alkcall/docs/architecture/decisions/015-call-protocol-stream-model.md

ADR-015: Call Protocol Stream Model

Status

Accepted

Context

The call protocol (alknet-call) operates on a QUIC connection with ALPN alk/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.

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.

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?"

Decision

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:

1. EventEnvelope on every stream — every bidirectional stream opened on the alk/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.

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.

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.

4. Top-level protocol operations — the call protocol defines four operations that map to EventEnvelope event patterns:

   • call: call.requested → call.responded (one response) or call.error
   • subscribe: call.requested → one or more call.responded → call.completed or call.aborted
   • batch: multiple call.requested events (with correlated IDs) → multiple call.responded events
   • schema: call.requested (name /services/list or /services/schema) → call.responded

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.

This resolves OQ-07: the call protocol's scope within a connection is the full operation registry. One alk/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.

Consequences

Positive:

• Simple mental model: one connection, full access, stream-agnostic correlation
• The protocol works the same way regardless of stream usage — no "right" way to use streams
• Bidirectional calls are natural — either side can open a stream and send call.requested
• PendingRequestMap from the reference implementation carries forward without modification
• QUIC's stream multiplexing provides natural flow control and head-of-line blocking avoidance
• The top-level operations (call, subscribe, batch, schema) are protocol primitives, not separate ALPNs

Negative:

• Clients that multiplex many operations on one stream must manage request IDs carefully — but this is standard RPC practice
• The PendingRequestMap requires timeout-based cleanup to prevent memory leaks from abandoned requests — but this is already implemented and tested in the reference
• No built-in stream-level backpressure per operation when multiple operations share a stream — but QUIC provides connection-level and stream-level flow control

References

• ADR-013: irpc as call protocol foundation
• ADR-004: ALPN string convention and connection model
• ADR-005: BiStream type definition
• OQ-07: Call protocol scope within a connection (resolved by this ADR)
• Reference implementation: /workspace/@alkdev/alknet-main/crates/alknet-core/src/call/