1. The Smithy model#

The Smithy model describes the Smithy semantic model and the files used to create it. Smithy models are used to describe services and data structures.

1.1. Smithy overview#

Smithy is a framework that consists of a semantic model, file formats used to define a model, and a build process used to validate models and facilitate model transformations.

Figure 1.1: Smithy framework concepts#

                ┌────────────────┐ part of          ┌────────────────┐
                │                │╲                ╱│                │
                │ Semantic Model │─○──────────────○─│   Model File   │
                │                │╱                ╲│                │
                └────────────────┘                  └────────────────┘
                                           split into       ╲│╱
                                                             ○
                                                             │
                ┌────────────────┐                           ┼
                │JSON AST (.json)│────────┐         ┌────────────────┐
                └────────────────┘        │         │                │
                                          ├────────▷│ Representation │
                ┌────────────────┐        │         │                │
                │ IDL (.smithy)  │────────┘         └────────────────┘
                └────────────────┘

Semantic model: The in-memory model used by tools. The semantic model may be serialized into one or more model file representations.

Model File

A file on the file system, in a particular representation. The model files that make up a semantic model MAY be split across multiple files to improve readability or modularity, and those files are not required to use the same representation. Model files do not explicitly include other model files; this responsibility is left to tooling to ensure that all necessary model files are merged together to form a valid semantic model.

One or more model files can be assembled (or merged) together to form a semantic model.

Representation

A particular model file format such as the Smithy IDL or JSON AST. Representations are loaded into the semantic model by mapping the representation to concepts in the semantic model.

Smithy IDL: a human-readable format that aims to streamline authoring and reading models.
JSON AST: a machine-readable JSON-based format.

1.2. The semantic model#

Smithy's semantic model is an in-memory model used by tools. It is independent of any particular serialized representation. The semantic model contains metadata and a graph of shapes connected by shape IDs.

Figure 1.2: The semantic model#

                                      ┌───────────────┐
                                      │Semantic Model │╲
                                      ├───────────────┤─○────────┐
                                      │metadata?      │╱         │
                                      │               │          │
                                      │               │          │
                                      └───────────────┘          │
                                              ┼     ┼ prelude    │
                                              │     ○────────────┘
                                              ○
                                       shapes╱│╲
    ┌───────────────┐                 ┌───────────────┐
    │ Applied Trait │╲          shape │  «abstract»   │
    ├───────────────┤─○──────────────┼│     Shape     │            ┌───────────────┐
    │               │╱                ├───────────────┤            │    ShapeID    │
    │               │                 │               │            ├───────────────┤
    │               │╲     applied-to │               │         id │namespace      │
    │               │─○──────────────┼│               │┼──────────┼│shape_name     │
    │               │╱traits          │               │            │member_name?   │
    └───────────────┘                 └───────────────┘            └───────────────┘

Shape: Shapes are named data definitions that describe the structure of an API. Shapes are referenced and connected by shape IDs. Relationships between shapes are formed by members that target other shapes, properties of shapes like the input and output properties of an operation, and applied traits that attach a trait to a shape.
Shape ID: A shape ID is used to identify shapes defined in a model. For example, smithy.example#MyShape, smithy.example#Foo$bar, and Baz are all different kinds of shape IDs.
Trait: Traits are specialized shapes that form the basis of Smithy's meta-model. Traits are applied to shapes to associate metadata to a shape. They are typically used by tools to influence validation, serialization, and code generation.
Applied trait: An applied trait is an instance of a trait applied to a shape, configured using a node value.
Model metadata: Metadata is a schema-less extensibility mechanism used to associate metadata to an entire model.
Prelude: The prelude defines various simple shapes and every trait defined in the core specification. All Smithy models automatically include the prelude.

1.3. Model metadata#

Metadata is a schema-less extensibility mechanism used to associate metadata to an entire model. For example, metadata is used to define validators and model-wide suppressions. Metadata is defined using a node value. The following example configures a model validator:

$version: "2"
metadata validators = [
    {
        name: "EmitEachSelector"
        id: "OperationInputName"
        message: "This shape is referenced as input but the name does not end with 'Input'"
        configuration: {
            selector: "operation -[input]-> :not([id|name$=Input i])"
        }
    }
]

Metadata conflicts

When a conflict occurs between top-level metadata key-value pairs, the following conflict resolution logic is used:

If both values are arrays, the values of both arrays are concatenated into a single array.
Otherwise, if both values are exactly equal, the conflict is ignored.
Otherwise, the conflict is invalid.

