README.md

Offchain Café: EVM

A simple-to-use EVM chain indexing made for Web3 applications.

Features

• Synchronize contract logs
• Serve the logs via GraphQL, including realtime subscriptions

Usage

To run Offchain Café: EVM, you'll need the following:

• A PostgreSQL database (DATABASE_URL)
• A Redis instance (REDIS_URL)
• An Ethereum RPC endpoint (ETH_CHAIN_ID, ETH_HTTP_RPC_URL and ETH_WS_RPC_URL (optional, but recommended))
• Addresses and deploy block numbers of the contracts you want to index

1. Prepare contracts.json (see contracts.example.json for an example)
2. Prepare environment (see env.example for an example)
3. Run pnpm install && pnpm build
4. Run pnpm db:migrate to create the database schema
5. Run pnpm start

You should now have the GraphQL API endpoint running at /graphql. Enjoy! ☕️

Client examples

This example uses @urql/core in NodeJS environment.

offchainCafe.ts

import { createClient, defaultExchanges } from "@urql/core";
import { yogaExchange } from "@graphql-yoga/urql-exchange";

const client = createClient({
  url: "http://<OFFCHAIN_SERVER_HOST>:<OFFCHAIN_SERVER_PORT>/graphql",
  exchanges: [...defaultExchanges, yogaExchange()],
});

export default client;

queryLogs.ts

import { gql } from "@urql/core";
import offchainCafe from "./offchainCafe";
import receiverAbi from "~/abi/receiver.json" assert { type: "json" };

const receiverAddress = "0x...";
const iface = new ethers.utils.Interface(receiverAbi);
const receiveEventTopic = iface.getEventTopic("Receive");

const fromBlock = 0; // 0 means contract creation block
const toBlock = -1; // -1 means the latest chain block

const logs = (
  await offchainCafe
    .query(
      gql`
        query ContractLogs(
          $fromBlock: Int!,
          $toBlock: Int!
        ) {
          contract(address: "${receiverAddress}") {
            logs(
              topics: [["${receiveEventTopic}"]],
              limit: 10,
              fromBlock: $fromBlock,
              toBlock: $toBlock
            ) {
                block {
                  number
                  timestamp
                }
                logIndex
                transaction {
                  hash
                }
                data
                topics
            }
          }
        }`,
      { fromBlock, toBlock }
    )
    .toPromise()
).data.contract.logs;

subscribeToLogs.ts

import { pipe, subscribe } from "wonka";
import { gql } from "@urql/core";
import offchainCafe from "./offchainCafe";
import receiverAbi from "~/abi/receiver.json" assert { type: "json" };

const receiverAddress = "0x...";
const iface = new ethers.utils.Interface(receiverAbi);
const receiveEventTopic = iface.getEventTopic("Receive");

const { unsubscribe } = pipe(
  offchainCafe.subscription(
    gql`
      subscription {
        log(
          contractAddress: "${receiverAddress}",
          topics: [["${receiveEventTopic}"]]
        ) {
          block {
            number
            timestamp
          }
          logIndex
          transaction {
            hash
          }
          data
          topics
        }
      }`,
    {}
  ),
  subscribe(async (result) => {
    const log = result.data!.log;
  })
);