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In the process of development, it is found that the API is unstable, and the same input will return HTTP 400Bad Request:Encountered an error (InternalServerError) from host runtime from time to time.

This mistake does not always recur.

After troubleshooting, we finally confirmed that the problem had nothing to do with our code calling API, so we reported a bug to Azure:

When analyzing positioning problems, I can't help but miss the convenience of doing development on ABAP On-Premise-most of the problems can be found by debugging on the ABAP application server.

This article records some technical discussions during the development of SAP CRM Fiori applications by the CRM development team of SAP Chengdu Research Institute in 2016, and the differences in response status codes for HTTP requests.

At that time, we opened the SAP Fiori application with Chrome, and in the network tab of the Chrome developer tool, we observed that some request response codes were HTTP 200, while others were HTTP 304.00.

There are a lot of theoretical differences between HTTP 200and HTTP 304on the Internet:

Https://ions/1665082/what-is-the-difference-between-http-status-code-200-cache-vs-status-code-304

In this paper, we start from a practical example to observe the circumstances under which the ABAP server returns the HTTP 200and 304status codes to help you deepen your understanding.

It is discussed in several situations.

The first case: HTTP 200OK

The second case: HTTP 304Not Modified

The third case: HTTP 200 (from Cache)

First, conduct the first round of tests.

Paste the url from the SAP UI5 standard library file into the browser to access:

Https://:7080/sap/bc/ui5_ui5/ui2/ushell/resources/~20160308134900~/sap/fiori/core-min-0.js

Get the HTTP 200status code:

Have you ever thought that the highlighted HTTP response header field, such as last-modified, is populated in which code on the ABAP server?

Flexible use of Jerry article SAP error message debugging seven weapons: so that all error messages can be located and introduced, successfully through debugging, to find the exact location as follows:

The logic of the above code:

(1) in line 9, the server attempts to extract the value of the field named If-Modified-Since from the header field of the HTTP request, because this is the first time I have requested the JavaScript file, and the value of this field should be logically equal to the value of the field named last-modified in the response structure returned from the server after the first request arrived at the server.

In my first round of tests, because this was the first time the file was requested, the HTTP request header did not contain an If-Modified-Since field, so the value parsed by the server was empty, that is, the variable lv_modified_since was empty.

(2) on the ABAP server I used, the time stamp of the last modification of the JavaScript file core-min-0.js is 20160316205045. Therefore, this value is appended to both variables lv_change_time_char and lv_change_time_string.

Line 20 below shows where the HTTP response field cache-control is populated in the screenshot of the previous HTTP 200 status code.

The second case: HTTP 304Not Modified

The url that was opened in the Chrome browser:

Https://:7080/sap/bc/ui5_ui5/ui2/ushell/resources/~20160308134900~/sap/fiori/core-min-0.js

Instead of closing the browser window, press F5 to refresh, and the response code received this time is no longer HTTP 200OK, but HTTP 304Not Modified.

Why is there this difference? After pressing F5, take a closer look at the request header and find that in the second request, the If-Modified-Since field in the HTTP request issued by the browser contains the last-modified field value returned from the server in the first request.

The request refreshed by F5 is sent to the server, and this time the ABAP server successfully parses the value of the request field If-Modified-Since:

Compare the If-Modified-Since timestamp sent by the client with the last modified timestamp of the file on the server (that is, the second judgment condition after the AND in line 26 below), and find that they are equal, so return HTTP 304 Not Modified on line 28.

The third case: HTTP 200 (from Cache)

Turn off the Chrome, open it, and then access the same url. At this point, the Chrome returns the JavaScript file directly from its own cache instead of initiating a request to the ABAP server. Therefore, all ABAP breakpoints on the server are not triggered.

Back to the Azure that Jerry encountered, the HTTP 400Bad request incident encountered by function to create API has not been resolved at the time of this article.

Although Azure's Function Host runtime is open source, it can't be debugged, and I can't do anything with this huge amount of code. Currently, there are as many as 967 open issue on Github.

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