Validating Requirements Determining Completeness and Correctness of Requirements Using the System Reference Model IV&V Workshop 16 September 2009

Overview Validation Purpose and Definitions A Correct and Complete SRM, and the Three Questions SRM Correlation Mapping Analysis of Correlation Results Correct, Complete, Incorrect, and Incomplete Examples Best practices, lessons learned, and challenges

IV&V’s Validation Definition The process of evaluating artifacts to ensure that the right behaviors have been defined in the artifacts. The right behaviors are those that adequately describe –what the system is supposed to do –what the system is not supposed to do, and –what the system is supposed to do under adverse conditions. Validation ensures that the software system performs to the user’s needs under operational conditions. –Contains the desired capabilities to accomplish the mission goals –Does not contain unspecified limitation that impedes the capabilities

Validation Goal To ensure that –The right behaviors have been defined Adequately describe –What the system is supposed to do –What the system is not supposed to do –What the system is supposed to do under adverse conditions Correct and Complete –The requirement specifications are of high quality Unambiguous, Consistent, and Verifiable

Correct (IVV 09-1) Applicable requirement(s) meet all or part of the goals and behaviors of the system –Note: not all requirements can be evaluated in isolation; it may require a set of requirements to be evaluated together in order to determine that a particular goal or behavior is being met). The requirements are an accurate elaboration of the defined objectives or goals –e.g., the use of temporal modal operators like “next”, “until”, “always”, and “eventually”, are appropriately used to reflect the desired behavior The requirements adequately refine the higher-level requirements Design or implementation-specific information is specified as constraints to the behaviors captured in the requirements

Complete (IVV 09-1) All the needed information to completely specify a desired behavior is identified (i.e., all preconditions, postconditions, and invariants are specified for the described behavior). Threads of behavior are represented by more than one requirement, versus one compound requirement that attempts to capture the entire thread (i.e., that each requirement specifies only one “thing”). The use of conjunctions (e.g., “and”, “or”) are restricted to preconditions, postconditions, and invariants.

How? “This goal is achieved through the development and application of a system reference model (SRM) that will include a formal specification. The SRM can then be used to show (e.g., validate) that the right system behaviors are specified and the associated requirements are unambiguous, correct, complete, consistent, and verifiable. The SRM can also be used to validate (or develop) a test design that will demonstrate that the software products meet the specification and the operational need.” – IVV 09-1

The System Reference Model Includes sets of Modeling Artifacts –Use cases –Activity Diagrams, Sequences Diagrams –Statecharts –Domain Models (Class Diagrams, Communication Diagrams) –Statechart Assertions –JUnit Test Cases A concise description of the IV&V team’s understanding of the problem –Analysis tool –Communication tool Captures expected system behaviors –3 Questions

What’s In the SRM? Validation and Verification involves answering the following three questions: 1. Will the System do what it is supposed to do? 2. Will the System not do what it is not supposed to do? 3. Will the System maintain operations under adverse conditions? Note, in order to answer these questions, we must first have an independent understanding of: –What the system is supposed to do –What the system is not supposed to do –What the system is supposed to do to maintain operations under adverse conditions This information can be found within different areas of our model.

SRM Product Dependencies

System Goals

Constraints, Actors, and Environment

Text Use Cases

Use Case Example What the system is supposed to do All parts within the Main Success Scenarios describe the actions that must take place to accomplish the goal(s). Adverse Conditions Extension Scenarios show how the system should react to adverse conditions to get back on the success path or transition to safe mode.

Activity Diagram Example Flight System Goal: Flight System precesses and damps nutation to point the High Gain Antenna at earth for communication and GRAV science Precondition: Engineering Instruments are calibrated Flight System Turn on IMUs Turn on Precession catalyst bed heaters Turn on X-band Transmitters Passive Nutation Damping Return Errors > Precess to Earth Point > Precess to Earth Point Fails Select High Gain Antenna Select Forward Low Gain Antenna Pulse RCS Thrusters Turn off IMU Turn off cat bed heaters Use Other Thrusters [IMU Does not turn on or malfunctions] [Cat bed heaters do not turn on] [turning to earth point] [not turning to earth point] [thrusters do not fire] What the system is supposed to do All parts within the Main Success Scenarios describe the actions that must take place to accomplish the goal(s). Adverse Conditions Extension Scenarios show how the system should react to adverse conditions to get back on the success path or transition to safe mode.

