Skip to main content
ADR 0003 · Accepted · Phase 3
Status: Accepted · Phase: 3 · Supersedes: none · Superseded by: none

​
Context

Arc settles in stablecoin and must not be wedded to one chain: cost, speed and liquidity differ per corridor and change over time. Settlement code also has to handle reorgs, stuck transactions, dropped transactions and reverts — none of which occur on demand against a real network.

​
Decision

A ChainDriver interface — broadcast, getTransaction, getConfirmations, estimateFee, head, subscribe — with a deterministic simulator as the default implementation. Five chains are modelled with genuinely different characteristics: block time, finality depth, reorg depth, fee model and failure rates. forceReorg(depth) injects a reorg at an exact moment.

​
Consequences

Good. The saga is written against finality, not against a vendor SDK. Failure paths are testable: a seeded run reproduces the same blocks, reorgs and transaction outcomes every time, so a reorg test is deterministic rather than flaky. Chain selection becomes a per-transfer decision instead of a deployment-time one. The modelling surfaces things a naive abstraction hides. Polygon has 2-second blocks but needs 128 confirmations, so it settles slowest despite looking fastest. And failed and dropped are different terminal states — a failed transaction was mined and consumed a fee; a dropped one never made it into a block and cost nothing. Compensation differs for each. Costs. The simulator is a model, and a model is wrong in ways you have not thought of. Real RPC behaviour — rate limits, inconsistent nodes, mempool visibility — is not represented at all.

​
Alternatives

A local testnet (Anvil/Hardhat) — more realistic execution, but adds a required external process, slows the suite, and still cannot produce a deep reorg on cue. Direct SDK calls per chain — rejected. Every chain-specific detail would leak into the saga, and the failure paths would remain untested. Simulator plus opt-in testnet driver — attractive and still open. The interface is the point; a second implementation is additive.