> ## Documentation Index > Fetch the complete documentation index at: https://arc-doc.mintlify.site/llms.txt > Use this file to discover all available pages before exploring further. # Architecture Decision Records: How Arc Was Designed > Numbered records capturing every significant design choice: the context that forced it, the consequences accepted, and the alternatives that were rejected. Decisions · ADRs An architecture decision record captures a choice **at the moment it was made**, with the context that made it reasonable and the alternatives that were rejected. Read later, an ADR answers the question every codebase eventually raises: *why on earth is it like this?* The value is in the parts that are uncomfortable to write. A record listing only the benefits is marketing. Each of these states what the decision costs. *** ## Accepted Every monetary value is a bigint count of minor units. No floats in monetary APIs, enforced by ESLint rather than convention. One deployable, six bounded contexts, and a boundary that fails the build rather than eroding. Settlement talks to an interface, not a chain SDK, with a seeded simulator so reorgs and stuck transactions are testable. Banks and blockchains do not enrol in distributed transactions, so every step carries a compensating action. A retry returns the original response; the same key with a different body is a conflict, not a cache hit. Events are staged in the same transaction as the state change, so delivery is at-least-once and handlers are idempotent. A tenant on every record, sandbox tenants by prefix, and environment-prefixed credentials. Balances derived from entries, entries append-only, and the balance rule enforced in Postgres as well as in code. *** ## The format Every record on this site uses the same four sections, and the order is deliberate: The forces in play **before** the decision. What made this a question at all? An ADR whose context does not explain why the obvious answer was insufficient is not describing a decision. What was chosen, stated plainly and in the present tense. One paragraph, not a design document. Both directions. What this makes easy, and what it makes hard or impossible. **The costs section is the one that earns the record its place.** What else was considered and specifically why it was rejected. Not strawmen: the version of each alternative its advocate would recognise. ADRs are **immutable once accepted**. A decision that changes gets a new record that supersedes the old one, and the old one stays in place marked as superseded. Editing an accepted ADR destroys the only thing it was for: the record of what was known and believed at the time. The same reasoning as the ledger's append-only entries: the record of what happened stays separate from the record of what was later thought better.