Difficulty level: ♦♦♦♦♢
-❝ FIXME ❞
— FIXME +❝ A ruffled mind makes a restless pillow. ❞
— Charlotte Brontë
FIXME +
The most important thing to understand about any type of web service is that network access is incredibly expensive. I don’t mean “dollars and cents” expensive (although bandwidth ain’t free). I mean that it takes an extraordinary long time to open a connection, send a request, and retrieve a response from a remote server. Even the fastest broadband connection is orders of magnitude slower than your local network, which in turn is orders of magnitude slower than you local disk. + +
HTTP is designed with caching in mind. There is an entire class of devices (called “caching proxies”) whose only job is to sit between you and the rest of the world and minimize network access. Your company or ISP almost certainly maintains caching proxies, even if you’re unaware of them. They work because caching built into the HTTP protocol. + +
Here’s a concrete example of how caching works. You visit diveintomark.org in your browser. That page includes a background image, wearehugh.com/m.jpg. When your browser downloads that image, the server includes the following HTTP headers:
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HTTP/1.1 200 OK
+Date: Sun, 31 May 2009 17:14:04 GMT
+Server: Apache
+Last-Modified: Fri, 22 Aug 2008 04:28:16 GMT
+ETag: "3075-ddc8d800"
+Accept-Ranges: bytes
+Content-Length: 12405
+Cache-Control: max-age=31536000, public
+Expires: Mon, 31 May 2010 17:14:04 GMT
+Connection: close
+Content-Type: image/jpegThe Cache-Control and Expires headers tell your browser (and any caching proxies between you and the server) that this image can be cached for up to a year. A year! And if, in the next year, you visit another page which also includes a link to this image, your browser will load the image from its cache without generating any network activity whatsoever.
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But wait, it gets better. Let’s say your browser purges the image from your local cache for some reason. Maybe it ran out of disk space; maybe you manually cleared the cache. Whatever. But the HTTP headers said that this data could be cached by public caching proxies (by virtue of that public keyword in the Cache-Control header). Caching proxies are designed to have tons of storage space, probably far more than your local browser has allocated.
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If your company or ISP maintain a caching proxy, the proxy may still have the image cached. When you visit diveintomark.org again, your browser will look in its local cache for the image, but it won’t find it, so it will make a network request to try to download it from the remote server. But if the caching proxy still has a copy of the image, it will intercept that request and serve the image from its cache. That means that your request will never reach the remote server; in fact, it will never leave your company’s network. That makes for a faster download (fewer network hops) and saves your company money (less data being downloaded from the outside world).
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HTTP caching only works when everybody does their part. On one side, servers need to send the correct headers in their response. On the other side, clients need to understand and respect those headers before they request the same data twice. The proxies in the middle are not a panacea; they can only be as smart as the servers and clients allow them to be. + +
Python’s HTTP libraries do not support caching, but httplib2 does.
Some data changes all the time. The home page of CNN.com is constantly updating every few minutes. On the other hand, the home page of Google.com may not change for days or even weeks (and then only when they put up a special holiday logo or advertise a new service). Web services are no different. The server knows when the data you’re requesting last changed, and HTTP provides a way for the server to include this last-modified date each time you request the data. +
Some data never changes, while other data changes all the time. In between, there is a vast field of data that might have changed, but hasn’t. CNN.com’s feed is updated every few minutes, but my weblog’s feed may not change for days or weeks at a time. In the latter case, I don’t want to tell clients to cache my feed for weeks at a time, because then when I do actually post something, people may not read it for weeks (because they’re respecting my cache headers which said “don’t bother checking this feed for weeks”). On the other hand, I don’t want clients downloading my entire feed once an hour if it hasn’t changed! -
If you ask for the same data a second (or third or fourth) time, you can tell the server the last-modified date that you got last time. You send an If-Modified-Since header with your request, with the date you got back from the server last time. If the data hasn’t changed since then, the server sends back a special HTTP status code 304, which means “this data hasn’t changed since the last time you asked for it.” Why is this an improvement? Because when the server sends a 304, it doesn’t re-send the data. All you get is the status code. So you don’t need to download the same data over and over again if it hasn’t changed; the server assumes you have the data cached locally.
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HTTP has a solution to this, too. When you request data for the first time, the server can send back a Last-Modified header. This is exactly what it sounds like: the date that the data was changed. That background image referenced from diveintomark.org included a Last-Modified header.
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All modern web browsers support last-modified date checking. If you’ve ever visited a page, re-visited the same page a day later and found that it hadn’t changed, and wondered why it loaded so quickly the second time — this could be why. Your web browser cached the contents of the page locally the first time, and when you visited the second time, your browser automatically sent the last-modified date it got from the server the first time. The server simply says 304: Not Modified, so your browser knows to load the page from its cache. Web services work the same way.
