Why Long polling and Server-Sent Events in HLD? - Purpose & Use Cases

The Big Idea

What if your app could magically get updates the moment they happen, without asking again and again?

The Scenario

Imagine you have a website that shows live sports scores. Without any special technique, the website must keep asking the server every few seconds, "Any new score?" This is like repeatedly knocking on a door to check if someone is home.

The Problem

This constant asking wastes time and energy. The server gets overwhelmed with many repeated requests, and users may see delays or outdated scores. It feels slow and clunky, like waiting in line to ask the same question over and over.

The Solution

Long polling and Server-Sent Events let the server tell the user immediately when there is new information. Instead of asking repeatedly, the user waits patiently, and the server sends updates as soon as they happen. This saves effort and makes updates feel instant.

Before vs After

✗ Before

while(true) {
  fetch('/score');
  await new Promise(resolve => setTimeout(resolve, 5000));
}

✓ After

const eventSource = new EventSource('/scores');
eventSource.onmessage = e => updateScore(e.data);

What It Enables

This approach enables real-time updates that feel smooth and instant, improving user experience and reducing server load.

Real Life Example

News websites use Server-Sent Events to push breaking news headlines instantly to readers without them refreshing the page.

Key Takeaways

Manual repeated requests cause delays and waste resources.

Long polling and Server-Sent Events let servers push updates instantly.

This makes apps faster, smoother, and more efficient.

Practice

1. What is the main difference between long polling and Server-Sent Events (SSE)?

easy

A. Long polling closes the connection after each response; SSE keeps the connection open.

B. Long polling uses WebSockets; SSE uses HTTP/2 only.

C. Long polling requires client to open multiple connections; SSE uses multiple connections.

D. Long polling pushes data continuously; SSE waits for client requests.

Solution

1. Step 1: Understand long polling behavior

   Long polling keeps the HTTP request open until the server has data, then closes it after sending the response.

2. Step 2: Understand SSE behavior

   SSE keeps a single HTTP connection open to continuously push updates without closing it.

3. Final Answer:

   Long polling closes the connection after each response; SSE keeps the connection open. -> Option A

4. Quick Check:

   Connection persistence = A [OK]

Hint: Remember: long polling reconnects, SSE stays connected [OK]

Common Mistakes:

• Confusing SSE with WebSockets
• Thinking SSE uses multiple connections
• Believing long polling pushes continuously

2. Which HTTP method is typically used by clients to initiate a Server-Sent Events connection?

easy

A. POST

B. GET

C. PUT

D. DELETE

Solution

1. Step 1: Identify HTTP method for SSE

   SSE connections are initiated by clients using the GET method to request a stream from the server.

2. Step 2: Confirm other methods are not used

   POST, PUT, DELETE are used for data modification, not for opening SSE streams.

3. Final Answer:

   GET -> Option B

4. Quick Check:

   SSE connection method = GET [OK]

Hint: SSE uses GET to open a streaming connection [OK]

Common Mistakes:

• Choosing POST because it sends data
• Confusing PUT or DELETE with SSE initiation
• Thinking SSE uses WebSocket handshake methods

3. Consider this simplified server behavior for long polling:

while True:
  wait_for_event()
  send_response_to_client()
  client_reconnects()

What is the main downside of this approach compared to SSE?

medium

A. It cannot send data to multiple clients simultaneously.

B. It keeps the connection open indefinitely, wasting server resources.

C. It uses UDP instead of TCP, causing unreliable delivery.

D. It requires the client to repeatedly reconnect, causing overhead.

Solution

1. Step 1: Analyze long polling loop

   The server waits for an event, sends a response, then the client reconnects to wait again.

2. Step 2: Identify overhead caused by reconnects

   Each reconnect requires new HTTP handshake, increasing latency and server load.

3. Final Answer:

   It requires the client to repeatedly reconnect, causing overhead. -> Option D

4. Quick Check:

   Repeated reconnects = overhead [OK]

Hint: Long polling reconnects cause overhead, SSE keeps one open [OK]

Common Mistakes:

• Thinking long polling keeps connection open indefinitely
• Confusing TCP/UDP protocols in HTTP context
• Assuming long polling can't handle multiple clients

4. A developer implements SSE but notices the client stops receiving updates after some time. What is a likely cause?

medium

A. The client is not reconnecting after each message.

B. The client is sending multiple GET requests simultaneously.

C. The server is closing the connection prematurely.

D. The server is using POST instead of GET for SSE.

Solution

1. Step 1: Understand SSE connection behavior

   SSE expects the server to keep the connection open to push updates continuously.

2. Step 2: Identify issue with connection closing

   If the server closes the connection early, the client stops receiving updates until it reconnects.

3. Final Answer:

   The server is closing the connection prematurely. -> Option C

4. Quick Check:

   Connection closed early = no updates [OK]

Hint: SSE needs server to keep connection open, not close early [OK]

Common Mistakes:

• Thinking client must reconnect after every message
• Confusing HTTP methods for SSE
• Assuming multiple GET requests improve SSE

5. You are designing a chat app that must support thousands of users receiving real-time messages. Which approach is best and why?
A) Use long polling for all clients.
B) Use Server-Sent Events for all clients.
C) Use WebSockets for all clients.
D) Use HTTP polling every 5 seconds.

hard

A. WebSockets provide full-duplex communication, best for chat scalability.

B. Long polling scales well and reduces server load compared to SSE.

C. SSE keeps one connection per client open, efficiently pushing updates.

D. HTTP polling every 5 seconds is the most real-time and efficient.

Solution

1. Step 1: Evaluate long polling and SSE for scale

   Long polling causes overhead reconnects; SSE keeps connections open but is unidirectional.

2. Step 2: Consider WebSockets for chat

   WebSockets allow two-way communication with a single persistent connection, ideal for chat apps.

3. Step 3: Assess HTTP polling

   Polling every 5 seconds causes high latency and unnecessary load.

4. Final Answer:

   WebSockets provide full-duplex communication, best for chat scalability. -> Option A

5. Quick Check:

   Best real-time chat = WebSockets [OK]

Hint: Chat needs two-way, low-latency: choose WebSockets [OK]

Common Mistakes:

• Choosing SSE for two-way chat communication
• Thinking polling is efficient for real-time
• Assuming long polling scales better than WebSockets