Solidity ABI
In traditional web development, interactions with data occur between applications and servers through APIs (Application Programming Interfaces). Servers act as centralized sources of information that feed data to the application upon request.
In the blockchain, there is no such data centralization. Nodes in the blockchain act as servers, and smart contracts are functions in the blockchain. Applications outside the blockchain need a way to communicate with smart contracts in the blockchain.
The Application Binary Interface (ABI) is the standard way to interact with contracts in the Ethereum ecosystem, both from outside the blockchain, and from contract-to-contract communication.
Smart contracts are low-level applications of the Ethereum Virtual Machine (EVM). The purpose of smart contracts is to execute transactions when certain conditions, defined in the contract, are met. These conditions can be events that happen both on and off the blockchain. Smart contracts are written in high-level languages like Solidity, but are stored in the EVM in executable byte code, which is a binary format.
Since byte code is not human readable, it needs to be interpreted in order to be understood. An ABI allows anyone writing a smart contract to communicate between the web application, written in a high-level language like Javascript, and the byte code that the EVM understands.
The ABI acts as a function selector which contains methods to identify which function in the contract to call. These methods and their associated data types are listed in a JSON RPC file that is created.
Unlike APIs, where a smart contract can be directly sent a request in JSON format, and the contract will respond, a smart contract only communicates with byte code. This information is encoded using ABI encoding in order to translate it into something the EVM will understand. This encoding includes the function signatures and variable declarations so that the EVM knows exactly which function within the smart contract to execute.
The first four bytes of the transaction (tx) payload in a call to a contract are usually used to distinguish which function in the contract is being called.
The responses from function calls made to a contract are also in byte code. This data also needs to be interpreted by the calling web application before it can be processed. This reverse translation from machine code is done through ABI Decoding.
If you are using tools like Hardhat/Truffle or an IDE like Remix, the contract ABI will be generated for you automatically. You can also generate the ABI manually using the Solidity Compiler NPM package. Once you have installed the package you can run the command `solcjs Contractname.sol -abi` in a terminal. This will generate a .abi file if successful.
Because the ABI acts as an interpreter between a website’s EVM byte code and Javascript, it is required any time you want to execute any function of a smart contract. In addition to the ABI, the contract’s blockchain address is also required.
On the Ethereum blockchain, you can view the ABI code for a deployed contract on the Etherscan website by inputting the contract’s address and going to the Contract tab.
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