicity let's say 10 people okay maybe I think I can write

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that down real quick max 10 members yeah

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and feel free to tell me

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if in the future we want to change this to like 99

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or 999 how that would change so maybe I think I can write

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that down um should be scalable to more than 100 plus members okay got it

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so maybe i'll think i'll start with 10 first

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and then let's see how we can uh make it more scalable kevin so yeah

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so maybe i'm just thinking of how i can tweak the existing design a bit to start supporting the group messaging.

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So here if you see when a message was sent,

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we had this key value pair with only one user and only one box.

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So what if we have a list of a map?

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So whenever a group will get created.

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So what would happen is a list will get created with

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them with a map having the key as the user and its associated box number.

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So by that way, what would happen is whenever a message is sent,

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the microservice or the messaging microservice will iterate over each of the elements in the list

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or rather the the key value pair it will check the you know the the you know the user name

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and its associated box number and it will exactly follow all the procedures

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that we have created for one does that make sense kevin yep that makes sense How can we ensure scalability here?

199

Okay, the problem here is that when we are sending a message, we don't want to send the message in sequential order.

200

We can actually parallel, you know, parallelize things here.

201

We can ensure that, like, say suppose there are 100 members in the list, we can separate them in a batch of 10 or 20.

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And we can have a main thread who will initiate a message, you know, who will initiate the group messaging, but the child thread will,

203

you know, equally distribute the number of items in the list and then send it to them parallel. By that way,

204

the other set of members in the list do not have to wait until the first set of members are done.

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Got it.

206

Cool.

207

All right.

208

That takes our group messaging.

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I have one more question before we wrap things up.

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So you have user A here connected to API Gateway 1, you have user B connected to API Gateway 2.

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If we have user C, where would user C be connected to?

212

Is it to their own API gateway or would they share a gateway?

213

No, they can share their gateway.

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It's not mandatory that they need to have their own gateway can definitely share the gateways okay

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and how would the how are the users assigned to the gateways is it uh randomly

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or is there some structure there uh well for now i don't think we need any structure

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but yeah they will be assigned randomly given okay got it

218

cool thanks roshan uh i think i understand where you're getting with this uh you somehow want to ensure

219

that not all the users end up in getting connected to one gateway right right

220

so i think yeah in order to solve

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that problem what we can do is we can have a

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limit on the number of users getting connected to a gateway

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so by that as soon as that

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that count reaches the max count we know

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that okay the gateway is fully occupied now we have to move to the next gateway yeah

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that makes sense um yeah thanks for clarifying

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that point for me all right cool um thanks rishan for

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your time i have a few pieces of pieces of feedback here

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that i'd love to point out

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and then i'd love it to turn it back to you

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maybe you can give some advice to the audience as well yeah

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so you did a really good job prioritizing the features in

233

the beginning figuring out what should be in the mvp you know whatsapp is a huge app

234

and you you were able to pick the ones that are most important to discuss for the MVP.

235

I think you also did a really good job of distinguishing some of the ambiguous parts of the question.

236

So, like sending and receiving messages, this could be anything from text to photos to videos to audio messages.

237

And you did a good job pointing out that ambiguity and helping to hone in the scope there around text and images.

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And the last point I wanted to make is

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that you did a really good job checking in with me and figuring out what questions I was trying to ask you, not only at the end there, but also in the beginning.

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After you finished talking about sending and receiving messages, you didn't just move on to the next part of the question.

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You paused and you asked me if I had any questions before moving on.

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So I really appreciate your mock interview here.

243

And before we wrap things up, do you have any pieces of advice for the audience if they have their own system design interview?

244

yeah of course thank you kevin thank you for acknowledging this mock interview

245

and for people who are you know watching this video right

246

now um maybe i think i would just summarize of whatever

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kevin has said just try to ask as many clarifying questions

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that you have

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because uh the only person to whom you can talk during the interview is your interviewer

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so they are the ones who who will who are you who are here to help you out

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so maybe you can ask as much clarifying questions

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and whenever you are designing something so whatever is going through your mind

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or whatever you are drawing just uh you know call it out loud

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so that the interviewer knows what is going through your mind

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and in case if you make any mistakes they will immediately stop you

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or you know they might tell you to uh you know tweak your solution a bit

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so yeah the key thing here is during the system design interview it's really really important

258

that you keep talking to your interviewer instead of you know just keeping it with yourself

259

so yeah makes sense thank you roshan thanks for the audience for watching

260

and good luck with your upcoming system design mock interview thanks

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so much for watching don't forget to hit the like

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and subscribe buttons below to let us know that this video is valuable for you.

