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.

  • The Smithy IDL is a human-readable format that aims to streamline authoring and reading models.
  • The JSON AST aims to provide a more machine-readable format to easily share models across language implementations and better integrate with JSON-based ecosystems.

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. Shapes#

Smithy models are made up of shapes. Shapes come in three kinds: simple, aggregate, and service. A simple shape defines atomic or primitive values such as integer and string. Aggregate shapes have members such as a list of strings or an Address structure. Service shapes have specific semantics, unlike the very generic simple and aggregate shapes, as they represent either a service, a resource managed by a service, or operations on services and resources.

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]?   │
┌──────────┐  │                    ├────│    Set     │  │    └─────────────────────────┘
│timestamp │──┤                    │    ├────────────┤  │    ┌─────────────────────────┐
└──────────┘  │                    │    │member      │  │    │        Operation        │
              │                    │    └────────────┘  │    ├─────────────────────────┤
      ┌───────────────┐            │    ┌────────────┐  │    │input: Structure         │
      │  «abstract»   │            ├────│    Map     │  ├────│output: Structure        │
      │    Number     │            │    ├────────────┤  │    │errors: [Structure]?     │
      └───────────────┘            │    │key         │  │    └─────────────────────────┘
              △                    │    │value       │  │    ┌─────────────────────────┐
┌──────────┐  │  ┌──────────┐      │    └────────────┘  │    │        Resource         │
│byte      │──┼──│short     │      │    ┌────────────┐  │    ├─────────────────────────┤
└──────────┘  │  └──────────┘      ├────│ Structure  │  │    │identifiers?             │
┌──────────┐  │  ┌──────────┐      │    └────────────┘  │    │create: Operation?       │
│integer   │──┼──│long      │      │    ┌────────────┐  │    │put: Operation?          │
└──────────┘  │  └──────────┘      └────│   Union    │  │    │read: Operation?         │
┌──────────┐  │  ┌──────────┐           └────────────┘  └────│update: Operation?       │
│float     │──┼──│double    │                                │delete: Operation?       │
└──────────┘  │  └──────────┘                                │list: : Operation?       │
┌──────────┐  │  ┌──────────┐                                │operations: [Operation]? │
│bigInteger│──┴──│bigDecimal│                                │collectionOperations:    │
└──────────┘     └──────────┘                                │    [Operation]?         │
                                                             │resources: [Resource]?   │
                                                             └─────────────────────────┘

1.3.1. Shape ID#

All shapes have an assigned shape ID. A shape ID is used to refer to shapes in the model. Shape IDs adhere to the following syntax:

com.foo.baz#ShapeName$memberName
\_________/ \_______/ \________/
     |          |          |
 Namespace  Shape name  Member name
Namespace
A namespace is a mechanism for logically grouping shapes in a way that makes them reusable alongside other models without naming conflicts. A semantic model MAY contain shapes defined across multiple namespaces. The IDL representation supports zero or one namespace per model file, while the JSON AST representation supports zero or more namespaces per model file.
Absolute shape ID
An absolute shape ID starts with a smithy:Namespace name, followed by "#", followed by a relative shape ID. All shape IDs in the semantic model MUST be absolute. For example, smithy.example#Foo and smithy.example#Foo$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.
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.

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

Best practices for defining shape names

  1. Use a strict form of PascalCase for shape names. 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.

    Recommended Not recommended
    UserId UserID
    ResourceArn ResourceARN
    IoChannel IOChannel
    HtmlEntity HTMLEntity
    HtmlEntity HTML_Entity

  2. Limit the number of namespaces used to model a single domain. Ideally only a single namespace is used 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.

1.3.2. 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.

See also

Merging model files for information on how conflicting shape definitions for the same shape ID are handled when assembling the semantic model from multiple model files.

1.4. Simple shapes#

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
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)
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 Represents an instant in time with no UTC offset or timezone. The serialization of a timestamp is an implementation detail that is determined by a protocol and MUST NOT have any effect on the types exposed by tooling to represent a timestamp value.
document Represents protocol-agnostic open content that functions as a kind of "any" type. Document types are represented by a JSON-like data model and can contain UTF-8 strings, arbitrary precision numbers, booleans, nulls, a list of these values, and a map of UTF-8 strings to these values. Open content is useful for modeling unstructured data that has no schema, data that can't be modeled using rigid types, or data that has a schema that evolves outside of the purview of a model. The serialization format of a document is an implementation detail of a protocol and MUST NOT have any effect on the types exposed by tooling to represent a document value.

