Pizza Inversion may sound like a cheesy name for a pattern, but it describes a recurring solution to a problem that is of fundamental significance to most software developers I know. I find myself using it much more frequently than anything in the now famous Design Patterns book^[1]. Keep reading and I'm certain you'll soon find yourself agreeing with me.

Intent

Also Known As

Motivation

The consumption of pizza (often in a group setting) is a commonly pursued remedy for addressing this important issue. Pizza often tastes, and smells, better than more traditional "fast foods," and is very rich in dairy-content and carbohydrates. Also, it is usually delivered, so you don't have to wait in line to order your food, or for it to be prepared and brought to your table.

Because efficiency and scheduling are always important concerns, developers frequently need to consume multiple slices of round pizza in a short period of time. The tomato sauce tends to be very hot, however, and the cheese on top of it adds to the problem by forming an insulating layer that locks in the heat. To make matters worse, cheese contains non-negligible levels of fat and, when melting, forms pockets of grease which can reach exceedingly high temperatures (a condition aggravated by fatty toppings like ground beef, pepperoni, and Italian sausage).

When trying to eat pizza quickly, the grease on the cheese and the hot tomato sauce underneath can severely burn the roof of one's mouth. This is not only painful, but may also damage the taste buds in this sensitive region of the mouth, rendering them useless for a short duration.

The naive approach of waiting for the pizza to cool may at first seem reasonable. But the resultant increase in the latency-time between when the pizza arrives and when it has cooled enough for human consumption imposes an unacceptable performance penalty that impedes programmer productivity.

Therefore, it is desirable to find a method for eating the pizza quickly without burning one's mouth. At the same time, one still wants to experience the full splendor that comes from the taste and smell of a savory pizza fresh from the oven.

One such method involves inverting one slice of pizza on top of another slice:

By inverting one of the pizza slices as shown above, the resulting design structure forms a layered hierarchy in which the crust encapsulates the cheese, sauce, and toppings from the top and bottom of the mouth. This insulates the mouth from the hot cheese and sauce while permitting the pizza to be consumed at approximately twice the usual rate.

Applicability

• you have multiple slices of hot, tasty pizza that you wish to consume;
• you want to eat the pizza quickly and cleanly, without burning the roof of your mouth; and
• you are in a very informal social setting.

Structure

Participants

• PizzaSlice
  -- objects of our desire that need to be quickly consumed

• Crust
  -- forms the flat, bread-based layer at the bottom of a PizzaSlice

• Sauce
  -- hot tomato-based liquid that sits atop the Crust

• Cheese
  -- melted mozzarella dairy-product resting above the sauce

• Toppings
  -- food items sprinkled over Cheese or Sauce to add flavor and texture

• Consumer
  -- hungry individual in a hurry

• Hands
  -- dexterous opposable appendages used by Consumer to manipulate PizzaSlices

• Mouth
  -- oral orifice of the consumer into which the pizza must enter before consumption

Collaborations

• Crust, Sauce, Cheese, and Toppings are respectively layered in bottom-up order to form PizzaSlices.

• Cheese layers of the two PizzaSlices are juxtaposed together with symmetric orientations.

• Hands take the Crust of one PizzaSlice and physically invert it by folding its Cheese-side over the top of the Cheese-side of other PizzaSlice, until the former is fully inverted, with each slice forming an approximate mirror-image of the other along the Cheese plane.

• Hands bring both resulting PizzaSlices simultaneously toward Mouth.

• Upon entry of PizzaSlices into Mouth, Mouth quickly chews PizzaSlices, then swallows them.

Consequences

• Consumed portions of PizzaSlices have Crust on the top and bottom, forming an effective layer of insulation that encapsulates the hot Cheese, Sauce, and Toppings from the temperature-sensitive roof of the Mouth.

• Through-put is increased by twice the normal rate because PizzaSlices enter the Mouth two slices at a time instead of one.

• Cheese, Sauce, and Toppings are each predominantly confined to their own domains in the resulting layered hierarchy. This "separation of concerns" forms a Facade^[1] which hides internal state information between the two Crusts, creating an Adaptor^[1] between the Mouth, and the surface of the PizzaSlices

• Latency time is reduced because Hands do not wait for PizzaSlices to cool before bringing them toward Mouth to access the PizzaSlice surface.

• Pizza with a particularly thick crust, or stuffed pizza (Chicago style, or otherwise) results in a very tall/deep hierarchy which may prove unwieldy for consumption.

• Excessive use of this pattern may cause "object bloat" for the Consumer.

• Trades off polymorphism in favor of endomorphism.

• The resultant intake of high levels of cholesterol may result in blockage of the arteries and cause a health-risk for the Consumer, necessitating the use of an EmergencyCardiacCare object.

