Quota Enforcement — Flow Diagram (rev 2.1)

Scope – this document explains how the free‑tier limits are enforced
inside the scanner service. For policy rationale and legal aspects see
33_333_QUOTA_OVERVIEW.md.

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0 · Key parameters (rev 2.1)

Symbol	Value	Meaning
L_anon	33	Daily ceiling for anonymous users
L_jwt	333	Daily ceiling for token holders
T_warn	200	Soft reminder threshold
D_soft	5 000 ms	Delay for first 30 over‑quota scans
D_hard	60 000 ms	Delay for all scans beyond the soft window

L_active is L_jwt if a valid token is present; else L_anon.

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1 · Sequence diagram

sequenceDiagram
    participant C as Client
    participant API as Scanner API
    participant REDIS as Redis (quota)
    C->>API: /scan
    API->>REDIS: INCR quota:
    REDIS-->>API: new_count
    alt new_count ≤ L_active
        API-->>C: 202 Accepted (no delay)
    else new_count ≤ L_active + 30
        API->>C: wait D_soft
        API-->>C: 202 Accepted
    else
        API->>C: wait D_hard
        API-->>C: 202 Accepted
    end

Counters auto‑expire 24 h after first increment (00:00 UTC reset).

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2 · Redis key layout

Key pattern	TTL	Description
quota:ip:<sha256>	24 h	Anonymous quota per hashed IP
quota:tid:<sha256>	24 h	Token quota per hashed token‑ID
quota:ip:<sha256>:ts	24 h	First‑seen timestamp (ISO 8601)

Keys share a common TTL for efficient mass expiry via redis-cli --scan.

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3 · Pseudocode (Go‑style)

func gate(key string, limit int) (delay time.Duration) {
    cnt, _ := rdb.Incr(ctx, key).Result()

    switch {
    case cnt <= limit:
        return 0             // under quota
    case cnt <= limit+30:
        return 5 * time.Second
    default:
        return 60 * time.Second
    }
}

The middleware applies time.Sleep(delay) before processing the scan request; it never returns HTTP 429 under the free tier.

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4 · Metrics & monitoring

Metric	PromQL sample	Alert
stella_quota_soft_hits_total	increase(...[5m]) > 50	Many users near limit
stella_quota_hard_hits_total	rate(...[1h]) > 0.1	Potential abuse
Average delay per request	histogram_quantile(0.95, sum(rate(...)))	P95 < 1 s expected

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Generated {{ “now” | date: “%Y‑%m‑%d” }} — values pulled from central constants.