Shadowing Practice: System Design Mock Interview: Design WhatsApp - Learn English Speaking with Video

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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

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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?

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So now let us talk about the group messaging.

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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?

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

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

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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,

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

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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.

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Cool.

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All right.

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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?

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

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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

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

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

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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

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

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

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

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

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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.

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

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

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and for people who are you know watching this video right

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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

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

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

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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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Vocabulary and speaking notes for this lesson

This C1 speaking lesson is built on the video “System Design Mock Interview: Design WhatsApp”. The speaker keeps coming back to these words: message, user, gateway, microservice, send. This video has 264 sentences and 3264 words to shadow. The speech runs for 22:22. The speaker talks at a steady 146 words per minute, a comfortable pace for shadowing. 86% of the words are among the 3,000 most common in English; the rest is worth studying before you start.

Key vocabulary in this video

The 15 most advanced words in the video, with pronunciation and meaning:

Word	Pronunciation	Meaning
gateway noun	/ˈɡeɪtˌweɪ/	A passage that can be closed by use of a gate.
microservice noun		A small independent process that communicates with other processes, used to break down a complex task into many simpler components.
acknowledgment noun	/əkˈnɑ.lɪd͡ʒ.mənt/	The act of acknowledging
receipt noun	/ɹɪˈsiːt/	The act of receiving, or the fact of having been received.
socket noun	/ˈsɑkɪt/	An opening into which a plug or other connecting part is designed to fit (e.g. a light bulb socket).
mock verb	/mɑk/	To mimic, to simulate.
scalable adjective	/ˈskeɪ.lə.bl̩/	Capable of being climbed.
clarify verb	/ˈklæɹ.ɪ.faɪ/	To make or become clear or bright by freeing from impurities or turbidity.
scalability noun	/ˌskeɪ.ləˈbɪl.ɪ.ti/	The property of being scalable.
interviewer noun	/ˈɪntɚvjuɚ/	One who interviews.
tick noun	/tɪk/	A tiny woodland arachnid of the suborder Ixodida.
schema noun	/ˈskiːmə/	An outline or image universally applicable to a general conception, under which it is likely to be presented to the mind (for example, a body schema).
summarize verb	/ˈsʌməˌɹaɪz/	To prepare a summary of (something).
tweak noun	/twiːk/	A sharp pinch or jerk; a twist or twitch.
assign verb	/əˈsaɪn/	To designate or set apart (something) for some purpose.

Phrasal verbs you will hear

Word	Meaning
go through verb	To travel from one end of something to the other.
figure out verb	To come to understand; to discover or find a solution; to deduce.
point out verb	To identify among a group of similar subjects, or in a scene where the subject might not be readily seen or noticed, with a gesture of the body.
check out verb	To record the departure or withdrawal of someone or something (such as guests, employees, books, etc.).
send back verb	To return (something) to its origin.

Grammar in this video

The structures the speaker uses most, with the exact words from the video:

Structure	In the video
Conditional sentences if + clause, will/would + verb — a condition and its result	If we have user C, where would user · if you make any mistakes they will immediately
Present perfect have/has + past participle — a past action that still matters now	has sent · has been · has been delivered
Passive voice be + past participle — the focus is on what happens, not who does it	has been delivered · would be connected · is connected
Relative clauses who / which + clause — extra information about a person or thing	people who can · people who are · ones who who

Pronunciation to watch

The speaker uses 28 contractions and reduced forms, such as don't, I'll, I'm. Say them the short way, as you hear them.

The “sh” and “zh” sounds: initiate /ɪˈnɪʃ.i.eɪt/, distinguish /dɪˈstɪŋ.ɡwɪʃ/, specially /ˈspɛʃəli/
Long words — get the stress right: acknowledgment /əkˈnɑ.lɪd͡ʒ.mənt/, scalability /ˌskeɪ.ləˈbɪl.ɪ.ti/, initiate /ɪˈnɪʃ.i.eɪt/, ambiguity /æm.bɪˈɡjuː.ɪ.ti/, prioritize /pɹaɪˈɒɹ.ə.taɪz/

How to practise with this video

Listen to the whole video once without speaking and note the words you do not know.
Shadow it sentence by sentence at normal speed, repeating each one until your rhythm matches the speaker.
Record yourself and compare with the original, paying attention to words like gateway, microservice, acknowledgment.

What is the Shadowing Technique?

Shadowing is a science-backed language learning technique originally developed for professional interpreter training and popularized by polyglot Dr. Alexander Arguelles. The method is simple but powerful: you listen to native English audio and immediately repeat it out loud — like a shadow following the speaker with just a 1–2 second delay. Unlike passive listening or grammar drills, shadowing forces your brain and mouth muscles to simultaneously process and reproduce real speech patterns. Research shows it significantly improves pronunciation accuracy, intonation, rhythm, connected speech, listening comprehension, and speaking fluency — making it one of the most effective methods for IELTS Speaking preparation and real-world English communication.

Shadowing technique: read the full step-by-step guide →

Shadowing Practice: System Design Mock Interview: Design WhatsApp - Learn English Speaking with Video

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Vocabulary and speaking notes for this lesson

Key vocabulary in this video

Phrasal verbs you will hear

Grammar in this video

Pronunciation to watch

How to practise with this video

What is the Shadowing Technique?