Server response 304 Not Modified

HTTP Status Code 304 (Not Modified)

The HTTP status code 304 (Not Modified) is a response from the server indicating that the requested resource has not been modified since the last request made by the client. This code is crucial in optimizing caching mechanisms, allowing servers to avoid transferring data that has remained unchanged.

Key Characteristics of Status Code 304

Definition and Purpose: The 304 status code informs the client that the cached version of the resource is still valid and can be used without fetching a new copy from the server.
How Caching Mechanism Works with 304: When a client makes a request for a resource, it can include cache-related headers. If the server finds that the resource has not changed, it responds with a 304 status, saving bandwidth and reducing load times.
Conditions for Server to Return 304: The server will send a 304 response if the client includes the appropriate headers, such as If-Modified-Since or If-None-Match, indicating that it wants to check if the resource has been altered.

Practical Examples of Using Status Code 304

Example with Image Caching on a Website

How the Browser Stores Cache: When a user visits a webpage containing images, the browser downloads these images and stores them in cache for future use.
Behavior on Subsequent Requests: On revisiting the same webpage, the browser sends a request for the images along with If-Modified-Since header. If the server finds that the images haven't changed, it responds with a 304 status, and the browser uses the cached images.

Example with API Requests

The 304 status code can also optimize requests made to an API. When a client queries a resource, it may pass cache validation headers. If the resource is unchanged, the server can respond with 304, eliminating unnecessary data transfer.

Example with If-Modified-Since and If-None-Match Headers

How These Headers Interact with 304: The If-Modified-Since header indicates the last modification date of the cached resource. The If-None-Match header contains an ETag (entity tag) value. The server checks these headers against the current state of the resource to determine if a 304 response is appropriate.

Fixing Issues with Status Code 304 in Different Programming Languages

PHP

Setting Cache Headers Correctly: Use the header function to send the appropriate cache headers before outputting any content.
Example Code for Handling Requests and Returning Status 304:

<?php
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
header("ETag: \"unique-id\"");
if (isset($_SERVER['HTTP_IF_NONE_MATCH']) && $_SERVER['HTTP_IF_NONE_MATCH'] === "\"unique-id\"") {
    header("HTTP/1.1 304 Not Modified");
    exit();
}
?>

Python (Flask)

Setting Up Caching and Handling Status 304: Use Flask's response decorators to manage cache headers.
Example Code for Checking Caching Conditions:

from flask import Flask, request, make_response
import datetime

app = Flask(__name__)

@app.route('/resource')
def resource():
    last_modified = datetime.datetime.utcnow()
    response = make_response("Resource Content")
    response.headers['Last-Modified'] = last_modified
    if request.if_modified_since and request.if_modified_since >= last_modified:
        response.status_code = 304
        return response
    return response

Node.js (Express)

Handling Requests with Status 304: Utilize Express middleware to check cache conditions and respond appropriately.
Example Code for Working with If-Modified-Since Headers:

const express = require('express');
const app = express();

app.get('/resource', (req, res) => {
    const lastModified = new Date().toUTCString();
    res.setHeader('Last-Modified', lastModified);
    if (req.headers['if-modified-since'] === lastModified) {
        res.status(304).end();
    } else {
        res.send("Resource Content");
    }
});

Common Errors and How to Resolve Them

Incorrect Cache Configuration on the Server: Ensure that cache settings are properly configured to utilize conditional requests.
Errors in Request Headers: Verify that the If-Modified-Since and If-None-Match headers are correctly formatted and present.
How to Check and Debug Status 304: Use network analysis tools in browsers or server logs to trace the request and response cycle.

Error Type	Possible Cause	Solution
Cache Not Used	Cache headers not set correctly	Review server cache settings
304 Not Returned	Missing conditional headers	Ensure headers are sent in requests
Unexpected 200 Status	Resource modified	Update client cache

Understanding and correctly implementing HTTP status code 304 can significantly enhance performance through efficient caching. By reducing unnecessary data transfers, applications can provide a smoother user experience and optimize resource usage.