Asynction

SocketIO python framework driven by the AsyncAPI specification. Built on top of Flask-SocketIO. Inspired by Connexion.

The purpose of Asynction is to empower a specification first approach when developing SocketIO APIs in Python. It guarantees that your API will work in accordance with its documentation.

Disclaimer: Asynction is still at a beta stage. Extensive testing is recommended when using this library in production.

Features

Registers all event and error handlers that are referenced within the API specification.
Provides out of the box validation on every Socket.IO interraction. In particular:
- Event validation (for both ingress and egress events), based on the specified message schemata
- HTTP request validation, upon connection, based on the channel binding schemata of each namespace
- Callback validation, upon the ACK of a message, based on the message x-ack schemata
Security à la Connexion. Handles OAuth2 and HTTP based authentication.
Generates HTML rendered docs, similar to the AsyncAPI playground. The docs get served through the GET {base_path}/docs route of the app.
Mock server support
CLI

A complete example can be found here (includes examples of both normal and mock server implementations).

Prerequisites

Python 3.7 (or higher)

Install

$ pip install asynction

With mock server support:

$ pip install asynction[mock]

With CLI support:

$ pip install asynction[cli]

The CLI can also be installed via Homebrew:

$ brew tap dedoussis/tap
$ brew install asynction

Usage (basic example)

Example event and error handler callables located at ./my_api/handlers.py:

# /user namespace

def user_sign_up(data):
    logger.info("Signing up user...")
    emit("metrics", "signup", namespace="/admin", broadcast=True, callback=cb)

def user_log_in(data):
    logger.info("Logging in user...")
    emit("metrics", "login", namespace="/admin", broadcast=True, callback=cb)
    return True  # Ack

def user_error(e):
    logger.error("Error: %s", e)

# /admin namespace

def authenticated_connect():
    token = request.args["token"]

def admin_error(e):
    logger.error("Admin error: %s", e)

Example specification located at ./docs/asyncapi.yaml:

asyncapi: 2.3.0

info:
  title: User Account Service
  version: 1.0.0
  description: This service is in charge of processing user accounts

servers:
  production:
    url: my-company.com/api/socket.io # Customizes the `path` kwarg that is fed into the `SocketIO` constructor
    protocol: wss

channels:
  /user: # A channel is essentially a SocketIO namespace
    publish:
      message:
        oneOf: # The oneOf Messages relationship expresses the supported events that a client may emit under the `/user` namespace
          - $ref: "#/components/messages/UserSignUp"
          - $ref: "#/components/messages/UserLogIn"
    x-handlers: # Default namespace handlers (such as connect, disconnect and error)
      error: my_api.handlers.user_error # Equivelant of: `@socketio.on_error("/user")`
  /admin:
    subscribe:
      message:
        oneOf:
          - "#/components/messages/Metrics"
    x-handlers:
      connect: my_api.handlers.authenticated_connect # Equivelant of: `@socketio.on("connect", namespace="/admin")`
      error: my_api.handlers.admin_error
    bindings: # Bindings are used to validate the HTTP request upon connection
      $ref: "#/components/channelBindings/AuthenticatedWsBindings"

components:
  messages:
    UserSignUp:
      name: sign up # The SocketIO event name. Use `message` or `json` for unnamed events.
      payload: # Asynction uses payload JSON Schemata for message validation
        type: object
      x-handler: my_api.handlers.user_sign_up # The handler that is to be registered. Equivelant of: `@socketio.on("sign up", namespace="/user")`
    UserLogIn:
      name: log in
      payload:
        type: object
      x-handler: my_api.handlers.user_log_in
      x-ack: # Specifies the structure of the ACK data that the client should expect
        args:
          type: boolean
    Metrics:
      name: metrics
      payload:
        type: string
        enum: [signup, login]
      x-ack: # Specifies the structure of the ACK data that the server expects
        args:
          type: string

  channelBindings:
    AuthenticatedWsBindings:
      ws:
        query:
          type: object
          properties:
            token:
              type: string
          required: [token]

