> For the complete documentation index, see [llms.txt](https://mau-eth.gitbook.io/mau/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mau-eth.gitbook.io/mau/account-abstraction/key-roles-and-their-interactions.md).

# Key Roles and Their Interactions

The ERC-4337 ecosystem introduces the UserOperation, a flexible alternative to typical Ethereum transactions.

## **UserOperation** 

It is like a blueprint that tells the system about a user's intended action. To avoid mix-ups, it's not just called a `transaction`, even though it shares similarities.

### **What does it have in common with regular transactions?**

Just like a typical transaction, a `UserOperation` has details such as:

* **`sender`**: Who's initiating it.
* **`to`**: The recipient or destination.
* **`calldata`**: The action or operation to be performed.
* **`maxFeePerGas`**: The maximum gas fee.
* **`maxPriorityFee`**: Priority fee for faster processing.
* **`signature`**: A cryptographic proof of authenticity.
* **`nonce`**: A unique number to prevent replay attacks.

### **How is it different?**

Apart from the common fields, a `UserOperation` contains additional unique fields which are explained separately.

The way the `signature` works in a `UserOperation` is unique. Instead of being pre-determined by the system, it's up to each individual account's rules to decide how it’s used.

In essence, while a `UserOperation` resembles a transaction in many ways, it also offers more flexibility and customization for users.

## **Sender** 

This refers to the client that initiates or sends out a `UserOperation`.

## **EntryPoint** 

A unique contract designed to process batches of `UserOperations`.

`Bundlers` or clients pick and choose which `EntryPoint` they want to work with, creating a whitelist of accepted EntryPoints.

## **Bundler (acting as the middleman, ensuring everything gets processed correctly into the blockchain)**

A type of node (or system participant) that specializes in managing `UserOperations`.

Responsibilities:

* Take `UserOperations` and convert them into a valid action, known as `EntryPoint.handleOps()`, to be added to a blockchain block.
* There are several ways this can happen:
  1. A `bundler` might take on the role of a block builder itself.
  2. If the bundler isn't acting as a block builder, it has to work in tandem with established block-building systems, like mev-boost or any other system that separates block proposal from block building (called PBS).
  3. Alternatively, a bundler can utilize an experimental system, if available, known as `eth_sendRawTransactionConditional RPC API`.

## **Aggregator (that assists in ensuring multiple signatures are legitimate)**

 A supportive contract that accounts rely on, especially to verify combined or aggregated signatures.

Just as with the `EntryPoint`, `bundlers` or clients create a whitelist for the `aggregators` they trust and want to work with.

## Recap

In simpler terms, when using the `UserOperation` system, you have:

* The **Sender** starting things off by initiating a `UserOperation`.
* The **EntryPoint** which is the go-to spot for processing groups of these operations.
* The **Bundler** acting as the middleman, ensuring everything gets processed correctly into the blockchain.
* And the **Aggregator** that assists in ensuring multiple signatures are legitimate.