Before you start

Make sure that you can compile and run core tests as described at Configure, Build and Install.

Take a look at the Writing your first HTTP server and make sure that you do realize the basic concepts.

Step by step guide

It is a common task to upload an image or even work with multiple HTML form parameters at once.

An OpenAPI scheme for such form could look like:

# yaml
requestBody:
  content: 
    multipart/form-data: # Media type
      schema:            # Request payload
        type: object
        properties:      # Request parts
          address:       # Part1 (object)
            type: object
            properties:
              street:
                type: string
              city:
                type: string
          profileImage:  # Part 2 (an image)
            type: string
            format: binary

Here we have multipart/form-data request with a JSON object and a file.

Let's implement a /v1/multipart handler for that scheme and do something with each of the request parts!

HTTP handler component

Just like in the Writing your first HTTP server the handler itself is a component inherited from server::handlers::HttpHandlerBase:

class Multipart final : public server::handlers::HttpHandlerBase {
public:
    // `kName` is used as the component name in static config
    static constexpr std::string_view kName = "handler-multipart-sample";
 
    // Component is valid after construction and is able to accept requests
    using HttpHandlerBase::HttpHandlerBase;
 
    std::string HandleRequestThrow(const server::http::HttpRequest& req, server::request::RequestContext&)
        const override;
};

The primary functionality of the handler should be located in HandleRequestThrow function. To work with the multipart/form-data parameters use the appropriate server::http::HttpRequest functions:

std::string Multipart::HandleRequestThrow(const server::http::HttpRequest& req, server::request::RequestContext&)
    const {
    const auto content_type = http::ContentType(req.GetHeader(http::headers::kContentType));
    if (content_type != "multipart/form-data") {
        req.GetHttpResponse().SetStatus(server::http::HttpStatus::kBadRequest);
        return "Expected 'multipart/form-data' content type";
    }
 
    const auto& image = req.GetFormDataArg("profileImage");
    static constexpr std::string_view kPngMagicBytes = "\x89PNG\r\n\x1a\n";
    if (!utils::text::StartsWith(image.value, kPngMagicBytes)) {
        req.GetHttpResponse().SetStatus(server::http::HttpStatus::kBadRequest);
        return "Expecting PNG image format";
    }
 
    req.GetHttpResponse().SetContentType(http::content_type::kApplicationJson);
    const auto& address = req.GetFormDataArg("address");
    auto json_addr = formats::json::FromString(address.value);
 
    return fmt::format("city={} image_size={}", json_addr["city"].As<std::string>(), image.value.size());
}

Note the work with the image in the above snippet. The image has a binary representation that require no additional decoding. The bytes of a received image match the image bytes on hard-drive.

JSON data is received as a string. FromString function converts it to DOM representation.

Warning: Handle* functions are invoked concurrently on the same instance of the handler class. Use synchronization primitives or do not modify shared data in Handle*.

Static config

Now we have to configure the service by providing task_processors and default_task_processor options for the components::ManagerControllerComponent and configuring each component in components section:

# yaml
components_manager:
    task_processors:                  # Task processor is an executor for coroutine tasks
        main-task-processor:          # Make a task processor for CPU-bound coroutine tasks.
            worker_threads: 4         # Process tasks in 4 threads.
 
        fs-task-processor:            # Make a separate task processor for filesystem bound tasks.
            worker_threads: 1
 
    default_task_processor: main-task-processor  # Task processor in which components start.
 
    components:                       # Configuring components that were registered via component_list
        server:
            listener:                 # configuring the main listening socket...
                port: 8080            # ...to listen on this port and...
                task_processor: main-task-processor    # ...process incoming requests on this task processor.
        logging:
            fs-task-processor: fs-task-processor
            loggers:
                default:
                    file_path: '@stderr'
                    level: debug
                    overflow_behavior: discard  # Drop logs if the system is too busy to write them down.
 
        handler-multipart-sample:
            path: /v1/multipart
            method: POST
            task_processor: main-task-processor

Note that all the components and handlers have their static options additionally described in docs.

int main()

Finally, we add our component to the components::MinimalServerComponentList(), and start the server with static configuration file passed from command line.

int main(int argc, char* argv[]) {
    const auto component_list = components::MinimalServerComponentList().Append<samples::Multipart>();
    return utils::DaemonMain(argc, argv, component_list);
}

Build and Run

To build the sample, execute the following build steps at the userver root directory:

mkdir build_release
cd build_release
cmake -DCMAKE_BUILD_TYPE=Release ..
make userver-samples-multipart_service

The sample could be started by running make start-userver-samples-hello_service. The command would invoke testsuite start target that sets proper paths in the configuration files and starts the service.

To start the service manually run ./samples/multipart_service/userver-samples-multipart_service -c </path/to/static_config.yaml>.

Note: Without file path to static_config.yaml userver-samples-multipart_service will look for a file with name config_dev.yaml; CMake doesn't copy static_config.yaml files from samples directory into build directory.

Now you can send a request to your server from another terminal:

bash
$ curl -v -F address='{"street": "3, Garden St", "city": "Hillsbery, UT"}' \
          -F "profileImage=@../scripts/docs/logo_in_circle.png" \
          http://localhost:8080/v1/multipart
*   Trying 127.0.0.1:8080...
* Connected to localhost (127.0.0.1) port 8080 (#0)
> POST /v1/multipart HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.81.0
> Accept: */*
> Content-Length: 10651
> Content-Type: multipart/form-data; boundary=------------------------048363632fdb9acc
> 
* We are completely uploaded and fine
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Date: Tue, 24 Oct 2023 09:31:42 UTC
< Content-Type: application/octet-stream
< Server: userver/2.0 (20231024091132; rv:unknown)
< X-YaRequestId: f7cb383a987248179e5683713b141cea
< X-YaTraceId: 7976203e08074091b20b08738cb7fadc
< X-YaSpanId: 29233f54bc7e5009
< Accept-Encoding: gzip, identity
< Connection: keep-alive
< Content-Length: 76
< 
* Connection #0 to host localhost left intact
city=Hillsbery, UT image_size=10173

Functional testing

Functional tests for the service could be implemented using the service_client fixture from pytest_userver.plugins.core in the following way:

import json
 
import aiohttp
 
 
async def test_ok(service_client, load_binary):
    form_data = aiohttp.FormData()
 
    # Uploading file
    image = load_binary('logo_in_circle.png')
    form_data.add_field('profileImage', image, filename='logo_in_circle.png')
 
    # Adding JSON payload
    address = {'street': '3, Garden St', 'city': 'Hillsbery, UT'}
    form_data.add_field(
        'address', json.dumps(address), content_type='application/json',
    )
 
    # Making a request and checking the result
    response = await service_client.post('/v1/multipart', data=form_data)
    assert response.status == 200
    assert 'application/json' in response.headers['Content-Type']
    assert response.text == f'city={address["city"]} image_size={len(image)}'

Do not forget to add the plugin in conftest.py:

 
pytest_plugins = ['pytest_userver.plugins.core']
 

Full sources

See the full example at:

⇦ Writing your WebSocket service | S3 client tutorial ⇨