Ethereum Execution Client: Geth Architecture Overview

The Ethereum network operates as a decentralized world computer, and at the heart of its execution layer lies Geth — one of the most widely used and battle-tested clients. This article explores the architecture of Geth (Go-Ethereum), offering developers and enthusiasts a structured framework to understand its design, core components, and startup process. Whether you're researching blockchain internals or preparing to contribute to client development, this guide provides a solid foundation.

Understanding Ethereum Clients Post-The Merge

Before The Merge, Ethereum operated under a single-client model where one software handled both transaction execution and consensus. After The Merge, Ethereum transitioned to a dual-layer architecture:

Execution Layer (EL): Handles transaction processing, state management, and smart contract execution.
Consensus Layer (CL): Manages proof-of-stake (PoS) consensus, block finality, and validator coordination.

These two layers communicate exclusively via the Engine API, ensuring modularity, security, and flexibility in client implementation.

Popular Execution Layer Clients

Geth: Built in Go, maintained by the Ethereum Foundation. Known for stability and wide adoption.
Nethermind: Developed in C#, supports advanced features like MEV tooling.
Besu: Java-based, part of Hyperledger, ideal for enterprise use.
Erigon: Optimized for fast sync and low disk usage; forked from Geth.
Reth: Rust-based, modular design with high performance; production-ready.

Leading Consensus Layer Clients

Prysm (Go)
Lighthouse (Rust)
Teku (Java)
Nimbus (Nim)

This architectural split allows innovation across different implementations while preserving network integrity.

The Execution Layer: A Transaction-Driven State Machine

At its core, the Ethereum execution layer functions as a state machine driven by transactions. Every transaction modifies the global state — balances, contract storage, or code — through deterministic execution in the Ethereum Virtual Machine (EVM).

Core Responsibilities of the Execution Layer

Execute transactions using the EVM
Maintain current state and historical data
Run P2P networking for peer discovery and data propagation
Manage a transaction pool (mempool)
Provide RPC interfaces for external interaction

Transactions are signed by users and broadcast across the network. Valid ones enter the mempool before being included in blocks. The actual execution order is determined by the consensus layer.

👉 Discover how modern blockchain clients handle real-time transaction validation

Communication Between Layers

The Engine API is the only interface between the execution and consensus layers. When a validator is selected to propose a block, the consensus layer instructs the execution client to build it. Conversely, when receiving a new block from the network, the consensus layer asks the execution client to validate it.

Key Modules in Geth’s Architecture

Geth's source code (go-ethereum) is extensive, but several core modules form the backbone of its functionality:

Module	Purpose
`core`	Blockchain logic: block/transaction processing, state transitions
`eth`	Full node protocol implementation: syncing, networking
`ethdb`	Database abstraction layer
`node`	Node lifecycle and service orchestration
`p2p`	Peer-to-peer networking stack
`rlp`	Recursive Length Prefix encoding for data serialization
`trie & triedb`	Merkle Patricia Trie for efficient state storage

These modules work together to ensure secure, scalable, and reliable operation.

High-Level Module Breakdown

External access to Geth occurs via two primary channels:

JSON-RPC APIs: For applications and tools
Console (JavaScript REPL): For node operators

Internally, these interfaces rely on layered abstractions that separate concerns and enhance maintainability.

Core Data Structures

`Ethereum` Struct

Defined in eth/backend.go, this struct encapsulates the entire execution node:

type Ethereum struct {
    config         *ethconfig.Config
    txPool         *txpool.TxPool
    blockchain     *core.BlockChain
    chainDb        ethdb.Database
    engine         consensus.Engine
    p2pServer      *p2p.Server
    // ... plus 15+ other fields
}

It includes components for transaction management, blockchain handling, database access, and consensus verification.

`Node` Struct

Located in node/node.go, this serves as the container for all services:

type Node struct {
    server       *p2p.Server
    lifecycles   []Lifecycle
    rpcAPIs      []rpc.API
    // ... manages startup/shutdown of all modules
}

This modular design enables flexible configuration and plugin-style extensibility.

