[Development required] basic cache concepts

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1. Client Cache

<Meta> tag, which is the most basic method for page caching.

1	<Meta http-equiv ="Expires" Content ="Mon, 25 Feb 2014 00:00:00 GMT"/>

2	<Meta http-equiv ="Pragma" Content ="No-Cache"/>

The meta tag sent to the HTML page of the browser tells the browser about the cache time and whether to enable the cache. Programa: No-cache content is not guaranteed, but most browsers follow this convention.

A better way is to use the header function to send HTTP headers.

1

<? PHP

2	Header ('Expires: Mon, 25 Feb 2014 00:00:00 gmt');

3	Header ('Pragma: No-cache');

4

?>

You can further use the cache-control header of http1.1:

1

<? PHP

2	Header ('Expires: Mon, 25 Feb 2014 00:00:00 gmt');

3	Header ('Cache-control: No-store, no-cache, must-revalidate');

4	Header ('Cache-control: Post-check = 0, pre-check = 0', False );

5	Header ('Pragma: No-cache');

6

?>

The problem with the above method is that the browser must download the page to read the meta tag. When the browser first requests the page, if there is no tag, it will continue to download the cached copy of the source file. If a cached Proxy Server exists between the browser and the server, the browser may get outdated pages. At the same time, the use of HTTP for Cache control is not completely guaranteed. cache-control and pragam are only added for insurance purposes, and the Expires header may fail due to the date setting error on the client computer.

The last-modified and if-modified-since headers can be used to better control the client cache based on client requests. This behavior is called to execute a conditional GET request.

This method requires that the last-modified header be sent every time you access the PHP file. the browser sends the IF-modified-since header containing the updated time on the next request page, the script checks whether the page content is updated after this time. If there is no update, a 304 status code is returned and then exited.

01

<? PHP

02	...// Get the filemtime

03	Header ('Last-modified :' .Gmdate('D, D m y h: I: s',$ Lastfilemtime).'Gmt');

04

05	$ Request =Getallheaders();

06	If (Isset ($ Requeset['If-modified-since'])

07

{

08	$ Modifiedsince =Explode(';' $ Request['If-modified-since']);

09	$ Modifiedsince =Strtotime($ Modifiedsince[0]);

10

}

11

Else

12

{

13	$ Modifiedsince = 0;

14

}

15	If ($ Lastmodified <=$ Ifmodifiedsince)

16

{

17	Header ('Http/1.1 304 not modified');

18

Exit();

19

}

20	....// Output content

21

?>

2. Server cache-Use PHP output buffer

The built-in caching mechanism of PHP is the output buffer. When Echo or print is used, you can use the output control function (beginning with OB) to store the content in the memory buffer.

First, you can store the requested page in the buffer before output. If an error occurs on the page, you can hide the error without being seen by the visitor.

Second, you can store the content of the output buffer to a file. In the next request, you can determine the modification time and then directly output the file without expiration to improve the website access performance. This is also a very good way to implement dynamic static website cache.

01

<? PHP

02	If (File_exist ('./Cache. page')&&$ Lasttime> =Filemtime('./Cache. page'))

03

{

04	Readfile ('./Cache. page');

05

Exit;

06

}

07

08	Ob_start ();

09

Echo <EOT

10

.......

11

.......

12

EOT;

13

14	$ Buffer = Ob_get_contents ();

15	Ob_end_flush ();

16

17	$ Fp =Fopen('./Cache. page','W');

18	Fwrite ($ Fp,$ Buffer);

19	Fclose ($ Fp);

At the same time, this method can be used to cache some infrequently changed parts of the page to different files, and then assemble the entire page during the request to return the response.

[Development required] basic cache concepts