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Indexing a Greeter Contract

Introduction​

This tutorial provides a step-by-step guide to indexing a simple Greeter smart contract deployed on multiple blockchains. You'll learn how to set up and run a multi-chain indexer using Envio's template system.

What is the Greeter Contract?​

The Greeter contract is a straightforward smart contract that allows users to store greeting messages on the blockchain. For this tutorial, we'll be indexing instances of this contract deployed on both Polygon and Linea networks.

What You'll Build​

By the end of this tutorial, you'll have:

  • A functioning multi-chain indexer that tracks greeting events
  • The ability to query these events through a GraphQL endpoint
  • Experience with Envio's core indexing functionality

Prerequisites​

Before starting, ensure you have the following installed:

Note: Docker is specifically required to run your indexer locally. You can skip Docker installation if you plan only to use Envio's hosted service.

Step 1: Initialize Your Project​

First, let's create a new project using Envio's Greeter template:

  1. Open your terminal and run:
pnpx envio init
  1. When prompted for a directory, you can press Enter to use the current directory or specify another path:
? Set the directory: (.) .
  1. Choose your preferred programming language for event handlers:
? Which language would you like to use?
> JavaScript
  TypeScript
  ReScript
  1. Select the Template initialization option:
? Choose an initialization option
> Template
  Contract Import
  1. Choose the Greeter template:
? Which template would you like to use?
> Greeter
  Erc20

After completing these steps, Envio will generate all the necessary files for your indexer project.

Step 2: Understanding the Generated Files​

Let's examine the key files that were created:

config.yaml​

This configuration file defines which networks and contracts to index:

# Partial example
envio_node:
  networks:
    - name: polygon
      # ... Polygon network settings
      contracts:
        - name: Greeter
          address: "0x9D02A17dE4E68545d3a58D3a20BbBE0399E05c9c"
          # ... contract settings
    - name: linea
      # ... Linea network settings
      contracts:
        - name: Greeter
          address: "0xdEe21B97AB77a16B4b236F952e586cf8408CF32A"
          # ... contract settings

schema.graphql​

This schema defines the data structures for the indexed events:

type Greeting {
  id: ID!
  user: String!
  greeting: String!
  blockNumber: Int!
  blockTimestamp: Int!
  transactionHash: String!
}

type User {
  id: ID!
  latestGreeting: String!
  numberOfGreetings: Int!
  greetings: [String!]!
}

src/EventHandlers.js (or .ts/.res)​

This file contains the logic to process events emitted by the Greeter contract.

Step 3: Start Your Indexer​

Important: Make sure Docker Desktop is running before proceeding.

  1. Start the indexer with:
pnpm dev

This command:

  • Launches Docker containers for the database and Hasura
  • Sets up your local development environment
  • Begins indexing data from the specified contracts
  • Opens a terminal UI to monitor indexing progress

The indexer will retrieve data from both Polygon and Linea blockchains, starting from the blocks specified in your config.yaml file.

Step 4: Interact with the Contracts​

To see your indexer in action, you can write new greetings to the blockchain:

For Polygon:​

  1. Visit the contract on Polygonscan
  2. Connect your wallet
  3. Use the setGreeting function to write a new greeting
  4. Submit the transaction

For Linea:​

  1. Visit the contract on Lineascan
  2. Connect your wallet
  3. Use the setGreeting function to write a new greeting
  4. Submit the transaction

Since this is a multi-chain example, you can interact with both contracts to see how Envio handles data from different blockchains simultaneously.

Step 5: Query the Indexed Data​

Now you can explore the data your indexer has captured:

  1. Open Hasura at http://localhost:8080
  2. When prompted for authentication, use the password: testing
  3. Navigate to the Data tab to browse the database tables
  4. Or use the API tab to write GraphQL queries

Example Query​

Try this query to see the latest greetings:

query GetGreetings {
  Greeting(limit: 10, order_by: { blockTimestamp: desc }) {
    id
    user
    greeting
    blockNumber
    blockTimestamp
    transactionHash
  }
}

Step 6: Deploy to Production (Optional)​

When you're ready to move from local development to production:

  1. Visit the Envio Hosted Service
  2. Follow the steps to deploy your indexer
  3. Get a production GraphQL endpoint for your application

For detailed deployment instructions, see the Hosted Service documentation.

What You've Learned​

By completing this tutorial, you've learned:

  • How to initialize an Envio project from a template
  • How indexers process data from multiple blockchains
  • How to query indexed data using GraphQL
  • The basic structure of an Envio indexing project

Next Steps​

Now that you've mastered the basics, you can:

  • Try the Contract Import feature to index any deployed contract
  • Customize the event handlers to implement more complex indexing logic
  • Add relationships between entities in your schema
  • Explore the Advanced Querying features
  • Create aggregated statistics from your indexed data

For more tutorials and examples, visit the Envio Documentation or join our Discord community for support.