# FIDL binary-compatibility (ABI) and source-compatibility (API) guide

Many changes to the Fuchsia platform involve changing FIDL APIs which have
already been published. Unless managed carefully, these changes risk breaking
existing usages. Failed changes manifest in the following ways:

* **Source incompatibility**: Users can no longer build against the generated
  code.
* **Binary incompatibility**: Consuming programs can no longer understand each
  other at runtime.

The Fuchsia project requires that changes to published FIDL libraries are both
source-compatible and binary-compatible for partners.

Note: Some changes are binary-compatible, yet require a specific transition path
to avoid runtime validation issues. Binary-compatibility indicates that two
peers have the same understanding of how to read or write the data, though these
two peers may disagree on which values are deemed valid. As an example, a
`uint32` is binary-compatible with a `enum : uint32`, even though the enum has
runtime validation which restricts the domain to only the specific values
identified by the enum members.

## Which changes to a FIDL API are safe?

For the purpose of describing interface compatibility, FIDL libraries are made
up of declarations. Each declaration has a name, type, attributes, and members.
Once an API is used outside of
[fuchsia.git](https://fuchsia.googlesource.com/fuchsia/) the safest assumption
is that all changes to it must be both binary-compatible and source-compatible
with current clients. This usually means evolving libraries using [soft
transitions], where the backwards-incompatible portions of a change are left to
the end when they will have no impact because all clients have already been
migrated. See [safely removing members](#safely-removing-members) for more
information on the most common soft transition pattern.

Note: Source-compatibility guarantees are only guaranteed under "normal"
circumstances. It is possible to write code which causes these guarantees to be
violated, e.g. static asserts.

### Safe changes to members

Aside from a declaration's name and attributes, all changes to its contract are
expressed in terms of changes to the declaration's members. This relationship
also means incompatible changes to a declaration become incompatible changes to
all the FIDL libraries which depend on that declaration, not just direct
consumers of the original library's generated bindings.

All operations are safe to perform if you are certain that all consumers can be
migrated atomically, i.e. they are all in the same source repository as the
library definition. Otherwise, these operations must be completed as the final
stage in a soft transition after all clients have been migrated away.

The table below summarizes various member changes and their respective safety
level when some clients cannot be migrated atomically:

| Parent   | Change Target | Reorder Lines | Add | Remove | Rename | Change Type | Change Ordinal | (Default) Value |
|----------|---------------|---------------|-----|--------|--------|-------------|----------------|-----------------|
| library  | declaration   | ✅ | ✅ | [⚠️](#library-declaration-remove) | ❌ | ❌ | -- | -- |
| protocol | method        | ✅ | [⚠️](#protocol-method-add) | [⚠️](#protocol-method-remove) | [⚠️](#protocol-method-rename) | ❌ | ❌ | -- |
| method   | parameter     | ❌ | ❌ | ❌ | [⚠️](#method-parameter-rename) | ❌ | -- | -- |
| struct   | field         | ❌ | ❌ | ❌ | ❌ | ❌ | -- | ✅ |
| table    | field         | ✅ | [⚠️](#table-field-add) | [⚠️](#table-field-remove) | [⚠️](#table-field-rename) | ❌ | ❌ | -- |
| union    | variant       | ✅ | [⚠️](#union-variant-add) | [⚠️](#union-variant-remove) | [⚠️](#union-variant-rename) | ❌ | ❌ | -- |
| enum     | member        | ✅ | [⚠️](#enum-member-add) | [⚠️](#enum-member-remove) | [⚠️](#enum-member-rename) | ❌ | -- | ✅ |
| bits     | member        | ✅ | [⚠️](#bits-member-add) | [⚠️](#bits-member-remove) | [⚠️](#bits-member-rename) | ❌ | -- | ✅ |
| const    | value         | -- | -- | -- | -- | ❌ | -- | [✅](#const-value-default-value) |
| _all_    | attribute     | -- | [⚠️](#attributes) | [⚠️](#attributes) | -- | -- | -- | -- |
| type     | constraint    | -- | [⚠️](#constraints) | [⚠️](#constraints) | -- | -- | -- | -- |
| _decl_   | modifier      | -- | [⚠️](#modifiers) | [⚠️](#modifiers) | -- | -- | -- | -- |

*Legend:*

* ✅ = *Safe*
* ⚠️ = *Careful (follow linked advice)*
* ❌ = *Unsafe*
* -- = *Not Applicable*

## Library {#library}

### Removing a library declaration {#library-declaration-remove}

ABI: It is binary-compatible to remove a library declaration.

