Skill Guide

On-chain data extraction and indexing (RPC calls, subgraphs, event logs, trace data)

The systematic process of querying blockchain node interfaces (RPC), deploying custom indexing schemas (subgraphs), and parsing low-level execution data (event logs, traces) to transform raw blockchain state into structured, queryable datasets.

This skill is the foundation for building data-driven DeFi protocols, NFT analytics platforms, and compliance tools. It directly impacts business outcomes by enabling real-time market intelligence, risk management, and the creation of novel financial products based on verifiable on-chain activity.

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How to Learn On-chain data extraction and indexing (RPC calls, subgraphs, event logs, trace data)

1. Master JSON-RPC API methods: `eth_getBlockByNumber`, `eth_getTransactionReceipt`, `eth_call`. 2. Understand Ethereum's data model: blocks, transactions, receipts, logs, and state trie. 3. Write basic scripts in Python (using `web3.py`) or JavaScript (using `ethers.js`) to fetch and parse transaction data from a public endpoint like Infura or Alchemy.

1. Deploy and query a custom subgraph using The Graph's hosted service to index a specific protocol's events (e.g., Uniswap V2 swaps). 2. Practice writing efficient GraphQL queries against your subgraph. 3. Learn to decode raw event log data using ABI and understand topics. Common mistake: failing to handle chain reorganizations (reorgs) when indexing events.

1. Architect a production-grade indexing pipeline that combines RPC polling for mempool data, subgraphs for event indexing, and trace calls (`debug_traceTransaction`, `trace_block`) for internal transaction analysis. 2. Implement backfilling strategies and handle multiple chain forks. 3. Optimize for cost and latency by mixing full-archive nodes with specialized data providers (e.g., Covalent, Dune).

Practice Projects

Beginner

Project

Wallet Transaction Historian

Scenario

Build a CLI tool that takes an Ethereum address and outputs its entire transaction history (last 100 txns) with decoded function names and token transfers.

How to Execute

1. Use `ethers.js` or `web3.py` to connect to an RPC provider. 2. Fetch transactions using `eth_getTransactionByHash` for a list of block ranges. 3. Decode input data using a local copy of the contract ABI for common contracts (ERC-20, Uniswap Router). 4. Parse and display logs from receipts to show token transfers.

Intermediate

Project

DEX Liquidity Pool Tracker

Scenario

Create a service that monitors Uniswap V2/V3 pair contracts for new liquidity events (Mint, Burn, Sync) and calculates real-time pool TVL and volume.

How to Execute

1. Define a subgraph schema for `Pair`, `Mint`, `Burn`, and `Sync` entities. 2. Write event handlers in AssemblyScript to process `Sync(address,uint112,uint112)` and `Mint(address,uint256,uint256)` events. 3. Deploy the subgraph to The Graph's hosted service. 4. Build a frontend dashboard that queries the subgraph's GraphQL endpoint for live stats.

Advanced

Project

MEV & Arbitrage Opportunity Scanner

Scenario

Design a system that scans mempool transactions and block traces to identify and analyze sandwich attacks, liquidations, and cross-DEX arbitrage opportunities in real-time.

How to Execute

1. Set up a streaming pipeline from an RPC node's `pendingTransactions` endpoint. 2. Simulate transactions using `eth_call` with state overrides to detect profitable paths. 3. Use `debug_traceBlockByNumber` with a custom tracer (javascript) to trace execution flow and internal calls of confirmed blocks. 4. Correlate observed patterns with known MEV bot addresses and profit calculations.

Tools & Frameworks

Node & RPC Providers

AlchemyInfuraQuickNodeSelf-hosted Geth/Erigon

Primary interfaces for querying blockchain data. Use managed providers for reliability and scalability; self-hosted (Erigon) for cost control and full data access (traces, archive).

Indexing Frameworks

The Graph (Subgraphs)GoldskyPonderSubQuery

For building and deploying custom APIs to index on-chain events. The Graph is the standard; newer tools like Ponder offer different developer experiences and hosting options.

Client Libraries

ethers.jsweb3.jsweb3.pyalloy (Rust)

For programmatically interacting with RPC endpoints. ethers.js (TypeScript) is the industry standard for frontend and script development.

Data Decoding & Analysis

ABI Encoding/DecodingDune AnalyticsTenderlyBlocknative

ABI is used to interpret raw call data and logs. Dune for SQL-based analytics on decoded data. Tenderly for transaction simulation and debugging.

Interview Questions

Answer Strategy

Contrast the raw, efficient, but stateless nature of `eth_getLogs` with the stateful, indexed, and query-optimized nature of a subgraph. Use a concrete example: `eth_getLogs` is good for a one-time historical backfill of a specific event across a block range. A subgraph is essential for building an application that needs to query aggregated state (e.g., 'give me all swaps for pair X with volume > $1M') in real-time.

Answer Strategy

Demonstrate understanding of subgraph health monitoring, backfilling, and architectural resilience. The core competency tested is operational reliability. The answer should show a systematic approach to diagnostics, recovery, and prevention.