Vercel Workflow Sdk

Vercel Workflow SDK

This skill documents how to use the Vercel Workflow SDK (workflow package) for building durable, long-running workflows.

Core Concepts

Workflows and Steps

Workflows ("use workflow" directive) are durable functions that orchestrate steps. They:

• Run in a sandboxed environment
• Must be deterministic - same inputs always produce same outputs
• Survive serverless timeouts and can resume after failures
• Should NOT perform side effects directly

Steps ("use step" directive) perform actual work. They:

• Have full Node.js/npm access
• Results are automatically persisted for replay
• Default to 3 retries (4 total attempts)
• Are where side effects (DB writes, API calls) should happen

export async function myWorkflow(input: string) {
  "use workflow";

  const result = await myStep(input);  // Step call
  return result;
}

async function myStep(input: string) {
  "use step";

  // Perform actual work here
  return await db.doSomething(input);
}

Key Constraint: Pass-by-Value

Parameters between workflows and steps are passed by value, not reference. Mutations inside steps don't propagate back - always return modified data.

Imports

import {
  getWorkflowMetadata,   // Workflow-level context
  getStepMetadata,       // Step-level context (stepId, attempt)
  sleep,                 // Durable sleep
  fetch,                 // Workflow-aware fetch with retries
  FatalError,            // Stop retries immediately
  RetryableError,        // Retry with custom delay
  createWebhook,         // Pause for external HTTP
} from "workflow";

import { start } from "workflow/api";  // Start workflows

Starting Workflows

import { start } from "workflow/api";

// Start and get Run object for tracking
const run = await start(myWorkflow, [arg1, arg2]);
console.log(run.runId);  // Unique run identifier

// For workflows with no arguments
const run = await start(myWorkflow);

The Run object provides:

• runId - Unique identifier
• status - Async getter for current status
• returnValue - Async, blocks until completion
• readable - ReadableStream for updates

Metadata Functions

getWorkflowMetadata()

Use inside workflow functions for workflow-level context:

export async function myWorkflow() {
  "use workflow";

  const { workflowRunId } = getWorkflowMetadata();
  log.info("Starting workflow", { workflowRunId });
}

getStepMetadata()

Use inside step functions for step-level context:

async function myStep() {
  "use step";

  const { stepId, attempt } = getStepMetadata();

  // stepId: Stable across retries, unique per step invocation
  // attempt: Current retry attempt number
}

Error Handling

Default Behavior

Steps automatically retry up to 3 times on errors. Configure with:

async function myStep() {
  "use step";
  // ...
}
myStep.maxRetries = 5;  // Set custom retry limit
myStep.maxRetries = 0;  // Disable retries

RetryableError

Signal that a step should retry with a specific delay:

import { RetryableError } from "workflow";

async function myStep() {
  "use step";

  const result = await someOperation();

  if (result.needsRetry) {
    throw new RetryableError("Temporary failure", {
      retryAfter: 5000,           // milliseconds
      // OR retryAfter: "5m",     // duration string
      // OR retryAfter: new Date() // specific time
    });
  }
}

FatalError

Signal that a step should NOT retry - permanent failure:

import { FatalError } from "workflow";

async function myStep() {
  "use step";

  const item = await db.findItem(id);

  if (!item) {
    throw new FatalError("Item not found");  // Don't retry
  }
}

When to use FatalError:

• Resource not found (404)
• Invalid input/state
• Permanent external failures
• Business logic rejections

Idempotency

Critical: When steps make external API calls with side effects, use stepId as an idempotency key:

async function chargeCustomer(amount: number) {
  "use step";

  const { stepId } = getStepMetadata();

  // stepId is stable across retries - prevents duplicate charges
  await stripe.charges.create({
    amount,
    idempotencyKey: `charge-${stepId}`,
  });
}

Control Flow Patterns

Sequential

const a = await stepA();
const b = await stepB(a);  // Depends on a
const c = await stepC(b);  // Depends on b

Parallel

// Independent steps run in parallel
const [a, b, c] = await Promise.all([
  stepA(),
  stepB(),
  stepC(),
]);

