alkcall/docs/architecture/decisions/047-openable-alpns-are-operations.md

# ADR-047: Openable ALPNs Are Operations

## Status

Accepted (amends ADR-037; refines ADR-044, ADR-046)

## Context

The channels spec (ADR-037) framed channel lifecycle as **one generic
`channel/open` operation** whose input carries the authorization-relevant
facts (`alpn`, `params`, `direction`) inside its JSON body. The call
protocol's ACL machinery (`AccessControl::check` on `OperationSpec`) runs
*before* the handler — but the generic op hides everything the ACL would
need to see inside one op's input, where the call ACL machinery can't
reach it.

The research findings
(`/workspace/@alkdev/alknet/docs/research/call-channels-unification/
findings.md`) surfaced the structural miss and the resolution together:

- **The miss.** Per-ALPN scopes (`tty:open` vs `tunnel:open`) have no
  enforcement home. ADR-011's `resource_id_path` (JSON pointer into input
  for the resource ID, checked by the ACL before the handler runs) can't
  work on a single generic op — the pointer differs per ALPN
  (`/params/container` for tty-docker, `/params/target` for tunnel). The
  two `direction` values are wildly different grants ("may you consume my
  TTY" vs "may you register a service I will consume as a client")
  squeezed under one ACL. The shape re-commits the structural miss
  ADR-024 (superseding ADR-023) was written to avoid: a parallel
  authorization system duplicating `AccessControl` one layer down.
- **The resolution.** An openable ALPN is an operation. Each openable
  ALPN registers its own ops on the call `OperationRegistry`, with its
  own `access_control`, `input_schema`, `resource_id_path`, and a
  `channel_open` marker that tells the channels layer "this op's stream
  is binary, not JSON." The call protocol's existing
  `AccessControl::check` is the ACL — unchanged. The `direction` field is
  gone; `OperationType` carries the direction (`Sub` = consumer
  subscribes, `Pub` = producer publishes), building on ADR-046.

This is the cheapest moment to amend: no channels code exists, no
deployments exist, the develop branch is pre-alpha. ADR-037's four op
names were declared one-way doors, but ADR-035/046 just demonstrated that
amendment is the normal mode here.

### What the research findings resolved (Gap A, E)

- **Gap A** (Pub handler shape) — resolved by ADR-046:
  `HandlerKind::Sink` / `SinkHandler` / `invoke_sink()`. The initiator
  streams *to* the responder via `call.published` events; the handler
  consumes the stream and returns a single `ResponseEnvelope`.
- **Gap E** (`OperationEnv::channel_manager()` couples call to channels)
  — resolved by the extension-trait pattern: an
  `alknet-channels-call`-local trait (`ChannelOperationEnv: OperationEnv`)
  adds the `channel_manager()` accessor, and the open-op wrapper
  downcasts `context.env` at invocation time. `alkcall` (the call crate)
  stays free of any channels types; the layering is preserved.

### What this ADR decides (Gap B, C, D, F, G)

- **Gap B** (hub broker) — **out of scope for alkcall.** The broker
  (topic registry, `Pub`↔`Sub` matching by `(op_name, params_hash)`,
  N-consumer fan-out) is a hub/consumer concern, not an alkcall concern.
  alkcall provides the `Pub`/`Sub` primitives (ADR-046) and the channel
  machinery (this ADR); the hub composes the broker on top, the same way
  the hub relay composes on top of call ops. This ADR names it as
  out-of-scope so it stops re-tangling every channels-control-plane
  conversation.
- **Gap C** (`from_call` relay wrapper) — the `from_call` forwarding
  handler for a marked op wraps with relay machinery (forward + allocate
  local-leg channel + record id mapping + byte-forward pumps) instead of
  the plain forwarding stub. The wrapping lives in a separate layer
  (`from_call` consumer code, e.g. the hub), not in alkcall's `from_call`
  core, preserving the layering. alkcall's `from_call` reconstructs the
  `channel_open` marker from discovery (Gap F) so the consumer can branch
  on it.
- **Gap D** (`channel_id` allocation in Pub case) — the real invariant is
  "the side that holds the `ChannelManager` allocates." In the `Sub` case
  that's the responder (the side that received the open op); in the `Pub`
  case that's the initiator (the side that sent the open op, which owns
  its own channels connection). ADR-037's "responder allocates" is
  amended to "connection-owner allocates."
- **Gap F** (`channel_open` marker wire format) — the marker is a boolean
  field `"channel_open": true` on the `services/schema` payload. The ALPN
  is derivable from the op name (`channels/<alpn>/sub` → ALPN
  `alknet/<alpn>`); the marker is the dispatch hint, not a carrier for
  the ALPN string. `spec_to_json` emits it; `rebuild_spec_for` parses it.
- **Gap G** (`resource_id_path` ACL vs handler ownership) — complementary,
  not redundant. The ACL check (via `resource_id_path` +
  `OwnershipProvider`) is the coarse gate ("may this identity touch
  resources of this type, and if a specific resource is targeted, does
  this identity own it?"). The handler's ownership check is ALPN-specific
  business logic the ACL can't express ("is this container in a state
  that allows TTY attachment?"). The two run at different layers and
  answer different questions.