Simple shapes are defined in the IDL using a simple_shape_statement.

Note

The prelude model contains pre-defined shapes for every simple type.

Simple shape examples

The following example defines a shape for each simple type in the smithy.example namespace.

namespace smithy.example

blob Blob
boolean Boolean
string String
byte Byte
short Short
integer Integer
long Long
float Float
double Double
bigInteger BigInteger
bigDecimal BigDecimal
timestamp Timestamp
document Document

Note

When defining shapes in the IDL, a namespace MUST first be declared.

1.5. Aggregate shapes#

Aggregate types define shapes that are composed of other shapes. Aggregate shapes reference other shapes using members.

Type Description
Member Defined in aggregate shapes to reference other shapes
List Ordered collection of homogeneous values
Set (Deprecated) Ordered collection of unique 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

1.5.1. Member#

Members are defined in aggregate shapes to reference other shapes using a shape ID. The shape referenced by a member is called its "target". A member MUST NOT target a trait, operation, resource, service, or member.

1.5.2. List#

The list type represents an ordered homogeneous collection of values. A list shape requires a single member named member. Lists are defined in the IDL using a list_statement. The following example defines a list with a string member from the prelude:

namespace smithy.example

list MyList {
    member: String
}

List member nullability

Lists are considered dense by default, meaning they MAY NOT contain null values. A list MAY be made sparse by applying the sparse trait. The box trait is not used to determine if a list is dense or sparse; a list with no @sparse trait is always considered dense. The following example defines a sparse list:

@sparse
list SparseList {
    member: String
}

If a client encounters a null value when deserializing a dense list returned from a service, the client MUST discard the null value. If a service receives a null value for a dense list from a client, it SHOULD reject the request.

List member shape ID

The shape ID of the member of a list is the list shape ID followed by $member. For example, the shape ID of the list member in the above example is smithy.example#MyList$member.

1.5.3. Set#

Danger

Sets are deprecated. Use a list with the uniqueItems trait instead.

The set type represents an ordered collection of unique values. A set shape requires a single member named member. Sets are implicitly considered to be marked with the uniqueItems trait, and as such MUST NOT transitively contain floats, doubles, or documents.

Important

Sets are considered sub-type of lists; any place a list is accepted, a set is accepted.

Sets are defined in the IDL using a set_statement. The following example defines a set of strings:

namespace smithy.example

set StringSet {
    member: String
}

Sets MUST NOT contain null values

The values contained in a set are not permitted to be null. null set values do not provide much, if any, utility, and set implementations across programming languages often do not support null values.

If a client encounters a null value when deserializing a set returned from a service, the client MUST discard the null value. If a service receives a null value for a set from a client, it SHOULD reject the request.

Set member shape ID

The shape ID of the member of a set is the set shape ID followed by $member. For example, the shape ID of the set member in the above example is smithy.example#StringSet$member.

Use insertion order

Implementations SHOULD use insertion ordered sets to ensure that clients and servers both agree on element ordering so that error messages about specific items in a set are actionable by clients. If a client and server don't agree on ordering, then pointing to where a validation error occurs becomes very challenging. Programming languages that do not support insertion ordered sets SHOULD store the values of a set in a list.

1.5.4. Map#

The map type represents a map data structure that maps string keys to homogeneous values. A map requires a member named key that MUST target a string shape and a member named value. Maps are defined in the IDL using a map_statement. The following example defines a map of strings to integers:

namespace smithy.example

map IntegerMap {
    key: String,
    value: Integer
}

Map keys MUST NOT be null

Map keys are not permitted to be null. Not all protocol serialization formats have a way to define null map keys, and map implementations across programming languages often do not allow null keys in maps.

Map value member nullability

Maps values are considered dense by default, meaning they MAY NOT contain null values. A map MAY be made sparse by applying the sparse trait. The box trait is not used to determine if a map is dense or sparse; a map with no @sparse trait is always considered dense. The following example defines a sparse map:

@sparse
map SparseMap {
    key: String,
    value: String
}

If a client encounters a null map value when deserializing a dense map returned from a service, the client MUST discard the null entry. If a service receives a null map value for a dense map from a client, it SHOULD reject the request.

