Several conditions need to be met for eTAG generation

  1. When the file doesn’t change,etagThe value stays the same. So you can’t just use itinode
  2. Easy to calculate, not special CPU consumption. It looks likehashNot particularly suitable
  3. Easy horizontal expansion, multiplenodeGenerated on theetagValue is consistent. It looks likeinodeRules out

For details on how to generate etags in servers, see HTTP: Generating etag Header

So how is eTag generated in Nginx?

ETag generation in nginx

I found some materials and source code on the Internet to understand the calculation method of ETAG. The calculation is shown in Python pseudocode as follows

etag = '{:x}-{:x}'.format(header.last_modified, header.content_lenth)Copy the code

Source: ngx_http_core_modules. C

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;Copy the code

Summary: The etag in nginx is a hexadecimal combination of last-Modified and Content-Length representations of the response header.

Find an Nginx service in my K8S cluster and 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: bytesCopy the code

Last-modified and Content-Length are calculated by eTAG and js as follows, and the results are consistent

> new Date(parseInt('5cbee66d', 16) * 1000). ToJSON () "2019-04-23t10:18:21.000z" > parseInt('264', 16Copy the code

Last-modified, ETag and negotiated cache

We know that there are two ways to negotiate a cache

  • Last-Modified/if-Modified-Since
  • ETag/If-None-Match

Since ETag in Nginx is made up of last-Modified and content-Length, it is a last-modified version. Where is the last-modified version?

Last-modified is represented by a Unix timestamp, which means it can only work on second-level changes

Next question: If the ETag value in the HTTP response header changes, does that mean that the file content must have changed

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