Etag

1、浏览器第一次请求图片资源文件，服务端计算出etag标识，并通过响应头告诉浏览器，如标红部分

1. Remote Address:
   10.70.128.75:8080
2. Request URL:
   http://wap.cmread.com/r/cover_file/8231/392268231/20140212102919/cover75100.jpg
3. Request Method:
   GET
4. Status Code:
   200 OK
5. Request Headersview parsed
1. GET http://wap.cmread.com/r/cover_file/8231/392268231/20140212102919/cover75100.jpg HTTP/1.1 Host: wap.cmread.com Proxy-Connection: keep-alive Cache-Control: no-cache Accept: image/webp,*/*;q=0.8 Pragma: no-cache User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/34.0.1847.132 Safari/537.36 Referer: http://wap.cmread.com/r/392268231/index.htm?nid=394224146&page=1&purl=%2Fr%2Fp%2Findex.jsp&srsc=1&vt=2&f=100557&pg=11&fraSeq=55&dataSrcId=22377209&sqId=L1 Accept-Encoding: gzip,deflate,sdch Accept-Language: zh-CN,zh;q=0.8,en;q=0.6 Cookie: JSESSIONID=91F9B74DEA37D0B23DC8F5919D3476EE; cookies_user_home_page=/r/p/index.jsp; cookies_user_wap_version=2; userVistorId=90458599284; cookies_user_wap_version=2; userPhone=18857188505
6. Response Headersview parsed
1. HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Accept-Ranges: bytesETag: W/"3817-1392213349000"Last-Modified: Wed, 12 Feb 2014 13:55:49 GMT Content-Type: image/jpeg;charset=UTF-8 Date: Tue, 03 Jun 2014 02:04:33 GMT Content-Length: 3817 Proxy-Connection: Keep-Alive Connection: Keep-Alive Age: 0

2、浏览器第二次请求图片资源文件，浏览器会将之前拿到的etag标识增加到请求头的if-none-match中，

      服务端解析到文件字段，则会取文件当前etag与请求头中的if-none-match内容进行对比，如果一致，则反馈304 not modified

1. Remote Address:
   10.70.128.75:8080
2. Request URL:
   http://wap.cmread.com/r/cover_file/8231/392268231/20140212102919/cover75100.jpg
3. Request Method:
   GET
4. Status Code:
   304 Not Modified
5. Request Headersview source
1. Accept:
   image/webp,*/*;q=0.8
2. Accept-Encoding:
   gzip,deflate,sdch
3. Accept-Language:
   zh-CN,zh;q=0.8,en;q=0.6
4. Cache-Control:
   max-age=0
5. Cookie:
   JSESSIONID=91F9B74DEA37D0B23DC8F5919D3476EE; cookies_user_home_page=/r/p/index.jsp; cookies_user_wap_version=2; userVistorId=90458599284; cookies_user_wap_version=2; userPhone=18857188505
6. Host:
   wap.cmread.com
7. If-Modified-Since:
   Wed, 12 Feb 2014 13:55:49 GMT
8. If-None-Match:
   W/"3817-1392213349000"
9. Pragma:
   no-cache
10. Proxy-Connection:
    keep-alive
11. Referer:
    http://wap.cmread.com/r/392268231/index.htm?nid=394224146&page=1&purl=%2Fr%2Fp%2Findex.jsp&srsc=1&vt=2&f=100557&pg=11&fraSeq=55&dataSrcId=22377209&sqId=L1
12. User-Agent:
    Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/34.0.1847.132 Safari/537.36
6. Response Headersview source
1. Accept-Ranges:
   bytes
2. Age:
   0
3. Connection:
   Keep-Alive
4. Content-Type:
   image/jpeg;charset=UTF-8
5. Date:
   Tue, 03 Jun 2014 02:14:58 GMT
6. ETag:
   W/"3817-1392213349000"
7. Last-Modified:
   Wed, 12 Feb 2014 13:55:49 GMT
8. Proxy-Connection:
   Keep-Alive
9. Server:
   Apache-Coyote/1.1

3、关于集群业务到底是否适合使用etag

      之前网上主流说法是采用集群方式部署的业务不适合用etag来进行缓存，原因是etag会根据inode进行计算，导致不同主机计算出来的etag可能不同。

      针对这个问题，我在内网进行验证发现不同主机（跨机房）对同一图片资源生成的Etag是完全相同的，与网上的说法并不一至。

      分析apache-tomcat-7.0.54源码发现，etag的计算方法如下：

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

apache-tomcat-7.0.54-src/java/org/apache/naming/resources/ResourceAttributes.java       /**      * Get ETag.      *       * @return strong ETag if available, else weak ETag.       */     public String getETag() {         String result = null;         if (attributes != null) {             Attribute attribute = attributes.get(ETAG);             if (attribute != null) {                 try {                     result = attribute.get().toString();                 } catch (NamingException e) {                     // No value for the attribute                 }             }         }         if (result == null) {             if (strongETag != null) {                 // The strong ETag must always be calculated by the resources                 result = strongETag;             } else {                 // The weakETag is contentLength + lastModified                 if (weakETag == null) {                     long contentLength = getContentLength();                     long lastModified = getLastModified();                     if ((contentLength >= 0) || (lastModified >= 0)) {                         weakETag = "W/"" + contentLength + "-" +                                    lastModified + """;                     }                 }                 result = weakETag;             }         }          return result;     }

    可以看出，etag仅根据contentLength和lastModified进行计算，并没有使用到文件inode，所以对于同一个文件只要文件大小 和最后修改时间完全一致，则在不同的主机上计算出来的etag是完全一致的。

来自为知笔记(Wiz)