Today when blogs are popular, some web applications need to parse a large number of RSS feeds. How to improve efficiency is a very important issue. This is listed in the Features of MagpieRSS: HTTP Conditional GETs Save bandwidth and speed up download times with intelligent use of Last-Modified and ETag.. The Etag here caught my attention.
What is Etag?
By reading RFC 2616, I got a little impression of Etag:
The ETag response-header field provides the current value of the entity tag for the requested variant......Entity tags are normally "strong validators," but the protocol provides a mechanism to tag an entity tag as "weak ." One can think of a strong validator as one that changes whenever the bits of an entity changes, while a weak value changes whenever the meaning of an entity changes. Alternatively, one can think of a strong validator as part of an identifier for a specific entity, while a weak validator is part of an identifier for a set of semantically equivalent entities.
From the above we can roughly know that Entity tags are essentially a "strong validator", but the HTTP protocol provides a "weak" mechanism by labeling Entity tags (similar to Verification code of content). Although this passage is explained in two ways later, it is still a bit obscure. After reading this passage, I just concluded that the "E" of Etag stands for "Entity".
There is an article mentioned on the Magpie homepage: HTTP Conditional Get for RSS Hackers. After reading it, it becomes much clearer. Let me first talk about the basic principle of HTTP Conditional GETs. It is very simple. That is, when fetching data from the Web server, if the file changes, give me a new file. If the file does not change, just tell the client that there is no change. Just make changes without having to get the files back. This can save a lot of network bandwidth and resources.
Etag and Last-Modified are concepts that are only available from HTTP 1.0 to HTTP 1.1. When we get a file from the Web server, we only need to read the Etag and Last-Modified fields of the HTTP response header. The specific contents of these two fields can be ignored (it may be weird, RFC 2616 There is no specific value definition for Etag), cache these two values locally, and compare the two values the next time you check whether the file has been updated. If there is no change, the server's response code is not HTTP 200 (OK), but 304.
As shown in the picture above. Currently OpenRSS has subscribed to more than 40 feeds, but the response speed is very good. In the process of using Gregarius (Lilina also applied ETag), I found that almost all of the feeds burned by FeedBurnrer used Etag (otherwise it is estimated that The server is going to be paralyzed, Hoho). Let’s test the response of the HTTP header:
$ curl -I http://feeds.feedburner.com/dbanotes HTTP/1.1 200 OK Date: Tue, 25 Oct 2005 11:34:15 GMT Server: Apache <b>Last-Modified: Tue, 25 Oct 2005 04:30:12 GMT</b> <b>ETag: U4q478bDKLqZ8UMMC8A5afZuHug</b> Content-Type: text/xml;charset=utf-8 $ curl -I http://feeds.feedburner.com/dbanotes HTTP/1.1 200 OK Date: Tue, 25 Oct 2005 11:34:21 GMT Server: Apache <b>Last-Modified: Tue, 25 Oct 2005 04:30:12 GMT </b> <b>ETag: U4q478bDKLqZ8UMMC8A5afZuHug</b> Content-Type: text/xml;charset=utf-8
During this period, my blog has not been updated. So Last-Modified and ETag return the same value. In this way, Gregarius does not have to re-parse. The domestic GreatNews supports HTTP Conditional GETs, which is even better Yes, it also supports gzip/deflate encoding. I don’t know about another RSS reading tool POPU (Zhou Botong).
The above are my notes, please correct me if there is any misunderstanding!
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