Bootstrap the AsynctionSocketIO server:

from asynction import AsynctionSocketIO
from flask import Flask

flask_app = Flask(__name__)

asio = AsynctionSocketIO.from_spec(
    spec_path="./docs/asyncapi.yaml",
    app=flask_app,
    message_queue="redis://localhost:6379",
    # or any other kwarg that the flask_socketio.SocketIO constructor accepts
)

if __name__ == "__main__":
    asio.run(app=flask_app)

The AsynctionSocketIO class extends the SocketIO class of the Flask-SocketIO library.
The above asio server object has all the event and error handlers registered, and is ready to run.
Validation of the message payloads, the channel bindings and the ack callbacks is also enabled by default.
Without Asynction, one would need to add additional boilerplate to register the handlers (as shown here) and implement the respective validators.

Security (Authentication and Authorization)

Asynction supports authentication of incoming connections through the security mechanisms specified in the AsyncAPI spec of an application. See this guide on how to add security as part of an API specification. To take advantage of this feature, a security handler callable should be attached to each security scheme definition under the components section. To attach a security handler(s), see the security specification extention section below.

The security handler callable(s) will be called upon every new client connection and MUST return a SecurityInfo typed dictionary (which allows extra keys). Asynction then validates this returned dictionary, refusing the connection to any unauthenticated/unauthorised requests. Finally, the validated SecurityInfo dictionary is passed to the connection handler as an extra token_info kwarg, to allow further/custom processing if needed.

Docs

API documentation is autogenerated by Asynction and served through the following routes of the app:

{base_path}/docs: Rendered HTML docs similar to the AsyncAPI playground.
{base_path}/docs/asyncapi.json: The raw specification data exposed for programmatic retrieval.

The base_path is determined automagically through the Socket.IO path argument. It essentially is the parent of that path. For example:

Socket.IO path	Base path	Docs path
`socket.io` (default)	`/`	`/docs`
`events/socket.io`	`/events`	`/events/docs`

Docs can be disabled by toggling the docs kwarg of the AsynctionSocketIO.from_spec factory method.

Emitting from an external process

All validation features of Asynction can be used when emitting/sending events from an external process. See the relevant Flask-SocketIO documentation. Note that the SocketIO instance of the external process needs to be constructed using the same AsynctionSocketIO.from_spec factory.

Mock server

Asynction can also create a fake "mock" based off an AsyncAPI document. This enables the consumers of a SocketIO API to interract with the API before it's even built.

from asynction import MockAsynctionSocketIO
from flask import Flask

flask_app = Flask(__name__)

mock_asio = MockAsynctionSocketIO.from_spec(
    spec_path="./docs/asyncapi.yaml",
    app=flask_app,
)

if __name__ == "__main__":
    mock_asio.run(app=flask_app)

The mock server:

Listens for all events defined in the given spec, returning fake acknowledgements where applicable.
Periodically emits events containing payloads of fake data, for the clients to listen on.

The fake data generation is fueled by Faker and Hypothesis, hence the use of the mock server functionality requires the installation of extra dependecies: pip install asynction[mock]

To make the fake generated data more realistic, one may attach faker providers to the string schemata of their spec using the format keyword of JSON Schema:

# example of a Message object
NewMessageReceived:
  name: new message
  payload:
    type: object
    properties:
      username:
        type: string
        format: first_name
      message:
        type: string
        format: sentence
    required: [username, message]

The formats supported are essentially all the faker providers that yield a string value.

CLI

For convenience, Asynction provides a command-line interface (CLI) that aims to be a toolbox of useful utilities for the development, testing and mocking of Asynction apps (ie any Socket.IO app driven by an AsyncAPI doc). For example, it allows one to run a "mock" instance of their Socket.IO server, only by passing the AsyncAPI YAML file, without even having to start the development of the server itself.

All commands support the –-help (or -h) argument to display additional information.

