fluxgate 0.9.4

A modern, composable circuit breaker library for Python

Fluxgate

A composable circuit breaker library for Python with first-class sync and async support.

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Why Fluxgate?

Circuit breakers prevent cascading failures by monitoring service health and temporarily blocking calls to failing dependencies. Most Python libraries trip on consecutive failures — brittle for any service with intermittent errors. Fluxgate trips on failure rates over a sliding window, and rules are first-class values you compose:

from fluxgate import CircuitBreaker
from fluxgate.trippers import MinRequests, FailureRate, SlowRate, FailureStreak

cb = CircuitBreaker(
    tripper=FailureStreak(5) | (MinRequests(20) & (
        FailureRate(0.5) | SlowRate(0.3, threshold=1.0)
    )),
)

Note: State is process-local and not thread-safe. For concurrency, use asyncio + AsyncCircuitBreaker, not threading.

Installation

pip install fluxgate                  # core, zero dependencies
pip install "fluxgate[prometheus]"    # +PrometheusListener
pip install "fluxgate[slack]"         # +SlackListener

Usage

import httpx
from fluxgate import AsyncCircuitBreaker
from fluxgate.trackers import TypeOf

cb = AsyncCircuitBreaker(
    tracker=TypeOf(httpx.ConnectError, httpx.TimeoutException),
    max_half_open_calls=10,
)

@cb
async def fetch(url):
    async with httpx.AsyncClient() as client:
        return (await client.get(url)).json()

CircuitBreaker mirrors this API for sync code. A tripped circuit raises CallNotPermittedError; pass @cb(fallback=...) to fall back gracefully.

How It Works

Fluxgate is a state machine. The core cycle is CLOSED → OPEN → HALF_OPEN:

stateDiagram-v2
    direction LR
    [*] --> CLOSED
    CLOSED --> OPEN: tripper
    OPEN --> HALF_OPEN: retry
    HALF_OPEN --> CLOSED: !tripper
    HALF_OPEN --> OPEN: tripper

Three additional states (METRICS_ONLY, DISABLED, FORCED_OPEN) are documented in Operational Controls.

Composable Rules

Every condition is a value and combines with & / |:

from fluxgate.trippers import (
    Closed, HalfOpened, MinRequests, FailureRate, SlowRate, FailureStreak,
)

# State-specific rules: stricter when probing recovery.
tripper = FailureStreak(5) | (MinRequests(20) & (
    (Closed()    & (FailureRate(0.5) | SlowRate(0.3, threshold=1.0))) |
    (HalfOpened() & (FailureRate(0.3) | SlowRate(0.2, threshold=1.0)))
))

Tracker (failure classification) follows the same pattern with & / | / ~.

Components

Component	Role	Examples
Window	Track recent calls (count- or time-based)	CountWindow(100), TimeWindow(60)
Tracker	Classify which exceptions count as failures	All(), TypeOf(HTTPError), Custom(fn); combine with & / | / ~
Tripper	Decide when to open the circuit	MinRequests, FailureRate, SlowRate, AvgLatency, FailureStreak, Closed/HalfOpened; combine with & / |
Retry	Trigger OPEN → HALF_OPEN	Cooldown, Backoff, Always, Never
Permit	Admit calls in HALF_OPEN	All, Random(p), RampUp(start, end, duration)
Listener	React to state transitions	LogListener, PrometheusListener, SlackListener

All components are abstract base classes (abc.ABC) with input validation — misconfigurations fail fast at construction time. Subclass to write your own.

Operational Controls

Beyond automatic trips, the breaker exposes hooks for safe rollouts and manual control:

• cb.metrics_only() — shadow mode: collect metrics without ever tripping. Ideal for validating thresholds in production before going live.
• cb.force_open() / cb.disable() — manual override during incident response or maintenance.
• cb.info() — snapshot of state, metrics, and reopen count.
• cb.reset() — return to CLOSED and clear metrics.

Monitoring

Pass listeners via listeners=... — built-in: LogListener, PrometheusListener (optional), SlackListener (optional). Each listener takes a name= identifier used in log lines, Prometheus labels, or Slack messages. State transitions emit Signal events; AsyncCircuitBreaker additionally accepts AsyncListener.

Documentation

• Full documentation — concepts, components, examples, API reference
• Comparison with other libraries — vs pybreaker, circuitbreaker, aiobreaker
• Changelog — version history and migration guides

Development

uv sync --all-extras --all-groups
uv run pytest