Map member shape IDs

The shape ID of the key member of a map is the map shape ID followed by $key, and the shape ID of the value member is the map shape ID followed by $value. For example, the shape ID of the key member in the above map is smithy.example#IntegerMap$key, and the value member is smithy.example#IntegerMap$value.

1.5.5. Structure#

The structure type represents a fixed set of named, unordered, heterogeneous values. A structure shape contains a set of named members, and each member name maps to exactly one member definition. Structures are defined in the IDL using a structure_statement.

The following example defines a structure with two members, one of which is marked with the required trait.

namespace smithy.example

structure MyStructure {
    foo: String,

    @required
    baz: Integer,
}

See also

Applying traits for a description of how to apply traits.

Adding new members

New members added to existing structures SHOULD be added to the end of the structure. This ensures that programming languages that require a specific data structure layout or alignment for code generated from Smithy models are able to maintain backward compatibility.

Structure member shape IDs

The shape ID of a member of a structure is the structure shape ID, followed by $, followed by the member name. For example, the shape ID of the foo member in the above example is smithy.example#MyStructure$foo.

1.5.5.1. Default structure member values#

The values provided for structure members are either always present and set to a default value when necessary or boxed, meaning a value is optionally present with no default value. Members are considered boxed if the member is marked with the box trait or the shape targeted by the member is marked with the box trait. Members that target strings, timestamps, and aggregate shapes are always considered boxed and have no default values.

  • The default value of a byte, short, integer, long, float, and double shape that is not boxed is zero.
  • The default value of a boolean shape that is not boxed is false.
  • All other shapes are always considered boxed and have no default value.

1.5.6. Union#

The union type represents a tagged union data structure that can take on several different, but fixed, types. Unions function similarly to structures except that only one member can be used at any one time. Each member in the union is a variant of the tagged union, where member names are the tags of each variant, and the shapes targeted by members are the values of each variant.

Unions are defined in the IDL using a union_statement. A union shape MUST contain one or more named members. The following example defines a union shape with several members:

namespace smithy.example

union MyUnion {
    i32: Integer,

    @length(min: 1, max: 100)
    string: String,

    time: Timestamp,
}

Unit types in unions

Some union members might not need any meaningful information beyond the tag itself. For these cases, union members MAY target Smithy's built-in unit type, smithy.api#Unit.

The following example defines a union for actions a player can take in a game.

union PlayerAction {
    /// Quit the game.
    quit: Unit,

    /// Move in a specific direction.
    move: DirectedAction,

    /// Jump in a specific direction.
    jump: DirectedAction
}

structure DirectedAction {
    @required
    direction: Integer
}

The quit action has no meaningful data associated with it, while move and jump both reference DirectedAction.

Union member presence

Exactly one member of a union MUST be set. The serialization of a union is defined by a protocol, but for example purposes, if unions were to be represented in a hypothetical JSON serialization, the following value would be valid for the PlayerAction union because a single member is present:

{
    "move": {
        "direction": 1
    }
}

The following value is invalid because multiple members are present:

{
    "quit": {},
    "move": {
        "direction": 1
    }
}

The following value is invalid because no members are present:

{}

Adding new members

New members added to existing unions SHOULD be added to the end of the union. This ensures that programming languages that require a specific data structure layout or alignment for code generated from Smithy models are able to maintain backward compatibility.

Union member shape IDs

The shape ID of a member of a union is the union shape ID, followed by $, followed by the member name. For example, the shape ID of the i32 member in the above example is smithy.example#MyUnion$i32.

1.5.7. 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.

See also

Prelude model, Union, Operation

1.5.8. Recursive shape definitions#

Smithy allows recursive shape definitions with the following limitations:

  1. The member of a list, set, or map cannot directly or transitively target its containing shape unless one or more members in the path from the container back to itself targets a structure or union shape. This ensures that shapes that are typically impossible to define in various programming languages are not defined in Smithy models (for example, you can't define a recursive list in Java List<List<List....).
  2. To ensure a value can be provided for a structure, recursive member relationship from a structure back to itself MUST NOT be made up of all required structure members.
  3. To ensure a value can be provided for a union, recursive unions MUST contain at least one path through its members that is not recursive or steps through a list, set, map, or optional structure member.

