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March 20, 2021

How to fetch Ethereum event logs in Ruby

Overview

Ethereum log records are very useful to understand and keep track of smart contract events. In this guide, we are going to learn how to fetch ethereum event logs in Ruby using ethereum.rb ruby gem.

Prerequisites

Ruby installed on your system (Ruby 2.x +)
Text editor.
An Ethereum node.
Terminal aka Command-Line.

What is Ruby?

Ruby is an open-source, interpreted, object-oriented programming language created by Yukihiro Matsumoto (aka Matz), who chose the gemstone's name to imply ruby is "a jewel of a language." Ruby is designed to be simple, complete, extensible, and portable. Developed mostly on Linux, Ruby works across most platforms, such as most UNIX-based platforms, DOS, Windows, and Mac. According to advocates, Ruby's simple syntax (partially inspired by Ada and Eiffel) makes it readable by anyone familiar with any modern programming language. Ruby is considered similar to Smalltalk and Perl.

What are Ethereum logs?

In event-driven languages like JavaScript, we often listen to events in order to perform other actions. Solidity provides the same paradigm. In addition to sending real-time events over WebSockets, the EVM (Ethereum Virtual Machine) actually stores a log of the events emitted by a particular smart contract for retrieval over HTTP. These logs are often used for debugging purposes, catching events, or telling the viewer of the records that something has happened.

It is common for log records to be used to describe a smart contract event, like a change of ownership or token transfer. Each log record has topics and data. Topics are 32-byte (256 bit) words that describe what’s going on in an event. Different types of opcodes (LOG0 … LOG1) are needed to describe the number of topics that need to be added to the log record. For example, LOG1 can have one topic, while LOG4 can have four topics. Therefore, the maximum number of topics a single log record can have is four. The first part of the log record has an array of topics, and the second part of a log record consists of additional data.

Topics and data work best when together as there are upsides and downsides to each. For example, while topics are searchable, data is not. Including data is cheaper than including topics. Topics are limited to 4 * 32 bytes, while event data is not, which means it can consist of large or complicated data like arrays or strings. Events give you a way to add relevant logging data to the logs of a block.

Contracts cannot read events created by smart contracts; however, they can be read outside the blockchain, and as events are immutable, they can be used to keep track of data changes.

Logs need to be decoded to be used outside of the blockchain. We’ll use ethereum.rb library to get and decode the logs of a smart contract event.

What is ethereum.rb?

ethereum.rb is a Ruby gem that makes it easier to interact with the Ethereum blockchain using Ruby.

Learn more about ethereum.rb in this guide on How to connect to the Ethereum network using Ruby.

We’ll first deploy a smart contract that will emit an event and then write a small Ruby code to fetch logs of that event.

Creating and Deploying a smart contract

We’ll deploy our contract on the Ropsten testnet. To get started, you will need the Metamask browser extension to create an ETH wallet and some test ETH, which you can get by going to the Ropsten faucet. You'll need to select Ropsten Test Network on your Metamask wallet and copy-paste the wallet address into the text field in the faucet, then click Send me test Ether.

Head over to the Ethereum Remix IDE and make a new Solidity file event.sol

Paste the following code into your new Solidity script:

// SPDX-License-Identifier: MIT
pragma solidity 0.8.1;

contract Counter {
    
    event ValueChanged(uint oldValue, uint256 newValue);
    
    // Private variable of type unsigned int to keep the number of counts
    uint256 private count = 0;

    // Function that increments our counter
    function increment() public {
        count += 1;
        emit ValueChanged(count - 1, count);
    }
    
    // Getter to get the count value
    function getCount() public view returns (uint256) {
        return count;
    }

}

Explanation of the code above

Line 1: Specifying SPDX license type, which is an addition after Solidity ^0.6.8; whenever the source code of a smart contract is made available to the public, these licenses can help resolve/avoid copyright issues. If you do not wish to specify any license type, you can use a special value UNLICENSED or simply skip the whole comment (it won’t result in an error, just a warning).

Line 2: Declaring the solidity version

Line 4: Starting our Contract named Counter

Line 6: Creating an event ValueChanged with two arguments, oldValue will be the input value of count and newValue which will be the output value of count.

Line 9: Declaring a private variable count with initial value zero.

Line 12-15: Creating a public function increment, increasing the value of count by 1, and emitting the event ValueChanged with oldValue = count’s current value minus 1 and newValue = count’s current value.

Line 18-19: Creating a public function getCount to return the value of count.

Compile the smart-contract and deploy it using Injected Web3; make sure to select the Ropsten network on your Metamask plugin before deploying the contract.

Also, copy the ABI of the contract from the compile tab. We’ll need it later.

You will see your contract under the "Deployed Contracts'' section in Remix. Open the deployed contract and click on getCount; you’ll see that the value returned is zero, which is expected.

Now click on increment to increase the value of count by one, confirm the transaction, and after receiving the transaction confirmation notification, click again on getCount. This time you’ll see that the value of the count has been changed.

We’ll fetch logs for this transaction. Copy the contract address from the copy button near the contract’s name under the “Deployed contracts'' section. Learn more about smart contracts and Solidity by going through the Solidity section of QuikNode guides.

Booting our Ethereum node

We could use pretty much any Ethereum client, such as Geth or OpenEthereum (fka Parity), for our purposes today. Since that is a bit too involved for fetching logs, we'll just grab a free endpoint from QuikNode to make this easy. We’ll need a Ropsten endpoint to get data from the chain as we’ve deployed our contract on the Ropsten testnet. After you've created your free ethereum endpoint, copy your HTTP Provider endpoint:

You'll need this later, so copy it and save it.