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HTTP/1.1 200 OK
+Date: Sun, 31 May 2009 17:14:04 GMT
+Server: Apache
+Last-Modified: Fri, 22 Aug 2008 04:28:16 GMT
+ETag: "3075-ddc8d800"
+Accept-Ranges: bytes
+Content-Length: 12405
+Cache-Control: max-age=31536000, public
+Expires: Mon, 31 May 2010 17:14:04 GMT
+Connection: close
+Content-Type: image/jpeg
+Python’s URL libraries have no built-in support for last-modified date checking, but httplib2 does.
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When you request the same data a second (or third or fourth) time, you can send an If-Modified-Since header with your request, with the date you got back from the server last time. If the data hasn’t changed since then, the server sends back a special HTTP 304 status code, which means “this data hasn’t changed since the last time you asked for it.” You can test this on the command line, using curl:
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+you@localhost:~$ curl -I -H "If-Modified-Since: Fri, 22 Aug 2008 04:28:16 GMT" http://wearehugh.com/m.jpg +HTTP/1.1 304 Not Modified +Date: Sun, 31 May 2009 18:04:39 GMT +Server: Apache +Connection: close +ETag: "3075-ddc8d800" +Expires: Mon, 31 May 2010 18:04:39 GMT +Cache-Control: max-age=31536000, public-
ETags are an alternate way to accomplish the same thing as the last-modified date checking. With Etags, the server sends a hash code in an ETag header along with the data you requested. (Exactly how this hash is determined is entirely up to the server. The only requirement is that it changes when the data changes.) The second time you request the same data, you include the ETag hash in an If-None-Match header of your request. If the data hasn’t changed, the server will send you back a 304 status code. As with the last-modified date checking, the server sends back only the 304 status code; it doesn’t send you the same data a second time. By including the ETag hash in your second request, you’re telling the server that there’s no need to re-send the same data if it still matches this hash, since you still have the data from the last time.
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Why is this an improvement? Because when the server sends a 304, it doesn’t re-send the data. All you get is the status code. Even after your cached copy has expired, last-modified checking ensures that you won’t download the same data twice if it hasn’t changed. (As an extra bonus, this 304 response also includes caching headers. Proxies will keep a copy of data even after it officially “expires,” in the hopes that the data hasn’t really changed and the next request responds with a 304 status code and updated cache information.)
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Python’s URL libraries have no built-in support for ETags, but httplib2 does.
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Python’s HTTP libraries do not support last-modified date checking, but httplib2 does.
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ETags are an alternate way to accomplish the same thing as the last-modified checking. With Etags, the server sends a hash code in an ETag header along with the data you requested. (Exactly how this hash is determined is entirely up to the server. The only requirement is that it changes when the data changes.) That background image referenced from diveintomark.org had an ETag header.
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HTTP/1.1 200 OK
+Date: Sun, 31 May 2009 17:14:04 GMT
+Server: Apache
+Last-Modified: Fri, 22 Aug 2008 04:28:16 GMT
+ETag: "3075-ddc8d800"
+Accept-Ranges: bytes
+Content-Length: 12405
+Cache-Control: max-age=31536000, public
+Expires: Mon, 31 May 2010 17:14:04 GMT
+Connection: close
+Content-Type: image/jpeg
+If-None-Match header of your request. If the data hasn’t changed, the server will send you back a 304 status code. As with the last-modified date checking, the server sends back only the 304 status code; it doesn’t send you the same data a second time. By including the ETag hash in your second request, you’re telling the server that there’s no need to re-send the same data if it still matches this hash, since you still have the data from the last time.
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+Python’s HTTP libraries do not support ETags, but httplib2 does.
When you talk about HTTP web services, you’re almost always talking about moving text-based data back and forth over the wire. Maybe it’s XML; maybe it’s JSON. Regardless of the format, text compresses well. When you request a resource over HTTP, you can ask the server to send it in compressed format. You include the Accept-encoding header in your request, and if the server supports compression, it will send you back compressed data and mark it with a Content-encoding header.
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When you talk about HTTP web services, you’re almost always talking about moving text-based data back and forth over the wire. Maybe it’s XML, maybe it’s JSON, maybe it’s just plain text. Regardless of the format, text compresses well. The example feed in the XML chapter is 3070 bytes uncompressed, but would be 941 bytes after gzip compression. That’s just 30% of the original size! -
HTTP supports several compression algorithms. The two most common types are gzip and deflate. +
HTTP supports several compression algorithms. The two most common types are gzip and deflate. When you request a resource over HTTP, you can ask the server to send it in compressed format. You include an Accept-encoding header in your request, and if the server supports compression, it will send you back compressed data with a Content-encoding header that tells you which compression algorithm it used. Then it’s up to you to decompress the data.
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Python’s URL libraries have no built-in support for compression, but httplib2 does.
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Python’s HTTP libraries do not support compression, but httplib2 does.
