Definition
Block building is the process by which a set of transactions is assembled into a structured data unit called a block, following a blockchain’s consensus and protocol rules. It involves choosing which transactions from the mempool to include, determining their ordering, and constructing a valid block template that satisfies size, fee, and validity constraints. The resulting candidate block is then ready for block proposal or block execution, depending on the network’s architecture.
In systems that separate roles, a block builder is the specialized actor responsible for performing block building, distinct from the entity that ultimately performs block proposal. Block building directly affects which transactions are confirmed, how quickly they are included, and how value such as fees or MEV is captured within the protocol. As a process, it is central to how a blockchain progresses from one block to the next while maintaining consensus integrity.
Context and Usage
Block building operates on top of the mempool, where pending transactions wait before being included in a block. The process transforms this unstructured pool into an ordered block template that can be validated and, if accepted, appended to the existing chain of blocks. In advanced designs, block building may be optimized for fee revenue, MEV extraction, or other protocol-specific objectives, while still remaining constrained by consensus rules.
The distinction between block building, block execution, and block proposal is important in architectures that modularize these responsibilities. Block building focuses on constructing the candidate block, block execution focuses on computing the resulting state transitions, and block proposal focuses on submitting the chosen block to the network for inclusion. Across designs, block building remains the core process that shapes the content and structure of each new block in the blockchain.