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FIDL versioning

This document describes FIDL's API versioning features. For a more technical specification, see RFC-0083: FIDL versioning. For guidance on how to evolve Fuchsia APIs, see Fuchsia API evolution guidelines.

Motivation

FIDL versioning lets you change a FIDL library over time while keeping the ability to generate bindings for older versions of the library. There are a number of ways you could do this manually:

  • Store .fidl files in a v1/ directory. To make a change, copy v1/ to v2/ and change files there. To generate bindings for the older version, use the v1/ library instead of v2/.

  • Store .fidl files in a git repository and make changes in commits. To generate bindings for an older version, check out an older revision of the repository.

The first solution is tedious and creates a lot of duplication. The second solution doesn't work well in a large repository that contains many things besides that specific FIDL library, such as the main Fuchsia repository.

FIDL versioning accomplishes the same thing, but without these shortcomings. When making a change, you use the @available attribute to describe when (i.e. at which version) the change occurs. To generate bindings for an older version, you pass the --available flag to fidlc and specify an older version.

There are two important things to keep in mind with FIDL versioning:

  • It affects API only. Versions exist only at compile time, and have no impact on runtime behavior.

  • It can represent any syntactically valid change. Just because you can represent a change with versioning doesn't mean that change is safe to make.

Concepts

The unit of versioning is a group of libraries, called a platform. By convention, libraries are named starting with the platform name. For example, the libraries fuchsia.mem and fuchsia.web belong to the fuchsia platform.

Each platform has a linear version history. A version is an integer from 1 to 2^63-1 (inclusive), or one of the special versions HEAD and LEGACY. The HEAD version is used for the latest unstable changes. The LEGACY version is like HEAD, but it also includes legacy elements.

All FIDL libraries are versioned. There is no need to support "unversioned libraries" because they would behave identically to libraries added at HEAD. The defaults are designed so that you can ignore versioning if you aren't using it: a library with no @available attribute is implicitly added at HEAD, and fidlc compiles libraries at HEAD by default.

Command line

The FIDL compiler accepts the --available flag to specify platform versions. For example, assuming example.fidl defines a library in the fuchsia platform with no dependencies, you can compile it at version 8 as follows:

fidlc --available fuchsia:8 --out example.json --files example.fidl

No matter what version you select, fidlc always validates all possible versions. For example, the above command can report an error even if the error only occurs in version 5.

If a library A has a dependency on a library B from a different platform, you can specify versions for both platforms using the --available flag twice. However, A must be compatible across its entire version history with the fixed version chosen for B.

Syntax

The @available attribute is allowed on any FIDL element. It takes the following arguments:

Argument Type Note
platform string Only allowed on library
added uint64 Integer or HEAD
deprecated uint64 Integer or HEAD
removed uint64 Integer or HEAD
note string Goes with deprecated
legacy boolean Goes with removed

All arguments are optional, but at least one must be provided. Argument values must be literals, not references to const declarations. The added, deprecated, removed, and legacy arguments inherit from the parent element by default. They must respect added <= deprecated < removed. For example:

@available(added=1, deprecated=2, removed=3, legacy=true)
const ANSWER uint64 = 42;

If @available is used anywhere in a library, it must also appear on the library declaration. For single-file libraries, this is straightforward. For libraries with two or more .fidl files, only one file can have its library declaration annotated. (The library is logically considered a single element with attributes merged from each file, so annotating more than one file results in a duplicate attribute error.) The FIDL style guide recommends creating a file named overview.fidl for this purpose.

