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corbanpro merged 1 commit into from
Nov 5, 2026
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docs(aip-134): discriminated-union merge-patch rules #3

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10 changes: 9 additions & 1 deletion aip/general/0126.md
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@
id: 126
state: approved
created: 2019-07-24
updated: 2026-10-22
updated: 2026-11-04
placement:
category: resource-design
order: 60
Expand Down Expand Up @@ -81,7 +81,15 @@ CallInitiationStatic:
format: int64
```

### Update (PATCH)

Partial updates of a discriminated-union field follow
[AIP-134](./0134.md#discriminated-unions-on-update): the patch object must
include `kind`; same-kind patches merge fields; a different `kind` replaces the
variant. Create, Replace, and responses still use the full variant shapes above.

## Further reading

- [Update and Replace](./0134.md)
- [Field names](./0140.md)

22 changes: 21 additions & 1 deletion aip/general/0134.md
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@
id: 134
state: approved
created: 2019-01-24
updated: 2026-10-27
updated: 2026-11-04
placement:
category: operations
order: 40
Expand Down Expand Up @@ -31,6 +31,9 @@ Public APIs use two write styles: partial Update and full Replace.
- Members omitted from the patch are left unchanged
- Nested objects are merged recursively
- Arrays are replaced as a whole (not merged element-wise). JSON `null` clears the array. `null` and `[]` are equivalent. Omitting the field leaves the current array unchanged
- [Discriminated unions](./0126.md) on Update follow
[Discriminated unions on Update](#discriminated-unions-on-update) (they are
not a plain recursive object merge across kinds)
- The patch object **must not** include read-only fields such as `id`,
`created_timestamp`, or `updated_timestamp`. If sent, servers **must**
ignore them or reject the request ([AIP-129](./0129.md)).
Expand All @@ -45,6 +48,22 @@ Public APIs use two write styles: partial Update and full Replace.
- State transitions that are product actions (pause, resume, cancel) **must
not** be modeled as PATCH field writes; use [custom methods](./0136.md).

#### Discriminated unions on Update

When a PATCH field is a [discriminated union](./0126.md):

- The patch object **must** include `kind`. Omitting `kind` is invalid.
- When `kind` matches the current value, servers **must** merge present members
into that variant. Members omitted from the patch are left unchanged.
- When `kind` differs from the current value, servers **must** replace the
variant. Omitted fields that have a product default **may** use that default
(for example a delay of `0`). Fields with no default that are required on
create for that variant **must** be present or the request is invalid.
- Members that do not belong to the stated `kind` **must** be rejected with a
clear client error naming the invalid member and the kind.
- Create, Replace, and response bodies still use the full variant shapes from
[AIP-126](./0126.md).

```http
PATCH /beta/books/AUIAE-DUXA
Content-Type: application/json
Expand Down Expand Up @@ -102,6 +121,7 @@ Content-Type: application/json

## Further reading

- [Enumerations and unions](./0126.md)
- [Singletons](./0156.md)
- [Custom methods](./0136.md)
- [Field behavior](./0203.md)