Request Computation

This tutorial shows you how to run computations on the Chainlink Functions Decentralized Oracle Network (DON). The example code computes the geometric mean of numbers in a list. After OCR completes off-chain computation and aggregation, it returns the result to your smart contract.

Before you begin

Complete the setup steps in the Getting Started guide: The Getting Started Guide shows you how to set up your environment with the necessary tools for these tutorials. You can re-use the same consumer contract for each of these tutorials.
Make sure your subscription has enough LINK to pay for your requests. Read Get Subscription details to learn how to check your subscription balance. If your subscription runs out of LINK, follow the Fund a Subscription guide.
Check out the correct branch before you try this tutorial: Each tutorial is stored in a separate branch of the Chainlink Functions Starter Kit repository.
```
git checkout tutorial-1
```

Tutorial

This tutorial is configured to get the average (geometric mean) from a list of numbers. For a detailed explanation of the code example, read the Explanation section.

Open Functions-request-config.js. Note that the args value is ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10"], so the source code will calculate the average (geometric mean) of “1,2,3,4,5,6,7,8,9,10”. You can change the args value to compute another set’s average. Read the request config explanation for more information about the request config file.
Open Functions-request-source.js to analyze the JavaScript source code. Read the source code explanation for a more detailed description of the source code.

Simulation

The Chainlink Functions hardhat starter kit includes a simulator to test your Functions code on your local machine. The functions-simulate command will execute your code in a local runtime environment and simulate an end-to-end fulfillment, helping you fix any issues before submitting your functions to a Decentralized Oracle Network.

Run the functions-simulate task to run the source code locally and make sure Functions-request-config.js and Functions-request-source.js are correctly written:

npx hardhat functions-simulate

Example:

$ npx hardhat functions-simulate
secp256k1 unavailable, reverting to browser version

__Compiling Contracts__
Nothing to compile
Duplicate definition of Transfer (Transfer(address,address,uint256,bytes), Transfer(address,address,uint256))

Executing JavaScript request source code locally...

__Console log messages from sandboxed code__
calculate geometric mean of 1,2,3,4,5,6,7,8,9,10
geometric mean is: 4.53

__Output from sandboxed source code__
Output represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000000001c5
Decoded as a uint256: 453

__Simulated On-Chain Response__
Response returned to client contract represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000000001c5
Decoded as a uint256: 453

Estimated transmission cost: 0.000043548320861207 LINK (This will vary based on gas price)
Base fee: 0.0 LINK
Total estimated cost: 0.000043548320861207 LINK

Reading the output of the example above, you can note that the geometric mean was correctly computed: 4.53. Because Solidity does not support decimals, we move the decimal point so that the value looks like an integer 453 before returning the bytes encoded value 0x00000000000000000000000000000000000000000000000000000000000001c5 in the callback. Note: Read the source code explanation for a more detailed explanation.

Request

Send a request to the Decentralized Oracle Network to compute the average (geometric mean). Run the functions-request task with the subid (subscription ID) and contract parameters. This task will pass the functions JavaScript source code and any arguments and secrets when calling the FunctionsConsumer’s executeRequest function. Read the functionsConsumer section for more details about the consumer contract.

npx hardhat functions-request --subid REPLACE_SUBSCRIPTION_ID --contract REPLACE_CONSUMER_CONTRACT_ADDRESS --network REPLACE_NETWORK

Example:

$ npx hardhat functions-request --subid 6 --contract 0xa9b286E892d579dc727c79D3be9b01949796240A  --network mumbai
secp256k1 unavailable, reverting to browser version
Simulating Functions request locally...

__Console log messages from sandboxed code__
calculate geometric mean of 1,2,3,4,5,6,7,8,9,10
geometric mean is: 4.53

__Output from sandboxed source code__
Output represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000000001c5
Decoded as a uint256: 453


If all 100000 callback gas is used, this request is estimated to cost 0.000052353682260389 LINK
Continue? (y) Yes / (n) No
y

Requesting new data for FunctionsConsumer contract 0xa9b286E892d579dc727c79D3be9b01949796240A on network mumbai
Waiting 2 blocks for transaction 0xa282664fc12c1cecca3607b506f7d4074cce102ae991101b88f4f32c412a56a4 to be confirmed...

Request 0x6ec358f19476daf07dba0f786f49f043b10449855c8a0b6a197776460db8bfc1 initiated
Waiting for fulfillment...

Request 0x6ec358f19476daf07dba0f786f49f043b10449855c8a0b6a197776460db8bfc1 fulfilled!
Response returned to client contract represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000000001c5
Decoded as a uint256: 453

Transmission cost: 0.000080952063590393 LINK
Base fee: 0.0 LINK
Total cost: 0.000080952063590393 LINK

The output of the example gives you the following information:

The executeRequest function was successfully called in the FunctionsConsumer contract. The transaction in this example is 0xa282664fc12c1cecca3607b506f7d4074cce102ae991101b88f4f32c412a56a4.
The request ID is 0x6ec358f19476daf07dba0f786f49f043b10449855c8a0b6a197776460db8bfc1.
The DON successfully fulfilled your request. The total cost was: 0.000080952063590393 LINK.
The consumer contract received a response in bytes with a value of 0x00000000000000000000000000000000000000000000000000000000000001c5. Decoding it off-chain to uint256 give you a result: 453.