Foundational Components: Network, Compute, Storage

We can view Ethereum as a distributed computer built on three pillars:

1. Network: devp2p

The devp2p protocol powers Ethereum’s P2P network. It enables:

Node discovery using a Kademlia-like DHT (Table struct in p2p/discover/table.go)
Secure communication between peers
Data exchange via subprotocols like eth/68 and snap

Each node is represented by an enr.Record, which stores identity, IP address, port, and supported protocols — encoded using RLP.

👉 Learn how decentralized networks achieve global consensus without central control

2. Compute: Ethereum Virtual Machine (EVM)

The EVM executes all transactions and smart contracts. It ensures deterministic outcomes regardless of underlying hardware — much like the JVM.

Key components:

EVM struct: Execution context and state database reference
EVMInterpreter: Executes bytecode instruction-by-instruction
Contract: Represents executing contracts with caller, gas, input data

All state changes must go through EVM execution — there is no alternative path.

3. Storage: ethdb

ethdb abstracts database operations behind a unified interface. It supports multiple backends like LevelDB and Pebble.

Two main interfaces:

KeyValueStore: For recent, frequently accessed data (e.g., latest blocks)
AncientStore: For immutable historical data (archival mode)

State data is stored using Merkle Patricia Tries, implemented in the trie package. Intermediate trie states are managed by triedb before final persistence.

Supporting Submodules

Additional functionality builds upon the core trio:

eth/protocols: Implements eth/68 (block sync, tx propagation) and snap (fast state sync)
rawdb / statedb: Higher-level wrappers over ethdb for structured data access
event: Publish-subscribe system for internal module communication
rpc: JSON-RPC and IPC endpoints for external interaction

These layers enable features like fast synchronization, filtering, and real-time event monitoring.

Geth Node Startup Process

Launching a Geth node involves two phases: initialization and activation.

Phase 1: Initialization

Occurs primarily in cmd/geth/config.go within makeFullNode():

Create the Node container
Initialize services:
- RPC servers (HTTP, WS, IPC)
- Account manager
- Database (chainDb)
- Consensus engine (for PoS validation)
- Blockchain instance
- Transaction pools (legacy + blob)
- Handler for P2P messages
Register protocols (eth, snap)
Register APIs (EthAPI, NetAPI, EngineAPI)
Register Ethereum as a lifecycle service

This phase sets up all dependencies without starting active networking or syncing.

Phase 2: Node Start

Once initialized:

Start all registered RPC servers
Launch lifecycle-managed services
Begin P2P discovery and data exchange

The node now accepts incoming connections, downloads blocks if needed, and participates in transaction propagation.

👉 See how leading blockchain platforms optimize node startup for enterprise deployment

Frequently Asked Questions

Q1: What is the role of Geth after The Merge?

Geth remains responsible for executing transactions and maintaining state. It no longer performs mining but validates blocks proposed by the consensus layer via the Engine API.

Q2: How does Geth ensure data consistency across nodes?

Through cryptographic hashing and deterministic EVM execution. All nodes independently verify each block’s state root, ensuring global consensus.

Q3: Can I run Geth on low-resource devices?

Yes — using light sync modes or snap sync reduces bandwidth and storage needs. However, full archival nodes require significant disk space.

Q4: Is Geth written entirely in Go?

Yes. Geth is implemented in Go (Golang), making it portable across operating systems and easy to compile from source.

Q5: What makes Geth different from other execution clients?

Geth has the longest track record of security and reliability. It's often the reference implementation during protocol upgrades due to its widespread use.

Q6: How do I extend Geth with custom functionality?

You can write plugins or middleware that interact via JSON-RPC or hook into internal events using filters and subscriptions.

Summary

Understanding Geth’s architecture reveals how Ethereum achieves decentralization at scale. As a transaction-driven state machine, it relies on three foundational elements:

Computation via the EVM
Storage via ethdb and Merkle trees
Networking via devp2p

By cleanly separating concerns through modular design — from node lifecycle management to P2P communication — Geth offers both robustness and extensibility.

With this high-level overview, developers can dive deeper into specific areas such as consensus integration, transaction pooling, or performance optimization — all within a well-documented and actively maintained codebase.

Whether you're auditing client behavior, contributing to open-source development, or building infrastructure tools, grasping Geth’s structure is essential for engaging with Ethereum at a technical level.