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Mercor Interview Preparation Guide

This guide is specifically curated for Software Engineer / Developer positions at Mercor. It covers multi-disciplinary topics including database design, system architecture, and behavioral decision-making.

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1. Database Design & Architecture

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Core Design Principles

When designing database architecture for user flows, consider:
  1. Data Flow Mapping: Map user actions to CRUD operations and plan transaction boundaries.
  2. Schema Design: Normalize data to reduce redundancy and design efficient indexes.
  3. Scalability: Choose between read-replicas, sharding, or microservices patterns.
Answer Framework:
  • Vertical Scaling: Increase CPU/RAM (simple but limited).
  • Horizontal Scaling: Add more servers (Scalable but complex).
  • Read Replicas: Distribute SELECT queries to slaves.
  • Sharding: Horizontally partition data across multiple database instances.
  • Caching: Use Redis to store hot query results.
Answer:
  • Atomicity: “All or nothing” - transaction fails if any part fails.
  • Consistency: Database follows rules (constraints) before and after.
  • Isolation: Concurrent transactions don’t interfere.
  • Durability: Committed data survives power loss/system crash.

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2. System Design: Load Balancing

Load balancing distributes incoming traffic across multiple servers to prevent bottlenecks.

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Algorithms

  • Round Robin: Sequential distribution.
  • Weighted Round Robin: Routes more traffic to powerful servers.
  • Least Connections: Best for long-lived connections (routes to least busy server).
  • IP Hash: Ensures a user stays on the same server (sticky sessions).

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Technical Implementation (Node.js)

Using the built-in cluster module to utilize all CPU cores: 
const cluster = require('cluster');
const numCPUs = require('os').cpus().length;
const express = require('express');

if (cluster.isMaster) {
    for (let i = 0; i < numCPUs; i++) cluster.fork();
    
    cluster.on('exit', (worker) => {
        console.log(`Worker ${worker.process.pid} died. Respawning...`);
        cluster.fork();
    });
} else {
    const app = express();
    app.get('/', (req, res) => res.send(`Handled by worker ${process.pid}`));
    app.listen(3000);
}

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3. Caching & Streaming

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Caching Patterns

  1. Cache-Aside: App checks cache; if miss, reads DB and updates cache.
  2. Write-Through: App writes to cache and DB simultaneously.
  3. Write-Behind: App writes to cache immediately; DB is updated asynchronously later.

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Real-time Communication

  • WebSockets: Full-duplex interactive communication (Best for chat).
  • SSE (Server-Sent Events): Uni-directional stream from server to client (Best for dashboards/news).
  • Long Polling: Legacy method where client holds connection until server has data.

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4. Behavioral & Decision-Making (Scenario-Based)

Mercor frequently uses Option A vs Option B scenarios to test engineering trade-offs.

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Scenario 1: Critical Deadline

Problem: Deadline is tomorrow, but feature isn’t ready.
  • Option A: Add more developers immediately.
  • Option B: Ask current team to work overtime.
Analysis:
  • Option A (The Trap): Brooks’ Law - “Adding people to a late project makes it later” due to onboarding overhead.
  • Choice: Option B is often safer for a 24-hour deadline, but Option A is better for a 2-week delay. Justify based on the Time Horizon.

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Scenario 2: Security vs Feature Launch

Problem: You found a vulnerability just before launch.
  • Option A: Delay launch and fix security.
  • Option B: Launch now, fix in next sprint.
Choice: Option A. Repercussions of a breach (legal, reputational) far outweigh the loss of a feature launch date.

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5. Technical Deep Dives (Sample Questions)

Answer:
  1. Logic: Map unique IDs (base62) to long URLs.
  2. Storage: NoSQL (Key-Value) or SQL with Index on the “short_code”.
  3. Scale: Use Redis for the most accessed URLs (10% of links get 90% of traffic).
Answer:
  • Use a Relationships Table (follower_id, following_id).
  • For the feed: SELECT * FROM posts WHERE user_id IN (SELECT following_id FROM relationships WHERE follower_id = ?).
  • Optimization: For celebs, use Push Model (precompute feed); for regular users, use Pull Model.
Answer: Load balancers check /health endpoints. If server returns 50x or times out, it is removed from the rotation.
async function healthCheck(server) {
    try {
        const res = await fetch(`${server.url}/health`);
        server.healthy = res.ok;
    } catch {
        server.healthy = false;
    }
}

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6. Preparation Checklist

  • STAR Method: Practice Situation, Task, Action, Result for behavioral answers.
  • Trade-offs: Always mention “Pros and Cons” before picking an Option (A or B).
  • Coding: Be ready to explain Express middleware, Redis integration, and SQL indexing.
  • Infrastructure: Understand Layer 4 vs Layer 7 load balancing.