Sequence Diagrams Example What the system is supposed to do The interactions of the Sequence Diagram describe the steps involved to accomplish the goal(s). 18 Adverse Conditions A sequence diagram can also be developed to show how the system should react to adverse condition.

SRM Validation Scenarios getEstimatedStateVector() estimateStateVector() AssertTrue() AssertFalse() estimateStateVector() AssertTrue() getEstimatedStateVector() public void testScenario2() { st.getEstimatedStateVector(); st.estimateStateVector(); assertTrue (st.isSuccess()); st.estimateStateVector(); assertTrue (st.isSuccess()); st.getEstimatedStateVector(); assertFalse (st.isSuccess()); } What the system is NOT supposed to do This information can be found in our Assertions and the Validation scenarios we create to test against them.

Validation WBS (IVV 09-1) 1.0 Validation 1.1 Obtain/Develop a System Reference Model (SRM) 1.2 Validate System Requirements 1.3 Validate Test Design

Validate System Requirements For each level of system decomposition, we need to determine –Sufficiency of the requirements Is there a corresponding requirement for every SRM behavior and Statechart assertion at that level? –Quality of the requirements Assess the quality of each requirement that has a corresponding SRM behavior or Statechart assertion at that level –Sufficiency of the SRM Is there any mission-critical, safety-critical requirement not covered by an SRM behavior or Statechart assertion at that level? A “correlation map” is built to capture these relationships

Validation Possibilities SRM BehaviorsRequirements SRM Correct? Requirements Complete? SRM Correct? Requirements In Scope and Valid? Validated Requirements Unambiguous, Correct, Consistent, & Verifiable Requirements

An Example Behavior Extensions – Q2 & Q3 Preconditions Main Success Scenario – Q1 & Q2 Constraints Post-conditions Goal References

Requirement Proxies

Other Analysis Tools Subject Requirement Child Requirements Parent Requirements Validation Findings Correlation Mapping

Requirement Data Subject Requirement Child Requirements Parent Requirements Validation Findings Correlation Mapping Subject Requirement

Parent Traces Subject Requirement Child Requirements Parent Requirements Validation Findings Correlation Mapping Parent Requirements

Child Traces Subject Requirement Parent Requirements Validation Findings Correlation Mapping Child Requirements

Correlation Mapping Subject Requirement Child Requirements Parent Requirements Validation Findings Correlation Mapping

Validation Findings Subject Requirement Child Requirements Parent Requirements Correlation Mapping Validation Findings

Requirement Proxies

Correlation Mapping & Requirement Evaluation

Correlation Map

Model Elements Requirement Mapping Findings/Issues

Correct (IVV 09-1) Applicable requirement(s) meet all or part of the goals and behaviors of the system –Note: not all requirements can be evaluated in isolation; it may require a set of requirements to be evaluated together in order to determine that a particular goal or behavior is being met). The requirements are an accurate elaboration of the defined objectives or goals –e.g., the use of temporal modal operators like “next”, “until”, “always”, and “eventually”, are appropriately used to reflect the desired behavior The requirements adequately refine the higher-level requirements Design or implementation-specific information is specified as constraints to the behaviors captured in the requirements Coverage of Model Consistency with Model Separation of Information

Complete (IVV 09-1) All the needed information to completely specify a desired behavior is identified (i.e., all preconditions, postconditions, and invariants are specified for the described behavior). Threads of behavior are represented by more than one requirement, versus one compound requirement that attempts to capture the entire thread (i.e., that each requirement specifies only one “thing”). The use of conjunctions (e.g., “and”, “or”) are restricted to preconditions, postconditions, and invariants. Coverage of Model

Correlation Mapping & Requirement Evaluation

Identifying Issues L2 Rqmnt - The Project shall generate, route, transport, store and execute a sequence containing any of the following types of time-tagged commands: absolute time, time relative to a sequence-external time value stored on-board, time relative to the execution of the previous command in the sequence. L3 Rqmnt - The sequence time tags shall be either an absolute execution time, time relative to a sequence external time value, or a time relative to the execution of the previous command or command file. L4 Rqmnt - The flight software shall provide the means for running onboard relative and absolute time, relative to a sequence external time value, tagged sequences.

Identifying Issues Model Elements Requirement Mapping Findings/Issues Model Requirements Findings

Lessons Learned & Best Practices

Challenges Varying levels of detail between the SRM and requirements being validated What vs. How – “Requirements Model” vs. “Design Model” Terminology differences between SRM behaviors and requirements Lack of adequate tools, work instructions, product descriptions/templates – particularly MKS Artifact Mapping capability

Questions?