Where Agile Meets Formal Methods Design by Contract Where Agile Meets Formal Methods

What is Design by Contract? Motivation History Concept Integrate with Agile Example Reviews Current State

What is Design by Contract? Motivation History Concept Integrate with Agile Example Reviews Current State

Motivation Improve software quality and reliability Second NASA Formal Methods Symposium (April 13 – 15, 2010) Verification and Validation of Flight-Critical Systems An Overview of Formal Methods at Intel Decision Engine for Software Analysis at Microsoft.

Motivation What are Formal Methods? “mathematically rigorous techniques and tools for developing specification, design and verification of software and hardware systems” -- Wikipedia

Motivation Agile: Are we building the right product? making sure clients get what they want won’t waste resources unnecessarily Formal: Are we building the product right? making sure code does exactly as specified separate process to determine right product

Motivation Is there a common ground? Alloy (MIT) Object Constraint Language (IBM) Design by Contract (Effiel)

Motivation Why should the Agile community care? Tests often insufficient to safeguard changes Documentations are poorly-written or outdated Collective code ownership  misinterpretation Why should the FM community care? Overemphasized full formalization of design Few people understand formal notations

What is Design by Contract? Motivation History Concept Integrate with Agile Example Reviews Current State

Design by Contract – History Bertrand Meyer developed DbC in 1986 “The quality factors on which we have concentrated — reusability, extendibility, compatibility — must not be attained at the expense of reliability (correctness and robustness).” -- Bertrand Meyer, Object Oriented Software Construction Correctness  Assertions Robustness  Exception Handling

What is Design by Contract? Motivation History Concept Integrate with Agile Example Reviews Current State

Design by Contract – Concept Example: a contract between an airline and a customer Obligations Benefits Customer (client) (Must ensure precondition) Be at the Pearson airport at least 5 minutes before scheduled departure time. Bring only acceptable baggage. Pay ticket price. (May benefit from postcondition) Reach Chicago. Airline (supplier) (Must ensure postcondition) Bring customer to Chicago. (May assume precondition) No need to carry passenger who is late, has unacceptable baggage, or has not paid ticket price.

Design by Contract – Concept Precondition: an obligation for the client and a benefit for the supplier Postcondition: a benefit for the client and an obligation for the supplier

Design by Contract – Concept Example: a contract between an airline and a customer Before Purchase After Purchase Airline (supplier) (Must ensure invariant) Ensures plane has certain number of free seats (Must ensure invariant) Number of free seats decreases appropriately Class Invariant: global property of the instances of a class, which must be preserved by all routines

Design by Contract – Concept Example: a contract between an airline and a customer Airline (General) Airline (Business) Customer (client) Be at the Pearson airport at least 5 minutes before scheduled departure time. Bring only acceptable baggage. Pay ticket price. Be at the Pearson airport at least 30 minutes before scheduled departure time. Bring only acceptable baggage. Pay ticket price. Airline (supplier) Bring customer to Chicago. Bring customer to O’Hare International Airport in Chicago.

Design by Contract – Concept Subcontract: Precondition can be weaken Postcondition can be strengthen

Design by Contract – Concept Example: a contract between an airline and a customer While Loading Baggage Handling Airline (supplier) Detected a snake in Customer A’s baggage Ask Customer A to leave the plane along with his baggage Exception: a run-time event that may cause a routine call to fail

What is Design by Contract? Motivation History Concept Integrate with Agile Example Reviews Current State

Design by Contract – Integrate "We will derive tremendous benefits from writing the assertions at the same time as we write the software, or indeed before we write the software." -- Bertran Meyer, Object Oriented Software Construction Differ from TDD, DbC checks for all cases ESC/Java2 checks spec consistency at runtime JMLUnit auto-generates test cases JMLDoc embeds JML in JavaDoc

What is Design by Contract? Motivation History Concept Integrate with Agile Example Reviews Current State

What is Design by Contract? Motivation History Concept Integrate with Agile Example Reviews Current State

Design by Contract – Reviews “I've tried to use [DbC] in a number of enterprise application settings, and I've found that in many situations it's as hard to write the pre and post conditions as it is to write the solution.” -- Martin Fowler “Design by Contract practice lessened the number of person-hours required to perform software testing.” -- An Experience With Design by Contract

What is Design by Contract? Motivation History Concept Integrate with Agile Example Reviews Current State

Design by Contract – Current State Effiel – built into the language Java – JCR305 submitted for JML Microsoft – Spec# and Boggie

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