OpenAPI Documentation Generation

OpenAPI is an open-source specification for describing RESTful API interface designs. It defines API request/response structures, parameters, return types, error codes, and other details in JSON or YAML format, making communication between clients and servers more explicit and standardized.

Originally the open-source version of the Swagger specification, OpenAPI has now become an independent project with support from many large enterprises and developers. Using the OpenAPI specification helps development teams collaborate better, reduce communication overhead, and improve efficiency. Additionally, OpenAPI provides tools for automatically generating API documentation, mock data, and test cases, facilitating development and testing workflows.

Salvo offers OpenAPI integration (adapted from utoipa). Leveraging its own features, Salvo elegantly extracts relevant OpenAPI data type information automatically from Handler. It also integrates several popular open-source OpenAPI interfaces like SwaggerUI, scalar, rapidoc, and redoc.

To address Rust's long type names that may not be suitable for OpenAPI usage, salvo-oapi provides the Namer type, allowing customization of rules to modify type names in OpenAPI.

Example Code

main.rs

Cargo.toml

oapi-hello/src/main.rs

use salvo::oapi::extract::*;
use salvo::prelude::*;

#[endpoint]
async fn hello(name: QueryParam<String, false>) -> String {
    format!("Hello, {}!", name.as_deref().unwrap_or("World"))
}

#[tokio::main]
async fn main() {
    tracing_subscriber::fmt().init();

    let router = Router::new().push(Router::with_path("hello").get(hello));

    let doc = OpenApi::new("test api", "0.0.1").merge_router(&router);

    let router = router
        .unshift(doc.into_router("/api-doc/openapi.json"))
        .unshift(SwaggerUi::new("/api-doc/openapi.json").into_router("/swagger-ui"));

    let acceptor = TcpListener::new("0.0.0.0:5800").bind().await;
    Server::new(acceptor).serve(router).await;
}

Enter http://localhost:5800/swagger-ui in your browser to view the Swagger UI page.

Salvo's OpenAPI integration is remarkably elegant. For the example above, compared to a regular Salvo project, we only needed to:

Enable the oapi feature in Cargo.toml: salvo = { workspace = true, features = ["oapi"] };
Replace #[handler] with #[endpoint];
Use name: QueryParam<String, false> to get query string values. When visiting http://localhost/hello?name=chris, this name query string will be parsed. Here, false indicates the parameter is optional—accessing http://localhost/hello won't trigger an error. Conversely, QueryParam<String, true> means the parameter is mandatory, and its absence will return an error.
Create OpenAPI and its corresponding Router. OpenApi::new("test api", "0.0.1").merge_router(&router) uses merge_router to indicate that this OpenAPI extracts necessary documentation information by parsing a specific route and its descendants. Routes whose Handler doesn't provide documentation generation info will be ignored (e.g., those defined with #[handler] instead of #[endpoint]). In real projects, you can choose not to generate OpenAPI docs (or generate them partially) for development speed. Later, you can incrementally add OpenAPI interfaces by simply changing #[handler] to #[endpoint] and adjusting function signatures.

Data Extractors

Import predefined common data extractors via use salvo::oapi::extract::*;. Extractors provide Salvo with essential information for OpenAPI documentation generation.

QueryParam<T, const REQUIRED: bool>: Extracts data from query strings. QueryParam<T, false> means the parameter is optional, while QueryParam<T, true> makes it mandatory (returns an error if missing).
HeaderParam<T, const REQUIRED: bool>: Extracts data from request headers. HeaderParam<T, false> indicates optionality; HeaderParam<T, true> enforces requirement.
CookieParam<T, const REQUIRED: bool>: Extracts data from request cookies. CookieParam<T, false> allows omission; CookieParam<T, true> requires presence.
PathParam<T>: Extracts path parameters from URLs. Absence prevents route matching, making this inherently non-optional.
FormBody<T>: Extracts information from form submissions.
JsonBody<T>: Extracts information from JSON payloads.

`#[endpoint]`

When generating OpenAPI docs, use the #[endpoint] macro instead of the regular #[handler] macro—it's an enhanced version.

It gathers OpenAPI-required information from function signatures.
For details inconvenient to provide via signatures, add attributes directly in #[endpoint]. These merge with signature-derived info, with attribute values overriding any conflicts.

You can mark a Handler as deprecated using Rust's built-in #[deprecated]. While this attribute supports reasons/versioning, OpenAPI ignores such details, only recognizing deprecation status.

Code documentation comments auto-populate OpenAPI: the first line becomes the summary, while the entire comment forms the description.

/// This is a summary of the operation
///
/// All lines of the doc comment will be included to operation description.
#[endpoint]
fn endpoint() {}

ToSchema

Define data structures with #[derive(ToSchema)]:

#[derive(ToSchema)]
struct Pet {
    id: u64,
    name: String,
}

Optional settings via #[salvo(schema(...))]:

example = ... can be json!(...). json!(...) parses to serde_json::Value via serde_json::json!.