Cool URIs don’t change, but many URIs are seriously uncool. Web sites get reorganized, pages move to new addresses. Even web services can reorganize. A syndicated feed at http://example.com/index.xml might be moved to http://example.com/xml/atom.xml. Or an entire domain might move, as an organization expands and reorganizes; http://www.example.com/index.xml becomes http://server-farm-1.example.com/index.xml.
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Cool URIs don’t change, but many URIs are seriously uncool. Web sites get reorganized, pages move to new addresses. Even web services can reorganize. A syndicated feed at http://example.com/index.xml might be moved to http://example.com/xml/atom.xml. Or an entire domain might move, as an organization expands and reorganizes; http://www.example.com/index.xml becomes http://server-farm-1.example.com/index.xml.
Every time you request any kind of resource from an HTTP server, the server includes a status code in its response. Status code 200 means “everything’s normal, here’s the page you asked for”. Status code 404 means “page not found”. (You’ve probably seen 404 errors while browsing the web.) Status codes in the 300’s indicate some form of redirection.
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HTTP has several different ways of signifying that a resource has moved. The two most common techiques are status codes 302 and 301. Status code 302 is a temporary redirect; it means “oops, that got moved over here temporarily” (and then gives the temporary address in a Location: header). Status code 301 is a permanent redirect; it means “oops, that got moved permanently” (and then gives the new address in a Location: header). If you get a 302 status code and a new address, the HTTP specification says you should use the new address to get what you asked for, but the next time you want to access the same resource, you should retry the old address. But if you get a 301 status code and a new address, you’re supposed to use the new address from then on.
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HTTP has several different ways of signifying that a resource has moved. The two most common techiques are status codes 302 and 301. Status code 302 is a temporary redirect; it means “oops, that got moved over here temporarily” (and then gives the temporary address in a Location header). Status code 301 is a permanent redirect; it means “oops, that got moved permanently” (and then gives the new address in a Location header). If you get a 302 status code and a new address, the HTTP specification says you should use the new address to get what you asked for, but the next time you want to access the same resource, you should retry the old address. But if you get a 301 status code and a new address, you’re supposed to use the new address from then on.
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The urllib module will automatically “follow” redirects when it receives the appropriate status code from the HTTP server, but unfortunately, it doesn’t tell you that it did so. You’ll end up getting data you asked for, but you’ll never know that the underlying library “helpfully” followed a redirect for you. So you’ll continue pounding away at the old address, and each time you’ll get redirected to the new address. That’s two round trips instead of one, which is bad for the service operator and bad for you.
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The urllib.request module automatically “follow” redirects when it receives the appropriate status code from the HTTP server, but it doesn’t tell you that it did so. You’ll end up getting data you asked for, but you’ll never know that the underlying library “helpfully” followed a redirect for you. So you’ll continue pounding away at the old address, and each time you’ll get redirected to the new address, and each time the urllib.request module will “helpfully” follow the redirect. In other words, it treats permanent redirects the same as temporary redirects. That means two round trips instead of one, which is bad for the server and bad for you.
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httplib2 handles permanent redirects for you. Not only will it tell you that a permanent redirect occurred, it will keep track of them locally and automatically rewrite redirected URLs before requesting them.
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httplib2 handles permanent redirects for you. Not only will it tell you that a permanent redirect occurred, it will keep track of them locally and automatically rewrite redirected URLs before requesting them.
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Let’s say you want to download a resource over HTTP, such as an Atom feed. Being a feed, you’re not just going to download it once; you’re going to download it over and over again. Let’s do it the quick-and-dirty way first, and then see how you can do better. +
Let’s say you want to download a resource over HTTP, such as an Atom feed. Being a feed, you’re not just going to download it once; you’re going to download it over and over again. (Most feed readers will check for changes once an hour.) Let’s do it the quick-and-dirty way first, and then see how you can do better.
>>> import urllib.request >>> data = urllib.request.urlopen('http://diveintopython3.org/examples/feed.xml').read() ① @@ -122,53 +188,73 @@ mark{display:inline}
urllib.request module has a handy urlopen() function that takes the address of the page you want, and returns a file-like object that you can just read() from to get the full contents of the page. It just can’t get any easier.
-So what’s wrong with this? Well, for a quick one-off during testing or development, there’s nothing wrong with it. I do it all the time. I wanted the contents of the feed, and I got the contents of the feed. The same technique works for any web page. But once you start thinking in terms of a web service that you want to access on a regular basis — and remember, you said you were planning on retrieving this syndicated feed once an hour — then you’re being inefficient, and you’re being rude. +
So what’s wrong with this? For a quick one-off during testing or development, there’s nothing wrong with it. I do it all the time. I wanted the contents of the feed, and I got the contents of the feed. The same technique works for any web page. But once you start thinking in terms of a web service that you want to access on a regular basis (e.g. requesting this feed once an hour), then you’re being inefficient, and you’re being rude.
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