Given the following two Smithy models:

model-a.smithy#

$version: "2"
metadata "foo" = ["baz", "bar"]
metadata "qux" = "test"
metadata "validConflict" = "hi!"

model-b.smithy#

$version: "2"
metadata "foo" = ["lorem", "ipsum"]
metadata "lorem" = "ipsum"
metadata "validConflict" = "hi!"

Merging model-a.smithy and model-b.smithy produces the following model:

$version: "2"
metadata "foo" = ["baz", "bar", "lorem", "ipsum"]
metadata "qux" = "test"
metadata "lorem" = "ipsum"
metadata "validConflict" = "hi!"

1.4. Node values#

Node values are JSON-like values used to define metadata and the value of an applied trait.

Figure 1.3: Node value types#

┌─────────────────┐                     ┌─────────────┐
│ Semantic Model  │                     │Applied Trait│
└─────────────────┘                     └─────────────┘
  │                                            │
  │                                            │
  │                                            ┼ nodeValue
  │                                     ┌─────────────┐
  │                                     │ «abstract»  │
  │                                     │    Value    │
  │metadata                             └─────────────┘
  │                                            △
  ○      ┌───────────────────┬─────────────────┼───────────────┬───────────────┐
  ┼      │                   │                 │               │               │
┌─────────────────┐ ┌─────────────────┐ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐
│     Object      │ │      Array      │ │   Number    │ │   Boolean   │ │   String    │
├─────────────────┤ ├─────────────────┤ └─────────────┘ └─────────────┘ └─────────────┘
│members:         │ │members: [Value] │
│  [String, Value]│ └─────────────────┘
└─────────────────┘

The following example defines metadata using a node value:

metadata foo = "hello"

The following example defines a trait using a node value:

$version: "2"
namespace smithy.example

@length(min: 1, max: 10)
string MyString

Node value types

Node values have the same data model as JSON; they consist of the following kinds of values:

Type	Description
null	The lack of a value
string	A UTF-8 string
number	A double precision floating point number
boolean	A Boolean, true or false value
array	A list of heterogeneous node values
object	A map of string keys to heterogeneous node values

1.5. Merging model files#

Multiple model files can be used to create a semantic model. Implementations MUST take the following steps when merging two or more model files:

Merge the metadata objects of all model files. If top-level metadata key-value pairs conflict, merge the metadata if possible or fail.
Shapes defined in a single model file are added to the semantic model as-is.
Shapes with the same shape ID defined in multiple model files are reconciled using the following rules:
1. All conflicting shapes MUST have the same shape type.
2. Conflicting aggregate shape types MUST contain the same members that target the same shapes.
3. Conflicting service shape types MUST contain the same properties and target the same shapes.
Conflicting traits defined in shape definitions or through apply statements are reconciled using trait conflict resolution.

1.6. Shapes#

Smithy models are made up of shapes. Shapes are named definitions of types.

Shapes are visualized using the following diagram:

