Software design refers to the thought process involved in planning and providing for a better solution during problem solving. Software design comes after the architecture is decided upon. Architecture is more closely related to the business needs of the project, and theoretically it does not concern the actual technology platform (such as J2EE or Microsoft .NET or PHP) on which the application will be built (although practically we can decide the platform either in parallel with working on the architecture of the application or before doing so). Software design deals with the high-level concepts related to the actual implementation of the architecture in our projects, which include tasks such as usability studies to make sure our project targets the right kind of users, deciding which design patterns to use to make our application scalable, secure and robust. During the design phase, we also decide on the implementation methodology to be used in the actual development phase (which comes after design and involves actual coding). The following diagram shows how architecture and design fit together and relate to each other:
As we can see in the diagram, the actual business requirements and scope of the project are the deciding factors when working on the application architecture. Software design and development come next and, based on the design, the actual development work gets executed. A single problem can have many possible solutions, some of which will be more efficient than others. Before a developer starts chunking out code for a particular business requirement, it would be prudent and beneficial to give some thought and select the best approach from the possible list of options to assure that code performance, scalability and maintainability is not sacrificed in the long run.
In order to understand all of this by way of a simple analogy, consider a car manufacturing plant as an example. The mechanical engineers developing the high-level blueprint of the car would be the architects, and the blueprint itself would be the architecture of the car. This blueprint would include high-level specifications such as:
Dimensions of the car and its components
Type of car (hatchback, sedan, or SUV)
Maximum passenger capacity, and load capacity
Minimum build strength
So the blueprint would specify the limitations as well as the conditions that need to be fulfilled for any design of that car, and besides the blueprint there would be additional constraints such as the budget for the production costs. But this blueprint would not include details of how exactly the engine would be designed, what quality of steel would be used, what type of tires would be used, what type of plastics would be used for the dashboard and other parts, and so on. All of this would actually be decided by the design engineers, who will make sure that their choices fit the blueprint specifications in the best possible way. The engineers will also consider production and design techniques that other car companies might have followed, so that they don't re-invent the wheel.
The actual assembly line production will follow the designs and techniques specified by the engineers and will involve tasks such as cutting metal, choosing the right machines, assembling the individual components, painting, safety tests, and so on, to create a complete working car. The following figure will correlate this example with the equivalent aspects of software development:
From the figure we can see how the car company example loosely translates to software architecture, design, and development. Now let us take another analogy, this time more closely related to the software industry. Consider a company that needs to build a bulk emailing program for its social networking website. A software architect will first understand the high-level requirements of the program, such as:
How many average emails need to be sent on a daily or hourly basis?
How often will the emails need to be sent?
Will there be attachments involved? If yes, what will be the biggest attachment size?
Does this program need to be extensible and re-usable (for other similar websites or applications in that company)?
Based on the answers to the above questions, the architect will come up with an application architecture which covers all aspects of the actual business needs. The architecture will decide how the emailing program should be developed: a Windows Service, or a Web Service, or a console utility, or some batch program run by a scheduler.
But the architecture would not include details such as:
How should the program handle exceptions?
How will we make sure that the code is efficient in terms of performance, and does not hang while sending bulk emails in a short period?
How should the program perform error logging?
How will the program be developed so that it is re-usable (if the architecture dictates it to be developed as a re-usable component)?
That's the part where design comes into the picture. The application architecture would define limits and boundaries within which the design would move around and improvise. So the architecture would neither go deep into the nitty-gritties of the design phase, nor would it dictate implementation guidelines and programming rules, as the architecture has no relation with programming at all. In fact, the architecture lays out specifications which are more aligned with business requirements, and makes sure that all business aspects are met and taken care of.
Coming back to our bulk email program, the term software design can be loosely translated into the process of designing the actual program, which involves using specific programming techniques (or design patterns, which we will study later) and laying out the basic solution framework. All coding would actually occur within that framework. We can have multiple design options for the same architectural specification, and it is up to the stakeholders to decide which one to go for, considering the overall efficiency and budget constraints.
Here is a simple diagram illustrating the basic process: