PHP uses HTTP caching protocol principle analysis and application analysis in Apache environment

There is also Etag for static pages.

1. Let’s look at the first situation: apache static page

The static page sent by apache to the client generally contains Last-Modified and Etag. These two tags The value comes from the modification time and inode of the static file.

The following is an interception of the header returned by apache to the client

XML/HTML code

Copy code The code is as follows:

 
Last-Modified: Fri, 26 Jan 2007 01:53:34 GMT 
ETag: "3f9f640-318-cb9f8380" 

The reason why search engines like static files Because of these two logos, you can determine whether the file has been updated

2. Dynamic pages such as PHP

Since PHP is dynamically generated, its content cannot The last modification date is determined based on the time of the PHP program, so by default PHP does not include any cache control when returning to the client. If you want to make good use of the cache, you must understand the cache mechanism and manage the interaction to reduce b and s, reduce bandwidth traffic, and reduce the load on the server. The burden...the benefits are many.

3. The specific meaning of cache control

First explain the meaning of these tags that I have tested and understood

Cache-Control: Specify The caching mechanism followed by requests and responses. Setting Cache-Control in a request message or response message does not modify the caching process during the processing of another message. The caching instructions during the request include no-cache, no-store, max-age, max-stale, min-fresh, only-if-cached, and the instructions in the response message include public, private, no-cache, no-store, no-transform, must-revalidate, proxy-revalidate, max-age.

The meaning of the instructions in each message is as follows:

Public indicates that the response can be cached by any cache area.

Private indicates that all or part of the response message for a single user cannot be processed by the shared cache. This allows the server to only describe a partial response from a user that is not valid for other users' requests.

no-cache indicates that the request or response message cannot be cached.

no-store is used to prevent important information from being released unintentionally. Sending it in the request message will cause both the request and response messages to use caching.

max-age indicates that the client can receive responses with a lifetime no longer than the specified time in seconds.

min-fresh indicates that the client can receive responses with a response time less than the current time plus the specified time.

max-stale indicates that the client can receive response messages beyond the timeout period. If you specify a value for max-stale messages, the client can receive response messages that exceed the specified value of the timeout period.

php usage:

Use header() before output, (if you use ob_start(), you can place the header anywhere in the program)

PHP code

Copy code The code is as follows:

header('Cache-Control: max-age=8');

max-age=8 means the maximum lifetime is 8 seconds. If it exceeds 8 seconds, the browser must go to the server to read it again. This time starts from the time the user reads the page, and Expires is an absolute time.

Expires: The absolute time for cache expiration. If it passes the specified time point, the browser will not recognize the cache and will go to the server to request the latest one again.

Last-Modified: The last modification time of the document. Its wonderful use is: 1

If it is a static file, the client will send the time in its cache, and apache will compare it. If it is found that there is no modification, a header will be returned directly. The status code is 304 and the number of bytes is very small. (The advanced version will also add a comparison Etag to determine whether the file has changed)

2 PHP dynamic file:

The client sends the comparison time, and PHP will determine whether to modify it. If the modification time is the same, only 1024 bytes will be returned. As for why 1024 is returned, it is unknown. If the file generated by your PHP is very large, it will only Returns 1024, so it saves bandwidth. The client will automatically display it from the cache file based on the modification time sent by the server.

Note: Cache-Control and Expires can also work without the Last-Modified header, but each request must return the real number of file bytes, not 1024

4. HOW?

You don’t need to worry about static pages. If you want to better control the caching of static pages, apache has several modules that can be well controlled, which will not be discussed here

php page:

There are two types here:

1. Pages that do not change frequently, similar to news releases. The characteristics of this type of page: there will be several times after the first release. Changes will basically not be modified over time. The control strategy should be: 1. Send Last-Modified for the first release, set max-age for 1 day, update Last-Modified after modification, and max-age time will be normal with the number of modifications. This seems more cumbersome, and you have to record the number of modifications. You can also predict the next possible modification time and use Expires to specify the approximate time to expire

PHP code

Copy code The code is as follows:

 
//header('Cache-Control: max-age=86400');//Cache for one day
header('Expires: Mon, 29 Jan 2007 08:56:01 GMT');/ /Specify expiration time
header('Last-Modified: '.gmdate('D, d M Y 01:01:01',$time).'GMT');//Greenwich time, $time is the time when the file is added Timestamp 

2 Frequently changed pages

are similar to bbs and forum programs. This kind of page updates faster. The main function of the cache is to prevent users from frequently refreshing the list, causing the server to The burden on the database must not only ensure the timeliness of updates, but also ensure that the cache can be utilized

This is generally controlled by Cache-Control, and max-age can be flexibly controlled according to the frequency of postings in the forum.

PHP code

Copy code The code is as follows:

 
header('Cache-Control: max-age =60');//Cache for one minute
header('Last-Modified: '.gmdate('D, d M Y 01:01:01',$time).'GMT');//Greenwich time, $time is the last updated timestamp of the post 

Five extra

1 The difference between refresh, go to, force refresh

Browser There are refresh and go buttons on the browser. Some browsers support using ctrl+F5 to force refresh the page. What is the difference between them?

Go to: When the user clicks on the link, it will go to it. It completely uses the caching mechanism. If there is Last-Modified, it will not communicate with the server. You can use the packet capture tool to see that the sent byte is 0byte. If the cache expires , then it will perform the F5 refresh action.

Refresh (F5): This refresh is also determined based on whether the cache has Last-Modified. If so, it will be transferred to 304 or 1024 (php). If there is no last update time, then go to the server to read it and return the true value. Document size

Forced refresh: completely abandon the cache mechanism, go to the server to read the latest document, and send the header to the server as follows

XML/HTML code

Copy code The code is as follows:

Cache-Control: no-cache

2 Debugging Tools

View browser and A better tool for server interaction is httpwatch pro, the current version 4.1, supports ie7

There are other proxy packet capture tools that can analyze, http debugging. I haven’t used it before, but there is also a tcp packet capture tool, the network that comes with 2000. There are also tcp packet capture tools. The network monitor that comes with 2000 is not specifically for http, but it is difficult to use.

PHP uses HTTP caching protocol principle analysis and application analysis in Apache environment_PHP tutorial