vdm/docs/adr/0011-admission-control-and-the-segment-budget.md

ADR 0011 — Admission control, the segment budget, and who counts what

Status: proposed · Date: 2026-09-09 · Lane: DAEMON, needs CORE sign-off Companion request: daemon/docs/core-requests-m1.md (the engine API this depends on) Blocks: daemon/src/sched/ — no scheduler code lands before this is accepted.

Context

Two lanes are each building a global concurrency governor, neither brief mentions the other, and they are counting different things.

• CORE is adding maxActiveSegments (default 32) inside the engine — a ceiling on segments actually transferring at once. It is the mechanism that makes the RSS target in docs/04 §8 hold (core/docs/buffer-sizing.md), so it is not optional.
• DAEMON is briefed to build "a concurrency governor (global max active, per-queue max, per-host caps)", backed by connection.maxConcurrentDownloads, Queue.maxConcurrent, schedules and queue order.

Left alone this lands in one of two states, and both are bad in a way that is hard to diagnose after the fact:

1. Double-throttling. Both lanes enforce a global ceiling, so the effective limit is the minimum of two numbers the user set independently. The link runs at half rate and it reads as a performance bug in the engine, not as a policy collision.
2. The gap. Each lane assumes the other holds the line. Nobody does, 20 downloads open 160 connections, and the RSS budget that maxActiveSegments exists to defend is gone.

There is a third problem underneath both. With maxActiveSegments = 32 and connection.maxSegmentsPerDownload capped at 32, one download can hold the entire segment budget. If the daemon admits a second task and the engine has no slot to give it, the task is running and transferring nothing: every DAEMON assumption that admission implies progress — stall detection, speed accounting, queue drain, "when queue completes" — is then wrong. CORE owns that fairness rule. DAEMON cannot write a governor without knowing what it is.

Decision

1. Every ceiling is enforced exactly once, by the lane that owns the unit it counts

This is the whole ADR in one line. The two governors stay two governors, on two axes, with strictly non-overlapping units:

Ceiling	Unit	Enforced by	Configured by
connection.maxConcurrentDownloads	tasks	DAEMON	user
Queue.maxConcurrent	tasks	DAEMON	user
per-host task cap	tasks	DAEMON	host table
schedules, windows, queue order, priority	tasks	DAEMON	user
connection.maxActiveSegments	segments	CORE	user
connection.maxSegmentsPerDownload	segments	CORE	user, per task
per-host segment cap	segments	CORE	host table, pushed by DAEMON
bufferBytes / maxTotalBufferBytes	bytes	CORE	user
speed limits (global → queue → task)	bytes/s	CORE	user, pushed by DAEMON

Corollaries, and these are the parts that actually prevent the two failure modes:

• DAEMON never counts segments to make an admission decision. Not directly, and not by inferring occupancy from a download count. Its governor sees tasks.
• CORE never refuses admission. start() always accepts. The engine paces the task inside the segment budget; it does not decide that the task should not be running. A refusal would be a policy decision, and policy lives in the daemon with the queues and the SQL behind it.
• The pattern generalises: DAEMON decides policy and configures; CORE enforces every ceiling counted in engine-internal units. Rate limiting (docs/04 §6) already works this way. Recording it here so it is not re-litigated per subsystem.

2. The one legitimate coupling — a clamp, not a second enforcement

DAEMON reads maxActiveSegments in exactly one place:

effective_max_running_tasks = min(connection.maxConcurrentDownloads,
                                  connection.maxActiveSegments)

with the same clamp applied per queue against that queue's share. The purpose is narrow: never admit more concurrently-running tasks than the segment budget can give one segment each. It is expressed in tasks, it throttles nothing that CORE also throttles, and it is the reason §3's fairness rule is satisfiable.

At the shipped defaults (maxConcurrentDownloads 5, maxActiveSegments 32) the clamp is not binding. It binds when a user raises concurrency to 64 or lowers the segment budget.

3. CORE's fairness rule — what DAEMON is allowed to assume

CORE owns this. It is recorded here because DAEMON's governor is built on top of it.

1. Min-1 before seconds. No task receives a second segment slot while any admitted task holds zero. A task's first slot always outranks another task's growth.
2. Beyond the first slot, remaining budget is distributed round-robin over running tasks in the priority order DAEMON supplies, up to each task's effective per-task cap: min(spec.segments ?? maxSegmentsPerDownload, per-host segment cap, 1 if not resumable).
3. Slots are released on segment completion, pause, and failure. A steal is slot-neutral — the stealing worker already holds the slot it re-ranges.
4. A paused task holds no slots.
5. Therefore admission implies progress: a task DAEMON starts gets at least one transferring segment, subject only to the connect timeout and the per-host cap.
6. Invariant: budget().tasks_starved == 0 in steady state. DAEMON asserts this. If it observes a non-zero value persisting past 2 s it logs a governor-invariant warning and surfaces it in velox ls --json; it does not compensate by throttling admission. Compensating is how the two governors would silently grow back into one.