Figure 1.4: Smithy shapes#

                                  ┌─────────────┐
                         members ╱│ «abstract»  │
                        ┌───────○─│    Shape    │
                        │        ╲│             │
                        │         └─────────────┘
                        │                △
              ┌─────────│────────────────┼────────────────────┐
              │         │                │                    │
      ┌───────────────┐ │         ┌─────────────┐      ┌─────────────┐
      │  «abstract»   │ │container│ «abstract»  │      │ «abstract»  │
      │    Simple     │ └────────┼│  Aggregate  │      │   Service   │
      └───────────────┘           └─────────────┘      └─────────────┘
              △                    △                    △
┌──────────┐  │  ┌──────────┐      │    ┌────────────┐  │    ┌─────────────────────────┐
│blob      │──┼──│boolean   │      ├────│    List    │  │    │         Service         │
└──────────┘  │  └──────────┘      │    ├────────────┤  │    ├─────────────────────────┤
┌──────────┐  │  ┌──────────┐      │    │member      │  │    │version                  │
│document  │──┼──│string    │      │    └────────────┘  ├────│operations: [Operation]? │
└──────────┘  │  └──────────┘      │                    │    │resources: [Resource]?   │
┌──────────┐  │       △            │    ┌────────────┐  │    └─────────────────────────┘
│timestamp │──┤       │            ├────│    Map     │  │    ┌─────────────────────────┐
└──────────┘  │  ┌──────────┐      │    ├────────────┤  │    │        Operation        │
              │  │enum      │      │    │key         │  │    ├─────────────────────────┤
              │  └──────────┘      │    │value       │  │    │input: Structure         │
      ┌───────────────┐            │    └────────────┘  ├────│output: Structure        │
      │  «abstract»   │            │                    |    │errors: [Structure]?     │
      │    Number     │            │    ┌────────────┐  │    └─────────────────────────┘
      └───────────────┘            ├────│ Structure  │  │    ┌─────────────────────────┐
              △                    │    └────────────┘  │    │        Resource         │
┌──────────┐  │  ┌──────────┐      │    ┌────────────┐  │    ├─────────────────────────┤
│float     │──┼──│double    │      └────│   Union    │  │    │identifiers?             │
└──────────┘  │  └──────────┘           └────────────┘  │    │create: Operation?       │
┌──────────┐  │  ┌──────────┐                           │    │put: Operation?          │
│bigInteger│──┼──│bigDecimal│                           │    │read: Operation?         │
└──────────┘  │  └──────────┘                           └────│update: Operation?       │
┌──────────┐  │  ┌──────────┐                                │delete: Operation?       │
│byte      │──┼──│short     │                                │list: : Operation?       │
└──────────┘  │  └──────────┘                                │operations: [Operation]? │
┌──────────┐  │  ┌──────────┐                                │collectionOperations:    │
│integer   │──┴──│long      │                                │    [Operation]?         │
└──────────┘     └──────────┘                                │resources: [Resource]?   │
     △                                                       └─────────────────────────┘
     │
┌──────────┐
│intEnum   │
└──────────┘

1.6.1. Shape types#

Shape types are grouped into three categories:

Simple types

Simple types are types that do not contain nested types or shape references.

Type	Description
blob	Uninterpreted binary data
boolean	Boolean value type
string	UTF-8 encoded string
enum	A string with a fixed set of values.
byte	8-bit signed integer ranging from -128 to 127 (inclusive)
short	16-bit signed integer ranging from -32,768 to 32,767 (inclusive)
integer	32-bit signed integer ranging from -2^31 to (2^31)-1 (inclusive)
intEnum	An integer with a fixed set of values.
long	64-bit signed integer ranging from -2^63 to (2^63)-1 (inclusive)
float	Single precision IEEE-754 floating point number
double	Double precision IEEE-754 floating point number
bigInteger	Arbitrarily large signed integer
bigDecimal	Arbitrary precision signed decimal number
timestamp	An instant in time with no UTC offset or timezone.
document	Open content that functions as a kind of "any" type.

Aggregate types

Aggregate types contain configurable member references to others shapes.

Type	Description
List	Ordered collection of homogeneous values
Map	Map data structure that maps string keys to homogeneous values
Structure	Fixed set of named heterogeneous members
Union	Tagged union data structure that can take on one of several different, but fixed, types

Service types

Types that define the organization and operations of a service.

Type	Description
service	Entry point of an API that aggregates resources and operations together
operation	Represents the input, output, and errors of an API operation
resource	Entity with an identity that has a set of operations

1.6.2. Member shapes#

Members are defined in shapes to reference other shapes using a shape ID. Members are found in enum, intEnum, list, map, structure, and union shapes. The shape referenced by a member is called its "target". A member MUST NOT target a trait, operation, resource, service, or member.

The following example defines a list that contains a member shape. Further examples can be found in the documentation for shape types.

$version: "2"
namespace smithy.example

list UserNameList {
    member: UserName
}

1.6.3. Shape ID#

A shape ID is used to refer to shapes in the model. All shapes have an assigned shape ID.

The following example defines a shape in the smithy.example namespace named MyString, giving the shape a shape ID of smithy.example#MyString:

$version: "2"
namespace smithy.example

string MyString

Shape IDs have the following syntax:

smithy.example.foo#ExampleShapeName$memberName
└─────────┬──────┘ └───────┬──────┘ └────┬───┘
     (Namespace)     (Shape name)  (Member name)
                   └──────────────┬────────────┘
                         (Relative shape ID)
└──────────────────────┬───────────────────────┘
              (Absolute shape ID)

Absolute shape ID

An absolute shape ID starts with a namespace, followed by "#", followed by a relative shape ID. For example, smithy.example#Foo and smithy.example#Foo$bar are absolute shape IDs.