## Decision

### 1. `channel/open` dissolves into per-ALPN ops

The generic `channel/open` operation (ADR-037) is **removed**. Each
openable ALPN registers its own ops on the call `OperationRegistry`,
named `channels/<alpn>/sub` and/or `channels/<alpn>/pub`:

| Op | `OperationType` | Initiator role | Responder role | Stream |
|----|-----------------|----------------|----------------|--------|
| `channels/<alpn>/sub` | `Sub` | consumer (subscribes) | producer (streams) | server→client |
| `channels/<alpn>/pub` | `Pub` | producer (publishes) | consumer (receives) | client→server |

An ALPN may register one or both ops. A peer that may subscribe is not
the same grant as a peer that may publish — separate op types, separate
ACLs.

The `direction` field from ADR-037 is **removed**. `OperationType`
carries the direction. The "who writes first" question is
ALPN-specific (determined by the handler's `params` contract), unchanged
from ADR-037's "the channels layer does not enforce write order."

**The generic ops stay:** `channel/close`, `channel/control`,
`channel/resources/subscribe` remain generic (keyed by `channel_id`),
registered by `channels-call` at assembly time. Only `channel/open`
dissolves.

### 2. `channel_open` marker on `OperationSpec`

`OperationSpec` gains an optional `channel_open` field — the marker that
tells the channels layer "this op's stream is binary, allocate a channel
for it":

```rust
pub struct OperationSpec {
    // ... existing fields ...
    pub channel_open: Option<ChannelOpenSpec>,
}

pub struct ChannelOpenSpec {
    pub alpn: &'static str,  // e.g., b"alknet/tty" as str
}
```

The marker is **registry metadata, not auth machinery** — parallel to
how `resource_id_path` tells ADR-011 where to find the resource ID. The
op's `access_control` is the ACL (unchanged); the marker is the dispatch
hint. The `OperationRegistry` is opaque to the marker; it's
`channels-call` that reads it.

**Wire-visible.** The marker survives discovery serialization — it is
part of the `services/schema` payload, not just the in-process struct.
`spec_to_json` emits `"channel_open": true` when set (boolean — the ALPN
is derivable from the op name). `rebuild_spec_for` parses it back. A
`from_call` importer can branch on the marker to wrap with relay
machinery (Gap C).

**Marked ops invoked outside a channels session.** `channels/tty/sub` is
registered on the call registry — which means it's also visible/invocable
on a bare top-level `alknet/call` connection, where there is no
`ChannelManager`. The open-op wrapper resolves `channel_manager()` at
invocation time via the extension trait (Gap E); if it returns `None`,
the wrapper returns `channel:no_channels_session`.

### 3. `ChannelCore` wrapper (the open-op composition seam)

The ALPN crate provides:
- The `OperationSpec` (with `channel_open` marker, `access_control`,
  `input_schema`, `resource_id_path`).
- The open handler (ALPN-specific work — validate params, consult
  ownership, prepare the backend, return a "channel plan" — a
  `ProtocolHandler` to spawn on the channel's `BiStream`).

`channels-call` provides:
- The `ChannelCore` (channel-id allocation, `ChannelManager` integration,
  per-connection opener ledger, `ChannelLifecyclePolicy` consultation,
  teardown hooks).
- A `register_openable(spec, open_handler, channel_core)` helper that
  wraps the ALPN's open handler with the channel machinery and registers
  the op on the call `OperationRegistry`.

The wrapper shape (not the invoke shape) is preferred: the ALPN's open
handler returns a channel plan; channels-call's wrapper does the
allocation/ledger/policy/spawn. The alternative (the handler calls
`context.channel_core.open(...)` itself) would require either
`OperationContext` to carry a `channel_core` reference (inverting the
call → channels-call dependency) or a generic extension mechanism on
`OperationContext` (more complexity than the wrapper). The exact API
shape of the channel plan is a two-way-door implementation detail; the
architectural point is the wrapper.