Consequence for the starvation question in the brief: one download cannot take the whole budget away from the next, because rule 1 makes the next task's first slot outrank the incumbent's second. A single download alone in the system does legitimately grow to 32 segments, and gives slots back as tasks arrive — growth is opportunistic, the first slot is guaranteed.

4. Per-host caps are split by unit, from one table

Both briefs say "per-host caps" and they are not the same cap.

• CORE enforces per-host segment caps — it owns the connections and is the only place segments are counted (docs/04 §3, "clamped per-host by settings").
• DAEMON enforces a per-host task cap, set to that host's segment cap, so it can never admit more tasks for one host than that host can be given one segment each. Without this, rule 3.1 is unsatisfiable: four tasks on a host capped at 4 connections is fine, five is a guaranteed starved task no fairness rule can fix.
• The table itself is DAEMON state (SQLite, settings), pushed into the engine via set_host_segment_cap(). One source of truth, two enforcement points, different units.

5. Probes do not consume segment slots

A download.probe is a HEAD or a one-byte ranged GET. Charging it against the segment budget would let a burst of probes starve transfers, and probes are on the latency path for capture.offer's 750 ms deadline. CORE bounds concurrent probes with its own small pool (proposed: 4) outside the segment budget. capture.offer never blocks on a probe under any circumstances — it answers ignore first and probes after.

6. Contract gap — connection.maxActiveSegments does not exist on the wire yet

Settings.schema.json has maxSegmentsPerDownload, bufferBytes, maxConcurrentDownloads; it has neither maxActiveSegments nor maxTotalBufferBytes. PROTO accepted the B2a follow-up bundle (contracts/proto-answers-m1.md) but it has not landed, and SettingKey is a closed enum against additionalProperties: false — a daemon cannot add the key locally without failing conformance.

Until PROTO lands it: DAEMON holds maxActiveSegments as a daemon-local value defaulted to 32, passes it to set_max_active_segments() at startup, and does not expose it through settings.get / settings.set. Filed as request C4 in daemon/docs/core-requests-m1.md (to PROTO, alongside CORE's existing ask).

Alternatives considered

DAEMON enforces both. The governor would have to predict each task's effective segment count to spend a segment budget in task units — but that count depends on the probe result, the per-host cap, the resumability demotion and live steals, all engine-internal and all changing continuously. Predicting it means either over-admitting (the gap) or leaving the link idle. Rejected: it asks the daemon to model the engine.

CORE enforces both. The engine would take every task and decide which run. That drags queues, schedules, priority, and "when queue completes" into core/, which the layering rule forbids and which would need SQL to be correct. Rejected.

A shared semaphore object handed to both lanes. Superficially the "one counter" answer, but it makes a mutable engine resource part of the daemon's API surface, inverts the dependency direction, and is the first thing that will deadlock under pause-during-steal. Rejected: one counter, one owner, read-only snapshots for everyone else.

Consequences

• daemon/src/sched/ may be written against a task-unit model only. A segment count appearing in an admission decision is a review-blocking defect.
• DAEMON needs an occupancy read-out rather than an inference — the engine API requested in daemon/docs/core-requests-m1.md §1 (budget(), segments_active(TaskId), a coalesced change callback). Without it, projecting TaskSummary.segments (which ADR 0010 pinned to the effective count) is guesswork.
• CORE's fairness rule needs a test that DAEMON can point at: N tasks admitted, budget smaller than N × their per-task caps, assert every task holds ≥ 1 segment.
• If CORE cannot honour min-1 for a reason not anticipated here, this ADR is wrong rather than incomplete — say so before sched/ exists, which is the entire point of settling it now.

Open, for CORE to confirm or amend

1. Min-1 before seconds (§3.1) — is it implementable inside the stealer without a priority inversion at slot release?
2. Probe pool outside the segment budget, size 4 (§5).
3. Does set_max_active_segments() apply live, draining as slots free rather than killing in-flight segments?
4. Priority shape: an integer, or DAEMON handing over an ordered task list per tick?
5. Coalescing rate for the budget-change callback — 4 Hz to match the event-batching rate?