# ADR-MONGO-002 — BSON representation is a pinned manifest

- **Status:** Accepted
- **Date:** 2026-08-13
- **Design source:** design §10, decision D-06

## Context

How a Java value is represented in BSON is a data contract, but nothing in the default toolchain
treats it as one. Spring Data and the MongoDB driver both have defaults, and those defaults have
changed across versions. A `BigDecimal` can land as a `Double`, a `String` or a `Decimal128`; a `UUID`
can land as `Binary` subtype 3 or subtype 4; an `Instant` can land as a `Date` or a `String`.

The consequences are asymmetric. A representation change is invisible in a value-equality test —
`12.30` looks like `12.30` whether it is a double or a `Decimal128` — but once a collection holds
production data, changing it is a full migration. And the UUID case is worse than a migration: legacy
Java representation byte-swaps two halves of the UUID, so a document written under one representation
and read under the other yields a *different, valid-looking* UUID. Nothing errors. You get the wrong
record.

## Decision

`MongoTypeRepresentationManifest` pins the representation for every type the platform maps, and
`MongoMappingConfiguration` builds the Spring Data converters from it. Nothing relies on a library
default.

| Java | BSON | Rationale |
|---|---|---|
| `UUID` | `Binary` subtype 4 (`STANDARD`) | Subtype 3 byte-swaps; cross-representation reads are silently wrong. |
| `BigDecimal` | `Decimal128` | A double cannot represent `12.30`; money compared as a double is eventually wrong by a cent. |
| `BigInteger` | `Decimal128`, or declared `String` when out of range | 34 significant digits; out of range fails on write instead of rounding. |
| `Instant` / `OffsetDateTime` / `ZonedDateTime` | UTC `Date` | One instant, one representation. |
| `LocalDate` | declared per field | A calendar day is not an instant. |
| `LocalDateTime` | **refused** | No offset: the stored value depends on the writing JVM's default zone. |
| `enum` | `String` name | Ordinals renumber when someone inserts a constant. |

Type metadata follows `MongoTypeMetadataPolicy` — `NONE`, `ALIAS` or `CLASS_NAME`. A
`@LongLivedMongoDocument` type may not use `CLASS_NAME`: writing a FQCN into a million documents makes
a package rename a data migration.

The manifest is enforced by a golden gate. `MongoBsonSnapshot` canonicalises a stored document,
preserving BSON types and keeping missing distinct from null, and
`MongoBsonSnapshotAssert.hasTypeSignature(...)` fails on any representation change. The registry
pins `UuidCodec(STANDARD)` explicitly rather than inheriting a default, since inheriting the default
is the exact drift the gate exists to catch.

## Consequences

**Positive.** A library upgrade cannot move a representation without failing a test. Money is exact.
UUIDs read back as themselves. Class moves stay refactors.

**Negative.** Every representation-affecting change requires updating a snapshot *and* writing a
migration. A new mapped type needs a manifest entry before it can be used. This is the intended
friction: the alternative is discovering the change in production.

**Rejected alternative — "snapshot the JSON."** JSON destroys exactly the distinctions the gate
protects: `Decimal128` and `String` both render as text, `Binary` UUID and `ObjectId` both render as
hex, and missing and null both disappear.