version: 1
id: error-budget-policy
title: Write an Error Budget Policy
summary: Calculate remaining error budget from an SLO burn and document freeze, slow-roll, and page gates for payments-api.
difficulty: intermediate
estimatedMinutes: 30
prerequisites: [slo-definition-basics]
image: alpine:3.21
shell: /bin/sh
setup:
  - "mkdir -p /workspace/slo /workspace/docs"
  - "printf 'slo_target_percent=99.9\\nsli_percent=99.7\\nwindow_days=30\\n' > /workspace/slo/inputs.env"
  - "printf 'Ticket: payments-api burned budget early.\\nCompute remaining budget and write freeze/slow-roll/page policy.\\n' > /workspace/slo/TICKET.md"
tasks:
  - id: budget-math
    title: Compute remaining budget
    description: 'Using inputs.env, allowed failure percent is (100 - 99.9) = 0.1. Actual failure percent is (100 - 99.7) = 0.3. Write allowed failure percent to /workspace/slo/allowed.txt as 0.1, actual failure percent to /workspace/slo/actual.txt as 0.3, and remaining budget percent to /workspace/slo/remaining.txt as -0.2 (actual exceeds allowed).'
    hints:
      - 'Tip code BUDGET_MATH: remaining ≈ allowed_failure - actual_failure (negative means over budget).'
      - 'allowed.txt should be 0.1; actual.txt should be 0.3; remaining.txt should be -0.2.'
      - 'Negative remaining means the freeze gate should engage.'
    checks:
      - type: file
        name: Allowed failure recorded
        path: /workspace/slo/allowed.txt
        value: "0.1"
      - type: file
        name: Actual failure recorded
        path: /workspace/slo/actual.txt
        value: "0.3"
      - type: file
        name: Remaining budget recorded
        path: /workspace/slo/remaining.txt
        value: "-0.2"
  - id: policy-gates
    title: Document freeze and page gates
    description: 'Create /workspace/docs/BUDGET_POLICY.md with headings Freeze, Slow-roll, and Page. Freeze must mention deploy or release freeze when over budget. Slow-roll must mention canary or reduced blast radius when budget is low. Page must mention burn or page when budget drains fast. Write /workspace/slo/STATUS.txt containing BUDGET_OK.'
    hints:
      - 'Tip code FREEZE_GATE: over budget means pause risky deploys until burn slows.'
      - 'Tip code BURN_FAST: fast budget drain pages on-call; slow drain opens a ticket.'
      - 'STATUS.txt should include BUDGET_OK.'
    checks:
      - type: file
        name: Freeze section present
        path: /workspace/docs/BUDGET_POLICY.md
        value: Freeze
      - type: file
        name: Slow-roll section present
        path: /workspace/docs/BUDGET_POLICY.md
        value: Slow-roll
      - type: file
        name: Page section present
        path: /workspace/docs/BUDGET_POLICY.md
        value: Page
      - type: command
        name: Freeze mentions deploy freeze
        command: "grep -A3 '^#* *Freeze' /workspace/docs/BUDGET_POLICY.md | grep -Eqi 'deploy|release|freeze'"
      - type: command
        name: Slow-roll mentions canary
        command: "grep -A3 '^#* *Slow-roll' /workspace/docs/BUDGET_POLICY.md | grep -Eqi 'canary|blast'"
      - type: command
        name: Page mentions burn
        command: "grep -A3 '^#* *Page' /workspace/docs/BUDGET_POLICY.md | grep -Eqi 'burn|page'"
      - type: file
        name: Budget status ok
        path: /workspace/slo/STATUS.txt
        value: BUDGET_OK
limits: {cpus: "0.5", memory: 128m, pids: 64, timeout: 1800, network: false}