Yield and Resume
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 (read our blog post!).
Contract can wait for 200 blocks - around 4 minutes - after which the yielded function will execute, receiving a "timeout error" as input
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.
- 🦀 Rust
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Creating a Yielded Promise
In the example above, we are creating a Promise 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.
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)
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.
- 🦀 Rust
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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.
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
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.
- 🦀 Rust
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In the example above, the return_external_response receives two parameters:
- A
request_id- passed on creation - which is used to remove the request from the state - A
response- passed when signaling to resume - which contains the external response, or aPromiseErrorif the contract timed out while waiting
The contract will be able to wait for 200 blocks - around 4 minutes - before timing out
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
