SAFRS: Python OpenAPI & JSON:API Framework

• Overview
• Installation
• JSON:API Interface
• Resource Objects
• Relationships
• Methods
  • Custom Methods
  • Class Methods
• Initialization
• HTTP Status Codes
• Security
• Endpoint Naming
• Configuration
• Exposing Existing Databases
• More Examples and Use Cases
• Advanced Usage
  • Custom Serialization
  • Custom Decorators
  • API Methods
  • Custom Swagger
• Limitations & TODOs
• References
• Thanks

Overview

SAFRS is an acronym for SqlAlchemy Flask-Restful Swagger. The purpose of this framework is to help python developers create a self-documenting JSON API for sqlalchemy database objects and relationships. These objects can be serialized to JSON and can be created, retrieved, updated and deleted through the JSON API. Optionally, custom resource object methods can be exposed and invoked using JSON. Class and method descriptions and examples can be provided in yaml syntax in the code comments. The description is parsed and shown in the swagger web interface.

The result is an easy-to-use swagger/OpenAPI and JSON:API compliant API implementation.

A LIVE DEMO is available, implementing the relationship example.

Installation

SAFRS can be installed as a pip package or by downloading the latest version from github, for example:

git clone https://github.com/thomaxxl/safrs
cd safrs
pip3 install -r requirements.txt --user
python3 setup.py install --user

The examples can then be started with

python3 examples/demo_relationship.py "your-interface-ip"

JSON:API Interface

Exposed resource objects can be queried using the JSON:API format. The API supports following HTTP operations:

• GET : Retrieve an object or a list of objects
• PATCH : Update an object.
• DELETE: Delete an object.
• POST : Create an object.

Please check the JSON:API spec for more implementation details. You can also check the interface in the live demo.

Resource Objects

Database objects are implemented as subclasses of the SAFRSBase and SQLAlchemy model classes. The SQLAlchemy columns are serialized to JSON when the corresponding REST API is invoked.

Following example from demo.py illustrates how the API is built and documented:

class User(SAFRSBase, db.Model):
    '''
        description: User description
    '''
    __tablename__ = 'Users'
    id = Column(String, primary_key=True)
    name = Column(String, default='')
    email = Column(String, default='')

The User class is implemented as a subclass of

• db.Model: SQLAlchemy base
• SAFRSBase: Implements JSON serialization for the object and generates (swagger) API documentation

This User object is then exposed through the web interface using the Api object

api.expose_object(User)

The User object REST methods are available on /User, the swagger schema is available on /api/swagger.json and the UI is available on /api/:

Relationships

Database object such as the User class from the demo.py example can be extended to include relationships with other objects. The demo_relationship.py contains following extension of the User class where a relationship with the Book class is implemented:

class User(SAFRSBase, db.Model):
    '''
        description: User description
    '''
    __tablename__ = 'Users'
    id = db.Column(db.String, primary_key=True)
    name = db.Column(db.String, default='')
    email = db.Column(db.String, default='')
    books = db.relationship('Book', back_populates="user")
...

A many-to-one database association is declared by the back_populates relationship argument. The Book class is simply another subclass of SAFRSBase and db.Model, similar to the previous User class:

class Book(SAFRSBase, db.Model):
    '''
        description: Book description
    '''
    __tablename__ = 'Books'
    id = db.Column(db.String, primary_key=True)
    name = db.Column(db.String, default='')
    user_id = db.Column(db.String, db.ForeignKey('Users.id'))
    user = db.relationship('User', back_populates='books')

The User.book relationship can be queried in the API through the following endpoints:

• POST adds an item to the relationship
• DELETE removes an item from the relationship
• GET retrieves a list of item ids

The relationship REST API works similarly for one-to-many relationships.

Methods

Custom Methods

Safrs allows the user to implement custom methods on the exposed objects. This methods can be invoked through the json API by sending an HTTP POST request to the method endpoint The following example implements a "send_mail" method fro example:

class User(SAFRSBase, db.Model):
    '''
        description: User description
    '''
    __tablename__ = 'Users'
    id = Column(String, primary_key=True)
    name = Column(String, default='')
    email = Column(String, default='')

    # Following method is exposed through the REST API 
    # This means it can be invoked with a HTTP POST
    @jsonapi_rpc(http_methods=['POST','GET'])
    def send_mail(self, email):
        '''
            description : Send an email
            args:
                email:
                    type : string 
                    example : test email
        '''
        content = 'Mail to {} : {}\n'.format(self.name, email)
        return { 'result' : 'sent {}'.format(content)}

This method shows up in the swagger interface:

The send_mail method is documented with the jsonapi_rpc decorator. This decorator generates a schema based on the function documentation. This documentation contains yaml specification of the API which is used by the swagger UI.

The yaml specification has to be in the first part of the function and class comments. These parts are delimited by four dashes ("----") . The rest of the comment may contain additional documentation.

