# Reserved Parameters

Table of Contents

Reserved parameters are part of an OIS object as part of the endpoints field (Airnode endpoints) and warrant a more in-depth explanation. They are part of an Airnode's defined endpoints in an OIS object but do not map to operation parameters (API parameters). They are used by Airnode for special purposes.

A requester can pass request parameters either by referencing a template that contains them, or as an argument of the request-making methods of AirnodeRrpV0.sol. In either case, these parameters are encoded using the AirnodeRrpV0 ABI. There are two types of parameters which are part of the OIS object:

  1. Endpoint parameters - Airnode endpoint parameters are mapped to API operation parameters.
  2. Reserved parameters - Reserved parameters perform a specific operation on the response before fulfilling the request. Reserved parameter names start with _.

# _type

Signifies what Solidity type the API response will be encoded to before fulfillment.

Support is provided for most common solidity types (opens new window), but the following are not supported.

  • Custom bits integer types - e.g. uint32 or uint8
  • Fixed point decimal numbers - e.g. fixed128x18 or ufixed128x18
  • Custom fixed size bytes - e.g. bytes4
  • Tuples - e.g. (int256, string)

On top of supported solidity types, there is support for a few "artificial" types created for special purposes that would otherwise be hard or impossible to represent.

You can also encode multiple values for one single API call - but this impacts all of the reserved parameters and is explained in the Encoding Multiple Values section below.

# Conversion and encoding behavior

Before the API response value is encoded for on chain use, it is parsed and converted. The conversion behaviors for any given type is explained in depth in the adapter package docs.

The converted value is then encoded internally by ethers ABI Coder (opens new window) using the following

ethers.utils.defaultAbiCoder.encode([solidityType], [value]);
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# Supported Primitive Values

  • int256
  • uint256
  • bool
  • bytes32
  • address
  • bytes
  • string

# string32 (encoded to bytes32 on chain)

The string32 is an artificial type that is not supported by solidity. It is instead encoded to bytes32 and provides a cheaper alternative to the regular string type for values with less than 32 characters.

Limitations

While using string32 is more efficient, decoding the original string from bytes32 on chain is both difficult and expensive.

Also bear in mind that this type is able to encode only strings shorter than 32 characters. If the value is longer, it will be trimmed and only first 31 characters will be encoded.

# timestamp (encoded to uint256 on chain)

The timestamp is an artificial type that is not supported by solidity. It is instead encoded to uint256 and specifies the UNIX timestamp value at the time when the transaction was encoded. You can use this value on chain to check the "freshness" of the Airnode response. This might be useful in certain scenarios, because Airnode cannot guarantee when a particular transaction will be mined on chain.

When using the timestamp type, the corresponding _path and _times variables must be empty strings or not provided.

# Arrays

Apart from the primitives defined above as well as all "artificial" types we created, you are free to use arrays with any of the above. Multidimensional arrays are supported as well. Solidity allows you to define fixed size arrays, which are more gas efficient to encode and you can use those as well.

For example

  • int256[] - regular integer array
  • uint256[8] - unsigned integer array with 8 elements
  • int256[][] - 2 dimensional integer array
  • string32[] - is an array of string32 values, which will be encoded to bytes32[] on chain
  • string[2][][3] - 3 dimensional string array, where first dimension contains 3 elements, second unboundedly many and last dimension only 2. Notice, that this definition is read backwards (opens new window) compared to C-style languages

# _path

Assuming that the API response will be a JSON object, defines the field to be used to fulfill the request using dot notation. For example, if the API returns

{
  "field1": {
    "fieldA": [
      "valueA1",
      "valueA2"
    ],
    "fieldB: "valueB"
  },
  "field2": {
    "fieldZ": "valueZ"
  }
}
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and _path is field1.fieldA.1, the response will be valueA2.

If the response is a literal value (i.e., not a JSON object) and _path is not provided or is an empty string (needed for encoding multiple values), Airnode will use the API response itself to fulfill the request.

Beware the separator

Make sure the keys in the path of the API response do not contain ., because it will be incorrectly considered as a separator.

{
  "strange.key": "123"
}
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The _path defined as "strange.key" will not work. As workaround you can escape the separator.

# Escaping Separators

In rare cases, when the _path to the API response would contain , or . (comma or a dot) things get a bit complicated. Those symbols have a very specific meaning when parsing the reserved parameters and they need to be escaped if they are to be considered as literals. For example, if the API provider response looks like the following

{
  "very,strange.key": "123"
}
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Then you need to escape those symbols, in this case _path="very//,strange\\.key".

# _times

If _type is int256 or uint256 and a valid _times parameter is provided Airnode multiplies the value returned by the API with the _times parameter before fulfilling the request. For example, if the API returns:

{
  "data": "1.238",
  "apiVersion": "1.0.4"
}
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and the reserved parameters are

_type: int256
_path: data
_times: "100"
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the request will be fulfilled with the value 123. Note that the number gets multiplied by 100, and then gets floored. This is because the result of the multiplication is cast to int256 afterwards.

Make sure to pass the _times parameter as string. Airnode will convert this string to number internally. You can also pass and empty string "" to _times parameter - this has the same effect as if the _times parameter was not provided. However, this is important when encoding multiple values.

The _times parameter also works in conjunction with arrays and multidimensional arrays. All elements of the API response array will be multiplied before they are encoded.

# Encoding Multiple Values

Solidity has support for decoding and "destructuring" multiple values. For example

function decodeMultipleParameters(bytes calldata data)
    public
    pure
    returns (string memory str, uint256 num, address addr)
{
    (str, num, addr) = abi.decode(data, (string, uint256, address));
}
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The example above demonstrates the decoding on chain of three values of types string, uint256 and address respectively. You can instruct Airnode to encode these values using the reserved parameters by separating the values using , (comma). For example using the following combination of reserved parameters

{
  _type: 'string,uint256,address',
  _path: 'pathToString,pathToFloat,pathToAddress',
  _times: ',10000,'
}
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Airnode will split the reserved parameters by , into "split values" and ensure they all contain the same number of them. It will extract and convert each of the "split values". Notice, that an "" (empty string) is used to specify that a certain reserved parameter should not be used for a certain "split value".

For example, let's say the API response looks like this

{
  "pathToString": "some string",
  "pathToFloat": "1234.567",
  "pathToAddress": "0xe021...08a74"
}
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Airnode will extract and convert each of the "split values" separately

  1. Combination of _type="string", _path="pathToString" and __times="" results in "some string"
  2. Combination of _type="uint256", _path="pathToFloat" and __times="10000" results in 12345670
  3. Combination of _type="address", _path="pathToAddress" and __times="" results in "0xe021...8a74"

All of these values are then together encoded to single bytes value that can be sent on chain. You can use testing gateway to inspect the raw API response, casting results and the final encoded value.

Last Updated: 8/9/2022, 3:28:25 PM