Fan-out with Concurrency Limit

import pLimit from "p-limit";

const limit = pLimit(5);  // Max 5 concurrent

const results = await Promise.all(
  items.map(item =>
    limit(() => processItem(item))
  )
);

Handling Errors in Parallel Processing

When processing multiple items where one failure shouldn't stop others:

const results = await Promise.all(
  items.map(item =>
    limit(async () => {
      try {
        return await processItem(item);
      } catch (error) {
        if (error instanceof FatalError) {
          return { id: item.id, success: false, error: error.message };
        }
        throw error;  // Unexpected errors propagate
      }
    })
  )
);

Sleep

Durable sleep that doesn't consume resources:

import { sleep } from "workflow";

// Duration strings
await sleep("30 seconds");
await sleep("5 minutes");
await sleep("1 hour");

// Date object
await sleep(new Date(Date.now() + 60000));

Webhooks

Pause workflow until external HTTP request:

import { createWebhook } from "workflow";

export async function orderWorkflow(orderId: string) {
  "use workflow";

  const webhook = createWebhook();

  // Store webhook.url for external system to call
  await saveWebhookUrl(orderId, webhook.url);

  // Workflow suspends here until webhook receives request
  const request = await webhook;

  return processWebhookData(request);
}

Best Practices

1. Keep Workflows Deterministic

// BAD - non-deterministic in workflow
export async function badWorkflow() {
  "use workflow";
  const random = Math.random();  // Different on replay!
}

// GOOD - move non-determinism to steps
export async function goodWorkflow() {
  "use workflow";
  const random = await generateRandom();  // Step handles it
}

Note: The SDK fixes Math.random() and Date during replay, but explicit determinism is preferred.

2. Use stepId for External API Idempotency

Always pass stepId when making API calls that have side effects.

3. Return Data from Steps

Don't mutate objects passed to steps - return modified data:

// BAD
async function badStep(data: Data) {
  "use step";
  data.processed = true;  // Won't propagate!
}

// GOOD
async function goodStep(data: Data) {
  "use step";
  return { ...data, processed: true };
}

4. Use FatalError for Permanent Failures

Don't waste retries on unrecoverable errors.

5. Separate Step Functions

Keep step functions in separate files or clearly organized to avoid bundler issues.

6. Use getWorkflowMetadata() for Run IDs

Don't generate custom IDs when the SDK provides them:

// Instead of passing ID as parameter
export async function myWorkflow() {
  "use workflow";
  const { workflowRunId } = getWorkflowMetadata();
  // Use workflowRunId for logging/tracking
}

Common Patterns

Retry with Exponential Backoff

async function stepWithBackoff() {
  "use step";

  const { attempt } = getStepMetadata();

  const result = await tryOperation();

  if (result.needsRetry) {
    // Exponential backoff: 5s, 10s, 20s, 40s...
    const delayMs = Math.min(5000 * Math.pow(2, attempt), 60000);
    throw new RetryableError("Retrying", { retryAfter: delayMs });
  }

  return result;
}
stepWithBackoff.maxRetries = 5;

Deterministic Jitter

When you need jitter for backoff but must stay deterministic:

function calculateBackoff(attempt: number): number {
  const baseDelay = Math.min(5 * Math.pow(2, attempt), 60);
  // Deterministic "jitter" based on attempt number
  const jitterPercent = ((attempt * 7) % 11) / 100;
  return baseDelay * (1 + jitterPercent);
}

Graceful Workflow Termination

export async function myWorkflow(id: string) {
  "use workflow";

  try {
    return await processWithRetries(id);
  } catch (error) {
    if (error instanceof FatalError) {
      return { success: false, reason: error.message };
    }
    // Retries exhausted
    return { success: false, reason: "max_attempts_reached" };
  }
}

Serialization

Supported types for workflow/step parameters and returns:

• JSON types: string, number, boolean, null, arrays, objects
• Date, URL, RegExp, BigInt
• Map, Set
• ArrayBuffer, typed arrays (Uint8Array, etc.)
• Headers, Request, Response

References

• Documentation: https://useworkflow.dev/docs
• API Reference: https://useworkflow.dev/docs/api-reference