---
title: Basic Workflow Patterns
description: Learn common patterns and techniques for building effective workflows
---

This guide explores common patterns you can use to build more complex workflows with workflows.

## Fan-out (Parallelism)

One of the most powerful features of workflows is the ability to run tasks in parallel:

```ts
import {
  createWorkflow,
  workflowEvent,
  getContext,
} from "@llamaindex/workflow-core";

// Define events
const startEvent = workflowEvent<string>();
const processItemEvent = workflowEvent<number>();
const resultEvent = workflowEvent<string>();
const completeEvent = workflowEvent<string[]>();

// Create workflow
const workflow = createWorkflow();

// Define a variable accessible within the handler scope to signal completion
let itemsToProcess = 10; // Total number of items
let itemsProcessed = 0;

// Process start event: fan out to multiple processItemEvent events
workflow.handle([startEvent], async (start) => {
  const { sendEvent, stream } = getContext();
  itemsProcessed = 0; // Reset counter for this execution context

  // Emit multiple events to be processed in parallel
  for (let i = 0; i < itemsToProcess; i++) {
    sendEvent(processItemEvent.with(i));
  }

  // Use an async IIFE to collect results and emit completeEvent
  try {
    const results = await stream
      .filter(resultEvent)
      .until(() => itemsProcessed === itemsToProcess)
      .toArray();
    // Send the final aggregated result
    sendEvent(completeEvent.with(results.map((event) => event.data)));
  } catch (err) {
    console.error("Error processing items:", err);
    // Handle error if needed
  }

  // Note: This handler finishes *before* the collection completes.
  // Returning nothing or a specific "processing started" event might be appropriate.
});

// Process each item
workflow.handle([processItemEvent], async (event) => {
  // Simulate async work
  await new Promise((resolve) => setTimeout(resolve, Math.random() * 100));
  const processedValue = `Processed: ${event.data}`;

  // Crucially, update the shared counter *after* processing
  itemsProcessed++;

  return resultEvent.with(processedValue);
});

// Example E2E Run Usage
async function runFanOut() {
  console.log("Running fan-out");
  const { stream, sendEvent } = workflow.createContext();
  sendEvent(startEvent.with("Start fan-out"));

  for await (const event of stream) {
    if (processItemEvent.include(event)) {
      console.log(`Processing item: ${event.data}`);
    } else if (resultEvent.include(event)) {
      console.log(`Result received: ${event.data}`);
    } else if (completeEvent.include(event)) {
      console.log("Final aggregated results:", event.data);
      break; // Stop processing the stream
    }
  }
}

runFanOut();
```

This pattern allows you to:

1. Emit multiple events to be processed in parallel
2. Collect results as they come in
3. Complete once all parallel tasks are finished

## Conditional Branching

You can implement conditional logic in your workflows:

```ts
import { createWorkflow, workflowEvent } from "@llamaindex/workflow-core";

const inputEvent = workflowEvent<number>();
const evenNumberEvent = workflowEvent<string>();
const oddNumberEvent = workflowEvent<string>();
const resultEvent = workflowEvent<string>();

const workflow = createWorkflow();

// Branch based on whether the number is even or odd
workflow.handle([inputEvent], (event) => {
  if (event.data % 2 === 0) {
    return evenNumberEvent.with(`${event.data} is even`);
  } else {
    return oddNumberEvent.with(`${event.data} is odd`);
  }
});

// Handle even numbers
workflow.handle([evenNumberEvent], (event) => {
  return resultEvent.with(`Even result: ${event.data}`);
});

// Handle odd numbers
workflow.handle([oddNumberEvent], (event) => {
  return resultEvent.with(`Odd result: ${event.data}`);
});

// Example E2E Run Usage
async function run(input_number: number) {
  // Create a workflow context and send the initial event
  const { stream, sendEvent } = workflow.createContext();
  sendEvent(inputEvent.with(input_number));

  // Collect all events until we get a stopEvent
  const allEvents = await stream.until(resultEvent).toArray();

  // The last event will be the stopEvent that was requested
  const finalEvent = allEvents[allEvents.length - 1];
  if (resultEvent.include(finalEvent)) {
    console.log(`Result: ${finalEvent.data}`);
  }
}

run(42);
run(43);
```

## Using Middleware

LlamaIndex workflows provide middleware that can enhance your workflows:

