alknet/docs/architecture/decisions/009-one-way-door-decision-framework.md

ADR-009: One-Way Door Decision Framework

Status

Accepted

Context

Not all architectural decisions carry the same reversal cost. Some decisions are easy to change later — if you pick the wrong data structure, you refactor. Other decisions are nearly impossible to reverse — if you build a type hierarchy that forecloses WASM compatibility, every handler written against that hierarchy must be rewritten.

This distinction matters especially during Phase 0 (exploration) and early Phase 1 (architecture). The project is post-pivot with foundational ADRs in place but no implementation code yet (except alknet-vault). Decisions made now shape the API surface that every handler depends on.

Without an explicit framework, one-way doors can be treated as casually as two-way doors, leading to costly rework. Or conversely, two-way doors can be over-analyzed, blocking progress on decisions that are cheap to reverse.

Decision

Classification

Every architectural decision is classified by reversal cost — how expensive it is to undo if you got it wrong:

One-way door — Reversing this decision requires rewriting significant code across multiple crates or permanently closes a capability door. Getting it wrong is expensive. Examples:

• BiStream as a concrete quinn type (closes WASM door permanently)
• alknet-vault pulled into alknet-core as a dependency (loses standalone property permanently)
• ProtocolHandler signature changes (every handler must be rewritten)

Two-way door — Reversing this decision is cheap or additive. Getting it wrong is recoverable. Examples:

• Static vs dynamic handler registration (can add ArcSwap later)
• Single transport vs multi-transport endpoint (can add transport trait later)
• Call protocol stream model (can add multiplexing later)

Process

• One-way doors require an ADR before implementation. If the right choice is unclear, validate with a POC before writing the ADR. If a POC can't resolve the uncertainty within a reasonable timebox, default to the option that keeps more doors open. One-way doors get the deliberation they deserve because getting them wrong is expensive.
• Two-way doors still require a decision — pick the simplest option that works, implement it, and move on. If it turns out wrong, revert and try the alternative. The decision is made; what's cheap is the reversal. Note the decision in a commit message or a brief ADR if the context is worth capturing, but don't block on it.
• When in doubt about which classification applies, classify up. If it's unclear whether a door is one-way or two-way, treat it as one-way until proven otherwise.

What this framework is NOT

This framework classifies decisions by reversal cost, not by urgency. It does not say "two-way doors can be deferred." A two-way door is a decision you make now and can revert later if needed — it's not a license to leave the decision unmade.

• Deferral is a separate concept: sometimes a decision genuinely doesn't need to be made yet because the use case isn't concrete (scope management). That's valid, but it's a scoping judgment, not a door-type classification.
• Conflating the two — using "it's a two-way door" as a reason to defer an architectural decision — leads to decisions that compound into a mess. The decision gets made by default (the implementation picks something), and downstream code builds on it, making the "cheap reversal" expensive.
• The architect's role: architecture decisions (one-way OR two-way) are for the architect to make, not the implementation agent. The implementation agent makes implementation decisions (variable names, loop order, which library to use for a parsed task). If a decision affects the system's structure, constraints, or API surface, it's an architecture decision regardless of its door type.

WASM as a design constraint

WASM compatibility is not an immediate implementation goal, but it is a design constraint on one-way doors. Decisions that would permanently prevent WASM targets from participating as peers require explicit justification. This means:

• Core types (BiStream, ProtocolHandler, AuthContext) must not assume tokio or quinn
• Protocol parsers that are pure data transformations should remain transport-agnostic
• The cost of keeping the WASM door open is low (trait vs concrete type, abstracted I/O) and the cost of closing it is high (impossible to reverse without rewriting every handler)

This is not "WASM support now." It's "don't close the WASM door accidentally."

Consequences

Positive:

• One-way doors get the deliberation they deserve — ADRs, POCs, explicit justification — because getting them wrong is expensive
• Two-way doors don't block progress — decide, implement, revert if needed — because getting them wrong is recoverable
• WASM compatibility is preserved as a constraint, not treated as an active deliverable
• The framework creates a shared vocabulary for discussing reversal cost ("is this a one-way door?")

Negative:

• Classification requires judgment — some decisions are genuinely ambiguous (mitigated: classify up when in doubt)
• POC timeboxing can feel constraining on genuine hard problems (mitigated: the timebox is "reasonable," not "arbitrary")
• The framework adds a step to every architectural discussion ("is this one-way or two-way?") — but this step is fast and prevents expensive mistakes

References

• ADR-007: BiStream type definition (one-way door: WASM compatibility)
• ADR-008: Secret service integration point (one-way door: standalone crate independence)
• SDD process: docs/sdd_process.md (Phase 0 exploration, POC specialist)
• Pivot proposal: docs/research/pivot/alpn-service-architecture.md