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A Simple Framework For Mobile System Design Interviews

The list of posts in this series:

• Part 1: A Simple Framework For Mobile System Design Interviews
• Part 2: Additional Topics For Mobile System Design Interviews
• Part 3: Mobile System Design Exercise: File Downloader Library
• Part 4: Mobile System Design Exercise: Caching Library
• Part 5: Mobile System Design Exercise: Image Library
• Part 6: Mobile System Design Exercise: Chat Application
• Part 7: Common Interview Mistakes

Below is a simple framework for Mobile System Design interviews. As an example, we are going to use the “Design Twitter Feed” question. The proposed solution is far from being perfect but it is not the point of a system design interview round: no one expects you to build a robust system in just 30 min — the interviewer is mostly looking for specific “signals” from your thought process and communication. Everything you say should showcase your strengths and help the interviewer to evaluate you as a candidate.

Disclaimer

The framework was heavily inspired by the similar “Scalable Backend Design” articles. Learning the framework does not guarantee success during the interview. The structure of the interview process depends on the personal style of the interviewer. The dialog is really important — make sure you understand what the interviewer is looking for. Make no assumptions and ask clarifying questions.

Interview Process (45–60 min)

• 2–5 min — acquaintances
• 5 min — defining the task and gathering requirements
• 10 min — high-level discussion
• 20–30 min — detailed discussion
• 5 min — questions to the interviewer

Acquaintances

Your interviewer tells you about themselves and you tell them about yourself. It’s better to keep it simple and short. For example, “My name is Alex, I’ve been working on iOS/Android since 2010 — mostly on frameworks and libraries. For the past 2.5 years, I’ve been leading a team on an XYZ project: my responsibilities include system design, planning, and technical mentoring.” The only purpose of the introduction is to break the ice and provide a gist of your background. The more time you spend on this, the less time you will have for the actual interview.

Defining The Task

The interviewer defines the task. For example: “Design Twitter Feed”. Your first step is to figure out the scope of the task:

• Client-side only — just a client-app: you have the backend and API available.
• Client-side + API — likely choice for most interviews: you need to design a client app and API.
• Client-side + API + Back-end — less likely choice since most mobile engineers would not have a proper backend experience. If the interviewer asks server-side questions — let them know that you’re most comfortable with the client-side and don’t have hands-on experience with backend infrastructure. It’s better to be honest than try to fake your knowledge. If the interviewer persists — let them know that everything you know comes from books, YouTube videos, and blog posts.

Gathering Requirements

Task requirements can be split into functional, non-functional, and out-of-scope. We’ll define them in the scope of the “Design Twitter Feed” task.

Functional requirements

Think about 3–5 features that would bring the biggest value to the business.

• Users should be able to scroll through an infinite list of tweets.
• Users should be able to like a tweet.
• Users should be able to open a tweet and see comments (read-only).

Non-functional requirements

Not immediately visible to a user but plays an important role for the product in general.

• Offline support.
• Real-time notifications.
• Optimal bandwidth and CPU/Battery usage.

Out of scope

Features that will be excluded from the task but would still be important in a real project.

• Login/Authentication.
• Tweet sending.
• Followers/Retweets.
• Analytics.

Providing the “signal”

System design questions are made ambiguous. The interviewer is more interested in seeing your thought process than the actual solution you produce:

• What assumptions did you make and how did you state them?
• What features did you choose?
• What clarifying questions did you ask?
• What concerns and trade-offs did you mention?

Your best bet is to ask many questions and cover as much ground as possible. Make it clear that you’re starting with the BFS approach and ask them which feature or component they want to dig in. Some interviewers would let you choose which topics you want to discuss: pick something you know best.

Clarifying Questions

Here are some of the questions you might ask during the task clarification step

• Do we need to support Emerging Markets?
  Publishing in a developing country brings additional challenges. The app size should be as small as possible due to the widespread use of low-end devices and the higher cost of cellular traffic. The app itself should limit the number and the frequency of network requests and heavily rely on caching.
• What number of users do we expect?
  This seems like an odd question for a mobile engineer but it can be very important: a large number of clients results in a higher back-end load — if you don’t design your API right, you can easily initiate a DDoS attack on your own servers. Make sure to discuss an Exponential Backoff and API Rate-Limiting with your interviewer.
• How big is the engineering team?
  This question might make sense for senior candidates. Building a product with a smaller team (2–4 engineers) is very different from building it with a larger team (20–100 engineers). The major concern is project structure and modularization. You need to design your system in a way that allows multiple people to work on it without stepping on each other’s toes.