On the library declaration, the @available attribute requires the added argument and allows the platform argument. If the platform is omitted, it defaults to the first component of the library name. For example:

// Equivalent to `@available(platform="fuchsia", added=1)`.
@available(added=1)
library fuchsia.examples.docs;

Inheritance

The arguments to @available flow from the library declaration to top-level declarations, and from each top-level declaration to its members. For example, if a table is added at version 5, there is no need to repeat this annotation on its members because they could not exist prior to the table itself. Here is a more complicated example of inheritance:

@available(added=2, deprecated=3)
protocol Versioned {
    // Equivalent to `@available(added=2, deprecated=3, removed=4, legacy=true)`.
    @available(removed=4, legacy=true)
    Removed(table {
        // Equivalent to `@available(added=3, deprecated=3, removed=4, legacy=true)`.
        @available(added=3)
        1: message string;
        // Equivalent to `@available(added=2, deprecated=3, removed=4, legacy=false)`.
        @available(legacy=false)
        2: count uint32;
    });
};

Deprecation

Deprecation is used to indicate that an element will be removed in the future. When you deprecate an element, you should add a # Deprecation section to the doc comment with a detailed explanation, and a note argument to the @available attribute with a brief instruction. For example:

protocol Example {
    // (Description of the method.)
    //
    // # Deprecation
    //
    // (Detailed explanation of why the method is deprecated, the timeline for
    // removing it, and what should be used instead.)
    @available(deprecated=5, removed=6, note="use Replacement")
    Deprecated();

    @available(added=5)
    Replacement();
};

As of May 2022 deprecation has no impact in bindings. However, the FIDL team plans to make it emit deprecation annotations in target languages. For instance, the example above could produce #[deprecated = "use Replacement"] in the Rust bindings.

Legacy

When removing an element, you can use legacy=true to keep it in the LEGACY version. This lets you preserve ABI for clients targeting API levels before its removal, since the Fuchsia system image is built against LEGACY FIDL bindings. For example:

protocol LegacyExample {
    @available(deprecated=5, removed=6, legacy=true, note="...")
    LegacyMethod();
};

Here, LegacyMethod does not appear in bindings at version 6 or higher nor at HEAD, but it gets added back in the LEGACY version.

For more information on legacy support, see RFC-0083: FIDL versioning.

References

There are a variety of ways one FIDL element can reference another. For example:

const VALUE uint32 = 5;
const REFERENCES_VALUE uint32 = VALUE;

type Type = struct {};
type ReferencesType = table {
    1: t Type;
};

alias ReferencesTypeAndValue = vector<Type>:VALUE;

When referencing elements, you must respect the @available attributes. For example, the following code is invalid because A exists from version 1 onward, but it tries to reference B which only exists at version 2:

// Does not compile!

@available(added=1)
const A bool = B;

@available(added=2, removed=3)
const B bool = true;

Similarly, it is invalid for a non-deprecated element to reference a deprecated element. For example, the following code is invalid at version 1 because A references B, but B is deprecated while A is not.

// Does not compile!

@available(deprecated=2)
const A bool = B;

@available(deprecated=1)
const B bool = true;

Swapping

Some parts of the FIDL language do not support attributes. For example, you cannot place the @available attribute directly on an enum's strict modifier. However, FIDL versioning can still represent this kind of change using a technique called swapping. Instead of changing the enum, you duplicate it, simultaneously removing the old copy and adding the new one. For example:

@available(removed=HEAD)
type Color = strict enum {
    RED = 1;
};

@available(added=HEAD)
type Color = flexible enum { // Note: flexible instead of strict
    RED = 1;
};

Taken to the extreme, swapping makes it possible to decompose a versioned library into a series of snapshots for each version. For example, given the protocol shown earlier:

@available(added=2, deprecated=3)
protocol Versioned {
    // Equivalent to `@available(added=2, deprecated=3, removed=4, legacy=true)`.
    @available(removed=4, legacy=true)
    Removed(table {
        // Equivalent to `@available(added=3, deprecated=3, removed=4, legacy=true)`.
        @available(added=3)
        1: message string;
        // Equivalent to `@available(added=2, deprecated=3, removed=4, legacy=false)`.
        @available(legacy=false)
        2: count uint32;
    });
};

We can decompose it by swapping the protocol at every version:

@available(added=2, removed=3)
protocol Versioned {
    Removed(table {});
};

@available(added=3, deprecated=3, removed=4)
protocol Versioned {
    Removed(table {
        1: message string;
    });
};

@available(added=4, deprecated=4)
protocol Versioned {};