Relative shape ID

A relative shape ID contains a shape name and an optional member name. The shape name and member name are separated by the "$" symbol if a member name is present. For example, Foo and Foo$bar are relative shape IDs.

Namespace

A namespace is a mechanism for logically grouping shapes in a way that makes them reusable alongside other models without naming conflicts. A semantic model MAY contain shapes defined across multiple namespaces. The IDL representation supports zero or one namespace per model file, while the JSON AST representation supports zero or more namespaces per model file.

Models SHOULD use a single namespace to model a single logical domain. Limiting the number of namespaces used to define a logical grouping of shapes limits the potential for ambiguity if the shapes are used by the same service or need to be referenced within the same model.

Shape name

The name of the shape within a namespace.

Consumers of a Smithy model MAY choose to inflect shape names, structure member names, and other facets of a Smithy model in order to expose a more idiomatic experience to particular programming languages. In order to make this easier for consumers of a model, model authors SHOULD utilize a strict form of PascalCase in which only the first letter of acronyms, abbreviations, and initialisms are capitalized. For example, prefer UserId over UserID, and Arn over ARN.

Root shape ID

A root shape ID is a shape ID that does not contain a member. For example, smithy.example#Foo and Foo are root shape IDs.

1.6.3.1. Shape ID ABNF#

Shape IDs are formally defined by the following ABNF:

ShapeId =
    RootShapeId [ShapeIdMember]

RootShapeId =
    AbsoluteRootShapeId / Identifier

AbsoluteRootShapeId =
    Namespace "#" Identifier

Namespace =
    Identifier *("." Identifier)

Identifier =
    IdentifierStart *IdentifierChars

IdentifierStart =
    (1*"_" (ALPHA / DIGIT)) / ALPHA

IdentifierChars =
    ALPHA / DIGIT / "_"

ShapeIdMember =
    "$" Identifier

1.6.3.2. Shape ID conflicts#

While shape ID references within the semantic model are case-sensitive, no two shapes in the semantic model can have the same case-insensitive shape ID. This restriction makes it easier to use Smithy models for code generation in programming languages that do not support case-sensitive identifiers or that perform some kind of normalization on generated identifiers (for example, a Python code generator might convert all member names to lower snake case). To illustrate, com.Foo#baz and com.foo#BAZ are not allowed in the same semantic model. This restriction also extends to member names: com.foo#Baz$bar and com.foo#Baz$BAR are in conflict.

1.7. Traits#

Traits are model components that can be attached to shapes to describe additional information about the shape; shapes provide the structure and layout of an API, while traits provide refinement and style.

1.7.1. Applying traits#

An instance of a trait applied to a shape is called an applied trait. Only a single instance of a trait can be applied to a shape. The way in which a trait is applied to a shape depends on the model file representation.

Traits are applied to shapes in the IDL using smithy:TraitStatements that immediately precede a shape. The following example applies the length trait and documentation trait to MyString:

$version: "2"
namespace smithy.example

@length(min: 1, max: 100)
@documentation("Contains a string")
string MyString

Refer to the IDL specification for a description of how traits are applied in the IDL.
Refer to the JSON AST specification for a description of how traits are applied in the JSON AST.

Scope of member traits

Traits that target members apply only in the context of the member shape and do not affect the shape targeted by the member. Traits applied to a member supersede traits applied to the shape targeted by the member and do not inherently conflict.

In the following example, the range trait applied to numberOfItems takes precedence over the trait applied to PositiveInteger.

structure ShoppingCart {
    // This trait supersedes the PositiveInteger trait.
    @range(min: 7, max:12)
    numberOfItems: PositiveInteger
}

@range(min: 1)
integer PositiveInteger

1.7.1.1. Applying traits externally#

Both the IDL and JSON AST model representations allow traits to be applied to shapes outside of a shape's definition. This is done using an apply statement in the IDL, or the apply type in the JSON AST. For example, this can be useful to allow different teams within the same organization to independently own different facets of a model; a service team could own the model that defines the shapes and traits of the API, and a documentation team could own a model that applies documentation traits to the shapes.