High-Level Diagram

Once your interviewer is satisfied with the clarification step and your choice for the system requirements — you should ask if they want to see a high-level diagram. Below is a possible solution for the “Twitter Feed” question.

High-Level Diagram

Server-side components:

• Backend
  Represents the whole server-sider infrastructure. Most likely, your interviewer won’t be interested in discussing it.
• Push Provider
  Represents the Mobile Push Provider infrastructure. Receives push payloads from the Backend and delivers them to clients.
• CDN (Content Delivery Network)
  Responsible for delivering static content to clients.

Client-side components:

• API Service
  Abstracts client-server communications from the rest of the system.
• Persistence
  A single source of truth. The data your system receives gets persisted on the disk first and then propagated to other components.
• Repository
  A mediator component between API Service and Persistence.
• Tweet Feed Flow
  Represents a set of components responsible for displaying an infinite scrollable list of tweets.
• Tweet Details Flow
  Represents a set of components responsible for displaying a single tweet’s details.
• DI Graph
  Dependency injection graph.
• Image Loader
  Responsible for loading and caching static images. Usually represented by a 3rd-party library.
• Coordinator
  Organizes flow logic between Tweet Feed and Tweet Details components. Helps decouple components of the system from each other.
• App Module
  An executable part of the system which “glues” components together.

Providing the “signal”

The interviewer might be looking for the following signals:

• The candidate can present the “big picture” without overloading it with unnecessary implementation details.
• The candidate can identify the major building blocks of the system and how they communicate with each other.
• The candidate has app modularity in mind and is capable of thinking in the scope of the entire team and not limiting themselves as a single contributor (this might be more important for senior candidates).

Frequently Asked Questions

Why using a high-level diagram is necessary? Can I skip it altogether or draw a detailed diagram right away?

A high-level diagram is by no means necessary — you can take any other approach which seems more appropriate for a specific interview case. However, there are some advantages in starting with a high-level approach:

• Time management — drawing a 30,000 feet view is quick and brings immediate topics for further discussion.
• Modularity — each high-level component can be potentially isolated in a separate module to allow a team of engineers to work on the project simultaneity without stepping on each other toes.
• Best practices — this approach is wildly used for the backend system design and closely resembles the C4 model for visualizing software architecture.

Still not sure? Ask your interviewer if they want you to draw a high-level diagram or skip it and jump to some concrete components.

Deep Dive: Tweet Feed Flow

After a high-level discussion, your interviewer might steer the conversation towards some specific component of the system. Let’s assume it was “Tweet Feed Flow”. Things you might want to talk about:

• Architecture patterns
  MVP, MVVM, MVI, etc. MVC is considered a poor choice these days. It’s better to select a well-known pattern since it makes it easier to onboard new hires (compared to some less known home-grown approaches).
• Pagination
  Essential for infinite scroll functionality. For more details see Pagination.
• Dependency injection
  Helps to build an isolated and testable module.
• Image Loading
  Low-res vs full-res image loading, scrolling performance, etc.

Tweet Feed Flow Diagram

Components

• Feed API Service — abstracts Twitter Feed API client: provides the functionality for requesting paginated data from the backend. Injected via DI-graph.
• Feed Persistence — abstract cached paginated data storage. Injected via DI-graph.
• Remote Mediator — triggers fetching the next/prev page of data. Redirects the newly fetched paged response into a persistence layer.
• Feed Repository — consolidates remote and cached responses into a Pager object through Remote Mediator.
• Pager — trigger data fetching from the Remote Mediator and exposes an observable stream of paged data to UI.
• “Tweet Like” and “Tweet Details” use cases — provide delegated implementation for “Like” and “Show Details” operations. Injected via DI-graph.
• Image Loader — abstracts image loading from the image loading library. Injected via DI-graph.

