slo-budget-tracker 0.1.0

SLO + error-budget tracker for Python services (FastAPI middleware + Prometheus exporter)

slo-budget-tracker

SLO + error-budget tracker for Python services — drop-in FastAPI middleware, Prometheus exporter, and a small standalone library you can wire into any ASGI app or background worker.

Built around the math in the Google SRE Workbook: one rolling window, multi-window burn-rate alerts (defaults to 1h + 6h at burn rate ≥ 14.4), and an explicit error-budget remaining gauge so dashboards stop lying about reliability.

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Why

Most "SLO dashboards" you find in the wild conflate availability with uptime and surface neither error budget nor burn rate. You can't tell, at a glance, whether the freshly deployed service is burning the next 30 days of error budget in the next 30 minutes. This library makes that visible by default.

Two things matter:

1. Error budget remaining — a [1.0 → ≤0] ratio on every dashboard.
2. Burn rate — (1 − actual_success_ratio) / (1 − target), sampled at short windows so fast-burn incidents page before the budget is spent.

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Install

pip install slo-budget-tracker
# or, with the FastAPI extras:
pip install "slo-budget-tracker[fastapi]"

Python 3.11+. Single runtime dep: prometheus-client.

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Quick start — standalone library

from slo_budget_tracker import SLODefinition, SLOTracker

tracker = SLOTracker(
    SLODefinition(
        name="availability",
        target=0.999,                # three nines
        window_seconds=30 * 24 * 3600,  # 30-day rolling window
        burn_rate_windows=(3600, 21600),  # alert on 1h and 6h
        burn_rate_threshold=14.4,         # SRE workbook fast-burn page
    )
)

# Hot path — O(1)
tracker.record_success()
tracker.record_failure()

snap = tracker.snapshot()
print(f"success ratio: {snap.success_ratio:.4f}")
print(f"budget left:   {snap.error_budget_remaining:.2%}")
print(f"burn rate:     {snap.burn_rate:.2f}")

if snap.is_budget_exhausted:
    print("Freeze deploys.")

for alert in tracker.check_burn_rate():
    print(f"FAST BURN over {alert.window_seconds}s: {alert.burn_rate:.1f}x budget")

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FastAPI middleware

SLOMiddleware auto-classifies every HTTP response — by default 5xx and unhandled exceptions are failures, everything else is a success. Override with your own classifier when 4xx (or specific routes) should burn budget.

from fastapi import FastAPI
from fastapi.responses import Response
from slo_budget_tracker import (
    PrometheusExporter,
    SLODefinition,
    SLOMiddleware,
    SLORegistry,
)

registry = SLORegistry()
registry.define(SLODefinition(name="availability", target=0.999))
registry.define(SLODefinition(name="freshness",    target=0.99))

app = FastAPI()
app.add_middleware(SLOMiddleware, registry=registry, slo_name="availability")

exporter = PrometheusExporter(registry)


@app.get("/metrics")
async def metrics() -> Response:
    body, content_type = exporter.render()
    return Response(content=body, media_type=content_type)


@app.get("/slo")
async def slo_snapshot() -> dict[str, object]:
    return {"slos": [s.__dict__ for s in registry.snapshot_all()]}

Point your Prometheus scrape at /metrics and you get:

slo_target{slo="availability"} 0.999
slo_success_ratio{slo="availability"} 0.9991
slo_error_budget_remaining{slo="availability"} 0.42
slo_burn_rate{slo="availability",window_seconds="3600"} 2.1
slo_burn_rate{slo="availability",window_seconds="21600"} 0.8
slo_breached{slo="availability"} 0.0

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Custom classification

Default: anything < 500 and no exception is a success. Want 4xx to burn budget? Pass classify=:

app.add_middleware(
    SLOMiddleware,
    registry=registry,
    slo_name="availability",
    classify=lambda status, exc: exc is None and status < 400,
)

The classifier receives (status_code, exception_or_None) and returns True for success.

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API surface

Object	Purpose
SLODefinition	Frozen dataclass: name, target, window, burn-rate windows + threshold. Validates at construction.
SLOTracker	Records observations, computes snapshots and burn-rate alerts.
SLORegistry	Holds many named trackers; supports snapshot_all() and check_burn_rates().
SLOMiddleware	ASGI middleware that auto-records HTTP outcomes against a tracker.
PrometheusExporter	Renders the registry as Prometheus text format on demand.
Observation	(timestamp, success) event.
SLOSnapshot	Point-in-time view: ratios, failures, budget remaining, burn rate.
BurnRateAlert	One short window has crossed the configured threshold.
BurnRateSample	One short-window measurement attached to a snapshot.

---

Burn-rate math

error_budget   = (1 - target) * total_requests_in_window
budget_used    = failures_in_window
remaining_pct  = (error_budget - budget_used) / error_budget

burn_rate(short_window) = (1 - success_ratio(short_window)) / (1 - target)

A burn_rate == 1.0 means the service is failing at exactly the rate the SLO allows. burn_rate == 14.4 means the next 30-day budget is being eaten in ~2 days. The default threshold of 14.4 follows the SRE Workbook fast-burn page.

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Storage backends

The default InMemoryStore keeps a thread-safe deque trimmed to the window. For services pushing > ~100 rps you'll want a sampling or bucketed backend — wire one in by passing store= to SLOTracker. The protocol is small:

class ObservationStore(Protocol):
    def record(self, observation: Observation) -> None: ...
    def window(self, now: float, seconds: int) -> list[Observation]: ...
    def trim(self, before: float) -> None: ...
    def __len__(self) -> int: ...

A Redis sorted-set backend is on the roadmap (ZADD/ZREMRANGEBYSCORE); contributions welcome.

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Tests

pip install -e ".[dev]"
ruff check src tests && ruff format --check src tests
mypy src
pytest -v

The CI matrix runs Python 3.11 / 3.12 / 3.13.

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Related work in this ecosystem

This is part of the Platform Reliability Stack — small, focused libraries that compose into a production reliability story:

• procurement-decision-api — drafts AI Procurement Decision Cards from vendor Suite documents.
• reliability-toolkit-rs — async rate-limit + circuit-breaker + retry + bulkhead in Rust (coming next).
• More at kineticgain.com.

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License

MIT. See LICENSE.