The following recursive shape definition is valid:

namespace smithy.example

list ValidList {
    member: IntermediateStructure
}

structure IntermediateStructure {
    foo: ValidList
}

The following recursive shape definition is invalid:

namespace smithy.example

list RecursiveList {
    member: RecursiveList
}

The following recursive shape definition is invalid due to mutual recursion and the required trait.

namespace smithy.example

structure RecursiveShape1 {
    @required
    recursiveMember: RecursiveShape2
}

structure RecursiveShape2 {
    @required
    recursiveMember: RecursiveShape1
}

1.6. Service shapes#

Service types have specific semantics and define services, resources, and operations.

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

1.6.1. Service#

A service is the entry point of an API that aggregates resources and operations together. The resources and operations of an API are bound within the closure of a service. A service is defined in the IDL using a service_statement.

The service shape supports the following properties:

Property Type Description
version string Defines the optional version of the service. The version can be provided in any format (e.g., 2017-02-11, 2.0, etc).
operations [string] Binds a set of operation shapes to the service. Each element in the given list MUST be a valid shape ID that targets an operation shape.
resources [string] Binds a set of resource shapes to the service. Each element in the given list MUST be a valid shape ID that targets a resource shape.
errors [string] Defines a list of common errors that every operation bound within the closure of the service can return. Each provided shape ID MUST target a structure shape that is marked with the error trait.
rename map of shape ID to string

Disambiguates shape name conflicts in the service closure. Map keys are shape IDs contained in the service, and map values are the disambiguated shape names to use in the context of the service. Each given shape ID MUST reference a shape contained in the closure of the service. Each given map value MUST match the smithy:Identifier production used for shape IDs. Renaming a shape does not give the shape a new shape ID.

  • No renamed shape name can case-insensitively match any other renamed shape name or the name of a non-renamed shape contained in the service.
  • Member shapes MAY NOT be renamed.
  • Resource and operation shapes MAY NOT be renamed. Renaming shapes is intended for incidental naming conflicts, not for renaming the fundamental concepts of a service.
  • Shapes from other namespaces marked as private MAY be renamed.
  • A rename MUST use a name that is case-sensitively different from the original shape ID name.

The following example defines a service with no operations or resources.

namespace smithy.example

service MyService {
    version: "2017-02-11"
}

The following example defines a service shape that defines a set of errors that are common to every operation in the service:

namespace smithy.example

service MyService {
    version: "2017-02-11",
    errors: [SomeError]
}

@error("client")
structure SomeError {}

1.6.1.1. Service operations#

Operation shapes can be bound to a service by adding the shape ID of an operation to the operations property of a service. Operations bound directly to a service are typically RPC-style operations that do not fit within a resource hierarchy.

namespace smithy.example

service MyService {
    version: "2017-02-11",
    operations: [GetServerTime],
}

@readonly
operation GetServerTime {
    output: GetServerTimeOutput
}

1.6.1.2. Service resources#

Resource shapes can be bound to a service by adding the shape ID of a resource to the resources property of a service.

namespace smithy.example

service MyService {
    version: "2017-02-11",
    resources: [MyResource],
}

resource MyResource {}

1.6.1.3. Service closure#

The closure of a service is the set of shapes connected to a service through resources, operations, and members.

Important

With some exceptions, the shapes that are referenced in the closure of a service MUST have case-insensitively unique names regardless of their namespace, and conflicts MUST be disambiguated using the rename property of a service.

By requiring unique names within a service, each service forms a ubiquitous language, making it easier for developers to understand the model and artifacts generated from the model, like code. For example, when using Java code generated from a Smithy model, a developer should not need to discern between BadRequestException classes across multiple packages that can be thrown by an operation. Uniqueness is required case-insensitively because many model transformations (like code generation) change the casing and inflection of shape names to make artifacts more idiomatic.

Shape types allowed to conflict in a closure

Simple types and lists or sets of compatible simple types are allowed to conflict because a conflict for these type would rarely have an impact on generated artifacts. These kinds of conflicts are only allowed if both conflicting shapes are the same type and have the exact same traits. In the case of a list or set, a conflict is only allowed if the members of the conflicting shapes target compatible shapes.

