1. Designing Scalable Architectures
Software Stability Models Introduction
by:
Mohamed E. Fayad,
and
Huáscar A. Sánchez
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Software Architecture Lecture

2. Software Architecture Lecture
So, What is Scalability?
Scalability to indicate the ability of architectures to easily adapt to evolving requirements without unnecessary effort.
Scalability can be further divided into two kinds: Vertical and Horizontal scalability.
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Software Architecture Lecture

3. Software Architecture Lecture
Scalability Types
Vertical Scalability is concerned with the ability of the architectures to scale Up and Down in order to adapt to evolving requirements.
Horizontal Scalability, on the other hand, emphasizes the ability of the architecture to extend their boundaries by establishing connections with other software architectures in an efficient manner.
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Software Architecture Lecture

4. Scalability Types – Vertical
Scaling Up and Down can also be referred to as “Upward and Downward Scalability.
Upward scalability is defined as the capacity of handling increasing demands by adding new layers, or functionalities.
Downward scalability is defined as the ability of the architecture to adapt itself to a more constrained environment, by disabling certain functionalities.
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Software Architecture Lecture

5. Scalability Types – Horizontal
Horizontal Scalability can be achieved in two directions, which we refer to as: Scaling Out (Extensibility) and Scaling In (Reduction).
Extensibility is the capacity of software architectures to glue external architectures (leaves) to their core structure, creating a synergy between these dissimilar architectures.
Reduction, on the other hand, is the action of ungluing those “leaves” from a particular architecture without provoking a reciprocal harmful impact on their internal structure.
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Software Architecture Lecture

6. Scalability is a major Concern
In most cases changes that cause software architectures to fail are:
(1) Natural Evolution of Architecture Business processes.
(2) Modification of underlying requirements to meet evolving needs.
When Business experience major increments in services demands, the main concern is their architecture’s ability to scale to accommodate these loads. Therefore:
(1) The architecture needs to efficiently scale and adapt to handle system
requirements changes.
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Software Architecture Lecture

7. Scalability Solutions Shortage
The problem of developing scalable architectures has been investigated in different contexts in software development. However…
Scalability in software architectures are emphasized merely on the achievement of Upward Scalability.
They rely on three elements: increasing speed and capacity, improving efficiency, and shifting or reducing the loads.
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Software Architecture Lecture

8. Scalability Solutions Shortage
Based upon these elements, designers/ developers need to pinpoint relevant information, along with the right techniques, to come up with a proper scalable architecture design.
Scalability, is therefore, limited to:
They are solely oriented to reach one side of Vertical Scalability (Upward).
Their validity usually relies on special hardware.
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Software Architecture Lecture

9. Software Architecture Lecture
Lesson Learned?
Implementing full scalability is not straightforward in software development, and especially with conventional approaches.
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Software Architecture Lecture

10. Scalability with Traditional Approaches
The Problem
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Software Architecture Lecture

11. Scalability with Conventional Approaches
A Bird’s eye view
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Software Architecture Lecture

12. Scalability with Conventional Approaches
A Bird’s eye view
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Software Architecture Lecture

13. Scalability with Conventional Approaches
Case Study: E-Commerce Application
The architecture contains the following objects: Customer, Catalog, Database, ShoppingCart, Product, Order, CreditCard.”
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Software Architecture Lecture

14. Scalability and Conventional Approaches
E-commerce Class Diagram
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Software Architecture Lecture

15. Scalability and Conventional Approaches
Adding Search, CyberCash, Echecks Classes
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16. Scalability and Conventional Approaches
Classes affected by Changes
Added
Affected
Affected
Affected
Added
Affected
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17. The Solution to Scalability: SSM
Software Stability Model (SSM) as a mean for Developing Scalable software architectures.
Software architectures that are flexible enough to scale the scope of their elements, methods, etc. when handling evolving requirements.
It provides the means for gluing together two or more heterogeneous architectures to perform one or more common tasks (Horizontal Scalability), or adding new functionality to the architecture’s structure to address more users needs (Vertical Scalability).
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Software Architecture Lecture

18. Software Architecture Lecture
SSM Result
Software architectures capable of evolving through time without the concern of a potential collapse.
These evolving architectures are referred to as Scalable architectures.
Scalable architectures are well-designed architectures, which structures remain constant, and are able to evolve proportionally with the introduction of new requirements.
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Software Architecture Lecture

19. Software Architecture Lecture
SSM Benefits
SSM will enhance the scalability of the architecture by identifying the changeable components in the system and organize them in such a way that their future changes do not require major changes in the other parts of the architecture.
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Software Architecture Lecture

20. Software Architecture Lecture
SSM Foundation
Software stability Models classify the developed system architecture into three layers: EBTs, BOs, and IOs.
EBTs are conceptual classes, which structure remain internally and externally stable.
BOs are classes that are internally stable but externally adaptable.
IOs are context specific classes, which structure is very volatile (internally and externally unstable).
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Software Architecture Lecture

21. Software Architecture Lecture
SSM Foundation
The synergy between EBTs and BOs form the basis for Scalable Architecture.
IOs are attached to the scalable architecture by means of certain extension points of the BOs, referred to as “Hooks.”
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Software Architecture Lecture

22. Software Architecture Lecture
SSM Foundation
These extension points can be added to or removed from a BO according to the problem needs and requirements.
BOs serve as the linking points for architectures to establish a connection with external architectures (Horizontal Scalability).
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Software Architecture Lecture

23. SSM Concept Visualization
Properties granted to
Software Architecture:
Stability
Scalability
Adaptability
Customizability
Traceability
Separation of Concerns
and more……
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Software Architecture Lecture

24. Software Architecture Lecture
Scalability with SSM
Case study: E-commerce Application
Identification Process:
First, we need to identify the purpose, and goals of this system (EBTs).
Then, we identify the core abstractions of the system’ processes (BOs).
Finally, we need to identify the physical entities of our model (IOs).
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Software Architecture Lecture

25. Software Architecture Lecture
Scalability with SSM
E-commerce Application: Stable Model
EBTs
BOs
IOs
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Software Architecture Lecture

26. Software Architecture Lecture
Scalability with SSM
Any single representation of EBTs along with their associated BOs and IOs, represent an independent functionality of the group of functionalities in the entire system’s architecture.
Each functionality, by its definition, is self-controlled. Nevertheless, it collaborates with other functionalities in order to accomplish one or more tasks in the architecture.
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27. Software Architecture Lecture
Scalability with SSM
Independent Functionality example
EBTs
BOs
IOs
EBTs
BOs
IOs
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Software Architecture Lecture

28. Vertical and Horizontal Scalability
Adding Customer Service, Content Management, Bidding Capacity
EBTs
BOs
IOs
Extensibility
Upward
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29. Vertical and Horizontal Scalability
Disabling Order Handling, and Content Management Application
EBTs
BOs
IOs
Downward
<EBT>
OrderHandling
Reduction X
<IO>
ContentManagement X
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30. Software Architecture Lecture
Conclusion
We have shown, through a simple case study, that developing architectures using the traditional approach does not allow the desired scalability to accommodate future requirements.
On the other hand, software stability approach partitions the entire functionality into several independent layers, and hence, we are able to incorporate scalability feature into the architectures.
Consequently, architectures can scale up or down, and scale out or in to accommodate evolving requirements, without the danger of being collapsed.
Such flexibility will improve the quality of the developed architectures while concurrently diminishing the cost and condensing the time of the development process.
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Software Architecture Lecture