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17.9 Serving Banner Ads17.9.1 ProblemYou want to display banner ads by choosing images on the fly from a set of images. 17.9.2 SolutionUse a script that selects a random row from an image table and sends the image to the client. 17.9.3 DiscussionThe display_image.pl script just shown assumes that the URL contains a parameter that names the image to be sent to the client. Another application might determine which image to display for itself. One popular image-related use for web programming is to serve banner advertisements for display in web pages. A simple way to do this is by means of a script that picks an image at random each time it is invoked. The following Python script, banner.py, shows how to do this, where the "ads" are the flag images in the image table: #! /usr/bin/python
# banner.py - server randomly chosen banner ad from image table
# (sends no response if no image can be found)
import sys
sys.path.insert (0, "/usr/local/apache/lib/python")
import MySQLdb
import Cookbook
conn = Cookbook.connect ( )
try:
query = "SELECT type, data FROM image ORDER BY RAND( ) LIMIT 1"
cursor = conn.cursor ( )
cursor.execute (query)
row = cursor.fetchone ( )
cursor.close ( )
if row is not None:
(type, data) = row
# Send image to client, preceded by Content-Type: and
# Content-Length: headers. The Expires:, Cache-Control:, and
# Pragma: headers help keep browsers from caching the image
# and reusing it for sucessive requests for this script.
print "Content-Type: %s" % type
print "Content-Length: %s" % len (data)
print "Expires: Sat, 01 Jan 2000 00:00:00 GMT"
print "Cache-Control: no-cache"
print "Pragma: no-cache"
print ""
print data
except MySQLdb.Error, e:
pass
conn.close ( )
banner.py sends a few headers in addition to the usual Content-Type: and Content-Length: headers. The extra headers help keep browsers from caching the image. Expires: specifies a date in the past to tell the browser that the image is out of date. The Cache-Control: and Pragma: headers tell the browser not to cache the image. The script sends both headers because some browsers understand one, and some the other. Why suppress caching? Because if you don't, the browser will send a request for banner.py only the first time it sees it in a link. On subsequent requests for the script, the brower will reuse the image, which rather defeats the intent of having each such link resolve to a randomly chosen image. Install the banner.py script in your cgi-bin directory. Then, to place a banner in a web page, use an <img> tag that invokes the script. For example, if the script is installed as /cgi-bin/banner.py, the following page references it to include an image below the introductory paragraph: <!-- bannertest1.html - page with single link to banner-ad script --> <html> <head> <title>Banner Ad Test Page 1</title> </head> <body bgcolor="white"> <p>You should see an image below this paragraph.</p> <img src="/cgi-bin/banner.py" /> </body> </html> If you request this page, it should display an image, and you should see a succession of randomly chosen images each time you reload the page. (I am assuming here that you have loaded several images into the image table by now using the load_image.pl script discussed in Recipe 17.7. Otherwise you may not see any images at all!) If you modify banner.py not to send the cache-related headers, you likely will see the same image each time you reload the page. The cache-control headers suppress caching for links to bannery.py that occur over the course of successive page requests. Another complication occurs if multiple links to the script occur within the same page. The following page illustrates what happens: <!-- bannertest2.html - page with multiple links to banner-ad script --> <html> <head> <title>Banner Ad Test Page 2</title> </head> <body bgcolor="white"> <p>You should see two images below this paragraph, and they probably will be the same.</p> <img src="/cgi-bin/banner.py" /> <img src="/cgi-bin/banner.py" /> <p>You should see two images below this paragraph, and they probably will be different.</p> <img src="/cgi-bin/banner.py?image1" /> <img src="/cgi-bin/banner.py?image2" /> </body> </html> The first two links to banner.py are identical. What you'll probably find when you request this page is that your browser will notice that fact, send only a single request to the web server, and use the image that is returned where both links appear in the page. As a result, the first two images displayed in the page will be identical. The second two links to banner.py show how to solve this problem. The links include some extra fluff at the end of the URLs that make them look different. banner.py doesn't use that information at all, but making the links look different fools the browser into sending two image requests. The result is that the second two images will differ from each other—unless banner.py happens to randomly select the same image both times, of course. 17.9.4 See AlsoIf you run a version of MySQL older than 3.23.2, the ORDER BY RAND( ) clause used in the image-selection query will fail. See Recipe 13.8 for a workaround. 
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