1. UserDAO

public class UserDAO(){
  static cache;
  static user_model;

  public static getInstance(CacheInterface cache, UserInterface user_model){
    this.cache = cache;
    this.user_model = user_model;
  }

  public static get(int Id){
    UserInterface from_cache = this.cache.get(this.cache.generateKeyHash(Id));
    if(from_cache){
      return from _cache;
    }else{
      UserInterface user = this.user_model.find(Id);
      return user;
    }
  }
}

2. UserInterface

public abstract class UserInterface{
  public static find(int Id);
  public static change_pass(String old_pass, String new_pass);
}

public class MongoUser extends UserInterface{
  public static find(Int Id){
    //I am a static get method where db instances are returned
    MongoUser user this.collection.findOne(Id);
    return User;
  }

  public function change_pass(String old_pass,String new_pass){
    //I am an instance method
    if(old_pass == this.pass){
      this.pass = new_pass;
      this.save();
    }
  }
}

//We could make other user interfaces, though

public class FileUser extends UserInterface{
  // same methods here, but hit filesys!
}

3. Cache

public abstract class Cache{
  public static function getKey();
  public static function setKey();
}

public class Memcache extends Cache{
  //do those two methods from memcached
}

public class FilesysCache extends Cache{
  //same thing but from filesys!
}

4. Controller

public class UserController{
  
  UserDAO DAO;

  public function Controller(){
    //different DAOs here can access totally different persistency or cache layers and the
    //whole system will still work!  Also dummy content for webservices, etc.  We could swap
    //the args to getInstance for any classes that inherit from cache or UserInterface
    
    this.DAO = UserDAO.getInstance(Memcache,MongoUser);
  }

  public function getRoot(int Id){
    //Call DAO methods instead of model directly!
    this.DAO.getUser(Id);
  }
}