capturing the problem: use case development and requirement analysis

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Peter Fox

Xinformatics

ITEC 4962/6961, ERTH 4963/6963, CSCI 4960/6960

Week 4, February 14, 2012

Capturing the problem: Use case development and requirement analysis

Contents• Discussion of reading

• Background on use cases

• Developing use cases

• Uncovering requirements

• Evaluation

• Assignment 2

• Next class(es)

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Software and wetware

• ‘Before you make the software interoperable, you need to make the people interoperable’: talk by Ian Jackson, chief of operations, British Geological Survey, presented at the American Geophysical Union, Dec. 2008

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Developed for NASA TIWG

Use Case

• Is a collection of possible sequences of interactions between the system under discussion and its Users (or Actors), relating to a particular goal.

• The collection of Use Cases should define all system behavior relevant to the actors to assure them that their goals will be carried out properly.

• Any system behavior that is irrelevant to the actors should not be included in the use cases.


Use Case

• is a prose description of a system's behavior when interacting with the outside world.

• is a technique for capturing functional requirements of business systems and, potentially, of an IT system to support the business system.


Use Case

• Must be documented (or it is useless)• Should be implemented (or it is not well

scoped)• Is used to identify: objects ~ resources,

processes, roles (aka actors), requirements, etc.

• Should iterate with as many actors as possible on wording and details at least once


Use Case Examples:• I have a gazillion images of the night sky from

a survey but there’s no way I (or all of the known professional galactic astronomers) can classify all those galaxies – what can I do?

• Provide browse and quick look access to a broad variety of climate, weather and ocean data.

• Provide web portal access to a federation of library catalogs with drill-down search and access of published articles


Use Case Examples:• Provide high-performance data transfer of

specific climate model data products into the CDAT tool for analysis independent of their storage format, organization or location on the internet

• Perform real-time MRI image analysis to detect abnormal tissue growth in adult humans.


Use Case Examples:

A US 9th grade teacher is preparing a lesson plan aimed at getting students to learn more about the ‘northern lights’, addressing NSES content standards in earth science. The teacher wants the students to learn the scientific terminology, where the phenomena occurs and retrieve some data or graphics for a recent occurrence. The goal of the lesson plan is the engage students, using authentic data from the aurora, as part of an inquiry-based program.


Elements of a Use Case

• http://wiki.esipfed.org/index.php/SolutionsUseCase_Template

• Start with the Plain Language Description• Short Definition• Purpose• Describe a scenario of expected use• Definition of Success

http://wiki.esipfed.org/index.php/SolutionsUseCase_Template



Short Definition

• Define the use case in plain sentences• Wherever possible avoid specifying

technical solutions or implementation choices

• Concentrate on the application aspects of the intended scenario

• Also note when the use case may be applicable to more than one application area


Purpose

• A plain language description of • why this use case exists,• what the problem is to be solved, and• what a successful outcome, and • what the impact may be.

• Often termed the ‘business case’


Scenario of expected use

• A verbose (more detailed) description of one instance of a problem to be solved• what resources are generally needed (if known)• what a successful outcome and impact may be• who might be expected to do the work or provide the

resources and • who might be expected to benefit from the work.

• List any performance or metric requirements for this use case and any other other considerations that a user would expect.


Definition of Success

• Quick test that would show whether or not the case is working as described.


At this stage

• Use case modelers should have a good sense of what the use case goal is.

• They proceed on to the next stage to extract details.

• They may contact other team members, e.g. domain experts, one-on-one for additional information.

But for Xinformatics?• Everything up to now can be considered

‘informational’ and is accessible to people

• It is intended to keep people in the loop

• Let’s discuss this use case:– I have a gazillion images of the night sky from a

survey but there’s no way I (or all of the known professional galactic astronomers) can classify all those galaxies – what can I do?

– What would you do?

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Formal Use Case Description

• Use Case Identification• Revision Information• Definition• Successful Outcomes• Failure Outcomes


Use Case Elaboration• Actors• Primary Actors• Other Actors

• Preconditions• Postconditions• Normal Flow (Process Model)• Alternative Flows• Special Functional Requirements• Extension Points


Non-functional requirements• Performance• Reliability• Scalability• Usability• Security• Other Non-functional Requirements• Repeatability?


