The point in question is why are you doing this on a new application? The sort of caching you describe is something you put in place after your site starts seeing a lot of traffic, and running queries for every request start to slow things down to the point where optimizing them doesn't help.

If you're designing your app well and running it on a solid stack, that should be something you need to worry about in year 3, after you're being TechCrunched on a regular basis. In the Rails/Django/AppEngine world, it seems to be the case that you need to resort to that level of caching just to see regular, day to day, 100 request/second traffic.

It raises red flags to see that.

  from google.appengine.api import memcache
  from google.appengine.ext import db

  class User(db.Model):
    email = db.StringProperty()
    password = db.StringProperty()
    sessionKey = db.StringProperty()

    def userKey(self):
      return "User.session="+self.sessionKey

    def put(self):
      super(User, self).put()
      memcache.delete(self.userKey())
      memcache.add(self.userKey(), self, 600)

    def getCurrent(self, request):
      user = memcache.get(self.userKey())
      if user is None:
        user = User.all().filter('sessionKey = ', request.header.cookie.get('user'))
        memcache.add(self.userKey(), user, 600)
      return user

The thing is, if you turn it on right away you never get a chance to see and fix a bunch of low hanging fruit optimizations that can really help you out. You don't know about them until you start to run up against scaling issues despite all your caching. Once you're there, it's not any fun to try to go back, find and fix those original bottlenecks, so you don't have much option but to start throwing hardware at it.

If, on the other hand you hold off and handle your first few scaling episodes by making your queries and code faster, only adding caching after you're not seeing returns from that other stuff anymore, you'll be able to go a lot farther before you have to start adding hardware.

  @Cached
  public class MyEntity {
      @Id Long id;
      ...
  }

That box is running about 20 sites, some big, some small, usually no more than one of them getting TechCrunched/Redditted at a time.

Here's a writeup of one particularly heavy day in the life of that box:

http://expatsoftware.com/articles/2008/03/6-million-hits-day...