Microservices in Flask

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I recently gave a talk about microservices in Flask on Wrocpy. This blog post is a translation of that talk into English.

Monolithic application

Microservices talk usually starts with a monolithic application. In my case, it is the same. I work on a project where I have a big monolithic application. If I wanted to take some part of it and make some microservice from for instance django app it would be impossible. There are too many internal imports from every part of application:

from app.users.models import UserSiteProfile
from app.utils.cache import cache_key_user
from app.sites.models import SiteProfile
from app.sites.utils import site_terms
from app.utils.users import get_homepage_url
from app.utils.views import ThemedFormView, ThemedTemplateView
from app.authentication import signals
from app.authentication.forms import AuthForm, EmailForm
from app.authentication.idp import (
    is_valid_idp, MultipleIDPError, saml_available,
    site_idp, user_idp_lookup)
from app.authentication.loginchecks import (
    check_account_expiration, get_site_login_errors,
    pre_login_checks
)
from app.saml.utils import site_specific_config_loader
from app.saml.views import _get_subject_id

Snippet from above presents exemplary imports of random python module in my project.

So where are these microservices? In my team, we decided to have new parts of the application made as a microservices. So right now from the architectural point of view I have a big monolithic application and small microservices that are around this big thing, like in this picture:

Flask microservices

We choose the flask as a tool that will be used in our microservices. It doesn’t set any boundaries and it’s flexible but don’t have batteries included. Let’s start with the first flask extension that helps us building microservices:

Flask-Script

Django has a set of powerful commands available. To enable such a feature in flask you need Flask-Script. It allows you to create commands such as runserver or shell. In listing below I created a runserver command:

from flask.ext.script import Manager, Shell, Server
from my_app.application import app

manager = Manager(app)
manager.add_command(
    'runserver',
    Server(host='0.0.0.0', port=7000, use_debugger=True)
)

Flask-RESTful

To communicate between microservices I use REST. To ease yourself when creating resources and endpoints you can use Flask-RESTful. It is superuseful when you need to create REST API. It is simple - you specify endpoint with resource and rest is done by Flask-RESTful. It also has request parsing and it is very simple to create other representations like xml. The snippet below shows it:

from flask_restful import Resource

class MyResource(Resource):
    def get():
        return {'OK'}

@api.representation('application/xml')
def output_xml(data, code, headers=None):
    response = make_response(dicttoxml.dicttoxml(data), code)
    inject_headers(response, headers)
    return response

Marshallow & flask-marshallow

To serialize or deserialize objects you can use flask-marshallow. In code below, I created a simple Schema with id, name and url. Then when the user enters /api/custom/1 I can easily serialize objects from a database and return JSON.

ma = Marshmallow(app)

class CustomSchema(ma.Schema):
    id = ma.Int(dump_only=True)
    name = ma.Str(required=True)
    url = ma.Url(relative=True)


    @app.route('/api/custom/<id>')
    def users():
        all_obj = Object.all()
        result = object_schema.dump(all_obj)
        return jsonify(result.data)

Flask-SQLAlchemy

Most of the modern frameworks have support for ORM- the same is with Flask. There is an extension called Flask-SQLAchemy that adds support for SQLAlchemy. Take this snippet for instance:

from flask.ext.sqlalchemy import SQLAlchemy
db = SQLAlchemy(app)

class MyModel(db.Model):
    GROUP = 'group'
    USER = 'user'
    TYPES = [
        (GROUP, GROUP),
        (USER, USER),
    ]

    __tablename__ = 'my_model'
    id = db.Column(db.Integer, primary_key=True)
    model_type = db.Column(ChoiceType(TYPES))
    model_value_id = db.Column(db.Integer, db.ForeignKey('model_value.id'))
    value = db.Column(db.String(1024))

    def __init__(self, model_type, model_value_id, value):
        self.share_type = share_type
        self.rule_value_id = rule_value_id
        self.value = value

I created MyModel class that will be translated to the table in a database. I also add columns to that table like model_type, model_value_id or value.

Flask SQLAlchemy is layer sitting on top of SQLAlchemy so you can use all advantages of ORM like having queries written in python.

Flask-Migrate

When your database keeps getting larger there is a need for database migrations to make sure that everyone has the same database structure. To accomplish that we use Flask-Migrate. It is using Alembic under the hood so you have to make sure that adjust your migration files after generation. Example migration file can look as follows:

def upgrade():
    op.create_table(
        'my_model',
        sa.Column('id', sa.Integer(), nullable=False),
        sa.Column(
            'model_type',
            sqlalchemy_utils.types.choice.ChoiceType(TYPES),
            nullable=True
        ),
        sa.PrimaryKeyConstraint('id'),
        sa.Column('value', sa.String(length=1024), nullable=True),
    )
    op.create_table(
        'my_model_values',
        sa.Column('id', sa.Integer(), nullable=False),
        sa.Column('model_value_id', sa.Integer(), nullable=True),
        sa.Column('value', sa.String(length=1024), nullable=True),
        sa.ForeignKeyConstraint(['model_value_id'], ['my_model.id']),
        sa.PrimaryKeyConstraint('id')
    )

In snipped above I created two tables: my_model and my_model_values with respective columns. Also my_model_values has ForeignKey relation to my_model by their ids.

Testing

During the development of microservices, we write unit tests as well as integration ones. Testing how well microservices behave with each other can be tricky: we mock whole external services. Because of that, we need to keep them up to date with real microservices. Nature of this system causes some difficulties while an error occurs: I got an error from external microservice in most cases with a form of HTTP status code and a small message in JSON or XML. Then I need to debug not only my microservice but also external one.

Deployment

After testing is done we deploy microservice using few tools:

Puppet

We use puppet for managing and provisioning our microservices. Especially we use an R10k module for puppet: gtihub link.

Cookiecutter

To make sure that every microservice has the same structure we also use cookiecutter. Thanks to that puppet knows that config file is always in this location or there will be logs stored there. Example microservice structure will look as follows:

└── flask_microservice
    ├── AUTHORS.rst
    ├── debian
    ├── docs
    │   ├── make.bat
    │   ├── Makefile
    │   └── source
    │       ├── authors.rst
    │       ├── conf.py
    │       ├── contributing.rst
    │       ├── history.rst
    │       ├── index.rst
    │       ├── readme.rst
    │       ├── technical_details.rst
    │       └── usage.rst
    |── HISTORY.rst
    ├── MANIFEST.in
    ├── README.rst
        ├── requirements.txt
        ├── setup.cfg
        ├── setup.py
        ├── src
        │   ├── flask_microservice
        │   │   ├── application.py
        │   │   ├── default_config.ini
        │   │   ├── __init__.py
        │   │   └── manage.py
        │   └── tests
        │       ├── conftest.py
        │       └── test_flask_microservice.py
        └── tox.ini

Dh-virtualenv

To distribute packages we use dh-virtualenv. This is taking python virtual enviroments and packing it to deb packages. So to have new code released we run jenkins job to create new deb.

That’s all for today! The slides from this presentation can be found here: presentation. Do you also use flask to build microservices?

Special thanks to Kasia for being editor for this post. Thank you.