#[derive(ToSchema)]
#[salvo(schema(example = json!({"name": "bob the cat", "id": 0})))]
struct Pet {
    id: u64,
    name: String,
}

xml(...) defines XML object properties:

#[derive(ToSchema)]
struct Pet {
    id: u64,
    #[salvo(schema(xml(name = "pet_name", prefix = "u")))]
    name: String,
}

ToParameters

Generates [path parameters][path_parameters] from struct fields.

This is the #[derive] implementation of the ToParameters trait.

Normally, path parameters need defining in the endpoint's [#[salvo_oapi::endpoint(...parameters(...))]][path_parameters]. But when using [struct][struct] for parameters, this step is unnecessary. However, descriptions or default configuration changes still require defining [primitive types][primitive] and [String][std_string] path parameters or [tuple]-style ones in parameters(...).

Mark fields as deprecated with Rust's #[deprecated], reflected in the generated OpenAPI spec.

While #[deprecated] supports reasons/versioning, OpenAPI only acknowledges boolean deprecation status. Reasons like #[deprecated = "There is better way to do this"] won't appear in OpenAPI.

Field doc comments become parameter descriptions in OpenAPI.

#[derive(salvo_oapi::ToParameters, serde::Deserialize)]
struct Query {
    /// Query todo items by name.
    name: String
}

ToParameters Container Attributes for`#[salvo(parameters(...))]`

These attributes apply to structs deriving ToParameters via #[salvo(parameters(…))]:

names(...): Defines comma-separated names for unnamed struct fields used as path parameters. Only for unnamed structs.
style = ...: Defines serialization for all parameters via [ParameterStyle][style]. Defaults based on parameter_in.
default_parameter_in = ...: Sets default parameter location from [parameter::ParameterIn][in_enum]. Defaults to query if unspecified.
rename_all = ...: Alternative to serde's rename_all with identical functionality.

Use names to name single unnamed parameters:

# use salvo_oapi::ToParameters;

#[derive(ToParameters, serde::Deserialize)]
#[salvo(parameters(names("id")))]
struct Id(u64);

Or multiple:

# use salvo_oapi::ToParameters;

#[derive(ToParameters, serde::Deserialize)]
#[salvo(parameters(names("id", "name")))]
struct IdAndName(u64, String);

ToParameters Field Attributes for`#[salvo(parameter(...))]`

Field-level attributes via #[salvo(parameter(...))]:

style = ...: Serialization via [ParameterStyle][style]. Defaults based on parameter_in.
parameter_in = ...: Parameter location from [parameter::ParameterIn][in_enum]. Defaults to query.
explode: Creates separate parameter=value pairs for each object/array item.
allow_reserved: Permits reserved characters :/?#[]@!$&'()*+,;= in values.
example = ...: Method reference or json!(...). Overrides underlying type's example.
value_type = ...: Overrides default OpenAPI field type. Useful for third-party types not implementing ToSchema or [primitive types][primitive]. Accepts any JSON-serializable Rust type or custom types like Object.
inline: Requires field type to implement ToSchema, inlining its definition.
default = ...: Method reference or json!(...).
format = ...: KnownFormat variant or open string value. Defaults derived from type per OpenAPI spec.
write_only: Restricts field to write operations (POST,PUT,PATCH).
read_only: Restricts field to read operations (GET).
nullable: Indicates if field can be null (distinct from optionality).
required = ...: Enforces mandatory parameters. See rules.
rename = ...: Alternative to serde's rename.
multiple_of = ...: Value must divide evenly by this (>0).
maximum = ...: Inclusive upper bound.
minimum = ...: Inclusive lower bound.
exclusive_maximum = ...: Exclusive upper bound.
exclusive_minimum = ...: Exclusive lower bound.
max_length = ...: Maximum string length.
min_length = ...: Minimum string length.
pattern = ...: ECMA-262 regex for valid values.
max_items = ...: Maximum array items (non-negative).
min_items = ...: Minimum array items (non-negative).
with_schema = ...: Uses function-generated schema instead of default. Function must satisfy fn() -> Into<RefOr<Schema>>.
additional_properties = ...: Defines free-form types for maps (e.g., HashMap/BTreeMap). Formats: additional_properties or additional_properties = true.

Field nullability and required rules

Rules for ToParameters field attributes mirror those for ToSchema. See rules.

Partial`#[serde(...)]` attributes support

ToParameters derivation supports select [serde attributes][serde attributes], reflected in OpenAPI docs:

rename_all = "..." (container-level)
rename = "..." (field-level only)
default (container/field-level per [serde][serde attributes])
skip_serializing_if = "..." (field-level only)
with = ... (field-level only)
skip_serializing = "..." (field/variant-level)
skip_deserializing = "..." (field/variant-level)
skip = "..." (field-level only)

Other serde attributes affect serialization but not OpenAPI generation.