Alternate form• Use case name• Summary• Activity diagram• Preconditions in tabular form• Triggers• Basic flow• Alternate flow• Post conditions


Comparison b/w formats

• Long format - intended to be a full capture of the use case and is used by domain contributors (first section) and technologists. Categories are more faceted.

• Short format - captures important basic and necessary elements of use case. Categories are more integrative.


Preconditions - data/model


Preconditions - event/application


Which format to use?

• Short (in document) format for:• Exploratory phase of a project where you want to

collect a lot of use cases • An example for others to use• Including in a proposal (or an assignment)

• Long (on wiki) format for:• Detailed documentation of the use case• Life cycle documentation for implementation• Asynchronous/ collaborative development


Table of Contents• ==Plain Language Description==• ===Short Definition===• ===Purpose===• ===Describe a scenario of expected use===• ===Definition of Success===• ==Formal Use Case Description==• === Use Case Identification===• ===Revision Information===• ===Definition===• ===Successful Outcomes===• ===Failure Outcomes===• ==General Diagrams==• ===Schematic of Use case===• ==Use Case Elaboration==• ===Actors===• ====Primary Actors====• ====Other Actors====• ===Preconditions===• ===Postconditions===• ===Normal Flow (Process Model)===• ===Alternative Flows===

• ===Special Functional Requirements===• ===Extension Points===• ==Diagrams==• ===Use Case Diagram===• ===State Diagram===• ===Activity Diagram===• ===Other Diagrams===• ==Non-Functional Requirements==• ===Performance===• ===Reliability===• ===Scalability===• ===Usability===• ===Security===• ===Other Non-functional Requirements===• ==Selected Technology==• ===Overall Technical Approach===• ===Architecture===• ===Technology A===• ====Description====• ====Benefits====• ====Limitations====• ===Technology B===• ====Description====• ====Benefits====• ====Limitations====• ==References==

• ===Special Functional Requirements===• ===Extension Points===• ==Diagrams==• ===Use Case Diagram===• ===State Diagram===• ===Activity Diagram===• ===Other Diagrams===• ==Non-Functional Requirements==• ===Performance===• ===Reliability===• ===Scalability===• ===Usability===• ===Security===• ===Other Non-functional Requirements===• ==Selected Technology==• ===Overall Technical Approach===• ===Architecture===• ===Technology A===• ====Description====• ====Benefits====• ====Limitations====• ===Technology B===• ====Description====• ====Benefits====• ====Limitations====• ==References==

Scoping

• Focus initially on:• Core functionality• What it takes to implement the use case, resist early

generalizations• May (will) have to iterate on use case and

requirements

• Acknowledge other important issues such as:• Required vs. optional• Non-functional requirements• Available personnel (skills) and resources

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Implementation Basics• Review your documented use case with team and

experts• Go into detail of your model; test it using the tools

you have• Look at the use case document and examine the

actors, process flow, artifacts, etc.• You will start to develop a design and an

architecture• Keep in mind that it is more flexible to examine

your interfaces, i.e. between layers and components in your architecture, i.e. between ‘users’ and ‘information’

Actors• The initial analysis will often have many human

actors• Begin to see where these can be replaced with

machine actors – may require additional encoding• If you are doing this in a team, take steps to

ensure that actors know their role and what inputs, outputs and preconditions are expected of them

• Often, you may be able to ‘run’ the use case (really the model) before you build anything

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Actors• Real people (round heads – smart

consumers of information) and computers (block heads – dump consumers)

• E.g. Data provider, end-user, data manager, alert service

• Primary – initiate (act on)

• Secondary – respond (acted upon)


What’s a pre-condition?

• defines all the conditions that must be true (i.e., describes the state of the system) for the trigger to meaningfully cause the initiation of the use case.

Preconditions• The preconditions are external to your

information system development and you may not understand how they fit in the implementation

• Some level of modeling of these preconditions may be required (often this will not be in your first pass encoding which focuses on the main process flow, goal, description, etc.)

• Beware of using another entities data and services: policies, access rights, registration, and ‘cost’

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What’s a post-condition?

