Application architecture

Invenio is at the core an application built on-top of the Flask web development framework, and fully understanding Invenio’s architectual design requires you to understand core concepts from Flask which will briefly be covered here.

The Flask application is exposed via different application interfaces depending on if the application is running in a webserver, CLI or job queue.

Invenio adds a powerful application factory on top of Flask, which takes care of dynamically assembling an Invenio application from the many individual modules that makes up Invenio, and which also allow you to easily extend Invenio with your own modules.

Core concepts

We will explain the core Flask concepts using simple Flask application:

from flask import Blueprint, Flask, request

# Blueprint
bp = Blueprint('bp', __name__)

@bp.route('/')
def my_user_agent():
    # Executing inside request context
    return request.headers['User-Agent']

# Extension
class MyExtension(object):
    def __init__(self, app=None):
        if app:
            self.init_app(app)

    def init_app(self, app):
        app.config.setdefault('MYCONF', True)

# Application
app = Flask(__name__)
ext = MyExtension(app)
app.register_blueprint(bp)

You can save above code in a file app.py and run the application:

$ pip install Flask
$ export FLASK_APP=app.py flask run

Application and blueprint

Invenio is a large application built up of many smaller individual modules. The way Flask allows you to build modular applications is via blueprints. In above example we have a small blueprint which just have one view (my_user_agent), which returns the browser’s user agent sting.

This blueprint is registered on the Flask application. This allow you to possible reuse the blueprint in another Flask application.

Flask extensions

Like blueprints allow you to modularise your Flask application’s views, then Flask extensions allow you to modularise non-view specific initialization of your application (e.g. providing database connectivity).

Flask extensions are just objects like the one in the example below, which has init_app method.

Application and request context

Code in a Flask application can be executed in two “states”:

  • Application context: when the application is e.g. being used via a CLI or running in a job queue (i.e. not handling requests).
  • Request context: when the application is handling a request from a user.

In above example e.g. the code inside the view my_user_agent is executed during a request, and thus you can have access to the browser’s user agent string. On the other hand, if you tried to access request.headers outside the view, the application would fail as no request is being processed.

The request object is a proxy object which points to the current request being processed. There is some magic happening behind the scenes in order to make this thread safe.

Interfaces: WSGI, CLI and Celery

Overall the Flask application is running via three different applications interfaces:

  • WSGI: The frontend webservers interfaces with Flask via Flask’s WSGI application.
  • CLI: The command-line interface is made using Click and takes care of executing commands inside the Flask application.
  • Celery: The disitrbuted job queue is made using Celery and takes care of executing jobs inside the Flask application.

Application assembly

In each of the above interfaces, a Flask application needs to be created. A common pattern for large Flask applications is to move the application creation into a factory function, named an application factory.

Invenio provides a powerful application factory for Flask which is capable of dynamically assembling an application. In order to illustrate the basics of what the Invenio application factory does, have a look at the following example:

from flask import Flask, Blueprint

# Module 1
bp1 = Blueprint(__name__, 'bp1')
@bp1.route('/')
def hello():
    return 'Hello'

# Module 2
bp2 = Blueprint(__name__, 'bp1')
@bp2.route('/')
def world():
    return 'World'

# Application factory
def create_app():
    app = Flask(__name__)
    app.register_blueprint(bp1)
    app.register_blueprint(bp2)
    return app

The example illustrates two blueprints, which are statically registered on the Flask application blueprint inside the application factory. It is essentially this part that the Invenio application factory takes care of for you. Invenio will automatically discover all your installed Invenio modules and register them on your application.

Assembly phases

The Invenio application factory assembles your application in five phases:

  1. Application creation: Besides creating the Flask application object, this phase will also ensure your instance folder exists, as well as route Python warnings through the Flask application logger.
  2. Configuration loading: In this phase your application will load your instance configuration. Your instance configuration is essentially all the configuration variables where you don’t want to use the default values, e.g. the database host configuration.
  3. URL converter loading: In this phase, the application will load any of your URL converts. This phase is usually only needed for some few specific cases.
  4. Flask extensions loading: In this phase all the Invenio modules which provides Flask extensions will initialize the extension. Usually the extensions will provide default configuration values they need, unless the user already set them.
  5. Blueprints loading: After all extensions have been loaded, the factory will end with registering all the blueprints provided by the Invenio modules on the application.

Understanding above application assembly phases, what they do, and how you can plug into them is essential for fully mastering Invenio development.

Note

No loading order within a phase

It’s very important to know, that within each phase, there is no order in how the Invenio modules are loaded. Say, with in the Flask extensions loading phase, there’s no way to specify that one extension has to be loaded before another extension.

You only have the order of the phases to work, so e.g. Flask extensions are loaded before any blueprints are loaded.

Module discovery

In each of the application assembly phases, the Invneio factory automatically discover your installed Invenio modules. The way this works, is via Python entry points. When you install the Python package for an Invenio module, the package describes via entry points which Flask extensions, blueprints etc. that this module provides. The section Extending Invenio describes in more detail how you use the entry points to extend Invenio.

WSGI: UI and REST

Each of the application interfaces (WSGI, CLI, Celery) may need slightly different Flask applications. The Invenio application factory is in charge of assembling these applications, which is done through the five assembly phases.

The WSGI application is however also split up into two Flask applications:

  • UI: Flask application responsible for processing all user facing views.
  • REST: Flask application responsible for processing all REST API requests.

The reason to split the frontend part of Invenio into two separate applications is partly

  • to be able to run the REST API in one domain (api.example.org) and the UI app on another domain (www.example.org)
  • because UI and REST API applications usually have vastly different requirements.

As an example, a 404 Not found HTTP error, usually needs to render a template in the UI application, but return a JSON response in the REST API application.

Implementation

The following Invenio modules are each responsible for implementing parts of above application architecture, and it is highly advisable to dig deeper into these modules if you want a better understanding of the Invenio application architecture:

  • Invenio-Base: Implements the Invenio application factory.
  • Invenio-Config: Implements the configuration loading phase.
  • Invenio-App: Implements default applications for WSGI, CLI and Celery.