1. API Paradigms: REST vs. RPC

When Server A talks to Server B, how do they structure the conversation?

REST (Representational State Transfer)

The language of the web.

• Concept: Resources (Nouns) are manipulated via HTTP Verbs.
• Format: Usually JSON.
• Example:
  • GET /users/123 (Get user)
  • POST /users (Create user)
• Pros:
  • Universal: Every language/browser supports HTTP/JSON.
  • Stateless: Easy to scale.
  • Debuggable: You can read the JSON.
• Cons:
  • Bloated: JSON contains repeated field names ({"name": "Alice"}).
  • Over-fetching: You might get more data than you need.
  • Text-based: Slower to parse than binary.

RPC (Remote Procedure Call) / gRPC

Calling a function on a remote server as if it were local.

• Concept: Actions (Verbs).
• Format: Binary (Protobuf).
• Example: getUser(123)
• gRPC (Google RPC): The modern standard built on HTTP/2.
• Pros:
  • Performance: Binary is much smaller and faster to parse than JSON.
  • Schema: Strict contracts (.proto files) prevent type errors.
  • Streaming: Supports bi-directional streaming out of the box.
• Cons:
  • Complexity: Requires code generation.
  • Not Browser Friendly: Hard to call directly from a web browser (needs a proxy).

Verdict: Use REST for public APIs (external). Use gRPC for internal microservices (server-to-server).

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2. Transport Layer: TCP vs. UDP

Underneath HTTP and gRPC lies the transport layer.

TCP (Transmission Control Protocol)

The reliable workhorse.

• Philosophy: "I guarantee the data will arrive, in order, and without errors."
• Mechanism (3-Way Handshake):
  1. SYN ("Hello?")
  2. SYN-ACK ("I hear you.")
  3. ACK ("Great, let's talk.")
• Features:
  • Retransmission: If a packet is lost, TCP resends it.
  • Ordering: If packets arrive out of order (3, 1, 2), TCP fixes them (1, 2, 3).
  • Congestion Control: Slows down if the network is busy.
• Use Case: Web browsing (HTTP), Email (SMTP), File Transfer (FTP). Accuracy > Speed.

UDP (User Datagram Protocol)

The speed demon.

• Philosophy: "I'm sending data. Good luck."
• Mechanism: Fire and forget. No handshake.
• Features:
  • No Guarantee: Packets might be lost, duplicated, or out of order.
  • Low Overhead: No handshake latency.
• Use Case:
  • Video Streaming / VoIP: If you lose a pixel in a video frame, you don't want to pause the video to wait for it. You just skip it.
  • Gaming: Real-time player positions. Old data is useless.
• Interview Tip: If building a Chat app, use TCP (messages must arrive). If building a Video Chat, use UDP (speed matters more).

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3. WebSockets vs. Long Polling

How does the server send data to the client without the client asking first? (e.g., a Chat message).

Long Polling (The Hack)

1. Client sends request: "Any new messages?"
2. Server holds the request open until a message arrives.
3. Server responds: "Yes, here."
4. Client immediately sends a new request.
5. Cons: Server resource heavy (holding connections).

WebSockets (The Solution)

1. Client sends HTTP Upgrade request.
2. Connection switches to a persistent, bi-directional TCP tunnel.
3. Server can push data anytime.
4. Pros: Real-time, low latency.
5. Use Case: Chat apps, Stock tickers, Collaborative editing (Google Docs).

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Summary

• REST is for public APIs (JSON, flexible).
• gRPC is for internal microservices (Binary, fast).
• TCP guarantees delivery (Web, Email).
• UDP guarantees speed (Video, Gaming).
• WebSockets enable real-time, bi-directional communication.

Next Up: Where do we store the data? We'll explore Databases: SQL vs NoSQL and Scaling Strategies.