• describes what the change in state of the system will be after the use case completes. Post-conditions are guaranteed to be true when the use case ends.


Success scenarios

• A re-statement of how the use case via its flows and actors and resources results in achieving the result

• Describe artifacts produced

• Describe impacts and metric values


Failure scenarios• A statement of how the use case via its flows

and actors and resources did not result in achieving the result

• Describe role of actors in failure• Describe role of resources in failure• Describe what artifacts were and were not

produced• Describe impacts of failure and any metric

values• And when you are doing science this is 80%

of the outcome!


Normal (process) flows

• A basis step of (usually) distinct steps that result when the use case is triggers (commences)

• Steps are often separated by actor intervention or represent modular parts of the flow (can encapsulate activities)

• Can have loops

• Should end with the final goal achieved

Process flow• Each element in the process flow usually denotes

a distinct stage in what will need to be implemented

• Often, actors mediate the process flow• Consider the activity diagram (and often a state

diagram) as a means to turn the written process flow into a visual one that your experts can review

• Make sure the artifacts and services have an entry in the resources section

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Alternate (process) flows• Variations from the main flow, often invoked

by valid but non-usual (or rules)

• Activity diagrams are useful in representing this part of the document

• Do not usually represent exceptions/ error flows

• Can often help to identify general patterns in the use case via similarities with the normal flow

• While many are possible, usually only include one - illustrative


Non-functional requirements• (from Wikipedia): requirements which specify

criteria that can be used to judge the operation of a system, rather than specific behaviors.

• This should be contrasted with functional requirements that specify specific behavior or functions.

• In general, functional requirements define what a system is supposed to do whereas non-functional requirements define how a system is supposed to be.


More - non-functional• (from Wikipedia): Non-functional requirements are

often called qualities of a system. Other terms for non-functional requirements are "constraints", "quality attributes", "quality goals" and "quality of service requirements".

• Qualities, aka. non-functional requirements, can be divided into two main categories.– Execution qualities, such as security and usability, are

observable at run time.– Evolution qualities, such as testability, maintainability,

extensibility and scalability, are embodied in the static structure of the software system.

Artifacts – things left behind• Add artifacts that the use case generates to the

resources list in the table• It is often useful to record which artifacts are critical

and which are of secondary importance• Be thinking of provenance and the way these were

produced, i.e. what went into them and produce suitable metadata or annotations

• Engage the actors to determine the names of these artifacts and who should have responsibility for them (usually you want the actors to have responsibility for evolution)

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General Diagrams

• Schematic of the Use case• Drawing diagrams:• Stick figures for actors (person or

computer)• Boxes to denote resources• Arrows to denote process flow• Concept maps are a useful tool


Schematic

Diagrams

• Use Case Diagram• State Diagram• Activity Diagram• Other Diagrams

Schematic

Reviewing the resources• Apart from the artifacts and actor resources, you may

find gaps• Define/ find the authoritative sources for data,

information, metadata, configuration• Your encodings can also be a resource, make it a first

class citizen, e.g. on the web give it a namespace and a URI

• Sometimes, a test-bed with local data is very useful as you start the implementation process, i.e. pull the data, maybe even implement their service (database, etc.)

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So far … Summary

• By now, the reality of going into complete detail for the design should be apparent

• Keeping it simple is also very important as you begin to implement

• Being prepared to iterate is really essential• Now is the time to validate your model with

domain experts and your team• The next stage would be to assess your

technology components and design (but we cover that later) 48

Interfaces

• Increasingly in tiered architectures there are numerous interfaces

• Information flow at interfaces and thus software engineering at those interfaces becomes a very important consideration

• Is often left to the ‘finish work’ but usually should not

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Metrics• Things you can measure (numerical)• Things that are categorical• Could not do before• Faster, more complete, less mistakes,

etc.• Wider range of users

• Measure or estimate the baseline before you start – use case!

