Host OpenAPI v2 spec through actix-web

With actix feature enabled, paperclip exports an experimental plugin for actix-web framework to host OpenAPI v2 spec for your APIs automatically. While it's not feature complete, you can rely on it to not break your actix-web flow.

Let's start with a simple actix-web application. It has actix-web and serde for JSON'ifying your APIs. Let's also add paperclip with actix feature.

# [package] ignored for brevity

[dependencies]
# actix-web 2.0 is supported through "actix2" and "actix2-nightly" features
actix-web = "3.0"
# The "actix-nightly" feature can be specified if you're using nightly compiler. Even though
# this plugin works smoothly with the nightly compiler, it also works in stable
# channel (replace "actix-nightly" feature with "actix" in that case). There maybe compilation errors,
# but those can be fixed.
paperclip = { version = "0.4", features = ["actix-nightly"] }
serde = "1.0"

Our main.rs looks like this:

use actix_web::{App, HttpServer, web::{self, Json}};
use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize)]
struct Pet {
    name: String,
    id: Option<i64>,
}

async fn echo_pet(body: Json<Pet>) -> Result<Json<Pet>, ()> {
    Ok(body)
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    HttpServer::new(|| App::new()
        .service(
            web::resource("/pets")
                .route(web::post().to(echo_pet))
        )
    ).bind("127.0.0.1:8080")?
    .run().await
}

Now, let's modify it to use the plugin!

use actix_web::{App, HttpServer};
use paperclip::actix::{
    // extension trait for actix_web::App and proc-macro attributes
    OpenApiExt, Apiv2Schema, api_v2_operation,
    // use this instead of actix_web::web
    web::{self, Json},
};
use serde::{Serialize, Deserialize};

// Mark containers (body, query, parameter, etc.) like so...
#[derive(Serialize, Deserialize, Apiv2Schema)]
struct Pet {
    name: String,
    id: Option<i64>,
}

// Mark operations like so...
#[api_v2_operation]
async fn echo_pet(body: Json<Pet>) -> Result<Json<Pet>, ()> {
    Ok(body)
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    HttpServer::new(|| App::new()
        // Record services and routes from this line.
        .wrap_api()
        // Add routes like you normally do...
        .service(
            web::resource("/pets")
                .route(web::post().to(echo_pet))
        )
        // Mount the JSON spec at this path.
        .with_json_spec_at("/api/spec")

        // ... or if you wish to build the spec by yourself...

        // .with_raw_json_spec(|app, spec| {
        //     app.route("/api/spec", web::get().to(move || {
        //         actix_web::HttpResponse::Ok().json(&spec)
        //     }))
        // })

        // IMPORTANT: Build the app!
        .build()
    ).bind("127.0.0.1:8080")?
    .run().await
}

We have:

• Imported OpenApiExt extension trait for actix_web::App along with Apiv2Schema derive macro and api_v2_operation proc macro attributes.
• Switched from actix_web::web to paperclip::actix::web.
• Marked our Pet struct and add_pet function as OpenAPI-compatible schema and operation using proc macro attributes.
• Transformed our actix_web::App to a wrapper using .wrap_api().
• Mounted the JSON spec at a relative path using .with_json_spec_at("/api/spec").
• Built (using .build()) and passed the original App back to actix-web.

Note that we never touched the service, resources, routes or anything else! This means that our original actix-web flow is unchanged.

Now you can check the API with the following cURL command:

curl -X POST http://localhost:8080/pets -H "Content-Type: application/json" -d '{"id":1,"name":"Felix"}'

And see the specs with this:

curl http://localhost:8080/api/spec

... we get the swagger spec as a JSON!

// NOTE: Formatted for clarity
{
  "swagger": "2.0",
  "definitions": {
    "Pet": {
      "type": "object",
      "properties": {
        "id": {
          "type": "integer",
          "format": "int64"
        },
        "name": {
          "type": "string"
        }
      },
      "required": ["name"]
    }
  },
  "paths": {
    "/pets": {
      "post": {
        "responses": {
          "200": {
            "description": "OK",
            "schema": {
              "$ref": "#/definitions/Pet"
            }
          }
        }
      },
      "parameters": [{
        "in": "body",
        "name": "body",
        "required": true,
        "schema": {
          "$ref": "#/definitions/Pet"
        }
      }]
    }
  },
  "info": {
    "version": "",
    "title": ""
  }
}

Similarly, if we were to use other extractors like web::Query<T>, web::Form<T> or web::Path, the plugin will emit the corresponding specification as expected.

Known limitations

• Enums: OpenAPI (v2) itself supports using simple enums (i.e., with unit variants), but Rust and serde has support for variants with fields and tuples. I still haven't looked deep enough either to say whether this can/cannot be done in OpenAPI or find an elegant way to represent this in OpenAPI.
• Functions returning abstractions: The plugin has no way to obtain any useful information from functions returning abstractions such as HttpResponse, impl Responder or containers such as Result<T, E> containing those abstractions. So currently, the plugin silently ignores these types, which results in an empty value in your hosted specification.

Missing features

At the time of this writing, this plugin didn't support a few OpenAPI v2 features:

Performance implications?

Even though we use some wrappers and generate schema structs for building the spec, we do this only once i.e., until the .build() function call. At runtime, it's basically a pointer read, which is quite fast!

We also add wrappers to blocks in functions tagged with #[api_v2_operation], but those wrappers follow the Newtype pattern and the code eventually gets optimized away anyway.