### 4. Per-connection `ChannelManager` resolution (Gap E)

The `register_openable` helper registers ops at assembly time (Layer 0,
curated, static per ADR-019). But the wrapper needs the
**per-connection** `ChannelManager` — the op arrives on channel 0 of one
specific channels connection, and the channel must be allocated on
*that* connection's manager. A globally-registered handler closing over
a static `ChannelCore` has no way to know which channels connection
invoked it.

The resolution: an extension trait in `channels-call` (not in the call
crate's core `OperationEnv` trait) adds the `channel_manager()` accessor:

```rust
// in channels-call
pub trait ChannelOperationEnv: OperationEnv {
    fn channel_manager(&self) -> Option<&ChannelManager>;
}
```

The wrapper handler downcasts `context.env` to
`&dyn ChannelOperationEnv` at invocation time — static registration,
dynamic resolution. If the downcast fails (no channels session), the
wrapper returns `channel:no_channels_session`. This keeps `alkcall`'s
call crate free of any channels types and preserves the layering
(ADR-044). The `OperationEnv` is already the integration point for
per-connection state (ADR-019); adding a `ChannelManager` accessor via
an extension trait is the natural extension.

Each channels connection's `OperationEnv` overlay carries its own
`ChannelManager` reference, so nested connections resolve correctly.

### 5. `channel_id` allocation: connection-owner allocates (Gap D)

ADR-037's "responder allocates" invariant is amended: **the side that
holds the `ChannelManager` for that connection allocates the
`channel_id`.** In the `Sub` case that's the responder (the side that
received the open op — it owns its channels connection). In the `Pub`
case that's the initiator (the side that sent the open op — it owns its
own channels connection).

This is the same invariant stated correctly: the connection owner
allocates. The "responder allocates" framing was a special case that
happened to hold for `Sub` (the common case) but broke for `Pub` (the
initiator owns the connection it's publishing from).

### 6. The discovery split

- **"What may I open"** (static, per-op): `services/list` (visibility-
  filtered + `AccessControl::check(calling_peer_identity)` server-side,
  per ADR-024 §6) + `services/schema` (per-op `access_control` and
  `channel_open` marker). The existing server-side ACL-filtered
  discovery is preserved; the spec is the authority. `channel:forbidden`
  on the open op is the real check; the preview is "here's what the spec
  says, you can fail fast."
- **"What is currently there"** (dynamic, ALPN-level):
  `channel/resources/subscribe`. Each ALPN crate that registers open ops
  also provides a resource enumerator (which containers are running,
  which TTY sessions are active). `channel/resources/subscribe`
  aggregates across all registered openable ALPNs. The data source lives
  in the ALPN crate, not in channels-call.

The `access` preview in `resources/subscribe` (ADR-037) becomes
redundant — it's on the op spec, available via `services/schema`. It is
dropped from the `resources/subscribe` payload; the spec is the
authority, and carrying a preview in a different shape invites
staleness.

### 7. Quota lifecycle: the opener ledger (Gap 2 from findings)

ADR-041's `ChannelLifecyclePolicy::on_close(&op_ctx.identity)` is called
from the `channel/close` handler with the **closer's** identity, not the
opener's — and is not called at all on transport drop. A peer whose
connection dies at cap is permanently at cap (a self-DoS).

The fix: `channels-call` keeps a per-connection opener ledger
(`channel_id → opener PeerId`). The decrement is keyed by the opener
(from the ledger), not the closer. The decrement is called from **every
teardown path** — close received, close sent locally, handler exit,
connection drop — not just `channel/close`. The ledger entry is removed
atomically with its decrement (teardown paths can race; a
double-decrement under-counts and weakens the cap).

The `ChannelLifecyclePolicy` trait shape (`check_open`, `on_close`)
survives; the change is where `on_close` is called from and where the
opener identity comes from. `channels-core` stays auth-blind (the
ledger lives in `channels-call`, not `channels-core`).

### 8. ALPN category reframe

ADR-086 §4 (alknet source) split the foundational handlers into
"channels data-channel ALPNs" and "SSH (endpoint ALPN wrapping
channels)." Under the unified model, the first category dissolves —
they're **call apps** (the same shape as docker). They register ops on
the call `OperationRegistry`. Some ops return JSON; some ops carry the
`channel_open` marker and produce a binary stream. The endpoint
(channels or bare call) determines the framing, not the app.

