Metadata in NFT

What is metadata ？

Metadata is the data of data . That might be a bit of a detour , Take an example to understand . An article has a title 、 author 、 Release time 、 Tags and so on , This data is the metadata of the article .

Look at a more intuitive example , We use OpenSea Medium One NFT For example ：

The picture on the left is the actual data , The metadata describing it looks like this , It's a json File format ：

{
    
   "name":"#1125",
   "symbol":"SC",
   "description":"Soul Café is a collection of 3333 randomly generated, unique and diverse women existing as NFTs on the Ethereum Blockchain.",
   "external_url":"https://soulcafe.io",
   "image":"https://arweave.net/l7lcr2Z45bVerur-8hO4OoOfd5HoJbvdnW_uVaxMn_g"
}

The metadata contains the name of the image 、 Symbol 、 Description information 、 Links to external websites and pictures . The most important thing is the picture link , The corresponding picture behind this link is the girl image on the left of the above figure , What ordinary users can directly feel is it , You can Click here Access the picture .

stay ERC721 The standard specifies ERC721 Metadata extension , Used to describe ERC721 In itself [3]：

/// @title ERC-721 Non-Fungible Token Standard, optional metadata extension
/// @dev See https://eips.ethereum.org/EIPS/eip-721
///  Note: the ERC-165 identifier for this interface is 0x5b5e139f.
interface ERC721Metadata /* is ERC721 */ {
    /// @notice A descriptive name for a collection of NFTs in this contract
    function name() external view returns (string _name);

    /// @notice An abbreviated name for NFTs in this contract
    function symbol() external view returns (string _symbol);

    /// @notice A distinct Uniform Resource Identifier (URI) for a given asset.
    /// @dev Throws if `_tokenId` is not a valid NFT. URIs are defined in RFC
    ///  3986. The URI may point to a JSON file that conforms to the "ERC721
    ///  Metadata JSON Schema".
    function tokenURI(uint256 _tokenId) external view returns (string);
}

accord with ERC721 A standard contract is a set of NFT Set . As in the above figure NFT Take girls' pictures for example , The set it belongs to is Soul Cafe Official .Soul Cafe Official It is through ERC721 Metadata extension name Function to get . Each in the set NFT Each has its own identifier tokenId And the corresponding point json Format metadata “ link ” tokenURI.

We can see this on the details page NFT Girl pictures tokenId by 1125：

take 1125 Enter in the query interface , You can get the corresponding tokenURI 了 ：

This results in a string of links that may look strange to you in format ：ipfs://QmT11YwPGdUv1ZVvEQ9tU7gH5Q3jWpDovMsT5NpQQec6VD/1125.json . This is actually IPFS Link to agreement , With our usual Baidu Links https://www.baidu.com similar , Baidu's link is in https start , The browser can easily recognize the link , And help you open Baidu website . And then ipfs At the beginning IPFS Protocol Links , It also needs special IPFS Tools to open , In order to let the browser directly recognize , We can manually turn it into https In the form of , So you get https://ipfs.io/ipfs/QmT11YwPGdUv1ZVvEQ9tU7gH5Q3jWpDovMsT5NpQQec6VD/1125.json , After opening, you will find that it is similar to the one shown above NFT Metadata of girl pictures json The content of the document is the same , But what I show in the article is the formatted and simplified version .

The role of metadata

Cost savings . Because every whole node of Ethereum must hold a complete backup of the blockchain , So the cost of blockchain storage like Ethereum is high , take NFT The cost of storing all the data in the chain will be very high . The conventional approach is to store the original data chain （ for example IPFS）, Only its metadata is stored on the chain . This can reduce the storage overhead on the chain , cost reduction .

Enrich NFT In itself . Metadata describes information about the original data , Therefore, the raw data can be enriched for other purposes . For example, for some NFT For game cards , Its metadata often contains levels 、 rarity 、 Quality and other attributes .

Record how to find the original data . In a narrow sense, it is a link to an image .

Metadata storage

tokenURI It can be http、ipfs、ar And so on . Different agreements , Different storage methods [1].

http Protocols mean that metadata is stored in centralized services , When you find your favorite NFT When metadata of is stored in this way , It needs to be remembered ： Even if you have the NFT, Its metadata can also be modified by the centralized organization that maintains the link at any time . You need to have full confidence that the agency will not act against you .

ipfs Protocol means that metadata is stored in distributed storage IPFS in . The protocol follows content addressing , The simple understanding is , alike ipfs The content obtained by opening the link of the protocol any number of times is consistent , So you don't have to worry about buying NFT after ,NFT Metadata of may be tampered with by some organization . It is worth noting that ,IPFS The protocol only promises content addressing , The data stored on it does not necessarily last forever .

ar Deal with the ipfs similar , The main difference is ar The protocol is based on Arweave Block chaining , And commitment Store your data forever at a low price , You don't have to worry about buying NFT after , Although metadata cannot be modified by centralized organizations , But the metadata is lost [4].

Use IPFS Store the sample ：Rarible: Example of uploading & using Metadata with IPFS

Reference resources

[1] OpenSea: Metadata Standards

[2] Rarible: Ethereum Metadata

[3] EIP-721

[4] Arweave Wiki