At any time, you can run the functions-read task with the contract parameter to read the latest received response.

npx hardhat functions-read  --contract REPLACE_CONSUMER_CONTRACT_ADDRESS --network REPLACE_NETWORK

Example:

$ npx hardhat functions-read  --contract 0xa9b286E892d579dc727c79D3be9b01949796240A --network mumbai
secp256k1 unavailable, reverting to browser version
Reading data from Functions client contract 0xa9b286E892d579dc727c79D3be9b01949796240A on network mumbai

On-chain response represented as a hex string: 0x00000000000000000000000000000000000000000000000000000000000001c5
Decoded as a uint256: 453

Explanation

FunctionsConsumer.sol

To write a Chainlink Functions consumer contract, your contract must import FunctionsClient.sol. You can read the API reference: FunctionsClient.

This contract is not available in an NPM package, so you must download and import it from within your project.
```
import "./dev/functions/FunctionsClient.sol";
```
Use the Functions.sol library to get all the functions needed for building a Chainlink Functions request. You can read the API reference: Functions.
```
using Functions for Functions.Request;
```
The latest request id, latest received response, and latest received error (if any) are defined as state variables. Note latestResponse and latestError are encoded as dynamically sized byte array bytes, so you will still need to decode them to read the response or error:
```
bytes32 public latestRequestId;
bytes public latestResponse;
bytes public latestError;
```
We define the OCRResponse event that your smart contract will emit during the callback
```
event OCRResponse(bytes32 indexed requestId, bytes result, bytes err);
```
Pass the oracle address for your network when you deploy the contract:
```
constructor(address oracle) FunctionsClient(oracle)
```
At any time, you can change the oracle address by calling the updateOracleAddress function.
The two remaining functions are:
- executeRequest for sending a request. It receives the JavaScript source code, encrypted secrets, list of arguments to pass to the source code, subscription id, and callback gas limit as parameters. Then:
  - It uses the Functionslibrary to initialize the request and add any passed encrypted secrets or arguments. You can read the API Reference for Initializing a request, adding secrets, and adding arguments.
```
Functions.Request memory req;
req.initializeRequest(Functions.Location.Inline, Functions.CodeLanguage.JavaScript, source);
if (secrets.length > 0) {
  if (secretsLocation == Functions.Location.Inline) {
     req.addInlineSecrets(secrets);
  } else {
    req.addRemoteSecrets(secrets);
  }
}
if (args.length > 0) req.addArgs(args);
```
  - It sends the request to the oracle by calling the FunctionsClient sendRequest function. You can read the API reference for sending a request. Finally, it stores the request id in latestRequestId.
```
bytes32 assignedReqID = sendRequest(req, subscriptionId, gasLimit);
latestRequestId = assignedReqID;
```
- fulfillRequest to be invoked during the callback. This function is defined in FunctionsClient as virtual (read fulfillRequest API reference). So, your smart contract must override the function to implement the callback. The implementation of the callback is straightforward: the contract stores the latest response and error in latestResponse and latestError before emitting the OCRResponse event.
```
latestResponse = response;
latestError = err;
emit OCRResponse(requestId, response, err);
```

Functions-request-config.js

Read the Request Configuration section for a detailed description of each setting. In this example, the settings are the following:

codeLocation: Location.Inline: The JavaScript code is provided within the request.
secretsLocation: Location.Inline: The encrypted secrets are provided within the request.
source: fs.readFileSync("./Functions-request-source.js").toString(): The source code must be a script object. That’s why we use fs.readFileSync to read Functions-request-source.js and then call toString() to get the content as a string object.
args: ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10"]: These arguments are passed to the source code. This example computes the average(geometric mean) of 1,2,3,4,5,6,7,8,9,10.
expectedReturnType: ReturnType.uint256: The response received by the DON is encoded in bytes. Because the average (geometric mean) is a uint256, define ReturnType.uint256 to inform users how to decode the response received by the DON.

Functions-request-source.js

Read the JavaScript code section for a detailed explanation of how to write compatible JavaScript source code. This JavaScript source code uses only vanilla Node.js and makes no API calls. The code is self-explanatory and has comments to help you understand all the steps. The main steps are:

Read the numbers provided as arguments in the args setting. Because args is an array of string, call parseInt to convert from string to number.
Calculate the average (geometric mean): First, compute the product of the numbers. Then, calculate the nth root of the product where n is the length of args.
Return the result as a buffer using the Functions.encodeUint256 helper function. Because solidity doesn’t support decimals, multiply the result by 100 and round the result to the nearest integer. Note: Read this article if you are new to Javascript Buffers and want to understand why they are important.

Request Computation

Before you begin

Tutorial

Simulation

Request

Explanation

FunctionsConsumer.sol

Functions-request-config.js

Functions-request-source.js

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