This project is an Ethereum adapter for Yggdrasil publisher/subscriber.

Small example

The following example uses Ethereum adapter to distribute messages:

We need to configure EthEvent which is the library use to request the events from contracts:

config :eth_event,
  node_url: "https://mainnet.infura.io/v3", # Can be a custom URL
  node_key: "" # Infura key or empty for custom URL

And then we declare the event using EthEvent library. In this example, we will declare a Transfer(address,address,uint256) event. These events are declared in ERC20 tokens and they are emitted when a successfull token transfers has occurred between wallets. In Solidity, we would have:

contract SomeToken is ERC20 {

  event Trasfer(
    address indexed from,
    address indexed to,
    uint256 value
  );

  (...)
}

Using EthEvent library, we can create an equivalent of the previous event in Elixir as follows:

iex(1)> defmodule Transfer do
iex(1)>   use EthEvent.Schema
iex(1)>
iex(1)>   event "Transfer" do
iex(1)>     address :from, indexed: true
iex(1)>     address :to, indexed: true
iex(1)>     uint256 :value
iex(1)>   end
iex(1)> end

We can now subscribe to any contract that has that type of event. For this example, we are going to subscribe to the Transfer events from OmiseGo contract. First we declare the channel:

iex(2)> contract = "0xd26114cd6EE289AccF82350c8d8487fedB8A0C07" # OmiseGo
iex(3)> channel = %Yggdrasil.Channel{
iex(3)>   name: {Transfer, [address: contract]},
iex(3)>   adapter: :ethereum
iex(3)> }

And then we subscribe to it:

iex(4)> Yggdrasil.subscribe(channel)
iex(5)> flush()
{:Y_CONNECTED, %Yggdrasil.Channel{(...)}}

If we wait enough, we should receive a message as follows:

iex(6)> flush()
{:Y_EVENT, %Yggdrasil.Channel{(...)}, %Transfer{(...)}}

Where the struct %Transfer{} contains the information of a token transfer between two wallet addresses.

When we want to stop receiving messages, then we can unsubscribe from the channel as follows:

iex(7)> Yggdrasil.unsubscribe(channel)
iex(8)> flush()
{:Y_DISCONNECTED, %Yggdrasil.Channel{(...)}}

Ethereum adapter

The Ethereum adapter has the following rules:

• The adapter name is identified by the atom :ethereum.
• The channel name must be a tuple with the module name of the event and the Keyword list of options e.g: :address, :from, :to, etc (depends on the event).
• The transformer should be always :default.
• Any backend can be used (by default is :default).

The following is an example of a valid channel for the subscribers:

%Yggdrasil.Channel{
  name: {Balance, [address: "0x3f5CE5FBFe3E9af3971dD833D26bA9b5C936f0bE"]},
  adapter: :ethereum
}

It will subscribe to the balance of that address.

Ethereum configuration

This adapter uses long-polling to get the information from the Ethereum node and the frequency of the requests can be configured with the :timeout parameter.

The following shows a configuration with and without namespace:

# Without namespace
config :yggdrasil,
  ethereum: [timeout: 5_000]

# With namespace
config :yggdrasil, EthereumOne,
  ethereum: [timeout: 15_000]

Additionally, we can configure EthEvent options:

• node_url - The Ethereum node URL.
• node_key - It’ll be appended to the URL at the end.

config :eth_event,
  node_url: "https://mainnet.infura.io/v3", # Can be a custom URL
  node_key: "some_key" # Empty for custom URL

All the options can be provided as OS environment variables. The available variables are:

• YGGDRASIL_ETHEREUM_TIMEOUT or <NAMESPACE>_YGGDRASIL_ETHEREUM_TIMEOUT.
• ETH_EVENT_NODE_URL.
• ETH_EVENT_NODE_KEY.

where <NAMESPACE> is the snakecase of the namespace chosen e.g. for the namespace EthereumTwo, you would use ETHEREUM_TWO as namespace in the OS environment variable.

Installation

Using this Ethereum adapter with Yggdrasil is a matter of adding the available hex package to your mix.exs file e.g:

def deps do
  [{:yggdrasil_ethereum, "~> 0.1"}]
end