@furlpay/resilience

@furlpay/resilience

Resilience primitives for financial clients. Zero runtime dependencies — native fetch, AbortController, crypto and standard collections only, so it runs unchanged on Node 20+ and Edge runtimes.

A payment path should not inherit a supply chain. Every dependency here would be another party with reach into money movement, so there are none.

What's in it

Module	Purpose
CircuitBreaker	Sliding-window breaker. Slow calls count as failures.
executeWithRetry	Full-jitter exponential backoff, Retry-After aware.
TokenBucketLimiter / SlidingWindowLimiter	Outbound rate limiting.
FailoverPool	Multi-endpoint RPC/provider failover with per-endpoint breakers.
ResilientHttpClient	The composition: limit → breaker → retry → timeout → fetch.
ResilientEventSource	SSE with heartbeat watchdog and Last-Event-ID resume.
Clock / ManualClock	Injectable time. Makes every test above deterministic.

Why some of it is shaped unusually

Slow calls trip the breaker. A provider answering 200 OK in 30 seconds does more damage than one returning 503 immediately, because callers keep waiting. A breaker that only counts errors never trips on the outage that actually hurts.

Full jitter, not exponential-plus-noise. When a provider 503s a thousand callers at once, fixed backoff retries all thousand in the same millisecond. Sampling uniformly from [0, ceiling) decorrelates them; partial jitter still leaves a spike at the interval's end.

POST is not retried without an idempotency key. A retry of a request whose response you never saw can charge twice. The request may well have succeeded and only the ACK was lost — silence is not failure.

The timeout uses a ref'd timer, not AbortSignal.timeout. Node unrefs that one, so when a request is the only pending work — the ordinary case in a serverless handler — the timeout never fires and the promise never settles. This was found by the test suite, not by review.

Each pool endpoint gets its own breaker. One shared breaker means the first bad provider trips it and the healthy ones become unreachable, turning a partial outage into a total one.

Nothing starts a background timer. FailoverPool.probe() is called by your scheduler. A library that spawns intervals keeps a serverless function alive and billing.

Usage

import { ResilientHttpClient, FailoverPool, TokenBucketLimiter } from "@furlpay/resilience";

const rpc = new FailoverPool({
  name: "solana",
  endpoints: [
    { id: "helius", url: "https://rpc-a.example", weight: 2 },
    { id: "quicknode", url: "https://rpc-b.example" },
  ],
  circuitBreaker: { failureRateThreshold: 0.5, slidingWindowSize: 20 },
});

const client = new ResilientHttpClient({
  baseUrl: "https://api.furlpay.com",
  timeoutMs: 5_000,
  rateLimiter: new TokenBucketLimiter({ capacity: 20, refillPerSecond: 10 }),
  circuitBreaker: { failureRateThreshold: 0.5 },
  retry: { maxRetries: 3, baseDelayMs: 200 },
  auth: {
    getAccessToken: () => loadToken(),
    refreshToken: () => refresh(),
  },
  telemetry: { onMetric: (m) => log.info("upstream", m) },
});

// A POST becomes retry-safe only with a key.
await client.post("/payments", { amount: 100 }, { idempotencyKey: crypto.randomUUID() });

Testing

Time is injected, not mocked. Tests drive a ManualClock directly, so "wait ten seconds for the breaker to half-open" is an assignment rather than a sleep:

const clock = new ManualClock();
const cb = new CircuitBreaker({ waitDurationInOpenStateMs: 10_000, clock });
await clock.advance(10_000);
assert.equal(cb.getState(), "HALF_OPEN");

This beats global fake timers: those are process-wide, leak between files when a restore is missed, and cannot represent two components on different clocks.

npm test   # 69 tests

MIT.