Server-Sent Events alternative in Flask - Step-by-Step Execution

Concept Flow - Server-Sent Events alternative

Client sends HTTP request

↓

Server holds connection open

↓

Server sends data chunks

↓

Client receives and processes data

↓

Connection closes or repeats

The server keeps the connection open and sends data chunks to the client as events happen, without the client needing to request repeatedly.

Execution Sample

Flask

from flask import Flask, Response
import time

app = Flask(__name__)

@app.route('/stream')
def stream():
    def event_stream():
        for i in range(3):
            yield f'data: Message {i}\n\n'
            time.sleep(1)
    return Response(event_stream(), mimetype='text/event-stream')

This Flask code streams three messages to the client, one every second, using Server-Sent Events.

Execution Table

Step	Action	Data Sent	Sleep Time (s)	Connection State
1	Start event_stream generator	None	0	Open
2	Yield 'data: Message 0\n\n'	Message 0	1	Open
3	Yield 'data: Message 1\n\n'	Message 1	1	Open
4	Yield 'data: Message 2\n\n'	Message 2	1	Open
5	Generator ends	None	0	Closed

💡 Generator finishes after sending 3 messages, connection closes.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	None	0	1	2	End
Connection State	Open	Open	Open	Open	Closed

Key Moments - 2 Insights

Why does the connection stay open after sending each message?

Because the generator yields data but does not end until all messages are sent, keeping the HTTP connection open as shown in steps 2-4 of the execution_table.

What causes the connection to close?

When the generator finishes yielding all messages (step 5), Flask closes the connection automatically.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'i' when the second message is sent?

A 0

B 1

C 2

D None

💡 Hint

Check the 'Data Sent' and 'Step' columns for when the second message is sent.

At which step does the connection close according to the execution_table?

A Step 3

B Step 4

C Step 5

D Step 2

💡 Hint

Look at the 'Connection State' column for when it changes to 'Closed'.

If we increase the range to 5 in the generator, how would the 'After 3' value of 'i' in variable_tracker change?

A It would be 2

B It would be 3

C It would be 4

D It would be 5

💡 Hint

The 'After 3' column shows the value after the third iteration, which is index 2 starting from 0.

Concept Snapshot

Server-Sent Events (SSE) let a server push updates to a client over HTTP.
In Flask, use a generator function yielding 'data:' lines.
Return Response with mimetype 'text/event-stream'.
Connection stays open until generator ends.
Client receives messages as they arrive without polling.

Full Transcript

Server-Sent Events (SSE) provide a way for a server to send real-time updates to a client by keeping an HTTP connection open and streaming data. In Flask, this is done by creating a generator function that yields messages prefixed with 'data:' and separated by double newlines. The Flask route returns a Response object with the mimetype set to 'text/event-stream'. The connection remains open while the generator yields messages, and closes when the generator finishes. This allows clients to receive updates without repeatedly asking the server, making it efficient for live data feeds.