Disambiguating shapes with rename

The rename property of a service is used to disambiguate conflicting shape names found in the closure of a service. The rename property is essentially a context map used to ensure that the service still presents a ubiquitous language despite bringing together shapes from multiple namespaces.

Note

Renames SHOULD be used sparingly. Renaming shapes is something typically only needed when aggregating models from multiple independent teams into a single service.

The following example defines a service that contains two shapes named "Widget" in its closure. The rename property is used to disambiguate the conflicting shapes.

namespace smithy.example

service MyService {
    version: "2017-02-11",
    operations: [GetSomething],
    rename: {
        "foo.example#Widget": "FooWidget"
    }
}

operation GetSomething {
    input: GetSomethingInput,
    output: GetSomethingOutput
}

@input
structure GetSomethingInput {}

@output
structure GetSomethingOutput {
    widget1: Widget,
    fooWidget: foo.example#Widget,
}

structure Widget {}

Resources and operations can be bound once

An operation or resource MUST NOT be bound to multiple shapes within the closure of a service. This constraint allows services to discern between operations and resources using only their shape name rather than a fully-qualified path from the service to the shape.

Undeclared operation inputs and outputs are not a part of the service closure

smithy.api#Unit is the shape that is implicitly targeted by operation inputs and outputs when they are not explicitly declared. This does not, however, add smithy.api#Unit to the service's closure, and does not require renaming to avoid conflicts with other shapes named Unit. Unions in the service closure with members targeting smithy.api#Unit, however, will cause smithy.api#Unit to be a part of the service closure.

1.6.2. Operation#

The operation type represents the input, output, and possible errors of an API operation. Operation shapes are bound to resource shapes and service shapes. An operation is defined in the IDL using an operation_statement.

An operation supports the following members:

Property Type Description
input string

The input of the operation defined using a shape ID that MUST target a structure.

  • Every operation SHOULD define a dedicated input shape marked with the input trait. Creating a dedicated input shape ensures that input members can be added in the future if needed.
  • Input defaults to smithy.api#Unit if no input is defined, indicating that the operation has no meaningful input.
output string

The output of the operation defined using a shape ID that MUST target a structure.

  • Every operation SHOULD define a dedicated output shape marked with the output trait. Creating a dedicated output shape ensures that output members can be added in the future if needed.
  • Output defaults to smithy.api#Unit if no output is defined, indicating that the operation has no meaningful output.
errors [string] The errors that an operation can return. Each string in the list is a shape ID that MUST target a structure shape marked with the error trait.

The following example defines an operation that accepts an input structure named MyOperationInput, returns an output structure named MyOperationOutput, and can potentially return the NotFound or BadRequest error structures.

namespace smithy.example

operation MyOperation {
    input: MyOperationInput,
    output: MyOperationOutput,
    errors: [NotFoundError, BadRequestError]
}

@input
structure MyOperationInput {}

@output
structure MyOperationOutput {}

While, input and output SHOULD be explicitly defined for every operation, omitting them is allowed. The default value for input and output is smithy.api#Unit, indicating that there is no meaningful value.

namespace smithy.example

operation MySideEffectOperation {}

The following example is equivalent, but more explicit in intent:

namespace smithy.example

operation MySideEffectOperation {
    input: Unit,
    output: Unit
}

Warning

Using the Unit shape for input or output removes flexibility in how an operation can evolve over time because members cannot be added to the input or output if ever needed.

1.6.3. Resource#

Smithy defines a resource as an entity with an identity that has a set of operations. A resource shape is defined in the IDL using a resource_statement.

A resource supports the following members:

Property Type Description
identifiers object Defines a map of identifier string names to Shape IDs used to identify the resource. Each shape ID MUST target a string shape.
create string Defines the lifecycle operation used to create a resource using one or more identifiers created by the service. The value MUST be a valid Shape ID that targets an operation shape.
put string Defines an idempotent lifecycle operation used to create a resource using identifiers provided by the client. The value MUST be a valid Shape ID that targets an operation shape.
read string Defines the lifecycle operation used to retrieve the resource. The value MUST be a valid Shape ID that targets an operation shape.
update string Defines the lifecycle operation used to update the resource. The value MUST be a valid Shape ID that targets an operation shape.
delete string Defines the lifecycle operation used to delete the resource. The value MUST be a valid Shape ID that targets an operation shape.
list string Defines the lifecycle operation used to list resources of this type. The value MUST be a valid Shape ID that targets an operation shape.
operations [string] Binds a list of non-lifecycle instance operations to the resource. Each value in the list MUST be a valid Shape ID that targets an operation shape.
collectionOperations [string] Binds a list of non-lifecycle collection operations to the resource. Each value in the list MUST be a valid Shape ID that targets an operation shape.
resources [string] Binds a list of resources to this resource as a child resource, forming a containment relationship. Each value in the list MUST be a valid Shape ID that targets a resource. The resources MUST NOT have a cyclical containment hierarchy, and a resource can not be bound more than once in the entire closure of a resource or service.

