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The SimpleRestJson protocol

Smithy4s provides a custom Json-in/Json-out protocol that smithy services can be annotated with.

Smithy4s comes with opt-in http4s-specific module, that contains functions that are aware of this protocol, and can be used to quickly derive http services and clients.

As for the json aspect of the protocol, jsoniter-scala is used for the (de)serialisation of the http bodies.


In this protocol, the values in shapes are bound to http metadata or body according to the specification of the Http Binding traits. However, the @mediaType trait has no incidence, and all bodies (when present) are serialised in JSON.

Example spec

namespace smithy4s.example

use alloy#simpleRestJson

service HelloWorldService {
version: "1.0.0"
// Indicates that all operations in `HelloWorldService`,
// here limited to the Hello operation, can return `GenericServerError`.
errors: [GenericServerError]
operations: [Hello]

structure GenericServerError {
message: String

@http(method: "POST", uri: "/{name}", code: 200)
operation Hello {
input: Person
output: Greeting

structure Person {
name: String

town: String

structure Greeting {
message: String

Supported traits

This protocol and its interpreters, are aware of the following traits provided out of the box:

For the full list, see below.

Decoding and encoding unions

The SimpleRestJson protocol supports 3 different union encodings :

  • tagged (default)
  • untagged
  • discriminated

See the section about unions for a detailed description.

Json Array Arity

  • By default there is a limit on the arity of an array, which is 1024. This is to prevent the server from being overloaded with a large array as this is a vector for attacks.
  • This limit can be changed by setting the maxArity smithy4s.http4s.SimpleRestJsonBuilder.withMaxArity(.) to the desired value.
  • an example can be seen in the client example

Explicit Null Encoding

By default, optional properties (headers, query parameters, structure fields) that are set to None and optional properties that are set to default value will be excluded during encoding process. If you wish to change this so that instead they are included and set to null explicitly, you can do so by calling .withExplicitDefaultsEncoding(true).

Supported traits

Here is the list of traits supported by SimpleRestJson

  • smithy.api#default
  • smithy.api#error
  • smithy.api#http
  • smithy.api#httpError
  • smithy.api#httpHeader
  • smithy.api#httpLabel
  • smithy.api#httpPayload
  • smithy.api#httpPrefixHeaders
  • smithy.api#httpQuery
  • smithy.api#httpQueryParams
  • smithy.api#httpResponseCode
  • smithy.api#jsonName
  • smithy.api#length
  • smithy.api#pattern
  • smithy.api#range
  • smithy.api#required
  • smithy.api#timestampFormat
  • alloy#uuidFormat
  • alloy#discriminated
  • alloy#nullable
  • alloy#untagged

Currently, @cors is not supported. This is because the @cors annotation is too restrictive. You can still use it in your model and configure your API using the information found in the generated code. See the Cors.scala file in the guides module for an example.

Structured Strings

As of smithy4s version 0.18.x, you are able to create strings which are parsed directly into structures for you. This can be accomplished using the alloy#structurePattern trait. For example:

@structurePattern(pattern: "{foo}_{bar}", target: FooBar)
string FooBarString

structure FooBar {
foo: String
bar: Integer

Now wherever FooBarString is used, it will really be parsing the string into the structure FooBar. As such, the generated code will replace instances of FooBarString with FooBar such that the parsing logic is abstracted away from your implementation. See the alloy documentation for more information.