API: Before removing a library declaration, ensure that no uses of this
declaration exists.

## Protocols {#protocol}

### Adding a method to a protocol {#protocol-method-add}

ABI: It is binary-compatible to add a method to a protocol.

API: To safely add a method to a protocol, mark the new method with
[`[Transitional]`][transitional]. Once all implementations of the new method are
in place, you can remove the [`[Transitional]`][transitional] attribute.

### Removing a method from a protocol {#protocol-method-remove}

ABI: It is binary-compatible to remove a method from a protocol.

API: To safely remove a method from a protocol, start by marking the method with
[`[Transitional]`][transitional]. Once this has fully propagated, you can remove
all implementations of the method, then remove the method from the FIDL
protocol.

Note: When using the Rust bindings, you need to manually add catch-all cases
(`_`) to all the match statements rather than rely on the
[`[Transitional]`][transitional] attribute. Read more about [how
`[Transitional]` impacts the Rust bindings][transitional-rust].

### Renaming a method {#protocol-method-rename}

ABI: Method renames can be made safe with use of the `[Selector = "..."]`
attribute.

API: It is not possible to rename a method in a source-compatible way.

## Method {#method}

### Renaming a method parameter {#method-parameter-rename}

ABI: It is binary-compatible to rename a method parameter.

API: Bindings typically rely on positional arguments, such that renaming a
method parameter is source-compatible.

## Table {#table}

### Adding a table field {#table-field-add}

ABI: It is binary-compatible to add a table field.

API: It is source-compatible to add a table field. However, all uses of the Rust
bindings must use the form `SomeTable { x: Member(1), ..SomeTable::empty() }` as
[described in the overview][rust-bindings-tables].

### Removing a table field {#table-field-remove}

ABI: It is binary-compatible to remove a table field.

API: There must not be any use of the field to ensure a source-compatible
removal. In Rust, this entails using the form `SomeTable { x: Member(1),
..SomeTable::empty() }` as [described in the overview][rust-bindings-tables].

### Renaming a table field {#table-field-rename}

ABI: It is binary-compatible to rename a table field.

API: It is not source-compatible to rename a table field.

## Union {#union}

### Adding a union variant {#union-variant-add}

ABI: It is binary-compatible to add a union variant. To ensure the added union
variant is not rejected during runtime validation, it must have propagated to
readers ahead of it being used by writers.

API: Care must be taken to transition [switches on the union
tag](#switch-evolvability).

### Removing a union variant {#union-variant-remove}

ABI: It is binary-compatible to remove a union variant. To ensure the removed
union variant is not rejected during runtime validation, no writer may use the
union variant when it is removed.

API: Care must be taken to transition [switches on the union
tag](#switch-evolvability).

### Renaming a union variant {#union-variant-rename}

ABI: It is binary-compatible to rename a union variant.

API: It is not source-compatible to rename a union variant.

## Enum {#enum}

### Adding an enum member {#enum-member-add}

ABI: It is binary-compatible to add an enum member. To ensure the added enum
member is not rejected during runtime validation, it must have propagated to
readers ahead of it being used by writers.

API: Care must be taken to transition [switches on the
enum](#switch-evolvability).

### Removing an enum member {#enum-member-remove}

ABI: It is binary-compatible to remove an enum member. To ensure the removed
enum member is not rejected during runtime validation, no writer may use the
enum member when it is removed.

API: Care must be taken to transition [switches on the
enum](#switch-evolvability). Ensure that no uses of this enum member exists.

### Renaming an enum member {#enum-member-rename}

ABI: It is binary-compatible to rename an enum member.

API: It is not source-compatible to rename an enum member.

## Bits {#bits}

### Adding a bits member {#bits-member-add}

ABI: It is binary-compatible to add a bits member. To ensure the added bits
member is not rejected during runtime validation, it must have propagated to
readers ahead of it being used by writers.

API: It is source-compatible to add a bits member.

### Removing a bits member {#bits-member-remove}

ABI: It is binary-compatible to remove a bits member. To ensure the removed bits
member is not rejected during runtime validation, no writer may use the bits
member when it is removed.

