Requirements specification

1
Requirements specification

• Why is this the first major stage of software
  development?
• Need to understand what customer wants first
• Goal of requirements gathering is to understand
  the customers problem
• Though customer may not fully understand it!
• Requirements specification should Describe
  explicitly how we will know when the job is
  done and the customer is satisfied.
• Why is this a non-trivial task?

2
"Editor with undo"

• A very simple, initial requirements spec
• Why might a small and concise initial spec be a
  good idea?
• Fosters initial buy-in and agreement by everyone
  on the team
• Real-world specs are usually much longer, though
• Whats the purpose of a purpose statement?
• Why is scope important?
• Why definitions?
• Other possible general requirements
• Business context organization sponsoring the
  development of the product
• User characteristics profile of user community,
  expected expertise with system domain
• User objectives wish list from users
  perspective, in line with business objectives
• Interface requirements GUI, API, communication
  interfaces, software interfaces

3
Use cases

• A "use case" is a set of cases or scenarios for
  using a system, tied together by a common user
  goal
• Essentially descriptive answers to questions that
  start with What does the system do if
• E.g., What does the auto-teller do if a customer
  has just deposited a check within 24 hours and
  theres not enough in the account without the
  check to provide the desired withdrawal?
• Use case describes what the auto-teller does in
  that situation
• Why are use cases good for brainstorming
  requirements?

4
Example use case(from Fowler and Scott, UML
Distilled)

• Use Case Buy a Product (Have we described this
  behavior is a users language?)
• Actors Customer, System (Why might it a be a
  good idea to define actors in a case?)
• Customer browsers through catalog and selects
  items to buy
• Customer goes to check out
• Customer fills in shipping information (address
  next-day or 3-day delivery)
• System presents full pricing information,
  including shipping
• Customer fills in credit card information
• System authorizes purchase
• System confirms sale immediately
• System sends confirming email to customer (Did
  we get the main scenario right?)
• Alternative Authorization Failure (When might
  this happen? I.e., at what step?)
• 6a. At step 6, system fails to authorize credit
  purchase
• Allow customer to re-enter credit card
  information and re-try
• Alternative Regular customer (At what step might
  this happen?)
• 3a. System displays current shipping information,
  pricing information,
• and last four digits of credit card information
• 3b. Customer may accept or override these
  defaults

5
Heuristics for writing use case text

• Avoid implementation specific language in use
  cases, such as IF-THEN-ELSE or GUI elements or
  even specific people or departments
• Which is better The clerk pushes the OK
  button. or The clerk signifies the
  transaction is done.
• The latter defers a UI consideration until
  design.
• Write use cases with the users vocabulary, the
  way a users would describe performing the task
• Use cases never initiate actions actors do.
• Actors can be people, computer systems or any
  external entity that initiate an action.
• A use case interaction produces something of
  value to an actor.
• Create use cases and requirements incrementally
  and iteratively
• Start with an outline or high-level description
• Work from a problem statement and statement of
  work (scope)
• Then broaden and deepen, then narrow and prune

6
Use Case exercise

• Write a use case for withdrawing cash from an ATM
  machine
• Assume customer has already logged into the ATM
  with PIN and sees menu
• Use Case Withdraw cash
• Actors Customer, ATM
• Customer selects Withdraw from menu
• ATM lists customers accounts to choose from
• Customer chooses an account
• ATM ask customer for an amount to withdraw
• Customer specifies an amount
• ATM subtracts this amount from the specified
  account
• ATM tells machine to disburse requested amount of
  cash
• Alternative Unit not 20s
• 5a. At step 5, customer specifies an amount not
  divisible by 20.00
• Allow customer to re-enter amount and repeat
  step 5
• Alternative Insufficient funds
• 6a. At step 6, amount requested is greater than
  amount in specified account

7
Advantages of use cases

• What do you think?
• Systematic and intuitive way to capture
  functional requirements?
• Facilitates communication between user and
  analyst?
• Thats why its important to write use cases in
  the users languages
• Drives the whole development process?
• Analysis starts with use cases
• Design and implementation realizes them
• How can use cases set up test plans?
• How can use cases help with writing a user
  manual?
• How can use cases help with user interface
  prototype?
• Can implementation details or language be added
  to use cases later in the life cycle?

8

• So, how will you do requirements analysis for
  your multimedia e-learning project?