tesco interview experience

You're developing a backend service for a real-time analytics dashboard that tracks user interactions across an e-commerce platform. Your service needs to:

• Process incoming event data from multiple sources
• Perform aggregation operations
• Provide API endpoints for the frontend dashboard with summarized data (given below)

Core Requirements:

Your service needs to fetch and combine data from three internal data sources:

• User Activity Service: Tracks page views, clicks, and session data
• Transaction Service: Tracks purchases, cart additions, and checkout events
• Inventory Service: Tracks product stock levels and alerts

Note that each service has different response characteristics, data formats, and reliability issues:

/**
 * Fetches recent user activity data
 * Returns detailed user interaction events
 * High volume data, occasionally returns partial results
 * ~150ms average response time with occasional spikes
 */
async function fetchUserActivity(timeWindow) {
    return new Promise((resolve, reject) => {
        const delay = 100 + Math.random() * 100; // 100-200ms
        setTimeout(() => {
            if (Math.random() < 0.15) {
                // Sometimes returns partial data with an error flag
                resolve({
                    events: generateRandomEvents(Math.floor(Math.random() * 10) + 5),
                    complete: false,
                    error: "Service degraded, partial data returned"
                });
            } else if (Math.random() < 0.05) {
                // Occasionally fails completely
                reject(new Error("User activity service temporarily unavailable"));
            } else {
                resolve({
                    events: generateRandomEvents(Math.floor(Math.random() * 20) + 10),
                    complete: true
                });
            }
        }, delay);
    });
function generateRandomEvents(count) {
    const events = [];
    const eventTypes = ["page_view", "click", "scroll", "search"];

    for (let i = 0; i < count; i++) {
        events.push({
            eventId: `evt-${Math.random().toString(36).substring(2, 10)}`,
            userId: `user-${Math.floor(Math.random() * 1000)}`,
            type: eventTypes[Math.floor(Math.random() * eventTypes.length)],
            timestamp: Date.now() - Math.floor(Math.random() * 60000),
            data: {
                page: Math.random() < 0.5 ? "/products" : "/checkout",
                duration: Math.floor(Math.random() * 300)
            }
        });
    }
    return events;
}

}
/**


Fetches recent transaction data


Returns financial transaction information


Slower but fairly reliable service


Data format differs from user activity
*/
async function fetchTransactions(timeWindow) {
return new Promise((resolve, reject) => {
const delay = 200 + Math.random() * 150; // 200-350ms
setTimeout(() => {
if (Math.random() < 0.08) {
reject(new Error("Transaction service unavailable"));
} else {
const transactions = [];
const count = Math.floor(Math.random() * 8) + 2;
         for (let i = 0; i < count; i++) {
             transactions.push({
                 transactionId: `tx-${Date.now()}-${Math.random().toString(36).substring(2, 7)}`,
                 customerData: {
                     id: `user-${Math.floor(Math.random() * 1000)}`,
                 },
                 amount: +(Math.random() * 200 + 10).toFixed(2),
                 status: Math.random() < 0.9 ? "completed" : "pending",
                 items: Math.floor(Math.random() * 5) + 1,
                 createdAt: new Date(Date.now() - Math.floor(Math.random() * 3600000)).toISOString()
             });
         }
         resolve({
             data: transactions,
             meta: {
                 total: transactions.length,
                 timeframe: timeWindow
             }
         });
     }
 }, delay);

});
}


/**


Fetches inventory status updates


Returns stock level alerts and changes


Fast but returns data in a completely different structure
*/
async function fetchInventoryAlerts(urgencyLevel = "all") {
return new Promise((resolve, reject) => {
const delay = 50 + Math.random() * 100; // 50-150ms
setTimeout(() => {
if (Math.random() < 0.1) {
reject(new Error("Inventory service connection refused"));
} else {
// Returns XML-like string that needs parsing
const alertCount = Math.floor(Math.random() * 5);
let xmlResponse = &lt;inventory-alerts generated="${new Date().toISOString()}" count="${alertCount}"&gt;;
         for (let i = 0; i < alertCount; i++) {
             const productId = Math.floor(Math.random() * 1000);
             const currentStock = Math.floor(Math.random() * 10);
             const threshold = 5;
             const urgency = currentStock < 2 ? "high" : "medium";

             if (urgencyLevel === "all" || urgencyLevel === urgency) {
                 xmlResponse += `
         &lt;alert type="stock" urgency="${urgency}"&gt;
           &lt;product id="${productId}" /&gt;
           &lt;current-stock&gt;${currentStock}&lt;/current-stock&gt;
           &lt;threshold&gt;${threshold}&lt;/threshold&gt;
           &lt;location warehouse="${Math.floor(Math.random() * 5) + 1}" /&gt;
         &lt;/alert&gt;
       `;
             }
         }

         xmlResponse += '&lt;/inventory-alerts&gt;';
         resolve(xmlResponse);
     }
 }, delay);

});
}

Task:

Create an Express.js API endpoint that:

• Implements a GET /api/dashboard endpoint that accepts a timeWindow query parameter (e.g., "15m", "1h", "24h")
• Fetches data from all three services concurrently
• Handles the different response formats, error scenarios, and partial data
• Aggregates the data into a unified dashboard response
• Implements proper timeout handling (maximum 300ms total response time)

Sample response :

{
"dashboard": {
"timeWindow": "15m", // or whatever was requested
"summary": {
"activeUsers": 42,
"revenueTotal": 1234.56,
"conversionRate": 2.7,
"inventoryAlerts": 3
},
"details": {
"userActivity": {
// Processed user activity data
"complete": true|false,
// Additional details
},
"transactions": {
// Processed transaction data
// Additional details
},
"inventory": {
// Processed inventory data
// Additional details
}
},
"status": {
"servicesResponded": ["userActivity", "transactions", "inventory"],
"servicesFailed": [],
"responseTime": 298
}
}
}