API: It is source-compatible to remove a bits member. Ensure that no uses of
this bits member exists.

### Renaming a bits member {#bits-member-rename}

ABI: It is binary-compatible to rename a bits member.

API: It is not source-compatible to rename a bits member.

## Constant {#const}

### Updating value of constants {#const-value-default-value}

It is safe to update the value of a `const` declaration. In rare circumstances,
such a change could cause source-compatibility issues if the constant is used in
static asserts which would fail with the updated value.

## General advice

### Safely removing members {#safely-removing-members}

Most [soft transitions] follow this basic shape:

1. Ensure that the element is not used or referenced
2. Remove the element

In a successful soft transition, only the second step is dangerous.

Note: Safely removing methods is more involved, see [removing a method from a
protocol](#protocol-method-remove).

### Renames {#renames}

Renaming declarations themselves is a source-incompatible change. Similarly,
renaming declaration members (e.g. a struct field) is source-incompatible.

Often, a source-compatible rename is possible following the long process of
adding a duplicate member with the desired name, switching all code to shift
from the old member to the new member, then deleting the old member. This
approach can be quite direct with table fields for instance.

Renames are binary-compatible, except in the case of libraries, protocols,
methods and events. See the `[Selector]` attribute for binary-compatible renames
of these.

### Attributes {#attributes}

Removing `[Discoverable]` is a source-incompatible change. You first need to
ensure that there are no references to the generated protocol name before
removing this attribute.

Adding or changing `[Selector]` is a binary-incompatible change on its own, but
can be used in the same change as method renames to preserve
binary-compatibility.

Removing [`[Transitional]`][transitional] is a source-incompatible change. You
first need to ensure that all implementations of the method are in place.

Adding or changing `[Transport]` is a source-incompatible and
binary-incompatible change.

Changes to the following attributes have no effect on compatibility, although
they often accompany other incompatible changes:

* `[Deprecated]` (although it may in the future if/when implemented)
* `[Doc]`
* `[MaxBytes]`
* `[MaxHandles]`

### Constraints {#constraints}

ABI: Relaxing or tightening constraints is binary-compatible. However, when
evolving constraints, care must be taken to transition readers or writers to
avoid runtime validation issues.

When relaxing a constraint (e.g. changing a vector's maximum allowable size to
grow from `vector<T>:128` to `vector<T>:256`), all readers must transition ahead
of writers to avoid values being rejected at runtime.

Conversely, when tightening a constraint, all writers must transition ahead of
readers to avoid emitting values which would then be rejected at runtime.

API: Relaxing or tightening constraints is source-compatible.

### Modifiers {#modifiers}

Adding or removing the `strict` modifier on an enum, bits, or union declaration
is a source-incompatible and binary-incompatible change.

Adding or removing the `resource` modifier on a struct, table, or union
declaration is a source-incompatible change.
<!-- TODO(fxbug.dev/59962): Binary compatible with one exception. -->

### Evolving switch on enums, or union tag {#switch-evolvability}

When adding an enum member (or adding a union variant), any switch on the enum
(respectively the union tag) must first evolve to handle the soon to be
added member (resp. variant). This is done by adding a `default` case for
instance, or a catch-all `_` match. Depending on compiler flags, this may
require additional attributes such as
[`#[allow(dead_code)]`](https://doc.rust-lang.org/stable/rust-by-example/attribute/unused.html).

Similarly, when removing an enum member (or removing a union variant), any
switch on the enum (respectively the union tag) must first evolve to replace the
soon to be removed member (resp. variant) by a default case.

Note: A union tag is the discriminator indicating which variant is currently
held by the union. This is often an enum in languages which do not support ADTs
like C++.

<!-- xrefs -->
[transitional]: /docs/reference/fidl/language/attributes.md#transitional
[transitional-rust]: /docs/reference/fidl/bindings/rust-bindings.md#transitional
[selector]: /docs/reference/fidl/language/attributes.md#selector
[soft transitions]: /docs/contribute/governance/rfcs/0002_platform_versioning.md#terminology
[Platform Versioning]: /docs/contribute/governance/rfcs/0002_platform_versioning.md
[rust-bindings-tables]: /docs/reference/fidl/bindings/rust-bindings.md#types-tables