Providing the “signal”

The interviewer might be looking for the following signals:

• The candidate is familiar with the most common MVx patterns.
• The candidate achieves a clear separation between business logic and UI.
• The candidate is familiar with dependency injection methods.
• The candidate is capable of designing self-contained isolated modules.

Frequently Asked Questions

How much detail should I provide in the deep-dive section?”

There’s no rule of thumb here. Work closely with the interviewer: ask them if you need to go deeper or move on to the next topic. If you have an in-person/video interview — watch their facial expressions. For example, if you see that the interviewer wants to interrupt you — stop talking and ask if they have any questions. The whole point is to work together — that provides a good signal for you as a team player/collaborator.

Why didn’t you mention specific classes (like RecyclerView/UICollectionView) and vendors (like Room, CoreDate, Realm, etc)?

• To make the guide stable and platform-agnostic: the libraries and the frameworks are constantly evolving — picking up a specific vendor can only be relevant for a short amount of time. Using an abstraction is more robust since you only concentrate on the functionality it provides without digging too much into the implementation details.
• Vendor selection is biased and depends on personal experience and current trends.
• Big tech companies (like FAANG) might not care much about vendors since they build their custom proprietary stacks.

API Design

The goal is to cover as much ground as possible — you won’t have enough time to cover every API call — just ask the interviewer if they are particularly interested in a specific part, or choose something you know best (in case they don’t have a strong preference).

Real-time notifications

We need to provide real-time notifications support as a part of the design. Below are some of the approaches you can mention during the discussion.

Push Notifications

Based on the operating system push notification service (OSPNS) and delivered from a 3rd-party push provider.

pros:

• easier to implement compared to a dedicated service.
• can wake the app in the background.

cons:

• not 100% reliable.
• may take up to a minute to arrive.
• relies on a 3rd-party service.

HTTP-polling

Polling requires the client to periodically ask the server for updates. The biggest concern is the amount of unnecessary network traffic and increased backend load.

Short HTTP-polling

The client sends lots of frequent requests to the server.

pros:

• simple and not as expensive (if the time between requests is long).
• no need to keep a persistent connection.

cons:

• the notification can be delayed for as long as the polling time interval.
• additional overhead due to TLS Handshake and HTTP-headers

Long HTTP-polling

A single request is sent to the server and the client is waiting for the response.

pros:

• instant notification (no additional delay).

cons:

• more complex and requires more server-side resources.
• keeps a persistent connection until the server replies.

Server-Sent Events

Allows the client to stream events over an HTTP/1.1 connection without polling.

pros:

• real-time traffic using a single connection.

cons:

• keeps a persistent connection.

Web-Sockets

Provide bi-directional communication between client and server.

pros:

• can transmit both binary and text data.

cons:

• more complex to set up compared to Polling/SSE.
• keeps a persistent connection.

The interviewer would expect you to pick a concrete approach most suitable for the design task at hand. One possible solution for the “Design Twitter Feed” question could be using a combination of SSE (a primary channel of receiving real-time updates on “likes”) with Push Notifications (sent if the client does not have an active connection to the backend).

Protocols

REST

A text-based stateless protocol is the most popular choice for CRUD (Create, Read, Update, and Delete) operations.

pros:

• easy to learn, understand, and implement.
• easy to cache using a built-in HTTP caching mechanism.
• loose coupling between client and server.

cons:

• less efficient on mobile platforms since every request requires a separate physical connection.
• schemaless — it’s hard to check data validity on the client.
• stateless — needs extra functionality to maintain a session.
• additional overhead — every request contains contextual metadata and headers.

GraphQL

A query language for working with API — allows clients to request data from several resources using a single endpoint (instead of making multiple requests in traditional RESTful apps).

pros:

• schema-based typed queries — clients can verify data integrity and format.
• highly customizable — clients can request specific data and reduce the amount of HTTP traffic.
• bi-directional communication with GraphQL Subscriptions (WebSocket based).

cons:

• more complex backend implementation.
• “leaky-abstraction” — clients become tightly coupled to the backend.
• the performance of a query is bound to the performance of the slowest service on the backend (in case the response data is federated between multiple services).

