provider-router

provider-router

Route one capability across interchangeable providers: preferred ordering, automatic failover on rate limits and transient errors, and a single normalized result contract.

Zero runtime dependencies. ESM, types included, Node 20+.

npm install provider-router

A Python port with identical semantics ships as provider-router on PyPI. Both are driven by the same shared test vectors, so they cannot quietly drift apart.

The problem

You have three ways to get the same thing — one public API, one that needs no key, one that is a scraper. They return different shapes, they fail in different ways, and the good one starts returning 429 on a Tuesday.

The failover part is easy — it's a list. The part that isn't easy is that every provider says "I'm rate limited" in a different language:

Provider	How it says rate limited
One API	HTTP 429 with a Retry-After header
Another	HTTP 200, and the payload says "rate limit exceeded"
A scraper	an unparseable block page
A geocoder	HTTP 200, and the field you needed is just missing

A router can't parse those. Your adapter can. That translation — provider dialect into one shared vocabulary — is what this library is actually for.

Adapters are your code

This package ships no adapters, and that is deliberate. An adapter encodes your providers, your failure dialects, and your normalization choices; shipping a catalog of them would mean a dependency per provider and a library that ages at the speed of other people's APIs.

What ships instead is the contract they implement, the router that drives them, and a conformance test that holds them honest. Adapters live in your app, usually 100–200 lines each. Reference implementations against three real keyless providers live in examples/ — copy them, don't install them.

Quickstart

import {
  type Attempt,
  type Deadline,
  type Failure,
  Outcome,
  rateLimited,
  Router,
  terminal,
  transient,
} from 'provider-router';

class Nominatim implements Provider<string, Coordinates> {
  readonly name = 'nominatim';

  supports(_address: string): boolean {
    return true;
  }

  async invoke(address: string, deadline: Deadline): Promise<Coordinates> {
    return geocode(address, { timeoutMs: deadline.remaining() * 1000 });
  }

  classify(error: unknown): Failure {
    if (error instanceof HttpError && error.status === 429) {
      return rateLimited(error.message, error.retryAfter, error);
    }
    if (error instanceof TypeError) return transient('fetch failed', error);
    return terminal(String(error), error);
  }

  assess(result: Coordinates, _attempt: Attempt): Outcome {
    // 200 with no coordinates is a success by every transport measure
    // and useless to the caller.
    return result.lat !== undefined ? Outcome.OK : Outcome.DEGRADED;
  }
}

const router = new Router([new GooglePlaces(), new Nominatim()], {
  defaultTimeout: 20,
});
const result = await router.invoke('1600 Amphitheatre Pkwy');

result.value; // the answer
result.provider; // who served it — callers comparing over time need this
result.degraded; // whether you got the good version

What a provider owes the router

Four methods. The router never learns what the provider is.

Method	Contract
supports(request)	Can you honor every constraint? Return false rather than dropping one.
invoke(request, deadline)	Do the work or throw. Bound your own retries by the deadline.
classify(error)	Translate your error into RATE_LIMITED / TRANSIENT / TERMINAL / UNSUPPORTED / BUDGET.
assess(result, attempt)	Judge your own success. DEGRADED if it's thin.

classify deliberately has no default on BaseProvider. Guessing that an unrecognized error is transient is how a permanent misconfiguration becomes an infinite failover loop.

Design decisions, and why

Order is preference. No hidden scoring. Want a different order, pass a different array.

Failover happens at the whole-invocation boundary, never mid-call. A router that could resume someone else's half-finished pagination would have to understand the payload — and then it isn't domain-agnostic any more.

A DEGRADED result is kept but not settled for. The router holds it and keeps trying lower-preference providers for an OK one. If none appears, the degraded result is returned rather than an error — thin beats nothing, and the caller is told which it got.

BUDGET aborts the route. Failing over after a spend ceiling trips would spend more against the ceiling that just tripped.

The answering provider is always reported. Callers that compare results over time — price history, anything with a threshold — need to know the source changed, or a failover reads as a real change in the data.

supports() returning false is not a failure. It's the router refusing to let a provider answer a different question than the one asked. A search that silently drops your filter hasn't failed; it has changed what the answer is an answer to, which is worse, because nothing downstream can tell.

Circuit breaker

Per-provider, Retry-After-driven, with exponential backoff and half-open probes.

import { breakerConfig, Router } from 'provider-router';

const router = new Router([primary, backup], {
  breakerConfig: breakerConfig({ failureThreshold: 3, baseCooldown: 5 }),
  providerConfigs: {
    nominatim: breakerConfig({ minInterval: 1 }), // usage policy
  },
});

A provider that advertises Retry-After opens the circuit immediately for exactly that long — no reason to make it say so three times. A failed half-open probe re-opens with a longer cooldown rather than needing another failureThreshold failures.

State lives behind a BreakerStore interface. The default is process-local; implement get/set against a shared store to coordinate several processes.

Testing your adapters

The contract test is reusable — point it at your own adapter:

import { assertProviderContract, FailureKind } from 'provider-router';

test('my adapter satisfies the contract', () => {
  assertProviderContract(new MyAdapter(), {
    sampleRequest: query,
    sampleResult: results,
    expectedClassifications: [
      [new HttpError(429), FailureKind.RATE_LIMITED],
      [new TypeError('fetch failed'), FailureKind.TRANSIENT],
      [new Error('bad input'), FailureKind.TERMINAL],
    ],
  });
});

It catches the failures that produce routing bugs which look like provider bugs: classify throwing or returning nothing, supports throwing, a rate limit reported as terminal (so the breaker never opens and you hammer a throttled provider forever).

ManualClock makes breaker cooldowns instant and deterministic — no timers, no flake:

import { ManualClock, Router } from 'provider-router';

const clock = new ManualClock();
const router = new Router([a, b], { clock });
await router.invoke(query);
clock.advance(31); // skip a 30-second cooldown

Observability

Events are a plain callback — no logging framework imposed. A sink that throws can never fail a route.

const router = new Router([a, b], {
  events: (e) => logger.info({ event: e.name, ...e.fields }, 'route'),
});

Names are a stable contract: router.route.selected, router.failover.triggered, router.provider.circuit_open, router.provider.skipped, router.route.aborted, and others on EventName.

Status

0.1.0 — the API is not frozen yet.

Idempotency is deliberately not addressed: failover re-issues a request against a different provider, which is safe for reads and not safe for writes. This library is built for read paths.

License

MIT