Rules of Workflows

A Workflow contains one or more steps. Each step is a self-contained, individually retriable component of a Workflow. Steps may emit (optional) state that allows a Workflow to persist and continue from that step, even if a Workflow fails due to a network or infrastructure issue.

This is a small guidebook on how to build more resilient and correct Workflows.

Ensure API/Binding calls are idempotent

Because a step might be retried multiple times, your steps should (ideally) be idempotent. For context, idempotency is a logical property where the operation (in this case a step), can be applied multiple times without changing the result beyond the initial application.

As an example, let us assume you have a Workflow that charges your customers, and you really do not want to charge them twice by accident. Before charging them, you should check if they were already charged:

export class MyWorkflow extends WorkflowEntrypoint {
  async run(event: WorkflowEvent<Params>, step: WorkflowStep) {
    const customer_id = 123456;
    // ✅ Good: Non-idempotent API/Binding calls are always done **after** checking if the operation is
    // still needed.
    await step.do(
      `charge ${customer_id} for it's montly subscription`,
      async () => {
        // API call to check if customer was already charged
        const subscription = await fetch(
          `https://payment.processor/subscriptions/${customer_id}`,
        ).then((res) => res.json());

        // return early if the customer was already charged, this can happen if the destination service dies
        // in the middle of the request but still commits it, or if the Workflows Engine restarts.
        if (subscription.charged) {
          return;
        }

        // non-idempotent call, this operation can fail and retry but still commit in the payment
        // processor - which means that, on retry, it would mischarge the customer again if the above checks
        // were not in place.
        return await fetch(
          `https://payment.processor/subscriptions/${customer_id}`,
          {
            method: "POST",
            body: JSON.stringify({ amount: 10.0 }),
          },
        );
      },
    );
  }
}

Make your steps granular

Steps should be as self-contained as possible. This allows your own logic to be more durable in case of failures in third-party APIs, network errors, and so on.

You can also think of it as a transaction, or a unit of work.

✅ Minimize the number of API/binding calls per step (unless you need multiple calls to prove idempotency).

export class MyWorkflow extends WorkflowEntrypoint {
  async run(event: WorkflowEvent<Params>, step: WorkflowStep) {
    // ✅ Good: Unrelated API/Binding calls are self-contained, so that in case one of them fails
    // it can retry them individually. It also has an extra advantage: you can control retry or
    // timeout policies for each granular step - you might not to want to overload http.cat in
    // case of it being down.
    const httpCat = await step.do("get cutest cat from KV", async () => {
      return await env.KV.get("cutest-http-cat");
    });

    const image = await step.do("fetch cat image from http.cat", async () => {
      return await fetch(`https://http.cat/${httpCat}`);
    });
  }
}

Otherwise, your entire Workflow might not be as durable as you might think, and you may encounter some undefined behaviour. You can avoid them by following the rules below:

🔴 Do not encapsulate your entire logic in one single step.
🔴 Do not call separate services in the same step (unless you need it to prove idempotency).
🔴 Do not make too many service calls in the same step (unless you need it to prove idempotency).
🔴 Do not do too much CPU-intensive work inside a single step - sometimes the engine may have to restart, and it will start over from the beginning of that step.

export class MyWorkflow extends WorkflowEntrypoint {
  async run(event: WorkflowEvent<Params>, step: WorkflowStep) {
    // 🔴 Bad: you're calling two seperate services from within the same step. This might cause
    // some extra calls to the first service in case the second one fails, and in some cases, makes
    // the step non-idempotent altogether
    const image = await step.do("get cutest cat from KV", async () => {
      const httpCat = await env.KV.get("cutest-http-cat");
      return fetch(`https://http.cat/${httpCat}`);
    });
  }
}

Do not rely on state outside of a step

Workflows may hibernate and lose all in-memory state. This will happen when engine detects that there is no pending work and can hibernate until it needs to wake-up (because of a sleep, retry, or event).

This means that you should not store state outside of a step:

function getRandomInt(min, max) {
  const minCeiled = Math.ceil(min);
  const maxFloored = Math.floor(max);
  return Math.floor(Math.random() * (maxFloored - minCeiled) + minCeiled); // The maximum is exclusive and the minimum is inclusive
}

export class MyWorkflow extends WorkflowEntrypoint {
  async run(event: WorkflowEvent<Params>, step: WorkflowStep) {
    // 🔴 Bad: `imageList` will be not persisted across engine's lifetimes. Which means that after hibernation,
    // `imageList` will be empty again, even though the following two steps have already ran.
    const imageList: string[] = [];

    await step.do("get first cutest cat from KV", async () => {
      const httpCat = await env.KV.get("cutest-http-cat-1");

      imageList.append(httpCat);
    });

    await step.do("get second cutest cat from KV", async () => {
      const httpCat = await env.KV.get("cutest-http-cat-2");

      imageList.append(httpCat);
    });

