What happens when you click the like button on YouTube?

Reading Marc Lou's blog post inspired me to ask this question to GPT-4o, below is the answer that once again fascinates me about the complexity and beauty of the Internet even in the tiniest operations:

When you click the "Like" button on YouTube, several processes happen under the hood at different levels of the internet infrastructure, from your local device to YouTube's global data centers. Here’s a breakdown of what happens:

1. Client-Side Event (Your Device & Browser)

• Your device (computer, smartphone, etc.) detects the button click via JavaScript.
• A JavaScript event handler is triggered in YouTube’s front-end code.
• The UI updates optimistically (i.e., it visually registers the "like" without waiting for confirmation from the server).

2. HTTPS Request to YouTube’s Servers

• The browser sends an asynchronous HTTPS request (typically an HTTP POST request) to YouTube's backend API (youtube.com/like/update or a similar endpoint).
• This request contains metadata such as:Your user ID (or session token if not logged in)Video IDAction type (like, unlike, etc.)CSRF token for securityPossibly fingerprinting details (IP, device info) for fraud prevention.

3. Network Transport (Internet Infrastructure)

The request travels through multiple network layers:

• Local Network (Wi-Fi or Mobile Network): The request first goes to your router or mobile carrier's nearest cell tower.
• ISP (Internet Service Provider): Your ISP forwards the request to Google's nearest edge server via peering agreements.
• Global Content Delivery Network (CDN) & Load Balancing:YouTube (Google) operates a massive network of global CDN edge servers.Google’s Anycast-based DNS resolves the request to the closest data center for fast response times.Load balancers determine which backend server should handle the request.

4. Backend Processing (YouTube's Data Centers)

• A backend microservice (likely part of YouTube’s Engagement API) receives the request.
• Authentication and validation occur (e.g., is the user logged in? Is the request genuine?).
• The like action is processed:It updates your profile's liked videos.It increments the video’s like count.It triggers real-time ranking algorithms for recommendations.
• This update is stored in multiple databases, possibly:Distributed NoSQL Databases (e.g., Google Spanner or Bigtable) for scalability.Cache systems (e.g., Google’s in-memory caching) to speed up response times.

5. Response Sent Back

• The server sends an HTTP response (200 OK or similar) confirming success.
• This response propagates back through the internet to your browser.

6. UI Update & Further Actions

• Your browser receives the response.
• If successful, the like button remains "liked"; otherwise, it may revert.
• YouTube may trigger additional background tasks:Update video rankings in personalized recommendation algorithms.Notify the video creator (if enabled).Adjust real-time analytics dashboards.

7. Background Data Synchronization

• YouTube ensures the like action is replicated across multiple servers to maintain consistency.
• Big data pipelines (e.g., Google Cloud Dataflow, Pub/Sub) may process and analyze the engagement for future trends.
• Machine learning models continuously use this engagement data for personalized recommendations.

Summary

Clicking "Like" on YouTube initiates:

1. Local UI interaction (frontend event handling).
2. Network request traveling through the internet infrastructure (ISP, CDN, data center).
3. Backend processing (authentication, database update, caching).
4. Response handling (UI confirmation and further processing).
5. Background updates affecting video rankings and recommendations.

This entire process happens in milliseconds to seconds, ensuring a seamless user experience. ??