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Lesson 8 Flyweight: consequences
ObjectiveUnderstand the Tradeoffs of the Flyweight Pattern

The Flyweight Pattern: Consequences and Trade-offs in Software Design

What are the consequences and tradeoffs of the Flyweight Pattern that a software architect must consider when designing software?
The Flyweight Pattern, a structural design pattern, revolves around the concept of optimizing memory usage by sharing as much data as possible with similar objects. While this pattern can significantly boost the efficiency of systems, especially those dealing with a large number of almost similar objects, it is not without its consequences and trade-offs. A software architect must prudently weigh these considerations when deciding to implement the Flyweight Pattern.

Consequences of the Flyweight Pattern:

  1. Memory Savings:
    • Pros: The most salient advantage of the Flyweight Pattern is its potential for substantial memory savings. By externalizing and sharing state, Flyweight minimizes the memory required for each object, making it invaluable for applications that instantiate numerous objects of a particular class.
    • Cons: The memory savings come at the cost of increased overhead for the shared state management, which might offset the benefits in systems with fewer objects.
  2. Increased Run-time Operations:
    • Pros: Flyweight can lead to more efficient memory use, making applications more scalable.
    • Cons: The pattern can introduce additional run-time costs due to the operations required to manage and look up flyweights. This can potentially slow down access times, especially if the system frequently accesses a vast number of flyweights.
  3. Complexity:
    • Pros: When used judiciously, the Flyweight Pattern can simplify the structure of hierarchies by removing redundant shared data.
    • Cons: The pattern can make the system more intricate, especially for developers unfamiliar with Flyweight. It introduces additional classes and necessitates the management of extrinsic states, adding layers of complexity.

Trade-offs to Consider

  1. Memory vs. Performance: While Flyweight optimizes memory usage, it can introduce overhead in terms of performance. The constant lookups and management of shared states can slow down operations, especially in scenarios where performance is more critical than memory optimization.
  2. Design Complexity vs. Scalability: Implementing the Flyweight Pattern can make the design more convoluted due to the separation of intrinsic and extrinsic states. However, this complexity can be justified if the system needs to be scalable, handling a vast number of objects efficiently.
  3. Initialization Costs: The initial setup of the Flyweight Pattern might be costlier in terms of development time and system resources. This upfront cost needs to be weighed against the long-term benefits of reduced memory consumption.
  4. State Management: As the Flyweight Pattern necessitates the externalization of some states, managing these states can become challenging. It's crucial to ensure that the extrinsic state doesn't grow too complex, or it might negate the benefits of the pattern.

In conclusion, while the Flyweight Pattern offers undeniable advantages in memory optimization and scalability, it's not a one-size-fits-all solution. Software architects must discerningly assess the system's requirements, the nature of the objects in play, and the overall architectural goals before deciding to employ this pattern. The judicious application of Flyweight can lead to efficient and scalable systems, but a hasty or ill-considered implementation can introduce unnecessary complexities and performance issues.
The main purpose of Flyweights is to save space (memory). Although performance/memory tradeoffs are a classic issue in program optimization, most of the time the impact on performance of using a Flyweight is negligible, especially if the objects are naturally immutable.
I have focused on immutable objects here, since they are definitely the simplest to work with and serve many purposes. However, even mutable objects can be represented by Flyweights if the mutable parts can be separated out and made extrinsic. For example, a drawing document might be required to store the position of each Flyweight shape rather than storing the position of the shape as part of the Shape class.
The disadvantage of this approach is that moving the state outside the object breaks encapsulation, and may be less efficient than keeping the state intrinsic. In these cases it may be necessary to decide whether performance or memory is more important.
Flyweights are based on pointers or references. Working with Flyweights is easy in a language like Java where all object variables are references and a garbage collector is responsible for removing old objects. Flyweights are just a little trickier in a language like C++ where objects can be allocated as local variables on the stack and destroyed as a result of programmer action.

Flyweight Pattern Described

A flyweight is an object that minimizes memory use by sharing as much data as possible with other similar objects; it is a way to use objects in large numbers when a simple repeated representation would use an unacceptable amount of memory. Often some parts of the object state can be shared, and it is common practice to hold them in external data structures and pass them to the flyweight objects temporarily when they are used. A classic example usage of the flyweight pattern is the data structures for graphical representation of characters in a word processor. It might be desirable to have, for each character in a document, a glyph object containing its font outline, font metrics, and other formatting data, but this would amount to hundreds or thousands of bytes for each character. Instead, for every character there might be a reference to a flyweight glyph object shared by every instance of the same character in the document; only the position of each character (in the document and/or the page) would need to be stored internally.