glm-5.2
cc470a363a
Port the call + channels architecture documentation from the alknet mono-repo into docs/architecture/, renumbered as alkcall ADR-001..045. Renumbering map (alknet -> alkcall): Core: 001,002,004,006,007,011,065,070,092,014,050,091 -> 001-012 Call: 005,064,012,023,015,022,024,016,049,017,028,029,030,032,066,069,067,068 -> 013-030 Shared: 003,009,013 -> 031-033 Channels: 071,093,072,073,074,075,076,094,079,080,081,089 -> 034-045 3 superseded/reversed ADRs kept for historical trail: - ADR-013 (irpc foundation, superseded by ADR-014) - ADR-023 (peer-scoped filtering, superseded by ADR-024) - ADR-077 (TTY inside channels, reversed by ADR-035 — not ported, TTY-only) Ported docs (11 spec files + README + open-questions): - call-README.md, call-protocol.md, operation-registry.md, client-and-adapters.md - channels-README.md, channels-overview.md, channels-wire.md, channels-connection.md, channels-adapter.md, channel-operations.md, channel-client.md - README.md (index with doc table, ADR table grouped by category, key principles) - open-questions.md (lean — 30 OQs, renumbered OQ-01..030; includes new OQ-22 for the pub/sub gap) Cross-reference rewriting: - All ADR-NNN references rewritten single-pass (no chaining bug) - Markdown link paths fixed - Title lines aligned with filenames - Non-ported ADR refs (052, 082, 086, etc.) left as-is with README note The open-questions.md includes OQ-22 (new): the call protocol pub/sub gap — subscribe exists but pub does not, needed for channels channel/resources/subscribe fan-out. This is the next ADR to write (alkcall ADR-046).
56 lines
4.2 KiB
Markdown
56 lines
4.2 KiB
Markdown
# ADR-015: Call Protocol Stream Model
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## Status
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Accepted
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## Context
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The call protocol (alknet-call) operates on a QUIC connection with ALPN `alknet/call`. Within that connection, QUIC provides bidirectional streams. The question is how the call protocol uses those streams and how it correlates requests with responses — especially when both sides can initiate calls.
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The reference implementation used `EventEnvelope` framing with a `PendingRequestMap` that correlates `call.requested` events to `call.responded` events by request ID, regardless of which stream carries them. This works well but the relationship between streams and operations was underspecified.
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OQ-07 asked: "What is the scope of the call protocol within a connection? Should operations be multiplexed within a single stream, or should each operation get its own stream?"
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## Decision
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The call protocol uses **bidirectional QUIC streams with EventEnvelope framing and ID-based correlation**. The protocol does not prescribe a stream usage pattern — it works with any arrangement:
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1. **EventEnvelope on every stream** — every bidirectional stream opened on the `alknet/call` connection carries length-prefixed JSON `EventEnvelope` messages. The five event types (`call.requested`, `call.responded`, `call.completed`, `call.aborted`, `call.error`) are the protocol primitives.
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2. **PendingRequestMap correlates by ID, not by stream** — the `id` field in `EventEnvelope` correlates requests with responses. A response on stream 5 can fulfill a request sent on stream 3. The PendingRequestMap is keyed by request ID.
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3. **Protocol is symmetric** — both sides of the connection can `open_bi()` to initiate calls and `accept_bi()` to receive them. The server calling a client operation uses the same EventEnvelope format and the same correlation mechanism.
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4. **Top-level protocol operations** — the call protocol defines four operations that map to EventEnvelope event patterns:
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- **call**: `call.requested` → `call.responded` (one response) or `call.error`
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- **subscribe**: `call.requested` → one or more `call.responded` → `call.completed` or `call.aborted`
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- **batch**: multiple `call.requested` events (with correlated IDs) → multiple `call.responded` events
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- **schema**: `call.requested` (name `/services/list` or `/services/schema`) → `call.responded`
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5. **Stream usage is the client's choice** — a client may open one stream per operation, one stream for all operations, or any mix. The protocol is stream-agnostic. The server accepts streams and processes EventEnvelopes regardless of which stream they arrive on.
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This resolves OQ-07: the call protocol's scope within a connection is the full operation registry. One `alknet/call` connection gives access to all operations (call, subscribe, batch, schema). QUIC's built-in stream multiplexing handles concurrency — the protocol doesn't need to impose additional multiplexing.
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## Consequences
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**Positive:**
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- Simple mental model: one connection, full access, stream-agnostic correlation
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- The protocol works the same way regardless of stream usage — no "right" way to use streams
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- Bidirectional calls are natural — either side can open a stream and send `call.requested`
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- PendingRequestMap from the reference implementation carries forward without modification
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- QUIC's stream multiplexing provides natural flow control and head-of-line blocking avoidance
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- The top-level operations (call, subscribe, batch, schema) are protocol primitives, not separate ALPNs
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**Negative:**
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- Clients that multiplex many operations on one stream must manage request IDs carefully — but this is standard RPC practice
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- The PendingRequestMap requires timeout-based cleanup to prevent memory leaks from abandoned requests — but this is already implemented and tested in the reference
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- No built-in stream-level backpressure per operation when multiple operations share a stream — but QUIC provides connection-level and stream-level flow control
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## References
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- ADR-013: irpc as call protocol foundation
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- ADR-004: ALPN string convention and connection model
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- ADR-005: BiStream type definition
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- OQ-07: Call protocol scope within a connection (resolved by this ADR)
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- Reference implementation: `/workspace/@alkdev/alknet-main/crates/alknet-core/src/call/` |