---
title: "builder"
description: "Block building, flashblock production, metering, and block publishing for Base."
source: https://basehub.org/crates/builder/overview/
---
The `builder` crate group produces blocks and flashblocks. It assembles flashblocks at 200ms intervals and publishes them to streaming clients.

## Sub-crates

| Crate | Package Name | Purpose |
|-------|-------------|---------|
| [core](/crates/builder/core/) | `base-builder-core` | Core block building logic and flashblock production |
| [metering](/crates/builder/metering/) | `base-builder-metering` | Resource metering for block building |
| [publish](/crates/builder/publish/) | `base-builder-publish` | Block/flashblock publishing and distribution |

## Role in the Architecture

The builder sits between the execution layer and the consensus layer. It:

1. Receives transactions from the transaction pool
2. Builds execution payloads using the EVM
3. Produces flashblocks at ~200ms intervals during the 2-second block time
4. Publishes flashblocks via WebSocket for real-time streaming
5. Finalizes the canonical block at the end of each slot

The `base-builder-core` crate contains the main building logic, `base-builder-metering` handles gas and resource accounting, and `base-builder-publish` handles distribution of produced blocks.

## Source

[`crates/builder/`](https://github.com/base/base/tree/main/crates/builder)