Long polling and Server-Sent Events in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is long polling in web communication?

Long polling is a technique where the client sends a request to the server and waits for the server to respond only when new data is available, keeping the connection open for a longer time instead of immediately responding.

intermediate

How do Server-Sent Events (SSE) differ from long polling?

Server-Sent Events allow the server to push updates to the client over a single, long-lived HTTP connection without the client repeatedly requesting data, unlike long polling where the client must send new requests after each response.

intermediate

Name one advantage of using Server-Sent Events over WebSockets.

Server-Sent Events are simpler to implement for unidirectional data flow from server to client and work over standard HTTP, making them easier to use with existing infrastructure and proxies.

beginner

What happens if the server has no new data during a long polling request?

The server holds the request open until new data is available or a timeout occurs, then responds to the client. The client then immediately sends a new request to continue waiting for updates.

advanced

Why might long polling cause more server load compared to Server-Sent Events?

Because long polling requires the client to repeatedly open new HTTP connections after each response, it can create more overhead and resource usage on the server compared to a single persistent connection used by Server-Sent Events.

Which technique keeps a single HTTP connection open for continuous data flow from server to client?

A Regular polling

B Long polling

C Server-Sent Events

D FTP

Server-Sent Events use a single long-lived HTTP connection to push data continuously from server to client.

In long polling, what does the client do after receiving a response from the server?

A Closes the connection and sends a new request

B Keeps the connection open indefinitely

C Switches to WebSocket protocol

D Sends no further requests

After receiving data, the client closes the connection and immediately sends a new request to wait for more data.

Which of the following is a limitation of Server-Sent Events?

A Only supports unidirectional communication from server to client

B Requires multiple HTTP connections

C Does not work over HTTP

D Cannot handle text data

SSE supports only server-to-client data flow, not client-to-server messages.

Why might long polling be less efficient than Server-Sent Events?

A Because it requires WebSocket support

B Because it uses multiple HTTP requests and responses

C Because it uses a single persistent connection

D Because it only works with FTP

Long polling repeatedly opens and closes HTTP connections, causing more overhead than SSE's single persistent connection.

Which scenario is best suited for Server-Sent Events?

A Downloading large files

B Two-way chat application

C File upload from client to server

D Live news feed updates from server to many clients

SSE is ideal for pushing live updates from server to clients in one direction.

Explain how long polling works and describe its request-response cycle.

Think about how the client waits for data and how the server responds.

Compare Server-Sent Events and long polling in terms of connection management and data flow.

Focus on how connections are kept open and how data is sent.