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API

Rate Limitsv1

How rate limiting works and how to build reliable backoff.

What to expect

Rate limits protect the API and keep tail latency predictable.

When you exceed a limit, you'll receive 429 Too Many Requests.

Design for 429s

Treat 429s as a normal condition. The happy path is: backoff, retry later, and avoid thundering herds.

  1. Add jittered backoff

Use exponential backoff with jitter.

  1. Cap retries

Retries are not infinite. Surface the failure when you hit a ceiling.

  1. Spread load

If you send in bursts, prefer batching (one transmission with multiple recipients) or smoothing.

Example backoff (TypeScript)

ts
function sleep(ms: number) {
  return new Promise((r) => setTimeout(r, ms));
}
 
function jitter(ms: number) {
  return Math.floor(ms * (0.5 + Math.random()));
}
 
for (let attempt = 0; attempt < 5; attempt++) {
  const res = await fetch("https://api.sendlib.com/v1/me", {
    headers: { Authorization: `Bearer ${process.env.SENDLIB_API_KEY}` },
  });
 
  if (res.status !== 429) {
    if (!res.ok) throw new Error(await res.text());
    console.log(await res.json());
    break;
  }
 
  const base = 250 * 2 ** attempt;
  await sleep(jitter(Math.min(base, 5000)));
}

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