GK One-Line Question & Answer

Software retirement decommissions a system and migrates its functions — criteria: maintenance cost exceeds business value delivered, replacement system has achieved equivalent functionality, user migration is complete, and data archival/legal retention requirements are satisfied

Rehosting ('lift and shift') moves to new infrastructure without code changes; replatforming makes minimal code changes to leverage cloud capabilities; refactoring restructures code for maintainability while preserving functionality — cost/risk increases in that order, as does modernisation value

MTBF is the average time between failures (reliability measure); MTTR is the average time to restore service after failure (recovery measure) — for high-availability systems, minimising MTTR often provides more availability improvement than reducing MTBF, as modern systems accept failures as inevitable

Re-engineering examines and alters an existing system to reconstruct it in a new form at a higher level of abstraction — it includes reverse engineering (understanding existing system), restructuring, and forward engineering, often transforming the system's architecture, not just code quality

Gradual code quality degradation due to uncoordinated changes increasing complexity and maintenance cost

Verification checks the product is built correctly per specs; validation checks the right product is built for user needs

Programming language used for implementation

Accumulated suboptimal design decisions for short-term gain that must be refactored at greater future cost

It must be continually adapted or it becomes progressively less satisfactory in its real-world environment

Maintainability

Complexity inherent to the problem being solved that cannot be designed away

Factors affecting the exponential scaling of effort with project size such as team cohesion and process maturity

Designing a national healthcare system where requirements are contradictory and no clear solution exists

Subtle differences in team skill, domain complexity, technology, and organisational context between reference and new project are hard to quantify and often overlooked

Inability of informal ad-hoc methods to reliably deliver large software on time, within budget, and with acceptable quality

Evolutionary/Iterative model

LOC incentivises verbose redundant code, penalises elegant solutions, varies with language, and counts non-functional code

Active and continuous user involvement throughout the development cycle

A process model is theoretical; a process is the actual enactment of activities in a specific project context

Spiral model

To establish project scope, eliminate highest-risk elements, define architecture baseline, and refine requirements

Adapting a standard process model to fit the specific constraints, size, domain, and team characteristics of a particular project

Process capability metrics such as defect removal efficiency and rework percentage across multiple projects

Loss of traceability, incomplete documentation, team confusion about roles, and contractual conflicts with fixed-scope agreements

Requirement errors in testing necessitate traversing back through design and implementation, causing exponential cost escalation due to Waterfall's sequential nature

Its emphasis on comprehensive upfront documentation and phase-gate reviews aligns with regulatory certification requirements like DO-178C

Waterfall requires complete stable requirements upfront; unknown UI preferences mean requirements will change after implementation, triggering costly rework

The Waterfall should include a preliminary iteration before main development, and feedback loops between phases are necessary

Requirements phase

Integration of subsystems developed in parallel reveals interface mismatches that cannot be detected until the integration phase, causing cascading rework

Complete and stable requirements can be elicited and frozen before any design or implementation begins

The broken windows theory (Hunt and Thomas) suggests that visible neglect (bad code, commented-out code, inconsistent style) signals that the codebase has no standards, inviting further neglect — empirically, modules with high existing defect density attract disproportionately more new defects

Maintenance cost models predict long-term maintenance effort — empirical research (Boehm, Jones, Lehman) consistently identifies the system's structural complexity (cyclomatic complexity, coupling, cohesion) and the quality of design documentation as the dominant maintenance cost drivers

Producing exhaustive documentation at each phase gate consumes significant effort and yields artefacts that become outdated and rarely consulted

System Design allocates requirements to hardware and software components and establishes overall architecture; Software Design details the internal structure of software modules

That the delivered system meets contractual requirements and is fit for deployment; owned by the customer/client

When accumulated risk is sufficiently low and the product satisfies stakeholder objectives

Spiral makes risk analysis an explicit mandatory activity driving iteration decisions; Incremental focuses on delivering planned functionality without formalised risk assessment

Small, well-understood projects with fixed budgets and tight deadlines where risk analysis overhead outweighs benefits

Cumulative cost incurred for the project up to that point