> ## Documentation Index
> Fetch the complete documentation index at: https://neardocs-docs-2854-nep-621-tokenized-vault.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Yield and Resume

> Wait for an external response and resume execution

export const Github = ({url, start, end, fname, language, withSourceLink = true}) => {
  const [code, setCode] = useState(null);
  function toRaw(ref) {
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      }
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        res = await (await fetch(url)).text();
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          localStorage.setItem(url, res);
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  }
  function buildSourceUrl(url, start, end) {
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  }
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  }, [url, start, end]);
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  const fileName = fname ?? sourceUrl.split('/').pop();
  return <div className="my-5">
      {code === null ? <div>Loading...</div> : <CodeBlock language={language} filename={fileName} lines>
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      {withSourceLink && <div className="flex justify-end" style={{
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            See code on GitHub
          </a>
        </div>}
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};

NEAR smart contracts can **yield** execution, until an **external** service **resumes** them. In practice, the contract yields a **cross-contract call** to itself, until an external service executes a function and the contract decides to resume.

This is a powerful feature that allows contracts to wait for external events, such as a response from an oracle, before continuing execution.

<Info>
  The contract can wait for 200 blocks - around 2 minutes - after which the yielded function will execute, receiving a "timeout error" as input
</Info>

***

## Yielding a Promise

Let's look at an example that takes a prompt from a user (e.g. "What is 2+2"), and yields the execution until an external service provides a response.

<Github fname="lib.rs" language="rust" url="https://github.com/near-examples/yield-resume/blob/main/contract/src/lib.rs" start="43" end="70" />

#### Creating a Yielded Promise

In the example above, we are creating a [`Promise`](./crosscontract#promises) to call the contract's function `return_external_response`.

Notice that we create the `Promise` using `env::promise_yield_create`, which will create an **identifier** for the yielded promise in the `YIELD_REGISTER`.

#### Retrieving the Yielded Promise ID

We read the `YIELD_REGISTER` to retrieve the `ID` of our yielded promise. We store the `yield_id` and the user's `prompt` so the external service query them (the contract exposes has a function to list all requests).

#### Returning the Promise

Finally, we return the `Promise`, which will **not execute immediately**, but will be **yielded** until the external service provides a response.

<Accordion title="What is that `self.request_id` in the code?">
  The `self.request_id` is an internal unique identifier that we use to keep track of stored requests. This way, we can delete the request once the external service provides a response (or the waiting times out)

  Since we only use it to simplify the process of keeping track of the requests, you can remove it if you have a different way of tracking requests (e.g. an indexer)
</Accordion>

***

## Signaling the Resume

The `env::promise_yield_resume` function allows us to signal which yielded promise should execute, as well as which parameters to pass to the resumed function.

<Github fname="lib.rs" language="rust" url="https://github.com/near-examples/yield-resume/blob/main/contract/src/lib.rs" start="72" end="75" />

In the example above, the `respond` function would be called by an external service, passing which promise should be resume (`yield_id`), and the response to the prompt.

<Warning>
  **Gatekeeping the Resume**

  Since the function used to signal the resume is public, developers must make sure to guard it properly to avoid unwanted calls. This can be done by simply checking the caller of the function
</Warning>

***

## The Function that Resumes

The function being resumed will have access to all parameters passed to it, including those passed during the yield creation, or the external service response.

<Github fname="lib.rs" language="rust" url="https://github.com/near-examples/yield-resume/blob/main/contract/src/lib.rs" start="77" end="89" />

In the example above, the `return_external_response` receives parameters:

1. A `request_id` - passed on [creation](#creating-a-yielded-promise) - which is used to remove the request from the state
2. A `response` - passed when [signaling to resume](#signaling-the-resume) - which contains the external response, or `None` if the contract timed out while waiting

<Tip>
  **There's plenty of time**

  The contract will be able to wait for 200 blocks - around 2 minutes - before timing out
</Tip>

<Info>
  Notice that, in this particular example, we choose to return a value both if there is a response or a time out

  The reason to not raise an error, is because we are changing the state (removing the request in line `#7`), and raising an error would revert this state change
</Info>

***

## Managing State

When using yield and resume, it's important that you carefully manage the contract's state. Because of its asynchronous execution, the contract function in which the contract yields and resumes are independent.

If you change the state of the contract in the function where you yield the promise (the `request` function here), then you need to make sure that you revert the state in the function that resumes (the `return_external_response` function here) in the case that the promise times out or the response is invalid.

It is best practice to check the validity of the response within the function where the resume is signaled (the `respond` function here) and panic if the response is not valid; the external service can attempt to respond again before the promise times out. You should not panic in `return_external_response` as this is only called when the promise has been resolved (it was resumed or timed out), meaning it can't be resumed again, and the state in `request` has been settled. You should gracefully complete the function and revert the state.

<Info>
  Read [asynchronous-call security](/smart-contracts/security/cross-contract-calls/callbacks) to avoid common pitfalls when dealing with callbacks and pending state.
</Info>