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Workflows

Workflows provide durable execution so you can write programs that are resilient to any failure. Workflows are comprised of steps, which are ordinary TypeScript functions annotated with @DBOS.step(). If a workflow is interrupted for any reason (e.g., an executor restarts or crashes), when your program restarts the workflow automatically resumes execution from the last completed step.

Here's an example workflow from the programming guide. It signs an online guestbook then records the signature in the database. Using a workflow guarantees that every guestbook signature is recorded in the database, even if execution is interrupted.

class Guestbook {

  @DBOS.workflow()
  static async greetingEndpoint(name: string): Promise<string> {
    await Guestbook.signGuestbook(name);
    await Guestbook.insertGreeting(name);
    return `Thank you for being awesome, ${name}!`;
  }
}

Reliability Guarantees​

Workflows provide the following reliability guarantees. These guarantees assume that the application and database may crash and go offline at any point in time, but are always restarted and return online.

  1. Workflows always run to completion. If a DBOS process crashes while executing a workflow and is restarted, it resumes the workflow from the last completed step.
  2. Steps are tried at least once but are never re-executed after they complete. If a failure occurs inside a step, the step may be retried, but once a step has completed (returned a value or thrown an exception to the calling workflow), it will never be re-executed.
  3. Transactions commit exactly once. Once a workflow commits a transaction, it will never retry that transaction.

Determinism​

Workflows are in most respects normal TypeScript functions. They can have loops, branches, conditionals, and so on. However, workflow functions must be deterministic: if called multiple times with the same inputs, it should invoke the same steps with the same inputs in the same order. If you need to perform a non-deterministic operation like accessing the database, calling a third-party API, generating a random number, or getting the local time, you shouldn't do it directly in a workflow function. Instead, you should do all database operations in transactions and all other non-deterministic operations in steps.

For example, don't do this:

class Example {
    @DBOS.workflow()
    static async exampleWorkflow() {
        // Don't make an HTTP request in a workflow function
        const body = await fetch("https://example.com").then(r => r.text()); 
        await Example.exampleTransaction(body);
    }
}

Do this instead:

class Example {
    @DBOS.step()
    static async fetchBody() {
      // Instead, make HTTP requests in steps
      return await fetch("https://example.com").then(r => r.text());
    }

    @DBOS.workflow()
    static async exampleWorkflow() {
        const body = await Example.fetchBody();
        await Example.exampleTransaction(body);
    }
}

Workflow IDs​

Every time you execute a workflow, that execution is assigned a unique ID, by default a UUID. You can access this ID through the DBOS.workflowID context variable. Workflow IDs are useful for communicating with workflows and developing interactive workflows.

Starting Workflows Asynchronously​

You can use DBOS.startWorkflow to start a workflow in the background without waiting for it to complete. This is useful for long-running or interactive workflows.

DBOS.startWorkflow returns a workflow handle, from which you can access information about the workflow or wait for it to complete and retrieve its result. The DBOS.startWorkflow method resolves after the handle is durably created; at this point the workflow is guaranteed to run to completion even if the app is interrupted.

Here's an example:

class Example {
    @DBOS.workflow()
    static async exampleWorkflow(var1: str, var2: str) {
        return var1 + var2;
    }
}

async function main() {
    // Start exampleWorkflow in the background
    const handle = await DBOS.startWorkflow(Example).exampleWorkflow("one", "two");
    // Wait for the workflow to complete and return its results
    const result = await handle.getResult();
}

You can also use DBOS.retrieve_workflow to retrieve a workflow's handle from its ID.

Workflow Events​

Workflows can emit events, which are key-value pairs associated with the workflow's ID. They are useful for publishing information about the state of an active workflow, for example to transmit information to the workflow's caller.

setEvent​

Any workflow can call DBOS.setEvent to publish a key-value pair, or update its value if has already been published.

DBOS.setEvent<T>(key: string, value: T): Promise<void>

getEvent​

You can call DBOS.getEvent to retrieve the value published by a particular workflow ID for a particular key. If the event does not yet exist, this call waits for it to be published, returning null if the wait times out.

DBOS.getEvent<T>(workflowID: string, key: string, timeoutSeconds?: number): Promise<T | null>

Events Example​

Events are especially useful for writing interactive workflows that communicate information to their caller. For example, in the e-commerce demo, the checkout workflow, after validating an order, directs the customer to a secure payments service to handle credit card processing. To communicate the payments URL to the customer, it uses events.

The checkout workflow emits the payments URL using setEvent():

  @DBOS.workflow()
  static async checkoutWorkflow(...): Promise<void> {
    ...
    const paymentsURL = ...
    await DBOS.setEvent(PAYMENT_URL, paymentsURL);
    ... 
  }

The HTTP handler that originally started the workflow uses getEvent() to await this URL, then redirects the customer to it:

  @DBOS.postApi('/api/checkout_session')
  static async webCheckout(...): Promise<void> {
    const handle = await DBOS.startWorkflow(Shop).checkoutWorkflow(...);
    const url = await DBOS.getEvent<string>(handle.workflowID, PAYMENT_URL);
    if (url === null) {
      DBOS.koaContext.redirect(`${origin}/checkout/cancel`);
    } else {
      DBOS.koaContext.redirect(url);
    }
  }

Reliability Guarantees​

All events are persisted to the database, so once an event is set, it is guaranteed to always be retrievable.

Workflow Messaging and Notifications​

You can send messages to a specific workflow ID. This is useful for sending notifications to an active workflow.

Send​

You can call DBOS.send() to send a message to a workflow. Messages can optionally be associated with a topic and are queued on the receiver per topic.

DBOS.send<T>(destinationID: string, message: T, topic?: string): Promise<void>;

Recv​

Workflows can call DBOS.recv() to receive messages sent to them, optionally for a particular topic. Each call to recv() waits for and consumes the next message to arrive in the queue for the specified topic, returning None if the wait times out. If the topic is not specified, this method only receives messages sent without a topic.

DBOS.recv<T>(topic?: string, timeoutSeconds?: number): Promise<T | null>

Messages Example​

Messages are especially useful for sending notifications to a workflow. For example, in the e-commerce demo, the checkout workflow, after redirecting customers to a secure payments service, must wait for a notification from that service that the payment has finished processing.

To wait for this notification, the payments workflow uses recv(), executing failure-handling code if the notification doesn't arrive in time:

@DBOS.workflow()
static async checkoutWorkflow(...): Promise<void> {
  ...
  const notification = await DBOS.recv<string>(PAYMENT_STATUS, timeout);
  if (notification) {
      ... // Handle the notification.
  } else {
      ... // Handle a timeout.
  }
}

A webhook waits for the payment processor to send the notification, then uses send() to forward it to the workflow:

@DBOS.postApi('/payment_webhook')
static async paymentWebhook(): Promise<void> {
  const notificationMessage = ... // Parse the notification.
  const workflowID = ... // Retrieve the workflow ID from notification metadata.
  await DBOS.send(workflow_id, notificationMessage, PAYMENT_STATUS);
}

Reliability Guarantees​

All messages are persisted to the database, so if send completes successfully, the destination workflow is guaranteed to be able to recv it. If you're sending a message from a workflow, DBOS guarantees exactly-once delivery because workflows are reliable. If you're sending a message from normal TypeScript code, you can specify an idempotency key for send or use DBOS.withNextWorkflowID to guarantee exactly-once delivery.