Getting logs in Ruby

Installing the ethereum.rb gem

Let’s first make sure we have Ruby installed on the system. To check just copy-paste and run the following in your terminal/cmd:

$ ruby -v

If not installed, follow the instructions on the download page of the official Ruby website, and Once we ensure ruby is installed and at the correct version (2.0+), let's move forward and install the ethereum.rb gem, You can install it from the command line using RubyGems, the package manager for ruby:

$ gem install ethereum.rb

If you're using macOS, you may encounter a permission-related problem while installing gem because the version of Ruby that ships with macOS is usually for Apple's use; you can always make few changes and get going, but I won't recommend that.

Instead of that, I'll suggest using rbenv or RVM (Ruby Version Manager) to manage a different Ruby version, which will be installed into a sandbox in your home directory, that you can make changes to without worrying about messing up the system Ruby.

Creating and running Ruby Script to fetch logs

Open your text editor and create a Ruby file log.rb, copy-paste the following in it.

require 'ostruct'
require 'ethereum.rb'

client = Ethereum::HttpClient.new("ADD_YOUR_ETHEREUM_NODE_URL")
abi = [
  {
    "inputs": [],
    "name": "increment",
    "outputs": [],
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "anonymous": false,
    "inputs": [
      {
        "indexed": false,
        "internalType": "uint256",
        "name": "oldValue",
        "type": "uint256"
      },
      {
        "indexed": false,
        "internalType": "uint256",
        "name": "newValue",
        "type": "uint256"
      }
    ],
    "name": "ValueChanged",
    "type": "event"
  },

  {
    "inputs": [],
    "name": "getCount",
    "outputs": [
      {
        "internalType": "uint256",
        "name": "",
        "type": "uint256"
      }
    ],
    "stateMutability": "view",
    "type": "function"
  }
]

contract = Ethereum::Contract.create(
  client: client,
  name: "event", 
  address: "CONTRACT_ADDRESS_FROM_REMIX", 
  abi: abi
)
event_abi = contract.abi.find {|a| a['name'] == 'ValueChanged'}
event_inputs = event_abi['inputs'].map {|i| OpenStruct.new(i)}
decoder = Ethereum::Decoder.new
filter_id = contract.new_filter.value_changed(
  {
    from_block: '0x0',
    to_block: 'latest',
    address: 'CONTRACT_ADDRESS_FROM_REMIX',
    topics: []
  }
)

events = contract.get_filter_logs.value_changed(filter_id)
events.each do |event|
  transaction_id = event[:transactionHash]
  transaction = client.eth_get_transaction_receipt(transaction_id)
  logs = transaction.dig('result', 'logs').find { |d| d['data'] != "0x" }
  data = logs.fetch('data')
  args = decoder.decode_arguments(event_inputs, data)
  eventlogs = transaction.inspect
  block = logs.fetch('blockNumber')
  puts "Transaction: #{eventlogs} with args: #{args}"
  puts "-" * 20
  puts "The Transaction was added to block: #{block.to_i(16)}" 
end

Make sure to replace `ADD_YOUR_ETHEREUM_NODE_URL` with the ropsten http provider and `CONTRACT_ADDRESS_FROM_REMIX` with the address of the deployed contract from the above sections

Explanation of the code above

Line 1-2: Importing necessary libraries.

Line 4: Setting up our Ropsten node URL.

Line 5-46: Paste your ABI, Which you copied in the previous section (if you used the same smart contract, you use the same ABI), saving the ABI in abi variables.

Line 48-53: Instantiating our contract object and getting the contract from the blockchain using the details, supplying our node to the client, Adding the name of our smart contract “event,” Replace the address of the deployed contract which you copied in the previous step and the abi.

Line 54: Finding the event ValueChanged from the ABI and storing it in the event_abi variable.

Line 55: Mapping our event_abi to an OpenStruct object to process hashes and storing it in the event_inputs variable.

Line 56: Creating a variable decoder and instantiating the Decoder method object.

Line 57-64: Putting a filter on the contract event value changed with necessary parameters and storing it in filter_id variable.

Line 66: Getting logs from the filter events and storing them in the events variable.

Line 67: Returning all elements of the events array using each method.

Line 68: Getting the transaction hash and storing it in the transaction_id variable.

Line 69: Getting transaction details using the eth_get_transaction_receipt method for transaction hash via our node.

Line 70: Searching the event logs for results and logs and storing it in a logs variable.

Line 71: Fetching data from logs and storing it in a data variable.

Line 72: Decoding the event_inputs and data and storing it in args variable.

Line 73: Getting the transaction details using inspect method and storing it in eventlogs variable.

Line 74: Fetching blockNumber from logs and storing it in block variable.

Line 75:Printing the logs with args which contains the input and output values of count from the smart contract.

Line 76: Printing 20 dashes to beautify the output.

Line 77: Printing the block number after converting it into a decimal number.

Now, save the file and run it as follows:

$ ruby log.rb

If everything goes right, your output must look like this:

The output has the transaction, and the event details, the later part of the output has the block number extracted from the data.

Conclusion

So here we learned about Ethereum logs and how to filter and extract information from them using Ruby.

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Overview
What is Ruby?
What are Ethereum logs?
What is ethereum.rb?
Creating and Deploying a smart contract
Booting our Ethereum node
Getting logs in Ruby
Conclusion

Sahil Sen

Web developer who believes in power of communities and is passionate about community building. Developer evangelist @ QuikNode.

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