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e.g. "templates" or "datastore"

# Using Memcache

Memcache is a high-performance, distributed memory object caching system, primarily intended for fast access to cached results of datastore queries.

- The Memcache Pattern
- Modifying guestbook.py to use Memcache

## The Memcache Pattern

Memcache is typically used with the following pattern:

- The application receives a query from the user or the application.
- The application checks whether the data needed to satisfy that query is in memcache.

- If the data is in memcache, the application uses that data.
- If the data is not in memcache, the application queries the datastore and stores the results in memcache for future requests.

The pseudocode below represents a typical memcache request:

    def get_data():
        data = memcache.get("key")
        if data is not None:
            return data
        else:
            data = self.query_for_data()
            memcache.add("key", data, 60)
            return data

## Modifying guestbook.py to use Memcache

The guestbook application in the Getting Started Guide queries the datastore on every request. You can modify the Guestbook application to use memcache before resorting to querying the datastore.

First we'll import the memcache module and create the method that checks memcache before running a query.

    from google.appengine.api import memcache

    def get_greetings(self):
        """get_greetings()

        Checks the cache to see if there are cached greetings.
        If not, call render_greetings and set the cache

        Returns:
          A string of HTML containing greetings.
        """
        greetings = memcache.get("greetings")
        if greetings is not None:
            return greetings
        else:
            greetings = self.render_greetings()
            if not memcache.add("greetings", greetings, 10):
                logging.error("Memcache set failed.")
            return greetings

Next we'll separate out the querying and creation of the HTML for the page. When we don't hit the cache, we'll call this method to query the datastore and build the HTML string that we'll store in memcache.

    def render_greetings(self):
        """render_greetings()

        Queries the database for greetings, iterate through the
        results and create the HTML.

        Returns:
          A string of HTML containing greetings
        """
        results = db.GqlQuery("SELECT * "
                              "FROM Greeting "
                              "ORDER BY date DESC").fetch(10)
        output = StringIO.StringIO()
        for result in results:
            if result.author:
                output.write("<b>%s</b> wrote:" % result.author.nickname())
            else:
                output.write("An anonymous person wrote:")
            output.write("<blockquote>%s</blockquote>" %
                          cgi.escape(result.content))
        return output.getvalue()

Finally we will update the `MainPage` handler to call the get_greetings() method and display some stats about the number of times the cache was hit or missed.

    import cgi
    import datetime
    import logging
    import StringIO

    from google.appengine.ext import db
    from google.appengine.api import users
    from google.appengine.ext import webapp
    from google.appengine.ext.webapp.util import run_wsgi_app
    from google.appengine.api import memcache

    logging.getLogger().setLevel(logging.DEBUG)

    class Greeting(db.Model):
        author = db.UserProperty()
        content = db.StringProperty(multiline=True)
        date = db.DateTimeProperty(auto_now_add=True)

    class MainPage(webapp.RequestHandler):
        def get(self):
          self.response.out.write("<html><body>")
          greetings = self.get_greetings()
          stats = memcache.get_stats()

          self.response.out.write("<b>Cache Hits:%s</b><br>" % stats['hits'])
          self.response.out.write("<b>Cache Misses:%s</b><br><br>" %
                                  stats['misses'])
          self.response.out.write(greetings)
          self.response.out.write("""
                <form action="/sign" method="post">
                  <div><textarea name="content" rows="3" cols="60"></textarea></div>
                  <div><input type="submit" value="Sign Guestbook"></div>
                </form>
              </body>
            </html>""")

        def get_greetings(self):
            """
            get_greetings()
            Checks the cache to see if there are cached greetings.
            If not, call render_greetings and set the cache

            Returns:
               A string of HTML containing greetings.
            """
            greetings = memcache.get("greetings")
            if greetings is not None:
                return greetings
            else:
                greetings = self.render_greetings()
                if not memcache.add("greetings", greetings, 10):
                    logging.error("Memcache set failed.")
                return greetings

        def render_greetings(self):
            """
            render_greetings()
            Queries the database for greetings, iterate through the
            results and create the HTML.

            Returns:
               A string of HTML containing greetings
            """
            results = db.GqlQuery("SELECT * "
                                  "FROM Greeting "
                                  "ORDER BY date DESC").fetch(10)
            output = StringIO.StringIO()
            for result in results:
                if result.author:
                    output.write("<b>%s</b> wrote:" % result.author.nickname())
                else:
                    output.write("An anonymous person wrote:")
                output.write("<blockquote>%s</blockquote>" %
                             cgi.escape(result.content))
            return output.getvalue()

    class Guestbook(webapp.RequestHandler):
        def post(self):
            greeting = Greeting()

            if users.get_current_user():
                greeting.author = users.get_current_user()

            greeting.content = self.request.get('content')
            greeting.put()
            self.redirect('/')

    application = webapp.WSGIApplication([
        ('/', MainPage),
        ('/sign', Guestbook)
    ], debug=True)

    def main():
        run_wsgi_app(application)

    if __name__ == '__main__':
        main()