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60 lines
2.8 KiB
Markdown
60 lines
2.8 KiB
Markdown
# Interface versioning and deprecation policy
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This explains how the MSD Host-Integration Interface (ADR-1239) evolves over
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time — what is an additive change, what is breaking, how `PROTOCOL_VERSION`
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bumps, and what deprecation window host-plugin authors can rely on. The policy
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exists because a published SDK is a long-term compatibility commitment (Hyrum's
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Law: once external hosts depend on it, the observable behavior is the contract).
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---
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## The version knob: `PROTOCOL_VERSION`
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`PROTOCOL_VERSION` is a positive integer carried by the engine and exchanged in
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every negotiation (in-process and serialized). It governs the negotiated
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capability **set** — which axes, values, and adapter shapes exist. A host
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declares the version it targets; the engine negotiates down to
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`min(host, engine)` and warns when a host declares a newer version than the
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engine (capabilities beyond the engine's version are not trusted).
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## Additive vs. breaking
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- **Additive (no version bump required):** adding a new *optional* axis value, a
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new adapter option, or a new reference profile. Existing host-plugins keep
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working — the engine negotiates to their declared version and ignores
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unknowns. Additive changes are the default; the interface prefers growing the
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vocabulary over changing it.
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- **Breaking (requires a `PROTOCOL_VERSION` bump):** removing or renaming an
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axis, changing an adapter's call/return shape, changing the negotiation result
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shape, or narrowing a value set. A breaking change is never silent — it lands
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behind a version bump so a host that declares the old version negotiates down
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and gets a warning, not a crash.
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## Deprecation window
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A value or adapter shape is **deprecated**, not removed, for at least **one
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minor release cycle**. During the window:
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1. The old form still works at its declared version.
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2. The reference docs mark it deprecated with the successor + the removal version.
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3. The negotiation emits a `warnings` entry when a host uses a deprecated value.
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Removal is a breaking change and follows the version-bump rule above. The
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`undocumented` sentinel is never deprecated — it is the permanent fail-closed
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fallback for a host that omits an axis.
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## What this means for host-plugin authors
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- Pin to a `PROTOCOL_VERSION` and assert on it in your host-plugin's test.
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- Treat unknown axis values / adapter options as additive (ignore, don't crash).
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- Watch negotiation `warnings` — they surface both "host declared a newer
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version" and "host used a deprecated value."
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- The serialized handshake's consistency with the in-process negotiation is
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itself part of the contract: a change that breaks that consistency is breaking.
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## See also
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- [Reference: the Host-Integration Interface](../reference/host-integration-interface.md)
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- ADR-1239 (Status: Accepted)
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