Examples

Demonstrates #[salvo(parameters(...))] container attributes with ToParameters for path parameters, inlining a query field:

use serde::Deserialize;
use salvo_core::prelude::*;
use salvo_oapi::{ToParameters, ToSchema};

#[derive(Deserialize, ToSchema)]
#[serde(rename_all = "snake_case")]
enum PetKind {
    Dog,
    Cat,
}

#[derive(Deserialize, ToParameters)]
struct PetQuery {
    /// Name of pet
    name: Option<String>,
    /// Age of pet
    age: Option<i32>,
    /// Kind of pet
    #[salvo(parameter(inline))]
    kind: PetKind
}

#[salvo_oapi::endpoint(
    parameters(PetQuery),
    responses(
        (status_code = 200, description = "success response")
    )
)]
async fn get_pet(query: PetQuery) {
    // ...
}

Overriding String with i64 via value_type:

# use salvo_oapi::ToParameters;

#[derive(ToParameters, serde::Deserialize)]
#[salvo(parameters(default_parameter_in = Query))]
struct Filter {
    #[salvo(parameter(value_type = i64))]
    id: String,
}

Overriding String with Object (renders as type:object in OpenAPI):

# use salvo_oapi::ToParameters;

#[derive(ToParameters, serde::Deserialize)]
#[salvo(parameters(default_parameter_in = Query))]
struct Filter {
    #[salvo(parameter(value_type = Object))]
    id: String,
}

Override with generics:

# use salvo_oapi::ToParameters;

#[derive(ToParameters, serde::Deserialize)]
#[salvo(parameters(default_parameter_in = Query))]
struct Filter {
    #[salvo(parameter(value_type = Option<String>))]
    id: String
}

Override Vec with another Vec:

# use salvo_oapi::ToParameters;

#[derive(ToParameters, serde::Deserialize)]
#[salvo(parameters(default_parameter_in = Query))]
struct Filter {
    #[salvo(parameter(value_type = Vec<i32>))]
    id: Vec<String>
}

Override with another ToSchema:

# use salvo_oapi::{ToParameters, ToSchema};

#[derive(ToSchema)]
struct Id {
    value: i64,
}

#[derive(ToParameters, serde::Deserialize)]
#[salvo(parameters(default_parameter_in = Query))]
struct Filter {
    #[salvo(parameter(value_type = Id))]
    id: String
}

Validation examples:

#[derive(salvo_oapi::ToParameters, serde::Deserialize)]
struct Item {
    #[salvo(parameter(maximum = 10, minimum = 5, multiple_of = 2.5))]
    id: i32,
    #[salvo(parameter(max_length = 10, min_length = 5, pattern = "[a-z]*"))]
    value: String,
    #[salvo(parameter(max_items = 5, min_items = 1))]
    items: Vec<String>,
}

Manual schema implementation via schema_with:

# use salvo_oapi::schema::Object;
fn custom_type() -> Object {
    Object::new()
        .schema_type(salvo_oapi::SchemaType::String)
        .format(salvo_oapi::SchemaFormat::Custom(
            "email".to_string(),
        ))
        .description("this is the description")
}

#[derive(salvo_oapi::ToParameters, serde::Deserialize)]
#[salvo(parameters(default_parameter_in = Query))]
struct Query {
    #[salvo(parameter(schema_with = custom_type))]
    email: String,
}

rename_all = ...: Supports serde-like syntax for renaming rules. If both #[serde(rename_all = "...")] and #[salvo(schema(rename_all = "..."))] exist, #[serde(rename_all = "...")] takes precedence.
symbol = ...: String literal defining the struct's OpenAPI schema path (e.g., #[salvo(schema(symbol = "path.to.Pet"))]).
default: Uses the struct's Default impl to populate field defaults.

Error Handling Approach

For typical applications, we define a global error type (AppError), implementing Writer or Scribe to send errors as web messages to clients.

For OpenAPI, we additionally implement EndpointOutRegister to expose error details:

use salvo::http::{StatusCode, StatusError};
use salvo::oapi::{self, EndpointOutRegister, ToSchema};

impl EndpointOutRegister for Error {
    fn register(components: &mut oapi::Components, operation: &mut oapi::Operation) {
        operation.responses.insert(
            StatusCode::INTERNAL_SERVER_ERROR.as_str(),
            oapi::Response::new("Internal server error").add_content("application/json", StatusError::to_schema(components)),
        );
        operation.responses.insert(
            StatusCode::NOT_FOUND.as_str(),
            oapi::Response::new("Not found").add_content("application/json", StatusError::to_schema(components)),
        );
        operation.responses.insert(
            StatusCode::BAD_REQUEST.as_str(),
            oapi::Response::new("Bad request").add_content("application/json", StatusError::to_schema(components)),
        );
    }
}

This centralizes all possible application errors. However, individual Handlers may only return specific subsets. Use status_codes to filter relevant errors:

#[endpoint(status_codes(201, 409))]
pub async fn create_todo(new_todo: JsonBody<Todo>) -> Result<StatusCode, Error> {
    Ok(StatusCode::CREATED)
}

On this page

#OpenAPI Documentation Generation

#Data Extractors

##[endpoint]

#ToSchema

#ToParameters

#ToParameters Container Attributes for#[salvo(parameters(...))]

#ToParameters Field Attributes for#[salvo(parameter(...))]

#Field nullability and required rules

#Partial#[serde(...)] attributes support

#Examples

#Error Handling Approach