WebSocket

Full-duplex communication over a single TCP connection.

pros:

• real-time bi-directional communication.
• provides both text-based and binary traffic.

cons:

• requires maintaining an active connection — might have poor performance on unstable cellular networks.
• schemaless — it’s hard to check data validity on the client.
• the number of active connections on a single server is limited to 65k.

gRPC

Remote Procedure Call framework which runs on top of HTTP/2. Supports bi-directional streaming using a single physical connection.

pros:

• lightweight binary messages (much smaller compared to text-based protocols).
• schema-based — built-in code generation with Protobuf.
• provides support of event-driven architecture: server-side streaming, client-side streaming, and bi-directional streaming
• multiple parallel requests.

cons:

• limited browser support.
• non-human-readable format.
• steeper learning curve.

The interviewer would expect you to pick a concrete approach most suitable for the design task at hand. Since the API layer for the “Design Twitter Feed” question is pretty simple and does not require much customization — we can select an approach based on REST.

Pagination

Endpoints that return a list of entities must support pagination. Without pagination, a single request could return a huge amount of results causing excessive network and memory usage.

Offset Pagination

Provides limit and offset query parameters. Example: GET /feed?offset=100&limit=20.

pros:

• easiest to implement — the request parameters can be passed directly to a SQL query.
• stateless on the server.

cons:

• bad performance on large offset values (the database needs to skip offset rows before returning the paginated result).
• inconsistent when adding new rows into the database (Page Drift).

Keyset Pagination

Uses the values from the last page to fetch the next set of items. Example: GET /feed?after=2021-05-25T00:00:00&limit=20.

pros:

• translates easily into a SQL query.
• good performance with large datasets.
• stateless on the server.

cons:

• “leaky abstraction” — the pagination mechanism becomes aware of the underlying database storage.
• only works on fields with a natural ordering (timestamps, etc).

Cursor/Seek Pagination

Operates with stable ids which are decoupled from the database SQL queries (usually, a selected field is encoded using base64 and encrypted on the backend side). Example: GET /feed?after_id=t1234xzy&limit=20.

pros:

• decouples pagination from SQL database.
• consistent ordering when new items are inserted.

cons:

• more complex backend implementation.
• does not work well if items get deleted (ids might become invalid).

You need to select a single approach after listing the possible options and discussing their pros and cons. We’ll pick Cursor Pagination in the scope of the “Design Twitter Feed” question. A sample API request might look like this:

GET /v1/feed?after_id=p1234xzy&limit=20
Authorization: Bearer <token>
{
  "data": {
    "items": [
      {
        "id": "t123",
        "author_id": "a123",
        "title": "Title",
        "description": "Description",
        "likes": 12345,
        "comments": 10,
        "attachments": {
          "media": [
            {
              "image_url": "https://static.cdn.com/image1234.webp",
              "thumb_url": "https://static.cdn.com/thumb1234.webp"
            },
            ...
          ]
        },
        "created_at": "2021-05-25T17:59:59.000Z"
      },
      ...
    ]
  },
  "cursor": {
    "count": 20,
    "next_id": "p1235xzy",
    "prev_id": null
  }
}

Authentication

Although we left it out of scope, it’s still beneficial to mention HTTP authentication. You can include theAuthorization header and discuss how to properly handle 401 Unauthorized response scenarios. Also, don't forget to talk about Rate-Limiting strategies (429 Too Many Requests).
Make sure to keep it brief and simple (without unnecessary details): your primary goal during a system design interview is to provide a "signal" and not to build a production-ready solution.

Providing the “signal”

The interviewer might be looking for the following signals:

• The candidate is aware of the challenges related to poor network conditions and expensive traffic.
• The candidate is familiar with the most common protocols for unidirectional and bi-directional communication.
• The candidate is familiar with REST-full API design.
• The candidate is familiar with authentication and security best practices.
• The candidate is familiar with network error handling and rate-limiting.

Data Storage

Key-Value Storage (UserDefaults/SharedPreferences/Property Lists)

Usually, baked by XML or binary files. Allows associating primitive data with string-based keys. Works best for simple, unstructured, non-sensitive data (settings, flags, etc).

pros:

• easy to use built-in API.

cons:

• insecure (Android provides EncryptedSharedPreferences; 3rd party wrapper libraries available on iOS).
• not suitable for storing large data.
• no schema support nor the ability to query data.
• no data migration support.
• poor performance.