### `withState` Middleware

The `withState` middleware adds a persistent state to your workflow context:

```ts
import { createStatefulMiddleware } from "@llamaindex/workflow-core/middleware/state";

// Define your state type
type CounterState = {
  count: number;
  history: number[];
};

// Create a workflow with state middleware
const { withState } = createStatefulMiddleware<CounterState>(() => ({
  count: 0,
  history: [],
}));

const workflow = withState(createWorkflow());

// Use the state in your handlers
workflow.handle([startEvent], () => {
  const { state } = getContext();
  state.count += 1;
  state.history.push(state.count);
  return incrementEvent.with(state.count);
});

workflow.handle([incrementEvent], (increment) => {
  const { state } = getContext();
  console.log("Current count:", state.count);
  console.log("History:", state.history);
  return resultEvent.with(state.count);
});

// Run the workflow
async function runWithState() {
  const { sendEvent, state } = workflow.createContext();

  // Send start event multiple times to see state update
  sendEvent(startEvent.with());
  sendEvent(startEvent.with());
  sendEvent(startEvent.with());

  // The state persists across events
  console.log("Final count:", state.count);
  console.log("Final history:", state.history);
}

runWithState();
```

You can also create a state with input:

```ts
const { withState } = createStatefulMiddleware(
  (input: { initialCount: number }) => ({
    count: input.initialCount,
    history: [input.initialCount],
  }),
);

const workflow = withState(createWorkflow());
const { state } = workflow.createContext({ initialCount: 10 });
```

### `withValidation` Middleware

The `withValidation` middleware adds compile-time and runtime validation to your workflows:

```ts
import { createWorkflow, workflowEvent } from "@llamaindex/workflow-core";
import { withValidation } from "@llamaindex/workflow-core/middleware/validation";

const startEvent = workflowEvent<string, "start">();
const processEvent = workflowEvent<number, "process">();
const resultEvent = workflowEvent<string, "result">();
const disallowedEvent = workflowEvent<void, "disallowed">();

// Create a workflow with validation middleware
// Define allowed event paths
const workflow = withValidation(createWorkflow(), [
  [[startEvent], [processEvent]], // startEvent can only lead to processEvent
  [[processEvent], [resultEvent]], // processEvent can only lead to resultEvent
]);

// This will pass validation
workflow.strictHandle([startEvent], (sendEvent, start) => {
  sendEvent(processEvent.with(123)); // ✅ This is allowed
});

// This would fail at compile time and runtime
workflow.strictHandle([startEvent], (sendEvent, start) => {
  // sendEvent(disallowedEvent.with("disallowed")); // ❌ This would cause an error
  // sendEvent(resultEvent.with("result")); // ❌ This would also cause an error
});
```

## Error Handling

LlamaIndex workflows provide built-in mechanisms for handling errors:

```ts
import {
  createWorkflow,
  workflowEvent,
  getContext,
} from "@llamaindex/workflow-core";

const startEvent = workflowEvent<string>();
const processEvent = workflowEvent<number>();
const errorEvent = workflowEvent<Error>();
const resultEvent = workflowEvent<string>();

const workflow = createWorkflow();

workflow.handle([startEvent], (start) => {
  try {
    const num = Number.parseInt(start.data, 10);
    if (isNaN(num)) {
      throw new Error("Invalid number");
    }
    return processEvent.with(num);
  } catch (err) {
    return errorEvent.with(err instanceof Error ? err : new Error(String(err)));
  }
});

workflow.handle([processEvent], (event) => {
  return resultEvent.with(`Result: ${event.data * 2}`);
});

workflow.handle([errorEvent], (event) => {
  return resultEvent.with(`Error: ${event.data.message}`);
});
```

You can also use the signal in `getContext()` to handle errors:

```ts
workflow.handle([processEvent], () => {
  const { signal } = getContext();

  signal.onabort = () => {
    console.error("Process aborted:", signal.reason);
    // Clean up resources
  };

  // Your processing logic here
});
```

## Connecting with Server Endpoints

Workflow can be used as middleware in server frameworks like Express, Hono, or Fastify:

```ts
import { Hono } from "hono";
import { serve } from "@hono/node-server";
import { createHonoHandler } from "@llamaindex/workflow-core/interrupter/hono";
import { createWorkflow, workflowEvent } from "@llamaindex/workflow-core";

// Define events
const queryEvent = workflowEvent<string>();
const responseEvent = workflowEvent<string>();

// Create workflow
const workflow = createWorkflow();

workflow.handle([queryEvent], (event) => {
  const response = `Processed: ${event.data}`;
  return responseEvent.with(response);
});

// Create Hono app
const app = new Hono();

// Set up workflow endpoint
app.post(
  "/workflow",
  createHonoHandler(
    workflow,
    async (ctx) => queryEvent.with(await ctx.req.text()),
    responseEvent,
  ),
);

// Start server
serve(app, ({ port }) => {
  console.log(`Server started at http://localhost:${port}`);
});
```

## Next Steps

Now that you've learned about basic workflow patterns, explore more advanced topics:

- [Streaming with Workflows](./streaming.mdx)
- [Advanced Event Handling](./advanced-events.mdx)