The following example applies the documentation trait and length trait to the smithy.example#MyString shape:

$version: "2"
namespace smithy.example

apply MyString @documentation("This is my string!")
apply MyString @length(min: 1, max: 10)

Note

In the semantic model, applying traits outside of a shape definition is treated exactly the same as applying the trait inside of a shape definition.

1.7.1.2. Trait conflict resolution#

Trait conflict resolution is used when the same trait is applied multiple times to a shape. Duplicate traits applied to shapes are allowed in the following cases:

If the trait is a list or set shape, then the conflicting trait values are concatenated into a single trait value.
If both values are exactly equal, then the conflict is ignored.

All other instances of trait collisions are prohibited.

The following model definition is valid because the length trait is duplicated on the MyList shape with the same values:

$version: "2"
namespace smithy.example

@length(min: 0, max: 10)
list MyList {
    member: String
}

apply MyList @length(min: 0, max: 10)

The following model definition is valid because the tags trait is a list. The resulting value assigned to the tags trait on the Hello shape is a list that contains "a", "b", and "c".

$version: "2"
namespace smithy.example

@tags(["a", "b"])
string Hello

apply Hello @tags(["c"])

The following model definition is invalid because the length trait is duplicated on the MyList shape with different values:

$version: "2"
namespace smithy.example

@length(min: 0, max: 10)
list MyList {
    member: String
}

apply MyList @length(min: 10, max: 20)

1.7.1.3. Trait node values#

The value provided for a trait MUST be compatible with the shape of the trait. The following table defines each shape type that is available to target from traits and how their values are defined in node values.

Smithy type	Node type	Description
blob	string	A `string` value that is base64 encoded.
boolean	boolean	Can be set to `true` or `false`.
byte	number	The value MUST fall within the range of -128 to 127
short	number	The value MUST fall within the range of -32,768 to 32,767
integer	number	The value MUST fall within the range of -2^31 to (2^31)-1.
long	number	The value MUST fall within the range of -2^63 to (2^63)-1.
float	string \| number	The value MUST be either a normal JSON number or one of the following string values: `"NaN"`, `"Infinity"`, `"-Infinity"`.
double	string \| number	The value MUST be either a normal JSON number or one of the following string values: `"NaN"`, `"Infinity"`, `"-Infinity"`.
bigDecimal	string \| number	bigDecimal values can be serialized as strings to avoid rounding issues when parsing a Smithy model in various languages.
bigInteger	string \| number	bigInteger values can be serialized as strings to avoid truncation issues when parsing a Smithy model in various languages.
string	string	The provided value SHOULD be compatible with the `mediaType` of the string shape if present; however, this is not validated by Smithy.
timestamp	number \| string	If a number is provided, it represents Unix epoch seconds with optional millisecond precision. If a string is provided, it MUST be a valid RFC 3339 string with no UTC offset and optional fractional precision (for example, `1985-04-12T23:20:50.52Z`).
list	array	Each value in the array MUST be compatible with the targeted member.
map	object	Each key MUST be compatible with the `key` member of the map, and each value MUST be compatible with the `value` member of the map.
structure	object	All members marked as required MUST be provided in a corresponding key-value pair. Each key MUST correspond to a single member name of the structure. Each value MUST be compatible with the member that corresponds to the member name.
union	object	The object MUST contain a single key-value pair. The key MUST be one of the member names of the union shape, and the value MUST be compatible with the corresponding shape.

Constraint traits

Trait values MUST be compatible with the required trait and any associated constraint traits.

1.7.2. Defining traits#

Traits are defined by applying smithy.api#trait to a shape. This trait can only be applied to simple types and aggregate types. By convention, trait shape names SHOULD use a lowercase name so that they visually stand out from normal shapes.

The following example defines a trait with a shape ID of smithy.example#myTraitName and applies it to smithy.example#MyString:

$version: "2"
namespace smithy.example

@trait(selector: "*")
structure myTraitName {}

@myTraitName
string MyString

The following example defines two custom traits: beta and structuredTrait:

$version: "2"
namespace smithy.example

/// A trait that can be applied to a member.
@trait(selector: "structure > member")
structure beta {}

/// A trait that has members.
@trait(selector: "string", conflicts: [beta])
structure structuredTrait {
    @required
    lorem: StringShape

    @required
    ipsum: StringShape

    dolor: StringShape
}

// Apply the "beta" trait to the "foo" member.
structure MyShape {
    @required
    @beta
    foo: StringShape
}

// Apply the structuredTrait to the string.
@structuredTrait(
    lorem: "This is a custom trait!"
    ipsum: "lorem and ipsum are both required values.")
string StringShape

Prelude traits

When using the IDL, built-in traits defined in the Smithy prelude namespace, smithy.api, are automatically available in every Smithy model and namespace through relative shape IDs.

