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While many documents delivered over HTTP are already fairly heavily compressed, including images (so long as they are not raw TIFF or BMP) and file formats like PDF (at the option of the document author), web pages themselves are written in verbose SGML dialects (see 10) that can consume much less bandwidth if subjected to generic textual compression. Similarly, CSS and JavaScript files also contain very stereotyped patterns of punctuation and repeated variable names, which is very amenable to compression. Web clients can make servers aware that they accept compressed documents by listing the formats they support in a request header, as in this example: Accept-Encoding: gzip For some reason, many sites seem to not offer compression unless the User-Agent: header specifies something they recognize. Thus, to convince Google to compress its Google News page, you have to use urllib2 something like this: >>> request = urllib2.Request('') >>> request.add_header('Accept-Encoding', 'gzip') >>> request.add_header('User-Agent', 'Mozilla/5.0') >>> info = -------------------------------------------------GET / HTTP/1.1 Host: User-Agent: Mozilla/5.0 Connection: close Accept-Encoding: gzip -------------------- Response -------------------HTTP/1.1 200 OK Content-Type: text/html; charset=UTF-8 ... Content-Encoding: gzip ... Remember that web servers do not have to perform compression, and that many will ignore your Accept-Encoding: header. Therefore, you should always check the content encoding of the response, and perform decompression only when the server declares that it is necessary: >>> info.headers['Content-Encoding'] == 'gzip' True >>> import gzip, StringIO >>> gzip.GzipFile(fileobj=StringIO.StringIO( '<!DOCTYPE HTML ...<html>...</html>' As you can see, Python does not let us pass the file-like info response object directly to the GzipFile class because, alas, it lacks a tell() method. In other words, it is not quite file-like enough. Here, we can perform the quick work-around of reading the whole compressed file into memory and then wrapping it in a StringIO object that does support tell().
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Many elements of a typical web site design are repeated on every page you visit, and your browsing would slow to a crawl if every image and decoration had to be downloaded separately for every page you viewed. Well-configured web servers therefore add headers to every HTTP response that allow browsers, as well as any proxy caches between the browser and the server, to continue using a copy of a downloaded resource for some period of time until it expires. You might think that adding a simple expiration date to each resource that could be cached and redisplayed would have been a sufficient innovation. However, given the real-world behaviors of servers, caches, and browsers, it was prudent for the HTTP specification to detail a much more complicated scheme involving several interacting headers. Several pages are expended, for example, on the specific question of how to determine how old a cached copy of a page is. I refer you to RFC 2616 for the real details, but I will cover a few of the most common cases here. There are two basic mechanisms by which servers can support client caching. In the first approach, an HTTP response includes an Expires: header that formats a date and time using the same format as the standard Date: header: Expires: Sun, 21 Jan 2010 17:06:12 GMT However, this requires the client to check its clock and many computers run clocks that are far ahead of or behind the real current date and time. This brings us to a second, more modern alternative, the Cache-Control header, that depends only on the client being able to correctly count seconds forward from the present. For example, to allow an image or page to be cached for an hour but then insist that it be refetched once the hour is up, a cache control header could be supplied like this: Cache-Control: max-age=3600, must-revalidate When the time comes to validate a cached resource, HTTP offers a very nice shortcut: the client can ask the server to retransmit the resource only if a new version has indeed been released. There are two fields that the client can supply. Either content type is sufficient to convince most servers to answer with only an HTTP header, but no content type or body, if the cached resource is still current. One possibility is to send back the value that the Last-modified: header had in the HTTP response that first requested the item: If-Modified-Since: Sun, 21 Jan 2010 14:06:12 GMT Alternatively, if the server tagged the resource version with a hash or version identifier in an Etag: header either approach will work, so long as the value always changes between versions of the resource then the client can send that value back: Etag: BFDS2Cpq/BM6w Note that all of this depends on getting some level of cooperation from the server. If a web server fails to provide any caching guidelines and also does not supply either a Last-modified: or Etag: header for a particular resource, then clients have no choice but to fetch the resource every time it needs to be displayed to a user. Caching is such a powerful technology that many web sites go ahead and put HTTP caches like Squid or Varnish in front of their server farms, so that frequent requests for the most popular parts of their site can be answered without loading down the main servers. Deploying caches geographically can also save bandwidth. In a celebrated question-and-answer session with the readers of Reddit about The Onion s then-recent migration to Django, the site maintainers who use a content delivery network (CDN) to transparently serve local caches of The Onion s web site all over the world indicated that they were able to reduce their server load by two-thirds by asking the CDN to cache 404 errors! You can read
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