Data Bus

This smart contract serves as an abstract data bus, which can be utilized to facilitate lightweight, inter-service communication.

Features

ability to send arbitrary events with arbitrary data
minimalistic implementation
low gas consumption
does not require support and administration

How it works

Each event in Ethereum consists of a list of topics and unindexed data. The first topic (topic 0) is usually the hash of the event signature (e.g., Deposit(address,uint256)), which helps identify the type of event. The other topics can be used to store the values of parameters that are marked as indexed in the event definition in the smart contract.

Thus, when we emit the event SomeEvent(address), it triggers a call SomeEvent(id("SomeEvent(address)"), address). This happens not explicitly, but internally within the EVM.

How the id function works:

flowchart LR
    A[Start of function id] --> B[Receive string value 'value']
    B --> C[Convert 'value' to bytes]
    C --> D[Apply KECCAK256 to bytes]
    D --> E[Return hash value]

Our idea is that we create an abstract event that explicitly takes as the first argument the hash of the event signature, the second argument is msg.sender, and the third argument is abstractly specified as bytes.

event Message(
    bytes32 indexed eventId,
    address indexed sender,
    bytes data
) anonymous;

As you might have noticed, we added an anonymous modifier to the event. This modifier records the event in the blockchain without the zeroth topic with the event signature (event Message(bytes32 indexed, address indexed, bytes)). Instead, the eventId we passed as the first argument will be recorded.

Next, we prepared the only method of the contract that emits the anonymous event.

function sendMessage(bytes32 _eventId, bytes calldata _data)

To call it, you need to encode your own event signature, for example: id(SomeEvent(address, bytes)). Then encode your second argument, depending on the type. Afterward, you can call the sendMessage method and the result will be recorded in the blockchain.

Most libraries for working with Ethereum out of the box will work with parsing event data, additional logic will be required if the second data argument represents something other than bytes. In such a case, you will need to do additional encode at the time of loading the event.

For convenience and as an example, we prepared DataBusClient. This is an implementation of methods for reading and sending events, which operates based on human-readable ABI. This library is developed for TS, but it can be easily implemented in any language.

You can familiarize yourself with the library implementation: library

Also with usage examples:

Building and testing

Create .env file

CHIADO_BLOCKSCOUT=<your key from https://gnosis-chiado.blockscout.com>
NODE_HOST=http://127.0.0.1:8888

Install the dependencies

yarn

Compile the smart contracts code

yarn compile

Run the tests

yarn test

Test environment

A pre-installed environment can be used to test the smart contract. These environments include

evm test node
event monitoring system
smart contract message spammer

To start the test environment, enter the command:

yarn stand:default

A test environment without message spammer is also provided, enter the command to run it:

yarn stand:monitoring

Deploy

Chiado — gnosis testnet

Already deployed: https://gnosis-chiado.blockscout.com/address/0x42E1DEfC18388E3AA1fCADa851499A11405cf37f?tab=txs

Create account

yarn create-account --network chiado

Visit website https://faucet.chiadochain.net/ and earn some xDAI
Deploy

yarn deploy --network chiado

(Optional) Run Testing Stand

The testing stand consists of:

transaction spammer
console monitoring

with the help of the testing stand you can visualize the principles of Data Bus operation