    // A long sleep can (and probably will) hibernate the engine which means that the first engine lifetime ends here
    await step.sleep("💤💤💤💤", "3 hours");

    // When this runs, it will be on the second engine lifetime - which means `imageList` will be empty.
    await step.do(
      "choose a random cat from the list and download it",
      async () => {
        const randomCat = imageList.at(getRandomInt(0, imageList.length));
        // this will fail since `randomCat` is undefined because `imageList` is empty
        return await fetch(`https://http.cat/${randomCat}`);
      },
    );
  }
}

Instead, you should build top-level state exclusively comprised of step.do returns:

function getRandomInt(min, max) {
  const minCeiled = Math.ceil(min);
  const maxFloored = Math.floor(max);
  return Math.floor(Math.random() * (maxFloored - minCeiled) + minCeiled); // The maximum is exclusive and the minimum is inclusive
}

export class MyWorkflow extends WorkflowEntrypoint {
  async run(event: WorkflowEvent<Params>, step: WorkflowStep) {
    // ✅ Good: imageList state is exclusively comprised of step returns - this means that in the event of
    // multiple engine lifetimes, imageList will be built accordingly
    const imageList: string[] = await Promise.all([
      step.do("get first cutest cat from KV", async () => {
        return await env.KV.get("cutest-http-cat-1");
      }),

      step.do("get second cutest cat from KV", async () => {
        return await env.KV.get("cutest-http-cat-2");
      }),
    ]);

    // A long sleep can (and probably will) hibernate the engine which means that the first engine lifetime ends here
    await step.sleep("💤💤💤💤", "3 hours");

    // When this runs, it will be on the second engine lifetime - but this time, imageList will contain
    // the two most cutest cats
    await step.do(
      "choose a random cat from the list and download it",
      async () => {
        const randomCat = imageList.at(getRandomInt(0, imageList.length));
        // this will eventually succeed since `randomCat` is defined
        return await fetch(`https://http.cat/${randomCat}`);
      },
    );
  }
}

Do not mutate your incoming events

The event passed to your Workflow's run method is immutable: changes you make to the event are not persisted across steps and/or Workflow restarts.

interface MyEvent {
  user: string;
  data: string;
}

export class MyWorkflow extends WorkflowEntrypoint {
  async run(event: WorkflowEvent<MyEvent>, step: WorkflowStep) {
    // 🔴 Bad: Mutating the event
    // This will not be persisted across steps and `event.data` will
    // take on its original value.
    await step.do("bad step that mutates the incoming event", async () => {
        let userData = await env.KV.get(event.user)
        event.data = userData
    })

    // ✅ Good: persist data by returning it as state from your step
    // Use that state in subsequent steps
    let userData = await step.do("good step that returns state", async () => {
      return await env.KV.get(event.user)
    })

    let someOtherData await step.do("following step that uses that state", async () => {
      // Access to userData here
      // Will always be the same if this step is retried
    })

Name steps deterministically

Steps should be named deterministically (even if dynamic). This ensures that their state is cached, and prevents the step from being rerun unnecessarily. Step names act as the "cache key" in your Workflow.

export class MyWorkflow extends WorkflowEntrypoint {
  async run(event: WorkflowEvent<Params>, step: WorkflowStep) {
    // 🔴 Bad: Dynamically naming the step prevents it from being cached
    // This will cause the step to be re-run if subsequent steps fail.
    await step.do(`step #1 running at: ${Date.now}`, async () => {
        let userData = await env.KV.get(event.user)
        event.data = userData
    })

    // ✅ Good: give steps a deterministic name.
    // Return dynamic values in your state, or log them instead.
    let state = await step.do("fetch user data from KV", async () => {
        let userData = await env.KV.get(event.user)
        console.log(`fetched at ${Date.now}`)
        return userData
    })

    // ✅ Good: steps that are dynamically named are constructed in a deterministic way.
    // In this case, `catList` is a step output, which is stable, and `catList` is
    // traversed in a deterministic fashion (no shuffles or random accesses) so,
    // it's fine to dynamically name steps (e.g: create a step per list entry).
    let catList = await step.do("get cat list from KV", async () => {
      return await env.KV.get("cat-list")
    })
    for(const cat of catList) {
      await step.do(`get cat: ${cat}`, async () => {
        return await env.KV.get(cat)
      })
    }

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