Class Methods

Two class-level methods have been defined to facilitate object retrieval:

• lookup : retrieve a list of objects that match the argument list. For example, following HTTP POST request to a container will retrieve a list of itemswhere the name is "thomas"

{
  "method": "lookup",
  "args": {
    "name": "thomas"
  }
}

• get_list : retrieve a list of the items with the specified ID's

Application Initialization

The API can be initialized like this:

api = SAFRSAPI(app, host=HOST, port=PORT, prefix=API_PREFIX)

Then you can expose objects with

api.expose_object(User)    

An example that uses the flask app factory pattern is implement in examples/mini_app.py

HTTP Status Codes

HTTP status codes are used to signal success or failure of a REST operation:

• 200 : OK
• 201 : The request has been fulfilled and resulted in a new resource being created.
• 204 : No Content, DELETE operation was successful
• 400 : The services raised an exception, for example in case of invalid input
• 500 : Internal Server Error

In case of errors( status codes 400+ ), the log file contains a stacktrace. Two custom exceptions are defined in errors.py: ValidationError and GenericError. In case of errors, the webservice will return a default HTTP status code 500 and a customizable error message, for example

{
  "error": "Failed to execute query Entity '<class 'C2_server.Image'>' has no property 'namex'"
}

Security

Authorization

Authorization can be implemented by applying decorators to the endpoints generated by the expose_objects method. Examples for flask_login and flask_jwt_extended can be found in the examples/authentication directory.

Logging

The default logging is configured to not show verbose error messages, so not to reveal information of the backend to the user, which may also mitigate script injection attacks in case the log messages are reflected in the user output. In case you do want verbose logging, you can set the log level to DEBUG:

app.config.update(DEBUG=True)

Endpoint Naming

As can be seen in the swagger UI:

• the endpoint collection path names are the SQLAlchemy __tablename__ properties (e.g. /Users )
• the parameter names are derived from the SAFRSBase class names (e.g. {UserId} )
• the the relationship names are the SAFRSBase class relationship names (e.g /books ) The URL path format is configurable

Configuration

Some configuration parameters can be set in config.py:

• USE_API_METHODS: set this to false in case you want to disable the jsonapi_rpc functionality
• INSTANCE_URL_FMT: This parameter declares the instance url path format
• RELATIONSHIP_URL_FMT: This parameter declares the relationship endpoint path format

Exposing Existing Databases

Safrs allows you to Expose existing databases as jsona:api services with the expose_existing.py script, for example:

python3 expose_existing.py mysql+pymysql://root:password@localhost/sakila --host localhost

This script uses sqlacodegen to generate a source file containing the SQLAlchemy and SAFRSBase database models and starts the API webservice.

More details here

More Examples and Use Cases

The examples folder contains more example scripts:

• Using a sha hash as primary key (id)
• CORS usage
• Flask-Admin integration example, eg.:

Advanced Usage

Custom Serialization

Serialization and deserialization are implemented by the SAFRSBase to_dict and __init__ : you can extend these methods as usual. For example, if you would like to add some attributes to the json payload of the User object, you can override the to_dict method:

class User(SAFRSBase, db.Model):
    '''
        description: User description
    '''
    __tablename__ = 'Users'
    id = db.Column(db.String, primary_key=True)
    name = db.Column(db.String, default='')
    email = db.Column(db.String, default='')
    books = db.relationship('Book', back_populates="user")

    def to_dict(self):
        result = SAFRSBase.to_dict(self)
        result['custom_field'] = 'custom'
        return result

This will add the custom_field attribute to the result attributes:

"attributes": {
    "custom_field": "custom",
    "email": "reader_email0",
    "name": "Reader 0"
}

Custom Decorators

The custom_decorators class attribute list can be used to add custom decorators to the instance HTTP endpoint. An example of this functionality is implemented in the authentication examples.

API Methods

Some additional API RPC methods are implemented in api_methods.py, e.g. mysql regex search.

Custom swagger

The swagger schema can be merged with a modified schema dictionary by supplying the to-be-merged dictionary as the custom_swagger argument to SAFRSAPI, e.g.

custom_swagger = {"info": {"title" : "New Title" }} # Customized swagger title will be merged
api = SAFRSAPI(app, host=swagger_host, port=PORT, prefix=OAS_PREFIX, api_spec_url=OAS_PREFIX+'/swagger',
               custom_swagger=custom_swagger, schemes=['http', 'https'], description=description)

Limitations & TODOs

This code was developed for a specific use-case and may not be flexible enough for everyone's needs. A lot of the functionality is available but not documented for the sake of brevity. Performance is reasonable for regular databases, but once you start exposing really big tables you may run into problems, for example: the count() for mysql innodb is slow on large(1M rows) tables, a workaround can be implemented by querying the sys tables or using werkzeug caching. Feel free to open an issue or drop me an email if you run into problems or something isn't clear!

References

• JSON:API specification
• OpenApi (Swagger)
• Flask
• SQLAlchemy

Thanks

I developed this code when I worked at Excellium Services. They allowed me to open source it when I stopped working there.