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Jen-Hsuan Hsieh
Jen-Hsuan Hsieh

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Day 35 of #100DaysOfCode: Review HTTP Cache

#cache #browser #webdev #100daysofcode

Introduction

That's review the HTTP cache for the browser.
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1. If you don't use cached files on the browser

Option 1. Progma: no-cache (HTTP 1.0 request header) 

Progma: no-cache
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  • Equals Cache-Control:no-cache

Option 2. Cache-Control: no-store (HTTP 1.1 request header) 

Cache-Control: no-store
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  1. The browser will never use the cache.
  2. The browser will always ask the server for files even though there are no new files. The server will always sends the entire file (status code: 200)

Option 3. Cache-Control: no-cache (HTTP 1.1 request header) 

Cache-Control: no-cache
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  1. The browser always checks the web cache.
  2. The browser will always ask the server for new files. The server will send the new file (status code: 200) or not (304)

2. Force to use HTTP cache (max-age > Expires)

(I) Cache according to the freshness

Option 1. Expires (HTTP response header) 

Date: {date}
Expires: {date}
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  • The browser will cache the file until the expires date. The expires date is the server's time and the browser will check it.
  • It's possible that it's different between the server's time and the browser's time

Option 2. Cache-Control: max-age (HTTP response header) 

Cache-Control: max-age (seconds)
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  • The browser will cache the file and expires on the specific date

Example

  • The browser will response 200 with from memory cache if the file isn't expired Alt Text

(II) Cache according to the latest edited date

Set the HTTP request header

  • Ask the server to check if the file’s content is the same or not. (hash) 
If-Modified-Since: {date}
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Set the HTTP response header 

Last-Modified: {date}
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  • The server will sends new file (status code: 200) or not (304)

(III) Get the new file if the content has any modifications

Set the HTTP request header

  • The browser will retrieve the new file after the file was expired 
If-None-Match: {hash value}
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Set the HTTP response header 

Etag: {hash value}
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  • The server will sends new file (status code: 200) or not (304)

Example

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3. The recommended cache strategy

  1. For the HTML file, the browser will always ask the server for the file. The HTML file will update if the CSS, or the JS file was changed.
  2. For the CSS file with the hashed name, the browser holds the cache for a long time but still can be updated immediately.
  3. For the JS file with the hashed name, the browser holds the cache for a long time but still can be updated immediately. private means the file can't be cached by CDNs. Alt Text

That's it!

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