References to traits

The only valid reference to a trait is through applying a trait to a shape. Members and references within a model MUST NOT target shapes.

1.7.2.1. `trait` trait#

Summary

Marks a shape as a trait.

Trait selector

:is(simpleType, list, map, set, structure, union)

This trait can only be applied to simple types, list, map, set, structure, and union shapes.

Value type

structure

Trait properties

smithy.api#trait is a structure that supports the following members:

Property	Type	Description
selector	`string`	A valid selector that defines where the trait can be applied. For example, a `selector` set to `:test(list, map)` means that the trait can be applied to a list or map shape. This value defaults to `*` if not set, meaning the trait can be applied to any shape.
conflicts	[`string`]	Defines the shape IDs of traits that MUST NOT be applied to the same shape as the trait being defined. This allows traits to be defined as mutually exclusive. Provided shape IDs MAY target unknown traits that are not defined in the model.
structurallyExclusive	`string`	One of "member" or "target". When set to "member", only a single member of a structure can be marked with the trait. When set to "target", only a single member of a structure can target a shape marked with this trait.
breakingChanges	[BreakingChangeRule]	Defines the backward compatibility rules of the trait.

1.7.2.2. Annotation traits#

A structure trait with no members is called an annotation trait. It's hard to predict what information a trait needs to capture when modeling a domain; a trait might start out as a simple annotation, but later might benefit from additional information. By defining an annotation trait rather than a boolean trait, the trait can safely add optional members over time as needed.

The following example defines an annotation trait named foo:

$version: "2"
namespace smithy.example

@trait
structure foo {}

A member can be safely added to an annotation trait if the member is not marked as required. The applications of the foo trait in the previous example and the following example are all valid even after adding a member to the foo trait:

$version: "2"
namespace smithy.example

@trait
structure foo {
    baz: String
}

@foo(baz: "bar")
string MyString4

1.7.2.3. Breaking change rules#

Backward compatibility rules of a trait can be defined in the breakingChanges member of a trait definition. This member is a list of diff rules. Smithy tooling that performs semantic diff analysis between two versions of the same model can use these rules to detect breaking or risky changes.

Note

Not every kind of breaking change can be described using the breakingChanges property. Such backward compatibility rules SHOULD instead be described through documentation and ideally enforced through custom diff tooling.

Property	Type	Description
change	`string`	Required. The type of change. This value can be set to one of the following: `add`: The trait or value at the given path was added. `remove`: The trait or value at the given path was removed. `update`: The trait or value at the given path was changed. `any`: The trait or value at the given path was added, removed, or changed. `presence`: The trait or value at the given path was either added or removed.
path	`string`	A JSON pointer as described in RFC 6901 that points to the values to compare from the original model to the updated model. If omitted or if an empty string is provided (`""`), the entire trait is used as the value for comparison. The provided pointer MUST correctly correspond to shapes in the model.
severity	`string`	Defines the severity of the change. This value can be set to: `ERROR`: The change is backward incompatible. This is the default assumed severity. `DANGER`: The change is very likely backward incompatible. `WARNING`: The change might be backward incompatible. `NOTE`: The change is likely ok, but should be noted during things like code reviews.
message	`string`	Provides an optional plain text message that provides information about why the detected change could be problematic.

It is a backward incompatible change to add the following trait to an existing shape:

@trait(breakingChanges: [{change: "add"}])
structure cannotAdd {}

Note

The above trait definition is equivalent to the following:

@trait(
    breakingChanges: [
        {
            change: "add",
            path: "",
            severity: "ERROR"
        }
    ]
)
structure cannotAdd {}

It is a backward incompatible change to add or remove the following trait from an existing shape:

@trait(breakingChanges: [{change: "presence"}])
structure cannotToAddOrRemove {}

It is very likely backward incompatible to change the "foo" member of the following trait or to remove the "baz" member:

@trait(
    breakingChanges: [
        {
            change: "update",
            path: "/foo",
            severity: "DANGER"
        },
        {
            change: "remove",
            path: "/baz",
            severity: "DANGER"
        }
    ]
)
structure fooBaz {
    foo: String,
    baz: String
}

So for example, if the following shape:

@fooBaz(foo: "a", baz: "b")
string Example

Is changed to:

@fooBaz(foo: "b")
string Example

Then the change to the foo member from "a" to "b" is backward incompatible, as is the removal of the baz member.

