How to temporarily respond to user requests when there is a problem with the origin site_PHP Tutorial

When there is a problem with the origin site, how can the user's request be temporarily responded to?

When there is a problem with the origin site and the requested content cannot be returned to the user, is there any way for the user to still get the requested content? Content, albeit expired content?
Method 1: Implemented by adding ats plug-in:
In order to prevent problems at the origin site, such as a timeout, resulting in data not being returned normally, and error codes such as 502 and 504 being returned, the ats proxy server will return directly Give users 502 and 504, which affects the user experience. ATS can add plug-in functions to achieve this. When ATS receives 502 and 504 returned by the origin site, if there is expired cache data locally in ATS, the expired cache data will be temporarily returned to the user. From the user's perspective, there are 200 successful visits.

Method 2: Use the processing mechanism of ats:
If the origin site cannot return data to ats, such as the origin site service is stopped or the machine is down, then if there is cache in ats at this time TCP_REFRESH_FAIL_HIT/200 will be returned, and it will always be TCP_HIT/200 when accessed again until the cache value of proxy.config.http.cache.max_stale_age is reached. In the future, TCP_REFRESH_FAIL_HIT/502 will be returned for subsequent requests.

The configuration in records.config Item:
proxy.config.http.cache.max_stale_age INT 604800
The meaning of the above configuration item: the maximum time that the old object can respond before the object cannot be cached (default is 604800s per week).

The premise for the above two methods to achieve the expected results is that there is a cache in ats and it has expired
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Test in the case of method 2 above:
The following is the difference between some response header information of the client:
1. The server provides services normally and responds normally (TCP_HIT/200):
Proxy-Connection: keep-alive
* Connection #0 to host 192.168.10.74 left intact
* Closing connection #0

2. Send the same request again under server stop (TCP_REFRESH_FAIL_HIT/200):
* HTTP/1.1 proxy connection set close!
* Closing connection #0

3. Based on the above 2, send multiple same requests again (TCP_HIT/200):
Proxy-Connection: keep-alive
* Connection #0 to host 192.168.10.74 left intact
* Closing connection #0

4. Server stop When the configuration value 604800s arrives, send the request again (TCP_REFRESH_FAIL_HIT/502):
Proxy-Connection: Keep-Alive
>
* Connection #0 to host 192.168.10.74 left intact
* Closing connection #0

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