LP-0031Finalexchangex-chaindagxvmassets

LP-031: Exchange VM

Abstract

The Exchange VM (XVM) runs the X-chain, Lux's asset creation and exchange chain. Unlike the linear P-chain and C-chain, the X-chain uses a DAG (Directed Acyclic Graph) structure where transactions are vertices that reference parent vertices. Conflicting transactions are resolved via FPC (LP-028). The DAG structure enables higher throughput by allowing non-conflicting transactions to be processed in parallel without block ordering.

Specification

DAG Structure

Each vertex contains:


Vertex {
    parentIDs   [][32]byte  // references to parent vertices (1-N)
    txs         []Tx        // transactions in this vertex
    height      uint64      // vertex height (max parent height + 1)
    epoch       uint64      // consensus epoch
}

Vertices form a DAG. A vertex is accepted when all its transactions are accepted and all parent vertices are accepted.

Transaction Types

| Tx Type | Description |

|---|---|

| CreateAssetTx | Create a new asset with name, symbol, denomination, and initial UTXOs |

| OperationTx | Operate on NFTs and custom assets (mint, burn, transfer) |

| BaseTx | Transfer fungible assets between addresses |

| ImportTx | Import UTXOs from P-chain or C-chain |

| ExportTx | Export UTXOs to P-chain or C-chain |

Asset Model

Assets on the X-chain are UTXO-based with flexible output types:

SECP256K1 Transfer Output: standard fungible transfer (threshold-of-N)
SECP256K1 Mint Output: minting authority for fungible assets
NFT Transfer Output: non-fungible token transfer
NFT Mint Output: minting authority for NFTs

Each asset has a unique 32-byte assetID (the hash of the CreateAssetTx).

Conflict Resolution

When two transactions spend the same UTXO:

1. Both transactions enter the DAG as competing vertices

2. FPC (LP-028) runs to decide which transaction is preferred

3. The losing transaction is rejected; its vertex is marked as abandoned

4. Dependent vertices of the abandoned vertex are also rejected

Throughput

The DAG structure enables parallelism:

Non-conflicting transactions are processed simultaneously
No block time bottleneck -- vertices are issued as soon as transactions arrive
Theoretical throughput: limited only by network bandwidth and UTXO set lookup speed
Practical throughput: >4,500 TPS on the X-chain

Security Considerations

1. Double-spend prevention: FPC guarantees that exactly one of two conflicting transactions is accepted, with finality in 300-800ms.

2. DAG consistency: a vertex is only accepted when all parents are accepted. This prevents orphan chains.

3. Asset creation spam: CreateAssetTx requires a transaction fee, preventing unbounded asset creation.

Reference

| Resource | Location |

|---|---|

| XVM implementation | github.com/luxfi/node/vms/xvm/ |

| DAG consensus (Nebula mode, LP-134) | github.com/luxfi/node/consensus/nebula/ |

| FPC protocol | LP-028 |

Copyright

Licensed under the MIT License.