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Iterating• Evolve, iterate, re-design, re-deploy• Small fixes• Decide what to do about the new use cases, or if

the goal is not met• Determine what design/ engineering is required

and who will do it (often participants in the evaluation may become domain experts in your methodology)

• Determine what new representations, features are required

• Assess need for an architectural re-design51

Evaluation?• Can be structured or less-structured• A good way to start is to get members of

your team (or someone else) to do a peer evaluation

• This is a professional exercise, treat it that way at all times

• Other possible techniques for moving forward on evolving the design, what to focus upon, priorities, etc.: SWOT, Porter’s 5 forces

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Evaluation References• Twidale, Randall and Bentley (1994) and

references therein• Scriven (1991, 1996)• Weston, Mc Alpine, and Bordonaro, (1995)• Worthen, Sanders, and Fitzpatrick, (1997)

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Result/ outcome• Refer to the use case document• Outcome (and value of it) is a combination of

data gathering processes, including surveys, interviews, focus groups, document analysis and observations that will yield both qualitative and quantitative results.

• Did you meet the goal?• Just listen… do not defend … if you start to

then: QTIP – quit taking it personally

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Evaluation (Twidale et al.)• An assessment of the overall effectiveness of

a piece of software, ideally yielding a numeric measure by which informed cost-benefit analysis of purchasing decisions can be made.

• An assessment of the degree to which the software fulfils its specification in terms of functionality, speed, size or whatever measures were pre-specified.

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Evaluation• An assessment of whether the software fulfils

the purpose for which it was intended.• An assessment of whether the ideas

embodied in the software have been proved to be superior to an alternative, where that alternative is frequently the traditional solution to the problem addressed.

• An assessment of whether the money allocated to a research project has been productively used, yielding useful generalizeable results. 56

Evaluation• An assessment of whether the software

proves acceptable to the intended end-users.• An assessment of whether end-users

continue to use it in their normal work.• An assessment of where the software fails to

perform as desired or as is now seen to be desirable.

• An assessment of the relative importance of the inadequacies of the software.

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(Orthogonal) Dimensions of evaluations

Structured Less structured

Quantitative Qualitative

Summative Formative

Controlled experiments Ethnographic observations

Formal and rigorous Informal and opportunistic

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• evaluation carried out for two reasons:• grading translations = summative evaluation• giving feedback = formative evaluation

Keep in mind• You need an evaluation plan that can lead to

improvements in what you have built• You need an evaluation to value what you

have built• You need an evaluation as part of your

preservation documentation – um, so that you might actually use the approach again, or um, reproduce it ;-)

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Requirements• Start with the actors and capture their

required actions, artifacts, outcomes, etc.• At each stage of the general (and alternate)

process flow, ask them what is required at the stage (and what is optional)

• Also ask about non-functional requirements (preferably without calling them that)

• This is required for human and computer actors! Yes, how do you ask a computer?• Read the documentation, try it out, or have

someone do that for you

Functional• ?

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Non-functional• ?

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When someone asks: “What is your use case”?

• Treat it like your ‘elevator pitch’• Know them, especially the ones you

have implemented• Tell them how you used it to develop

a solution FOR use

If you have not developed one

• Try reverse engineering • Start with a personal example• E.g. balancing your checkbook (hah,

that’s SO 80’s)• What’s an example that comes to

mind?

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Resources• http://alistair.cockburn.us/index.php/Use_cases,_ten_years_later• http://members.aol.com/acockburn/papers/AltIntro.htm• http://alistair.cockburn.us/index.php/Resources_for_writing_use_cases• http://alistair.cockburn.us/images/Usecasesintheoryandpractice180.ppt• http://alistair.cockburn.us/images/Agileusecases1dy.ppt• http://alistair.cockburn.us/index.php/Structuring_use_cases_with_goals• http://www.foruse.com/publications/bibliographies/usecases.htm• http://en.wikipedia.org/wiki/Use_case• http://www.ddj.com/dept/architect/184414701• Omnigraffle (Mac) or • Cmap• ESIP wiki template


Discussion• About use cases?

• Requirements?

• Backward assignment (previous weeks)– Detect the use cases in the presentations (if they

are not explicit)– Drive your friends crazy but asking them for their

use case

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Assignment 2• Surprise: use case development

• Due in ~ three weeks

• Let’s take a closer look

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What is next• Week 5 – presentations – be prepared

• Reading for this week is review

• Reading for week 5 is in preparation for week 6!

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capturing the problem: use case development and requirement analysis

Documents