Sending ETH, providing liquidity on an AMM, or deploying a new smart contract? Every action you take on the Ethereum blockchain leaves an on-chain record. But what exactly goes into a transaction?
This guide breaks down the atomic components of an Ethereum transaction, helping you understand the metadata, cache, and data that form the backbone of every interaction with the world computer.
What Is an Ethereum Transaction?
Ethereum is often described as a world computer—a globally shared infrastructure existing across thousands of networked devices. Users interact with it through wallets like MetaMask, which create and send transactions to the network. Once received, these transactions are written into blocks.
Each transaction is composed of three core parts:
- Metadata: Information about the transaction, such as sender/receiver, value transferred, and gas details.
- Cache: A list of accounts and storage keys the transaction is expected to access.
- Data: The payload of the transaction, which can include smart contract code or API calls.
Let’s explore each of these in detail.
Metadata: Transaction Context
Metadata forms the foundational layer of any Ethereum transaction. It includes basic but essential information that validators and users need to verify and process the transaction.
Here are the key metadata fields:
- chainid: Introduced through EIP-155, this field protects the chain from replay attacks across networks like Ethereum Classic.
- type: Two primary types exist—0x0 for new contract deployments and 0x2 for all other transactions. EIP-2718 introduced type 0x2 as an envelope for future transaction types without altering core protocol rules.
- nonce: A count of transactions sent from a specific address. It increments after each transaction is mined, preventing replay attacks.
- to: The recipient's address, which can be a wallet or a smart contract.
- value: The amount of ETH being transferred (note: this applies only to ETH, not other tokens).
- gas: The total gas units allocated for the transaction.
- maxFeePerGas: The maximum fee per gas unit (in Wei) the user is willing to pay. This includes both the base fee and the priority fee.
- maxPriorityFeePerGas: The additional tip per gas unit (in Wei) paid to miners/validators beyond the base fee, incentivizing faster inclusion.
- gasPrice: The cost per gas unit for the transaction (mainly used in legacy transaction types).
- (r, s, v): These values form the user’s digital signature, verifying that the transaction was authorized by the sender before execution.
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Cache: Access Lists
The cache component includes the accessList, a predefined set of addresses and storage keys that the transaction intends to use. While transactions can access resources outside this list, doing so incurs higher gas costs.
EIP-2929 introduced access lists to allow clients to pre-fetch and cache data during transaction execution. Currently, accessing data within the list offers a 10% discount on gas fees. This discount may expand in the future as Ethereum continues to support light clients and scalability improvements.
Data: The Transaction Payload
The data field carries the core payload of the transaction. It can be structured in three ways:
- ETH Transfer: Left blank for simple ETH transfers.
- Smart Contract API Call: Contains function names and parameters for interactions with deployed contracts.
- New Smart Contract Deployment: Includes the compiled bytecode of a new contract.
Although stored in binary form on-chain, this data can be decoded into human-readable format. It exists on the blockchain but is not part of the Ethereum Virtual Machine (EVM) state. It serves only as input during transaction execution and cannot be directly tracked or used at the consensus layer.
This characteristic is useful for applications that need to store historical data on-chain for later retrieval without requiring direct EVM access. Rollups, for example, leverage this feature extensively by storing batched transaction data in a cost-effective manner.
Frequently Asked Questions
What is the purpose of the nonce in an Ethereum transaction?
The nonce is a sequential number assigned to each transaction from a specific address. It ensures transaction order and prevents replay attacks, where a malicious actor might rebroadcast a previously executed transaction.
How does an access list reduce gas fees?
An access list specifies which accounts and storage keys a transaction will use, allowing nodes to pre-load and cache this data. Accesses within the list receive a gas discount, making execution more efficient and cheaper.
Can the data field be modified after a transaction is sent?
No. Once a transaction is broadcast and included in a block, all its components—metadata, cache, and data—are immutable. This ensures transparency and security across the network.
What is the difference between value and data in a transaction?
The value field specifies the amount of ETH being sent to the recipient, while the data field contains additional input information, such as function calls or contract deployment code.
Why are transactions signed with (r, s, v) values?
These values form a digital signature using Elliptic Curve Digital Signature Algorithm (ECDSA). They prove that the transaction was authorized by the holder of the private key associated with the sending address.
How do rollups use the data field?
Rollups batch multiple transactions off-chain and submit compressed data via the data field. This approach reduces congestion and gas costs on the main Ethereum chain while maintaining security through on-chain data availability.
Conclusion
Understanding the structure of an Ethereum transaction empowers users and developers to interact with the blockchain more effectively. From metadata defining the transaction context to access lists optimizing execution costs and data fields enabling complex operations, each component plays a critical role in the network's functionality.
As Ethereum continues to evolve, features like access lists and data fields will become even more significant in supporting scalability and innovation. Whether you're a beginner or an experienced user, knowing what goes on under the hood can enhance your blockchain experience.