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This article mainly introduces "how the etag in nginx is generated". In the daily operation, I believe that many people have doubts about how the etag in nginx is generated. The editor consulted all kinds of materials and sorted out a simple and easy-to-use method of operation. I hope it will be helpful to answer the doubt of "how is the etag in nginx generated?" Next, please follow the editor to study!

Generation of ETag in nginx

I found some information and source code on the Internet to learn about the calculation method of etag. Represented by python pseudo code, the calculation method is as follows

Etag ='{: X}-{: X} '.format (header.last_modified, header.content_lenth)

Source code: ngx_http_core_modules.c [2]

Etag- > value.len = ngx_sprintf (etag- > value.data, "\"% xT-%xO\ ""

R-> headers_out.last_modified_time

R-> headers_out.content_length_n)

-etag- > value.data

Summary: etag in nginx consists of a combination of Last-Modified and Content-Length in the response header as hexadecimal.

Find a nginx service in my K8s cluster to test it.

$curl-- head 10.97.109.49

HTTP/1.1 200 OK

Server: nginx/1.16.0

Date: Tue, 10 Dec 2019 06:45:24 GMT

Content-Type: text/html

Content-Length: 612

Last-Modified: Tue, 23 Apr 2019 10:18:21 GMT

Connection: keep-alive

ETag: "5cbee66d-264"

Accept-Ranges: bytes

Last-Modified and Content-Length are calculated by etag and calculated using js as follows. The results are consistent.

> new Date (parseInt ('5cbee66dink, 16) * 1000). ToJSON ()

2019-04-23T10:18:21.000Z

> parseInt ('264 years, 16)

612 Last-Modified,ETag and negotiation caching

We know that there are two ways to negotiate caching

Last-Modified/ if-Modified-SinceETag/ If-None-Match

Since ETag is made up of Last-Modified and Content-Length in nginx, it can be regarded as an enhanced version of Last-Modified, so where is the enhancement?

* * Last-Modified is represented by a unix timestamp, which means that it can only act on changes in seconds * *

Then the next question: if the ETag value in the http response header has changed, does it mean that the content of the file must have changed [3]

At this point, the study on "how the etag in nginx is generated" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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