SUMMARY

Flask replicated is a Flask extension, designed to work with SqlAlchemy. It's purpose it to provide more or less automatic master-slave replication. On each request, extension determines database usage intention (to read or to write into a database). Then, it picks right database url inside overriden db.get_engine() whenever request handler tries to access database depending on REST verb used.

Flask replicated comes with a security kill-switch to enable developper mode to easily keep control on the feature AUTO_READ_ON_SLAVE true by default, once deactivated only master database will be used unless explicit use of the decorators.

INSTALLATION

1. Install flask replicated distribution using pip install flask_replicated. Or add flask-replicated==1.4 in requirements.txt and requirements pip3 install -r requirements.txt.

2. Import library from flask_replicated import FlaskReplicated or import flask_replicated depending on how you want to call the functions or decorators

3. In flask app.config configure your database bindings a standard way:

   AUTO_READ_ON_SLAVE = True
   SQLALCHEMY_DATABASE_URI = '%(schema)s://%(user)s:%(password)s@%(master_host)s/%(database)s'
   SQLALCHEMY_BINDS = {
       'master': SQLALCHEMY_DATABASE_URI,
       'slave': '%(schema)s://%(user)s:%(password)s@%(slave_host)s/%(database)s'
   }
   

4. Register app extension:

   app = Flask(...)
   ...
   FlaskReplicated(app)
   

USAGE

Flask replicated routes SQL queries into different databases based on request method. If method is one of READONLY_METHODS which are defined as set(['GET', 'HEAD']) and config AUTO_READ_ON_SLAVE has not been set

While this is usually enough there are cases when DB access is not controlled explicitly by your business logic. Good examples are implicit creation of sessions on first access, writing some bookkeeping info, implicit registration of a user account somewhere inside the system. These things can happen at arbitrary moments of time, including during GET requests.

To handle these situations wrap appropriate view function with @flask_replicated.use_master_database decorator. It will mark function to always use master database url.

Conversely, wrap the view function with the @flask_replicated.use_slave_database decorator if you want to ensure that it always uses the slave replica.

GET after POST

There is a special case that needs addressing when working with asynchronous replication scheme. Replicas can lag behind a master database on receiving updates. In practice this mean that after submitting a POST form that redirects to a page with updated data this page may be requested from a slave replica that wasn't updated yet. And the user will have an impression that the submit didn't work.