1. Overview

Purpose

This document compares how five major blockchains handle transaction validation before consensus to establish a clear reference for the Nomos mempool design.

Objectives

• Compare validation order, scope, and state access across five blockchains.
• Examine how different systems protect against denial-of-service and maintain consistent validation between their mempools and consensus layers.

Scope

Focuses on Bitcoin, Ethereum, Polkadot, Cosmos, and Cardano as implemented in Q1 2025.

Covers:

• transaction validation flow and ordering

• ledger access during validation

• queueing and eviction logic

• propagation and caching behavior

Key points

• All reviewed systems validate transactions from cheap → expensive stages to minimize wasted computation.
• Ledger reads during validation are always read-only with respect to committed state.
• Mempool and consensus components often reuse cached results to avoid repeated signature or proof checks.
• Queueing separates transactions that are ready to include from those waiting on dependencies.
• Pull-based transaction fetch offers stronger protection against inbound spam and resource floods.