• The emphasis on efficiency causes the Consumer to miss out on some (but not all) of the visual and olfactory experience of pizza eating: Rather than taking in the sight and full aroma of the cheese and the colorful array of toppings, all you see is crust. The pizza still tastes great, but it's less thrilling.

Implementation

1. Data Compression and Loss: One needs to be extra careful when using only a single slice and/or when the pizza has extra cheese because then excess cheese can easily ooze out the side and burn your mouth. In this case, use your Hands to press the pieces (or halves) together to and detach the excess Cheese, Toppings, and Sauce, before inserting into Mouth.

2. Singleton Slice: If you have a single slice of pizza you can envision an imaginary line lengthwise down the center of the slice that divides it into two symmetrical halves. Fold one half of the slice over (and on top of) the other half. Another variation that works with a single slice (assuming you have a round pizza) is to fold the pointed end of the slice towards the crust end. You don't get full slice coverage this way but some have expressed a preference for this particular implementation.

3. Olfactory Method: One of the desirable aspects of eating pizza isn't just the taste, but also the smell of the steaming hot cheeses, toppings, and fresh-baked crust. Folding one slice over another slice lessens the effect of this pleasing olfactory sensation. One commonly employed compromise is to take a brief moment at the very beginning to fully enjoy the sight and smell of the pizza before commencing with its consumption. Spend a good half-minute or so observing and inhaling the full sight and aroma of the tomatoes, mozzarella, fresh basil, garlic, onion and other spices, and all of the toppings. Once the visual and olfactory organs have been used to employ Observer^[1] and Reflection^[2] for the non-oral aspects of your pizza-eating experience, begin folding and then consuming the PizzaSlices promptly thereafter. This imposes a small but tolerable performance penalty due to increased latency-time prior to oral access.

4. Multi-Slicing: A number of adjacent and attached slices may be folded over onto an equivalent number of adjacent slices. Hence one could fold two pieces onto two pieces, three pieces onto three pieces, and so on. For "individual size" pizzas, simply fold the entire half of the pizza over onto the other half.

5. Mix-In Slices: This works particularly well for collaborations involving large numbers of consuming objects. Rather than trying to satisfy each Consumer's favorite combination of Toppings, simply order relatively few Toppings on each pizza and let each Consumer use PizzaSlices from different pizzas to create their own custom configuration of Toppings.

Known Uses

• Used frequently at Domino's, Little Ceasar's, Pizza Hut, and Papa John's Pizza
• Various scenes from the movie "Mystic Pizza"
• The opening sequence of the movie "Saturday Night Fever"
• A "calzone" is basically a pre-fabricated instance of Pizza Inversion.

The historical origins influencing this method of pizza consumption may be traced back to the "Pita" and the Earl of Sandwich.

Related Patterns

• Adapter^[1] is used to create a Facade^[1] that protects the Mouth from the surface of the PizzaSlices

• Hands play the role of Mediator^[1] between the PizzaSlices and the Mouth.

• Observer^[1] and Reflection^[2] participate in the Olfactory Method implementation of this pattern.

• Hans Rohnert^[3] describes the relationship between Pizza Inversion, and the Layers^[2] and Whole-Part^[2] patterns:

  [Pizza Inversion is related to] the Relaxed Layered System variant of Layers because the respective toppings and especially the cheeses from the two slices cannot maintain a clear separation between the layers. Rather, there is direct interaction between different components in both layers. We even can safely assume that objects will migrate, mostly from the top to the lower layer.

  In a way Pizza Inversion seems related to Whole-Part as well .... Intuitively speaking two slices layered on top of each other are the two constituent parts. But Whole-Part explicitly states that direct access to the parts is not possible.

• A whole pizza is a Composite^[1] of PizzaSlices.

• Iterator^[1] may be used to apply Pizza Inversion for an entire pizza, or a collection of pizzas.

• Abstract Factory^[1] and Builder^[1] are commonly used by pizza providers to create the pizza, and Decorator^[1] is commonly used to apply the Toppings.

• A Visitor^[1] is frequently employed to deliver the pizza.

• Not related: Flyweight^[1]

Acknowledgements

• Robert Martin encouraged me to document and disseminate Pizza Inversion after I described it to him during lunch at Tonelli's in the summer of 1995.

• Several subscribers to the patterns-discussion mailing list provided me with many suggestions for improvement: Eleanor J. Barnes, John Hartley, Richard Gabriel, Kent Beck, Thomas Gerth, Eric Pearl, Linda Rising, Jens Coldeway, Hans Wegener, and Hans Rohnert.

• Participants in the PLoP'97 example writer's workshop for this pattern made numerous useful comments: Brian Foote, Dirk Riehle, Linda Rising, Doug Schmidt, and Bobby Woolf.

References