Available commands

mock run

$ asynction --spec ./docs/asyncapi.yml mock run --port 5001 --debugger
* Restarting with stat
* Debugger is active!
* Debugger PIN: 339-844-897
(71320) wsgi starting up on http://0.0.0.0:5001
...

scaffold (coming soon)

$ asynction --spec ./docs/asyncapi.yml scaffold
✨ Successfully generated app.py

Dockerised

The CLI can be installed via pip or Homebrew (see the install section) but is also available through docker, negating the need for a local python environment:

$ docker run -v ${PWD}/docs/asyncapi.yml:/opt/asynction/asyncapi.yml dedoussis/asynction mock run --debugger
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 339-844-897
(71320) wsgi starting up on http://0.0.0.0:5000
...

Further resources

Specification Extentions

Asynction has extended the AsyncAPI 2.x.x specification to provide support for coupling SocketIO semantical entities (such as namespaces, events and acks) to python objects (such as handler callabes or other flask_socketio.SocketIO methods). Some of the extentions below are necessary to express the Socket.IO protocol semantics, while others are solely needed for the programmatic purposes of Asynction. The extentions introduced adhere to the Specification Extention guidelines of the AsyncAPI spec.

For further guidance on how to generally express a SocketIO API using AsyncAPI, refer to this article: https://dedouss.is/posts/2021-07-14-documenting-socketio-part-2.html

Event handler

The x-handler field MAY be defined as an additional property of the Message Object. The value of this field MUST be of string type, expressing a dot joint path to a python callable (the event handler).

Message Objects listed under a subscribe operation MUST include the x-handler field.
Message Objects listed under a publish operation SHOULD NOT include the x-handler field.

Default namespace handlers

The x-handlers field MAY be defined as an additional property of the Channel Item Object. The value of this field SHOULD be a Channel Handlers Object.

Channel Handlers Object

Field Name	Type	Description
connect	`string`	Dot joint path to the python connect handler callable
disconnect	`string`	Dot joint path to the python disconnect handler callable
error	`string`	Dot joint path to the python error handler callable

ACK packet

The basic unit of information in the Socket.IO protocol is the packet. There are 7 distinct packet types. The publish and subscribe Message Objects expressed in the A2S YAML above correspond to the EVENT and BINARY_EVENT packet types. These are essentially the packets that are transmitted when the Socket.IO sender invokes the emit or send API functions of the Socket.IO library (regardless of implementation). In turn, the Socket.IO event receiver handles the received event using the on API function of the Socket.IO library. As part of the on handler, the receiver may choose to return an acknowledgement of the received message. This acknowledgement is conveyed back to the transmitter via the ACK and BINARY_ACK packet types. This ack data is passed as input into the callback that the message transmitter has provided through the emit/send invocation.

In order to express the above acknowledgement semantics, the A2S specification needs to be extended as follows:

Message Objects MAY include the x-ack field. The value of this field SHOULD be a Message Ack Object.
Components Object MAY include the x-messageAcks field. The value of this field should be of type: Map[string, Message Ack Object | Reference Object]

Although Asynction uses these fields to validate the input args of the callback functions, these ACK extentions are necessary to express semantics of the Socket.IO protocol, regardless of any tooling used for automation / code generation.

Message Ack Object

Field Name	Type	Description
args	Schema Object	Schema of the arguments that are passed as input to the acknowledgement callback function. In the case of multiple arguments, use the `array` type to express the tuple.

In the future, the Message Ack Object may be extended with extra fields to enable additional documentation of the callback.

Security handers

In order to support the AuthN/AuthZ functionality of asynction, the Security Scheme Object needs to be extended as follows:

A Security Scheme Object of oauth2 type MUST include the x-tokenInfoFunc field.
A Security Scheme Object of oauth2 type MAY include the x-scopeValidateFunc field.
A Security Scheme Object of basic http type MUST include the x-basicInfoFunc field.
A Security Scheme Object of bearer http type MUST include the x-basicBearerInfoFunc field.
A Security Scheme Object of bearer apiKey type MUST include the x-apiKeyInfoFunc field.

The value of all these fields MUST be of string type, expressing a dot joint path to a python callable (the security handler).

Per Namespace Security

The x-security field MAY be defined as an additional property of the Channel Item Object. The value of this field MUST be an array of Security Requirement Objects which is the same format used to specify Server Security Requirements. If a namespace specifies x-security the security requirements specified for that namespace will overwrite any security requirements specified in the Server Object.

dedoussis / asynction

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