The ALPN crates served under channels (tty, tunnel, socks5, fs, sftp)
stop being "just ALPNs" and become call apps. They inherit call's
auth/composition/identity by construction (they *are* call apps). The
binary-stream part is the `channel_open` marker on the op spec. SSH
stays distinct (endpoint ALPN wrapping channels).

## Consequences

**Positive:**

- Per-ALPN ACLs have an enforcement home: each openable ALPN's op has
  its own `access_control`, checked by `OperationRegistry::invoke`
  before the handler runs, like every other op. No new auth machinery.
- ADR-011's `resource_id_path` works for channel-open ops: the path is
  per-op (`/params/container` for tty-docker), not per-ALPN-branch-of-a-
  generic-op. The coarse ownership gate runs before the handler.
- The `direction` field is gone; `OperationType` (`Sub`/`Pub`) carries
  the direction. Separate op types → separate ACLs. The hub-as-proxy
  pattern (worker publishes, hub owns, consumers subscribe) composes on
  top.
- The `channel_open` marker is the dispatch hint — orthogonal to the
  ACL. The same `Pub`/`Sub` model works for JSON streams (marker absent)
  and binary streams (marker present).
- The ALPN crates are call apps — the gating is a consequence, not the
  definition. They inherit call's auth by construction.
- The per-connection opener ledger fixes the quota self-DoS (Gap 2).
- The extension-trait pattern (Gap E) keeps the call crate free of
  channels types.

**Negative:**

- `OperationSpec` gains a field (`channel_open: Option<ChannelOpenSpec>`,
  defaults `None`). Every spec-constructing site adds the field,
  defaulting to `None`. This is a mechanical, additive change — the
  `Option`/`None`-default keeps it non-breaking.
- The ALPN→path-segment mapping (`alknet/tty` → `tty`) needs pinning.
  The op name `channels/tty/sub` implies the path segment is `tty`;
  non-`alknet/*` ALPNs need a rule. The rule: the path segment is the
  ALPN with the `alknet/` prefix stripped; ALPNs without that prefix
  use their full ALPN string as the path segment (rare case, two-way-
  door).
- The `from_call` relay wrapper (Gap C) is a consumer concern, not in
  alkcall's `from_call` core. The consumer (hub) wraps marked ops with
  relay machinery after `from_call` returns. This preserves the
  layering but means the hub has more to do than a plain
  `register_imported_all`.
- The broker (Gap B) is out of scope. The `Pub`/`Sub` primitives are the
  building blocks; the hub composes the broker on top. Designing the
  broker blind would produce a half-design.

## Door type

**One-way (control-plane shape).** The per-ALPN op names
(`channels/<alpn>/sub`, `channels/<alpn>/pub`), the `channel_open`
marker on `OperationSpec`, and the removal of `channel/open` /
`direction` are one-way: once consumers register open ops and clients
call them by name, changing the shape requires a protocol migration.
The `ChannelCore` wrapper shape, the extension-trait pattern, and the
opener ledger are two-way-door implementation details within the
one-way decision.

The `channel_open` marker's wire shape (`"channel_open": true` boolean)
is one-way: `services/schema` consumers will read it, and changing the
shape would break them. The `ChannelOpenSpec` in-process struct carrying
the ALPN is a two-way-door detail (the wire shape is boolean; the
in-process struct is free to evolve).

## References

- ADR-037: channel lifecycle operations (amended by this ADR —
  `channel/open` dissolves; generic ops stay; `direction` removed)
- ADR-041: per-identity channel cap (amended by this ADR — the opener
  ledger; the trait shape survives)
- ADR-046: Publish operation type and `HandlerKind::Sink` (the
  `Pub`/`Sub` primitives this ADR builds on; Gap A resolved)
- ADR-024: peer-graph routing model (the `AccessControl::check` path
  this ADR preserves; the precedent for avoiding a parallel auth
  system)
- ADR-011: dynamic resource ownership (`resource_id_path` works again
  under per-ALPN ops)
- ADR-019: operation registry layering (`OperationEnv` as the
  integration point; the extension-trait pattern extends it)
- ADR-044: channels sub-crate decomposition (the layering this ADR
  preserves — channels types stay out of the call crate)
- ADR-042: hub relay (the relay contract unchanged in shape; the op
  name changes, the marker replaces prefix-matching)
- `/workspace/@alkdev/alknet/docs/research/call-channels-unification/
  findings.md` — the research that surfaced the unification and the
  gaps this ADR resolves