Spiral Model — major processes & flow maps

Each pass through the four quadrants is one spiral. Risk analysis (Q2) gates commitment to build (Q3).

Guide · Updated · Source

1. Spiral quadrant cycle

Spiral quadrant cycle

How each spiral pass cycles four quadrants from objectives through risk-gated build to evaluation.

  1. StartA new spiral begins with the current product and risk posture.
  2. Core steps (see walkthrough below)Q1 sets direction, Q2 manages risk, Q3 produces work, Q4 evaluates and plans.
  3. OutcomeThe spiral closes with stakeholder evaluation and the next spiral's entry criteria.

Each pass through the four quadrants is one spiral. Risk analysis (Q2) gates commitment to build (Q3).

2. Risk-driven decision (Q2 detail)

Risk-driven Q2 gate

How Q2 risk analysis decides whether the spiral continues build work or refines approach.

  1. Current stateThe spiral's scope, known threats, and unresolved uncertainties are visible.
  2. Checkpoint / gateRisk analysis evaluates whether threats are acceptable for current commitment.
  3. refine or escalateUnacceptable risk triggers refinement, scope change, or escalation.
  4. Continue flowAcceptable risk authorizes Q3 development within the bounded spiral scope.

3. Anchor-point milestone sequence

Anchor-point milestone gates

How LCO, LCA, and IOC commitment gates pace spiral investment across the program.

  1. StartThe program begins toward the first anchor-point commitment gate.
  2. Core steps (see walkthrough below)LCO, LCA, and IOC gates confirm worth, architecture fitness, and operational readiness.
  3. OutcomePassed gates authorize deeper spirals; failed gates halt or redirect investment.

Milestone count and spiral numbering are illustrative; actual projects adjust based on risk and scope.

4. Spiral scope evolution

Spiral scope evolution

How spiral scope and engineering intensity grow as confidence and risk acceptance increase.

  1. StartEarly spirals explore objectives with minimal committed scope.
  2. Core steps (see walkthrough below)Each spiral expands scope and verification depth as confidence grows.
  3. OutcomeProgressive commitment yields a mature product without premature large-scale investment.

5. Phases A–F (Spiral mapping per spiral)

Blueprint phase Typical Spiral locus Notes
A Shape Q1: Objectives, constraints Revisited each spiral with increasing detail
B Plan Q1 + Q4: Spiral planning, risk approach Plan adapts to current risk profile
C Build Q3: Development, prototyping Scope matches spiral maturity
D Verify Q3: Testing against spiral objectives Verification depth grows with product maturity
E Release Q4: Milestone review, stakeholder commit Early releases are internal (demos); later are operational
F Learn Q4 → next Q1: Feedback loop Systematic learning drives next spiral

6. Flow details (walkthrough)

Quadrant cycle — The four quadrants are not sequential phases but a repeated cycle. Q1 sets direction, Q2 manages risk, Q3 produces work, Q4 evaluates and plans. The model's power is in the Q2 gate: no commitment to expensive build work without understanding risks.

Risk-driven decision — Q2 produces actionable risk resolution: build a prototype to test feasibility, add safeguards for known threats, accept monitored risks, or change approach to avoid them entirely. The cycle repeats until risk is acceptable for the current spiral's scope.

Anchor-point milestones — LCO confirms the project is worth doing. LCA confirms the architecture can support it. IOC confirms the system is ready for users. These are commitment gates — stakeholders decide whether to invest in the next phase of spirals.

Scope evolution — Early spirals are cheap, exploratory, and may produce throwaway prototypes. As confidence grows, spirals become more engineering-intensive. This progressive commitment is the model's core risk management strategy.

7. Authoritative sources & further reading

Full curated list: External reference URLs (methodology guides).