When building modern web apps, logging user behavior or system events reliably is essential. However, network conditions are unpredictable—so sending analytics events must be delayed, serialized, and resilient to transient failures.
Let's build an SDK that:
- Queues events via
logEvent() - Sends events one at a time , every 1 second
- Simulates failure every 5th event
- Retries once on failure
✅ Desired Output:
Analytics sent event 1
Analytics sent event 2
...
-----------------------
Failed to send event 5
Retrying sending event 5
-----------------------
Analytics sent event 5
...
✅ Final Code
class SDK { constructor() { this.queue = []; this.isSending = false; this.sendCount = 0; } // Enqueue an event for later processing logEvent(event) { this.queue.push(event); console.log(`Logged event: ${event}`); return this; } // Sleep utility for delays sleep(ms) { return new Promise(resolve => setTimeout(resolve, ms)); } // Attempt to send a single event, with retry on failure async sendEventWithRetry(event) { await this.sleep(1000); // Initial send delay if (this.sendCount % 5 === 0) { // Simulated failure console.log('-----------------------'); console.log(`Failed to send ${event}`); console.log(`Retrying sending ${event}`); console.log('-----------------------'); await this.sleep(1000); // Retry delay } console.log(`Analytics sent ${event}`); } // Process the queue serially async send() { if (this.isSending || this.queue.length === 0) return; this.isSending = true; while (this.queue.length > 0) { const event = this.queue.shift(); this.sendCount++; await this.sendEventWithRetry(event); } this.isSending = false; } } // ------------------------- // 🧪 Usage Example // ------------------------- const runExample = async () => { const sdk = new SDK(); // Log 10 events for (let i = 1; i <= 10; i++) { sdk.logEvent(`event ${i}`); } // Start sending await sdk.send(); }; runExample();
🔍 How It Works
1. Queueing
Every call to logEvent() pushes the event into an internal array.
2. Controlled Sending
The send() method ensures:
- Only one sending loop runs at a time (
this.isSending) - Events are dequeued one-by-one (
this.queue.shift()) - Each event waits 1 second before sending
3. Simulated Failures
Every 5th event (sendCount % 5 === 0) fails, logs a failure, waits another second, and retries sending.
4. Retry Guarantee
The logic does not proceed to the next event until the current one (including its retry) completes.
🔧 Why This is Good Engineering
- Reliable delivery : Events are serialized and retried.
- Robust simulation : Failures are explicit and testable.
- Reusable utilities :
sleep()is abstracted for clarity. - Separation of concerns : Queue management, retry logic, and delay handling are well-isolated.
Quick Quiz
Test your understanding with 3 quick questions
Q1Why is `this.isSending` used in the Analytics SDK?
Q2What happens when the 5th event fails to send in this SDK implementation?
Q3What does `this.queue.shift()` do in the send() method?