Skip to main content

SDE-II Backend Engineer – Role Understanding

Goal of this file: Make it crystal clear what an SDE-II Backend Engineer is expected to do in interviews and in a real team so you can align your preparation and projects accordingly.

What Is Expected From an SDE-II Backend Engineer?​

  • Scope & Ownership

    • Own medium-to-large services or features end-to-end (design → implementation → deployment → monitoring).
    • Work with imperfect requirements, clarify with stakeholders, and propose technical options with trade-offs.
    • Unblock juniors, review code, and contribute to engineering standards (coding, testing, observability, security).

  • Technical Depth

    • Strong in TypeScript/JavaScript and Node.js internals.
    • Solid understanding of HTTP, REST APIs, databases (MongoDB/MySQL), caching (Redis), queues, and distributed systems basics.
    • Can design and implement robust, observable, secure services that run in production.

  • System Design & Architecture

    • Design modular, scalable, maintainable service architectures.
    • Understand HLD (high-level design) and LLD (low-level design): APIs, data models, class design, patterns.
    • Communicate designs with diagrams, explain trade-offs, and respond to feedback.

  • Quality & Reliability

    • Write clean, testable code with unit, integration, and API tests.
    • Integrate with CI/CD pipelines, use feature flags, and understand rollback strategies.
    • Proactively consider failure modes, implement timeouts, retries, idempotency, and rate limiting.

  • Collaboration & Leadership

    • Mentor junior engineers, give constructive code reviews.
    • Communicate effectively with PMs, QAs, DevOps, and other teams.
    • Drive small technical initiatives (e.g., logging standardization, error handling improvements).

Expectations in Interviews​

1. Coding + DSA (Backend-Relevant)​

  • What they test

    • Ability to write correct, efficient, readable code under time pressure.
    • Comfort with arrays, strings, hash maps, sets, trees, graphs (practical), heaps, sliding window, two pointers.
    • Clean handling of edge cases, error cases, and input validation.

  • How it differs from SDE-I

    • Higher expectations for code quality: modular functions, good names, clear invariants, tests (if time).
    • Better time/space complexity analysis and ability to explain trade-offs (e.g., why use a heap vs sort).
    • Owning the problem-solving process: you drive the conversation.

2. Machine Coding / LLD Rounds​

  • What they test

    • Ability to translate a product requirement into:
      • Well-defined APIs or classes
      • Data models (DB schemas, entities)
      • Appropriate design patterns (Strategy, Factory, Observer, etc.)
    • Code that is extensible and maintainable, not just "works once".

  • Typical problems

    • Parking lot, Splitwise, Rate limiter, In-memory cache, Pub/Sub, In-memory DB, Notification service.

3. System Design (HLD + LLD)​

  • What they test

    • Can you design a backend system for millions of users?
    • Do you understand bottlenecks, scaling strategies, data modeling, consistency vs availability?
    • Can you discuss APIs, storage choices, caching, queues, rate limiting, load balancing?

  • Examples

    • Design Swiggy/Uber (location tracking, matching, pricing, surge).
    • Design Instagram/Notification service/Rate limiter/File storage service.

4. Backend Fundamentals & Theory​

  • What they test
    • Basic but solid understanding of:
      • Networking: TCP vs UDP, HTTP, DNS, proxies, SSL/TLS.
      • OS: processes vs threads, context switching, scheduling basics, memory management.
      • DBMS: indexing, normalization, ACID, transactions, isolation levels.
      • Distributed systems basics: CAP, eventual consistency, replication, sharding.

5. Behavioral & Ownership​

  • What they test
    • Examples of owning a project end-to-end.
    • Handling production incidents, debugging, and post-mortems.
    • Ability to work with conflict, communicate clearly, and learn from mistakes.

Expectations in Real Teams​

Responsibility Scope​

  • Medium-sized services or components

    • Own one or more services (e.g., payments-service, notification-service).
    • Drive feature development from idea to production, coordinate with PM/QA/DevOps.

  • Example

You are the owner of "Notification Service":
  - Understand requirements: email, SMS, push, in-app.
  - Propose architecture: API gateway → notification-service → providers (SES, Twilio, FCM).
  - Design DB schema: notifications, templates, user preferences.
  - Implement APIs: sendNotification, getNotificationStatus.
  - Add observability: logs, metrics, traces.
  - Handle incidents: retries, DLQ, alerts.

Day-to-Day Activities​

  • Design

    • Participate in design discussions and write design docs for new features.
    • Review other engineers’ designs and suggest improvements.

  • Implementation

    • Convert requirements into tickets and tasks.
    • Write TypeScript/Node.js code with tests and documentation.
    • Integrate with databases, caches, queues, third-party APIs.

  • Quality & Operations

    • Add and maintain metrics, logs, and alerts.
    • Participate in on-call rotations, help debug production issues.
    • Suggest and implement performance improvements (e.g., caching, query optimization).

  • Collaboration

    • Mentor juniors via pair-programming, code reviews, and knowledge-sharing sessions.
    • Communicate progress and risks clearly to leads and managers.

Skills Required to Be Productive From Day 1​

Use this as a target checklist for your preparation.

  • Language & Runtime

    • TypeScript/JavaScript
      • Comfortable with ES6+ features, async/await, promises, prototypes, closures, modules.
      • Understand TypeScript types, interfaces, generics, utility types, type narrowing.
    • Node.js
      • Understand event loop, libuv, async IO, clustering, worker threads.
      • Experience with Express/Fastify, middleware, routing, error handling.

  • Backend Fundamentals

    • HTTP & REST
      • HTTP verbs, status codes, headers, cookies, sessions, pagination, versioning.
    • API Design
      • Consistent resource naming, request/response contracts, and error formats.
    • Authentication & Authorization
      • JWT, sessions, OAuth2 basics, role-based access control, permission checks.

  • Data & Storage

    • MongoDB
      • Collections, documents, schema design, indexes, aggregation.
    • MySQL
      • Tables, normalization, joins, indexes, transactions.
    • Redis
      • Basic data structures, caching patterns (write-through, write-back), rate limiting.

  • Distributed Systems & Infrastructure

    • Caching, Queues, Rate Limiting, Load Balancing
    • Basics of AWS (EC2, RDS, S3, SQS, Lambda, API Gateway).
    • Understanding of CI/CD, containers (Docker), and basic Kubernetes concepts (optional but valuable).

  • Reliability & Observability

    • Logging best practices (structured logs, correlation IDs).
    • Metrics (latency, throughput, error rate, saturation).
    • Tracing basics, dashboards, and alerting.

  • Design & Patterns

    • Familiarity with SOLID principles and patterns (Factory, Strategy, Adapter, Observer, Repository, etc.).
    • Ability to create clean LLD: classes, interfaces, modules, layering (controller/service/repository).

ASCII View: SDE-II Backend Skill Map​

                  +---------------------------+
                  |   SDE-II Backend (Node)  |
                  +---------------------------+
                     /        |        \
                    /         |         \
           Language/Runtime   |      System Design
                 /            |           \
         JS/TS, Node.js   Backend Fundamentals    Distributed Systems
                         (HTTP, APIs, Auth)    (Caching, Queues, Scaling)

              Data & Storage        Quality & Ops        Leadership
           (Mongo, MySQL, Redis)   (Testing, CI/CD,     (Mentoring, Reviews,
                                     Monitoring)         Communication)

Use this file as a north star: every day of your preparation plan should move you closer to this SDE-II profile.