Referring to list members

The JSON pointer can path into the members of a list using a member segment.

In the following example, it is a breaking change to change values of lists or sets in instances of the names trait:

@trait(
    breakingChanges: [
        {
            change: "update",
            path: "/names/member"
        }
    ]
)
structure names {
    names: NameList
}

@private
list NameList {
    member: String
}

So for example, if the following shape:

@names(names: ["Han", "Luke"])
string Example

Is changed to:

@names(names: ["Han", "Chewy"])
string Example

Then the change to the second value of the names member is backward incompatible because it changed from Luke to Chewy.

Referring to map members

Members of a map shape can be referenced in a JSON pointer using key and value.

The following example defines a trait where it is backward incompatible to remove a key value pair from a map:

@trait(
    breakingChanges: [
        {
            change: "remove",
            path: "/key"
        }
    ]
)
map jobs {
    key: String,
    value: String
}

So for example, if the following shape:

@jobs(Han: "Smuggler", Luke: "Jedi")
string Example

Is changed to:

@jobs(Luke: "Jedi")
string Example

Then the removal of the "Han" entry of the map is flagged as backward incompatible.

The following example detects when values of a map change.

@trait(
    breakingChanges: [
        {
            change: "update",
            path: "/value"
        }
    ]
)
map jobs {
    key: String,
    value: String
}

So for example, if the following shape:

@jobs(Han: "Smuggler", Luke: "Jedi")
string Example

Is changed to:

@jobs(Han: "Smuggler", Luke: "Ghost")
string Example

Then the change to Luke's mapping from "Jedi" to "Ghost" is backward incompatible.

Note

Using the "update" change type with a map key has no effect.
Using any change type other than "update" with map values has no effect.

1.8. Prelude#

All Smithy models automatically include a prelude. The prelude defines various simple shapes and every trait defined in the core specification. When using the IDL, shapes defined in the prelude can be referenced from within any namespace using a relative shape ID.

Public prelude shapes#

$version: "2"
namespace smithy.api

string String

blob Blob

bigInteger BigInteger

bigDecimal BigDecimal

timestamp Timestamp

document Document

boolean Boolean

byte Byte

short Short

integer Integer

long Long

float Float

double Double

/// The single unit type shape, similar to Void and None in other
/// languages, used to represent no meaningful value.
@unitType
structure Unit {}

@default(false)
boolean PrimitiveBoolean

@default(0)
byte PrimitiveByte

@default(0)
short PrimitiveShort

@default(0)
integer PrimitiveInteger

@default(0)
long PrimitiveLong

@default(0)
float PrimitiveFloat

@default(0)
double PrimitiveDouble

1.8.1. Unit type#

Smithy provides a singular unit type named smithy.api#Unit. The unit type in Smithy is similar to Void and None in other languages. It is used when the input or output of an operation has no meaningful value or if a union member has no meaningful value. smithy.api#Unit MUST NOT be referenced in any other context.

The smithy.api#Unit shape is defined in Smithy's prelude as a structure shape marked with the smithy.api#unitType trait to differentiate it from other structures. It is the only such structure in the model that can be marked with the smithy.api#unitType trait.

1. The Smithy model#

1.1. Smithy overview#

1.2. The semantic model#

1.3. Model metadata#

1.4. Node values#

1.5. Merging model files#

1.6. Shapes#

1.6.1. Shape types#

1.6.2. Member shapes#

1.6.3. Shape ID#

1.6.3.1. Shape ID ABNF#

1.6.3.2. Shape ID conflicts#

1.7. Traits#

1.7.1. Applying traits#

1.7.1.1. Applying traits externally#

1.7.1.2. Trait conflict resolution#

1.7.1.3. Trait node values#

1.7.2. Defining traits#

1.7.2.1. trait trait#

1.7.2.2. Annotation traits#

1.7.2.3. Breaking change rules#

1.8. Prelude#

1.8.1. Unit type#

1.7.2.1. `trait` trait#