Cross-Zone Atomic Coordination Protocol (CACP)

Introduction

Cross-zone transactions on the Logos Blockchain involve operations across two distinct zones that must be committed atomically. Without explicit coordination between the sequencers responsible for each zone, there is no guarantee that both operations succeed or fail together — a sequencer has no visibility into whether its counterpart has committed, failed, or is still pending.

This document specifies the Cross-Zone Atomic Coordination Protocol (CACP), a two-party off-chain protocol through which a pair of sequencers reach agreement on a joint MantleTx and ensure its atomic submission. CACP operates over the Synchronous Messaging mechanism defined in the Cross-Channel Messaging framework, and draws on the structure of Two-Phase Commit (2PC), adapted to the constraints of the Mantle execution model.

For this stage of the research, we adopt the simplifying assumption that each Zone is served by a single sequencer (committee of size one), and that the two sequencers involved in any cross-zone transaction are distinct, mutually known entities. Under this assumption, the question of who participates is trivial. The open research question is entirely how the two parties coordinate under partial failure.

The main result of this specification is as follows: CACP guarantees that a cross-zone transaction is either committed atomically across both zones, or not committed at all. A key property is that, upon completing Phase 3 (signature exchange), both sequencers independently hold a complete, valid signed transaction. This removes the single point of failure inherent in designs where only one coordinator is responsible for final submission.

Overview

CACP proceeds in five phases:

1. Intent Proposal — the Initiator proposes the cross-zone operation to the Counterparty.
2. Verification — the Counterparty validates the proposal against its own zone's state.
3. Signature Exchange — both parties sign the joint transaction; each party holds the complete signed transaction by the end of this phase.
4. Submission — the Initiator submits the signed transaction to Bedrock; the Counterparty retains a copy and acts as fallback if the Initiator becomes unavailable.
5. Timeout and Abort — at any phase prior to Phase 3 completing, either party may abort with no on-chain effect.

No on-chain state is modified before Phase 3 completes. Any failure before this point leaves both zones unchanged. There is nothing to roll back.

Protocol

Actors and Assumptions