1.6.4. Resource Identifiers#

Identifiers are used to refer to a specific resource within a service. The identifiers property of a resource is a map of identifier names to shape IDs that MUST target string shapes.

For example, the following model defines a Forecast resource with a single identifier named forecastId that targets the ForecastId shape:

namespace smithy.example

resource Forecast {
    identifiers: {
        forecastId: ForecastId
    }
}

string ForecastId

When a resource is bound as a child to another resource using the "resources" property, all of the identifiers of the parent resource MUST be repeated verbatim in the child resource, and the child resource MAY introduce any number of additional identifiers.

Parent identifiers are the identifiers of the parent of a resource. All parent identifiers MUST be bound as identifiers in the input of every operation bound as a child to a resource. Child identifiers are the identifiers that a child resource contains that are not present in the parent identifiers.

For example, given the following model,

resource ResourceA {
    identifiers: {
        a: String
    },
    resources: [ResourceB],
}

resource ResourceB {
    identifiers: {
        a: String,
        b: String,
    },
    resources: [ResourceC],
}

resource ResourceC {
    identifiers: {
        a: String,
        b: String,
        c: String,
    }
}

ResourceB is a valid child of ResourceA and contains a child identifier of "b". ResourceC is a valid child of ResourceB and contains a child identifier of "c".

However, the following defines two invalid child resources that do not define an identifiers property that is compatible with their parents:

resource ResourceA {
    identifiers: {
        a: String,
        b: String,
    },
    resources: [Invalid1, Invalid2],
}

resource Invalid1 {
    // Invalid: missing "a".
    identifiers: {
        b: String,
    },
}

resource Invalid2 {
    identifiers: {
        a: String,
        // Invalid: does not target the same shape.
        b: SomeOtherString,
    },
}

1.6.4.1. Binding identifiers to operations#

Identifier bindings indicate which top-level members of the input or output structure of an operation provide values for the identifiers of a resource.

Identifier binding validation

  • Child resources MUST provide identifier bindings for all of its parent's identifiers.
  • Identifier bindings are only formed on input or output structure members that are marked as required.
  • Resource operations MUST form a valid instance operation or collection operation.

Instance operations are formed when all of the identifiers of a resource are bound to the input structure of an operation or when a resource has no identifiers. The put, read, update, and delete lifecycle operations are examples of instance operations. An operation bound to a resource using operations MUST form a valid instance operation.

Collection operations are used when an operation is meant to operate on a collection of resources rather than a specific resource. Collection operations are formed when an operation is bound to a resource with collectionOperations, or when bound to the list or create lifecycle operations. A collection operation MUST omit one or more identifiers of the resource it is bound to, but MUST bind all identifiers of any parent resource.

1.6.4.2. Implicit identifier bindings#

Implicit identifier bindings are formed when the input or output of an operation contains member names that target the same shapes that are defined in the "identifiers" property of the resource to which an operation is bound.

For example, given the following model,

resource Forecast {
    identifiers: {
        forecastId: ForecastId,
    },
    read: GetForecast,
}

@readonly
operation GetForecast {
    input: GetForecastInput,
    output: GetForecastOutput
}

@input
structure GetForecastInput {
    @required
    forecastId: ForecastId,
}

@output
structure GetForecastOutput {
    @required
    weather: WeatherData,
}

GetForecast forms a valid instance operation because the operation is not marked with the collection trait and GetForecastInput provides implicit identifier bindings by defining a required "forecastId" member that targets the same shape as the "forecastId" identifier of the resource.