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And of course, check out hundreds more videos just like this at tryexponent.com.

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Thanks for watching and good luck on your upcoming interview.

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Лексика и советы по произношению к этому уроку

Этот урок разговорной практики уровня C1 построен на видео «System Design Mock Interview: Design WhatsApp». Чаще всего повторяются слова: message, user, gateway, microservice, send. В этом видео 264 предложений и 3264 слов для шедоуинга. Речь длится 22:22. Говорящий держит ровный темп — около 146 слов в минуту, удобный для шедоуинга. 86% слов входят в 3000 самых частых слов английского языка; остальные стоит разобрать перед занятием.

Ключевая лексика этого видео

Самые сложные слова из видео (15), с произношением и значением:

Слово	Произношение	Значение
microservice существительное		микросервис
acknowledgment существительное	/əkˈnɑ.lɪd͡ʒ.mənt/	призна́ние, подтвержде́ние
receipt существительное	/ɹɪˈsiːt/	получе́ние, приём
socket существительное	/ˈsɑkɪt/	розе́тка, гнездо́
mock глагол	/mɑk/	подража́ть, имити́ровать
scalable прилагательное	/ˈskeɪ.lə.bl̩/	масштаби́руемый, расширя́емый
clarify глагол	/ˈklæɹ.ɪ.faɪ/	очища́ть, чи́стить
scalability существительное	/ˌskeɪ.ləˈbɪl.ɪ.ti/	масштабируемость
interviewer существительное	/ˈɪntɚvjuɚ/	интервью́ер, опра́шивающий
tick существительное	/tɪk/	клещ
schema существительное	/ˈskiːmə/	схе́ма
summarize глагол	/ˈsʌməˌɹaɪz/	обобща́ть, обобщи́ть
tweak существительное	/twiːk/	щипо́к
assign глагол	/əˈsaɪn/	назнача́ть, назна́чить
initiate глагол	/ɪˈnɪʃ.i.eɪt/	инициировать

Фразовые глаголы, которые вы услышите

Слово	Значение
figure out глагол	разбира́ться, разобра́ться
point out глагол	ука́зывать, указа́ть
check out глагол	проверя́ть, прове́рить

Грамматика в этом видео

Конструкции, которые говорящий использует чаще всего, с точными словами из видео:

Конструкция	В видео
Условные предложения if + предложение, will/would + глагол — условие и его результат	If we have user C, where would user · if you make any mistakes they will immediately
Present Perfect have/has + причастие прошедшего времени — прошлое действие, важное сейчас	has sent · has been · has been delivered
Пассивный залог be + причастие прошедшего времени — важно, что происходит, а не кто это делает	has been delivered · would be connected · is connected
Придаточные определительные who / which + предложение — уточнение о человеке или предмете	people who can · people who are · ones who who

Произношение, на которое стоит обратить внимание

Говорящий использует 28 сокращённых и редуцированных форм, например don't, I'll, I'm. Произносите их коротко, так, как слышите.

Звуки «sh» и «zh»: initiate /ɪˈnɪʃ.i.eɪt/, distinguish /dɪˈstɪŋ.ɡwɪʃ/, specially /ˈspɛʃəli/
Длинные слова — следите за ударением: acknowledgment /əkˈnɑ.lɪd͡ʒ.mənt/, scalability /ˌskeɪ.ləˈbɪl.ɪ.ti/, initiate /ɪˈnɪʃ.i.eɪt/, ambiguity /æm.bɪˈɡjuː.ɪ.ti/, prioritize /pɹaɪˈɒɹ.ə.taɪz/

Звуки, трудные для русскоязычных:

/w/ — губы округлены, это не /v/: gateway /ˈɡeɪtˌweɪ/, tweak /twiːk/, sequential /səˈkwɛntʃəl/, distinguish /dɪˈstɪŋ.ɡwɪʃ/, sequel /ˈsiːkwəl/
Звонкие согласные в конце слова — не оглушайте их: summarize /ˈsʌməˌɹaɪz/, decode /dɪˈkəʊd/, prioritize /pɹaɪˈɒɹ.ə.taɪz/, pause /pɔːz/, subscribe /səbˈskɹaɪb/

Как заниматься с этим видео

Прослушайте всё видео один раз молча и выпишите незнакомые слова.
Повторяйте предложение за предложением на обычной скорости, пока ваш ритм не совпадёт с ритмом говорящего.
Запишите себя и сравните с оригиналом, обращая внимание на такие слова, как microservice, acknowledgment, receipt.

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