Database/ORM (Sqlite/Room/Core Data/Realm/etc)

Based on a relational database. Perfect for large amounts of structured data that needs complex querying logic.

pros:

• object-relational mapping support.
• schema and querying support.
• data migration support.

cons:

• more complex setup.
• insecure (wrapper libraries available on iOS/Android).
• bigger memory footprint.

Custom/Binary (DataStore/NSCoding/Codable/etc)

Handles storing and loading data on a low level. Works best when you need to customize the data storage pipeline.

pros:

• performant.

cons:

• no schema/migration support.
• lots of manual effort.

On-Device Secure Storage (Keystore/Key Chain)

Use OS-encrypted storage for creating/storing encryption keys and key-value data.

pros:

• secure (still not 100% unless provided by the hardware).

cons:

• not optimized for storing anything but encryption keys.
• encryption/decryption performance overhead.
• no schema/migration support.

Storage Location

• Internal
  Sand-boxed by the app and not readable to other apps (with few exceptions).
• External
  Publicly visible and most likely not deleted when your app is deleted.
• Media/Scoped
  A special type of storage for media files.

Storage Type

• Documents (Automatically Backed Up)
  User-generated data cannot be easily re-generated and will be automatically backed up.
• Cache
  Data that can be downloaded again or regenerated. Can be deleted by the user to free-up space.
• Temp
  Data that is only used temporarily and should be deleted when no longer needed.

Best Practices

• Store as little sensitive data as possible.
• Use encrypted storage if you can’t avoid storing sensitive data.
• Do not allow your app storage to grow uncontrollably. Make sure that cleaning up cached files won’t affect app functionality.

Approach

You need to select a single approach after listing options and discussing their pros and cons. Don’t worry about building a complete solution — just try to lay out a high-level idea without going too deep into details.
A possible breakdown for the “Design Twitter Feed” question might look like this:

Feed Pagination Table

Create a “feed” database table for storing paginated feed responses:

item_id:        String
author_id:      String
title:          String
description:    String
likes:          Int
comments:       Int
attachments:    String # comma separated list
created_at:     Date   # also used for sorting
cursor_next_id: String # points to the next cursor page
cursor_prev_id: String # points to the prev cursor page

• Limit the total number of entries to 500 in order to control local storage size.
• Flatten attachments into a comma-separated list. Alternatively, you can create an attachments table and join it with the feed table on item_id.
• Explicitly store next/prev cursor id with each item to simplify paged navigation.

Attachments Storage

• Store attachments as files in Internal Cached storage.
• Use attachment URLs as cache keys.
• Delete attachments after deleting corresponding items from the feed table.
• Limit the cache size to 200–400 Mb.

A possible follow-up question might require you to handle sensitive media data (private accounts, etc). In this case, you can selectively encrypt image files with encryption keys stored in Keystore/KeyChain.

Providing the “signal”

The interviewer might be looking for the following signals:

• The candidate is aware of mobile storage types, security, limitations, and compatibility.
• The candidate is capable of designing a storage solution for the most common scenarios.

Conclusion

There’s a significant amount of randomness during a system design interview. The process and the structure can vary depending on the company and the interviewer.

Things you can control

• Your attitude — always be friendly no matter how the interview goes. Don’t be pushy and don’t argue with the interviewer — this might provide a bad “signal”.
• Your preparation — the better your preparation is, the bigger the chance of a positive outcome. Practice mock design interviews with your peers (you can find people on Teamblind).
• Your knowledge — the more knowledge you have, the better your chances are.
• Gain more experience.
• Study popular open-source projects: iOS, Android
• Read development blogs from tech companies
• Your resume — make sure to list all your accomplishments with measurable impact.

Things you cannot control

• Your interviewer’s attitude — they might have a bad day or simply dislike you.
• Your competition — sometimes there’s simply a better candidate.
• The hiring committee — would make a decision based on the interviewers’ reports and your resume.

Judging the outcome

You can influence the outcome but you can’t control it. Don’t let minor setbacks determine your self-worth.

Looking for more content?

• Join the “Mobile System Design” Discord server for general discussions and feedback.
• Check out the mock interview archive or sign-up to be a candidate yourself!

Originally published at: https://github.com/weeeBox/mobile-system-design