Implicit identifier bindings for collection operations are created in a similar way to an instance operation, but MUST NOT contain identifier bindings for all child identifiers of the resource on an input shape.

Given the following model,

resource Forecast {
    identifiers: {
        forecastId: ForecastId,
    },
    collectionOperations: [BatchPutForecasts],
}

operation BatchPutForecasts {
    input: BatchPutForecastsInput,
    output: BatchPutForecastsOutput
}

@input
structure BatchPutForecastsInput {
    @required
    forecasts: BatchPutForecastList,
}

BatchPutForecasts forms a valid collection operation with implicit identifier bindings because BatchPutForecastsInput does not require an input member named "forecastId" that targets ForecastId.

1.6.4.3. Explicit identifier bindings#

Explicit identifier bindings are defined by applying the resourceIdentifier trait to a member of the input of for an operation bound to a resource. Explicit bindings are necessary when the name of the input structure member differs from the name of the resource identifier to which the input member corresponds.

For example, given the following,

resource Forecast {
    // continued from above
    resources: [HistoricalForecast],
}

resource HistoricalForecast {
    identifiers: {
        forecastId: ForecastId,
        historicalId: HistoricalForecastId,
    },
    read: GetHistoricalForecast,
    list: ListHistoricalForecasts,
}

@readonly
operation GetHistoricalForecast {
    input: GetHistoricalForecastInput,
    output: GetHistoricalForecastOutput
}

@input
structure GetHistoricalForecastInput {
    @required
    @resourceIdentifier("forecastId")
    customForecastIdName: ForecastId,

    @required
    @resourceIdentifier("historicalId")
    customHistoricalIdName: String
}

the resourceIdentifier trait on GetHistoricalForecastInput$customForecastIdName maps it to the "forecastId" identifier is provided by the "customForecastIdName" member, and the resourceIdentifier trait on GetHistoricalForecastInput$customHistoricalIdName maps that member to the "historicalId" identifier.

If an operation input supplies both an explicit and an implicit identifier binding, the explicit identifier binding is utilized.

1.6.5. Resource lifecycle operations#

Lifecycle operations are used to transition the state of a resource using well-defined semantics. Lifecycle operations are defined by providing a shape ID to the put, create, read, update, delete, and list properties of a resource. Each shape ID MUST target an operation that is compatible with the semantics of the lifecycle.

The following example defines a resource with each lifecycle method:

namespace smithy.example

resource Forecast {
    identifiers: {
        forecastId: ForecastId,
    },
    put: PutForecast,
    create: CreateForecast,
    read: GetForecast,
    update: UpdateForecast,
    delete: DeleteForecast,
    list: ListForecasts,
}

1.6.5.1. Put lifecycle#

The put lifecycle operation is used to create a resource using identifiers provided by the client.

The following example defines the PutForecast operation.

@idempotent
operation PutForecast {
    input: PutForecastInput,
    output: PutForecastOutput
}

@input
structure PutForecastInput {
    // The client provides the resource identifier.
    @required
    forecastId: ForecastId,

    chanceOfRain: Float
}

Put semantics

The semantics of a put lifecycle operation are similar to the semantics of a HTTP PUT method as described in section 4.3.4 of [RFC7231]:

The PUT method requests that the state of the target resource be created or replaced ...

The noReplace trait can be applied to resources that define a put lifecycle operation to indicate that a resource cannot be replaced using the put operation.

1.6.5.2. Create lifecycle#

The create operation is used to create a resource using one or more identifiers created by the service.

The following example defines the CreateForecast operation.

operation CreateForecast {
    input: CreateForecastInput,
    output: CreateForecastOutput
}

operation CreateForecast {
    input: CreateForecastInput,
    output: CreateForecastOutput
}

@input
structure CreateForecastInput {
    // No identifier is provided by the client, so the service is
    // responsible for providing the identifier of the resource.
    chanceOfRain: Float,
}

1.6.5.3. Read lifecycle#

The read operation is the canonical operation used to retrieve the current representation of a resource.

For example:

@readonly
operation GetForecast {
    input: GetForecastInput,
    output: GetForecastOutput,
    errors: [ResourceNotFound]
}

@input
structure GetForecastInput {
    @required
    forecastId: ForecastId,
}

1.6.5.4. Update lifecycle#

The update operation is the canonical operation used to update a resource.

