Block Builder Module (BBM)

The Block Builder Module (BBM) is an external block builder operating under a model analogous to Proposer–Builder Separation (PBS).

BBM constructs the complete block outside the validator using the current network execution state and hands the finished block to the validator for propagation and participation in consensus.

Purpose

BBM is responsible for block construction under contention and MEV:

  • transaction and bundle selection

  • write-account conflict resolution

  • transaction inclusion and ordering

  • construction of a ready-to-propose block

  • reduction of execution uncertainty by operating on up-to-date state

BBM does not execute transactions and does not participate in consensus.

PBS interaction model

BBM and the validator perform strictly separated roles.

  • BBM (builder) Constructs the entire block, performing transaction selection, inclusion, and ordering, producing a ready-to-propose block.

  • Validator (proposer) Accepts the prepared block, performs standard protocol verification, propagates the block, and participates in consensus.

BBM does not modify protocol rules and does not interfere with voting logic.

Execution-aware block construction

BBM operates on the current execution state, synchronized with the network.

Inclusion and ordering decisions are made as close as possible to execution, which:

  • reduces execution failure probability

  • limits degradation of auctions into EV-based selection

  • improves predictability of inclusion outcomes

Execution proximity is an architectural property rather than a tunable parameter.

Inclusion and ordering semantics

BBM separates two decision layers:

  • Inclusion — whether a transaction is admitted into the block

  • Ordering — its position relative to other transactions

This separation allows state conflicts to be handled explicitly without coupling execution risk to prioritization logic, enabling deterministic block construction prior to propagation.

Conflict and failure handling

BBM treats execution failure as a systemic property rather than an edge case.

The module:

  • accounts for write-account conflicts

  • avoids speculative transaction inclusion

  • bounds the impact of failure probability on ordering and inclusion

This prevents hidden EV-driven behavior from dominating auction outcomes.

Scope and non-goals

BBM:

  • operates externally to the validator

  • is not an RPC service

  • does not custody funds

Its responsibility is to deterministically construct the complete block before propagation.

Role in the Shiroi system

BBM is the central component of Shiroi’s execution-close block production pipeline.

It composes with other systems, including Shredstream and Tips, forming a PBS-style infrastructure for block construction and delivery.

Summary

  • BBM is an external block builder

  • BBM constructs the entire block

  • The validator acts as the proposer

  • Consensus and protocol rules remain unchanged

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