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The main content of this article is "will the response change when ETag changes?", interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "will the response change when the ETag changes?"

Not necessarily, it is determined by the ETag generation algorithm in the server. Do you know how the ETag in the http response header is generated?

For example, etag in nginx is composed of last_modified and content_length, while last_modified is made up of mtime.

When you edit a file without changing the contents of the file, the mtime also changes, and the etag changes, but the contents of the file remain unchanged.

What is the basis for generating the Last-Modified of static files in http services

The mtime of the file is generally selected to indicate the modification time of the file content.

Nginx is also handled in this way. For source code, please see ngx_http_static_module.c [1].

R-> headers_out.status = NGX_HTTP_OK

R-> headers_out.content_length_n = of.size

R-> headers_out.last_modified_time = of.mtime

Then why use mtime instead of ctime?

What do mtime and ctime refer to in the file system, and what are the differences

In linux

Mtime: modified time refers to the timestamp of the change of file content ctime: change time refers to the timestamp of the change of file attributes, including mtime. On windows, it means creation time.

So ctime will be a little larger than mtime. Use stat to view the file properties as follows

$stat hello.txt

File: 'hello.txt'

Size: 30 Blocks: 8 IO Block: 4096 regular file

Device: fd01h/64769d Inode: 917526 Links: 1

Access: (0644) Uid: (0 / root) Gid: (0 / root)

Access: 2019-12-10 16 purl 15purl 55.253325208 + 0800

Modify: 2019-12-10 16-15-15-52.740653330 + 0800

Change: 2019-12-10 16 purl 15purl 52.742653069 + 0800

Birth:-

When http services choose Last_Modified, they usually choose mtime.

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