For example:

operation UpdateForecast {
    input: UpdateForecastInput,
    output: UpdateForecastOutput,
    errors: [ResourceNotFound]
}

@input
structure UpdateForecastInput {
    @required
    forecastId: ForecastId,

    chanceOfRain: Float,
}

1.6.5.5. Delete lifecycle#

The delete operation is canonical operation used to delete a resource.

For example:

@idempotent
operation DeleteForecast {
    input: DeleteForecastInput,
    output: DeleteForecastOutput,
    errors: [ResourceNotFound]
}

@input
structure DeleteForecastInput {
    @required
    forecastId: ForecastId,
}

1.6.5.6. List lifecycle#

The list operation is the canonical operation used to list a collection of resources.

  • List operations MUST form valid collection operations.
  • List operations MUST be marked with the readonly trait.
  • The output of a list operation SHOULD contain references to the resource being listed.
  • List operations SHOULD be paginated.

For example:

@readonly @paginated
operation ListForecasts {
    input: ListForecastsInput,
    output: ListForecastsOutput
}

@input
structure ListForecastsInput {
    maxResults: Integer,
    nextToken: String
}

@output
structure ListForecastsOutput {
    nextToken: String,
    @required
    forecasts: ForecastList
}

list ForecastList {
    member: ForecastId
}

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 to shapes#

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:

namespace smithy.example

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

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.

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:

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.

1.7.2. 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:

  1. If the trait is a list or set shape, then the conflicting trait values are concatenated into a single trait value.
  2. 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:

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".

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:

namespace smithy.example

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

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

1.7.3. 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 and set 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 any constraint traits found related to the shape being validated.

1.7.4. Defining traits#

Traits are defined inside of a namespace by applying smithy.api#trait to a shape. This trait can only be applied to simple types, list, map, set, structure, and union shapes.

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

namespace smithy.example

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

@myTraitName
string MyString

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.

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

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.

Naming traits

By convention, trait shape names SHOULD use a lowercase name so that they visually stand out from normal shapes.

1.7.4.1. 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:

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:

namespace smithy.example

@trait
structure foo {
    baz: String,
}

@foo(baz: "bar")
string MyString4

1.7.4.2. Trait 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 and set members

The JSON pointer can path into the members of a list or set 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. 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 an object node value.

1.8.1. Merging metadata#

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

  1. If a metadata key is only present in one model, then the entry is valid and added to the merged model.
  2. If both models contain the same key and both values are arrays, then the entry is valid; the values of both arrays are concatenated into a single array and added to the merged model.
  3. If both models contain the same key and both values are exactly equal, then the conflict is ignored and the value is added to the merged model.
  4. If both models contain the same key, the values do not both map to arrays, and the values are not equal, then the key is invalid and there is a metadata conflict error.

Given the following two Smithy models:

model-a.smithy#
metadata "foo" = ["baz", "bar"]
metadata "qux" = "test"
metadata "validConflict" = "hi!"
model-b.smithy#
metadata "foo" = ["lorem", "ipsum"]
metadata "lorem" = "ipsum"
metadata "validConflict" = "hi!"

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

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

1.9. Node values#

Node values are JSON-like values used in the following places in the semantic model:

  • metadata: Metadata is defined as a node value object.
  • applied trait: The value of a trait applied to a shape is defined using a node value.
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:

namespace smithy.example

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

1.9.1. 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

Shape IDs, text blocks, et al.

There is no specific node value type for shape IDs, text blocks, or other higher-level features of the IDL; these values are stored and treated in the semantic model as simply opaque strings, and their validation happens before the creation of the model.

1.10. Merging model files#

Implementations MUST take the following steps when merging two or more model files to form a semantic model:

  1. Merge the metadata objects of all model files using the steps defined in Merging metadata.
  2. Shapes defined in a single model file are added to the semantic model as-is.
  3. 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 shapes MUST contain the same members that target the same shapes.
    3. Conflicting service shapes MUST contain the same properties and target the same shapes.
  4. Conflicting traits defined in shape definitions or through apply statements are reconciled using trait conflict resolution.

Note

The following guidance is non-normative. Because the Smithy IDL allows forward references to shapes that have not yet been defined or shapes that are defined in another model file, implementations likely need to defer resolving relative shape IDs to absolute shape IDs until all model files are loaded.