Customer.io Rate Limits

Prerequisites

• Current provider limit documentation/plan, a measured baseline, idempotency design, and a named capacity owner.
• Synthetic test payloads and alerting for 429s, queue age, delivery failure, and duplicate-event risk.

Output

• A rate-aware client/workflow configuration with bounded concurrency, backoff, idempotency, monitoring, and a safe replay decision path.
• An aggregate load-test or incident receipt showing limit behavior without recipient payloads.

Examples

Send synthetic events at gradually increasing concurrency below the approved limit, record 429s and queue age, and apply exponential backoff with jitter using a stable idempotency key. If throttling occurs, reduce concurrency and allow recovery; do not retry every failed event immediately or widen sends to compensate.

Overview

Understand Customer.io's API rate limits and implement proper throttling: token bucket limiters, exponential backoff with jitter, queue-based processing, and 429 response handling.

Rate Limit Reference

These are approximate. Customer.io uses sliding window rate limiting. When exceeded, you get a 429 Too Many Requests response.

Instructions

Step 1: Token Bucket Rate Limiter

// lib/rate-limiter.ts
export class TokenBucket {
  private tokens: number;
  private lastRefill: number;

  constructor(
    private readonly maxTokens: number = 80,  // Stay under 100/sec limit
    private readonly refillRate: number = 80   // Tokens per second
  ) {
    this.tokens = maxTokens;
    this.lastRefill = Date.now();
  }

  private refill(): void {
    const now = Date.now();
    const elapsed = (now - this.lastRefill) / 1000;
    this.tokens = Math.min(this.maxTokens, this.tokens + elapsed * this.refillRate);
    this.lastRefill = now;
  }

  async acquire(): Promise<void> {
    this.refill();
    if (this.tokens >= 1) {
      this.tokens -= 1;
      return;
    }
    // Wait until a token is available
    const waitMs = ((1 - this.tokens) / this.refillRate) * 1000;
    await new Promise((r) => setTimeout(r, Math.ceil(waitMs)));
    this.tokens = 0;
    this.lastRefill = Date.now();
  }
}

Step 2: Exponential Backoff with Jitter

// lib/backoff.ts
interface BackoffOptions {
  maxRetries: number;
  baseDelayMs: number;
  maxDelayMs: number;
  jitter: number;         // 0 to 1
}

const DEFAULTS: BackoffOptions = {
  maxRetries: 4,
  baseDelayMs: 1000,
  maxDelayMs: 60000,
  jitter: 0.25,
};

export async function withBackoff<T>(
  fn: () => Promise<T>,
  opts: Partial<BackoffOptions> = {}
): Promise<T> {
  const { maxRetries, baseDelayMs, maxDelayMs, jitter } = { ...DEFAULTS, ...opts };
  let lastErr: Error | undefined;

  for (let attempt = 0; attempt <= maxRetries; attempt++) {
    try {
      return await fn();
    } catch (err: any) {
      lastErr = err;
      const status = err.statusCode ?? err.status;

      // Don't retry 4xx errors (except 429)
      if (status >= 400 && status < 500 && status !== 429) throw err;

      if (attempt === maxRetries) break;

      // Check Retry-After header (429 responses)
      const retryAfter = err.headers?.["retry-after"];
      let delay: number;

      if (retryAfter) {
        delay = parseInt(retryAfter) * 1000;
      } else {
        delay = Math.min(baseDelayMs * Math.pow(2, attempt), maxDelayMs);
      }

      // Add jitter to prevent thundering herd
      delay += delay * jitter * Math.random();
      console.warn(`CIO retry ${attempt + 1}/${maxRetries} in ${Math.round(delay)}ms`);
      await new Promise((r) => setTimeout(r, delay));
    }
  }
  throw lastErr;
}

Step 3: Rate-Limited Client

// lib/customerio-rate-limited.ts
import { TrackClient, RegionUS } from "customerio-node";
import { TokenBucket } from "./rate-limiter";
import { withBackoff } from "./backoff";

export class RateLimitedCioClient {
  private client: TrackClient;
  private limiter: TokenBucket;

  constructor(siteId: string, apiKey: string, ratePerSec: number = 80) {
    this.client = new TrackClient(siteId, apiKey, { region: RegionUS });
    this.limiter = new TokenBucket(ratePerSec, ratePerSec);
  }

  async identify(userId: string, attrs: Record<string, any>): Promise<void> {
    await this.limiter.acquire();
    return withBackoff(() => this.client.identify(userId, attrs));
  }

  async track(userId: string, event: { name: string; data?: any }): Promise<void> {
    await this.limiter.acquire();
    return withBackoff(() => this.client.track(userId, event));
  }

  async trackAnonymous(event: {
    anonymous_id: string;
    name: string;
    data?: any;
  }): Promise<void> {
    await this.limiter.acquire();
    return withBackoff(() => this.client.trackAnonymous(event));
  }

  async suppress(userId: string): Promise<void> {
    await this.limiter.acquire();
    return withBackoff(() => this.client.suppress(userId));
  }

  async destroy(userId: string): Promise<void> {
    await this.limiter.acquire();
    return withBackoff(() => this.client.destroy(userId));
  }
}

Step 4: Queue-Based Processing with p-queue

For sustained high volume, use p-queue for cleaner concurrency control:

// lib/customerio-queued.ts
import PQueue from "p-queue";
import { TrackClient, RegionUS } from "customerio-node";

const cio = new TrackClient(
  process.env.CUSTOMERIO_SITE_ID!,
  process.env.CUSTOMERIO_TRACK_API_KEY!,
  { region: RegionUS }
);

// Process at most 80 requests per second with max 10 concurrent
const queue = new PQueue({
  concurrency: 10,
  interval: 1000,
  intervalCap: 80,
});

// Queue operations instead of calling directly
export function queueIdentify(userId: string, attrs: Record<string, any>) {
  return queue.add(() => cio.identify(userId, attrs));
}

export function queueTrack(userId: string, name: string, data?: any) {
  return queue.add(() => cio.track(userId, { name, data }));
}

// Monitor queue health
setInterval(() => {
  console.log(
    `CIO queue: pending=${queue.pending} size=${queue.size}`
  );
}, 10000);

Install: npm install p-queue

Step 5: Bulk Import Strategy

For large data imports (>10K users), avoid hitting rate limits with controlled batching:

// scripts/bulk-import.ts
import { RateLimitedCioClient } from "../lib/customerio-rate-limited";

async function bulkImport(users: { id: string; attrs: Record<string, any> }[]) {
  const client = new RateLimitedCioClient(
    process.env.CUSTOMERIO_SITE_ID!,
    process.env.CUSTOMERIO_TRACK_API_KEY!,
    50  // Conservative rate — 50/sec for imports
  );

  let processed = 0;
  let errors = 0;

  for (const user of users) {
    try {
      await client.identify(user.id, user.attrs);
      processed++;
    } catch (err: any) {
      errors++;
      console.error(`Failed user ${user.id}: ${err.message}`);
    }

    if (processed % 1000 === 0) {
      console.log(`Progress: ${processed}/${users.length} (${errors} errors)`);
    }
  }

  console.log(`Done: ${processed} processed, ${errors} errors`);
}

Error Handling

Resources

• Track API Limits
• App API Reference
• p-queue npm

Next Steps

After implementing rate limits, proceed to customerio-security-basics for security best practices.