Viewflow Internals http://viewflow.io

1. Viewflow
internals
0.6.0 beta

2. Intro
Reusable workflow library
➔ Django friendly
➔ Easy to start
➔ Helps to grow
➔ Business applications
➔ People collaboration

3. Intro
Highly technical talk
Don’t panic. Library is only about 3k SLOC
If anything unclear - ask kmmbvnr@gmail.com

4. Intro
➔ Flow
◆ Nodes
◆ This object
➔ Models
➔ Nodes
◆ Views
◆ Jobs
◆ Gates
➔ Activation
◆ Livecycle
➔ Parallel tasks execution
◆ Token
◆ Locking
➔ Error handling
◆ Activation context
➔ Viewsite
◆ Forms
Viewflow internals

5. Flow
➔ CRUD and Flow Separation
➔ Task dependencies graph
➔ Boilerplate: Permissions, URLs

6. Models
➔ Process/Task tables
➔ Multi-table inheritance
➔ Abstract base classes

7. Views
➔ Function based Views ➔ Class based Views
URL: <flow_cls>/<process_pk>/<task_name>/<task_pk>

8. Jobs
➔ Never miss the broken task
➔ Celery out of the box
➔ Easy to add another libraries support:
rq, huye

9. Gates
➔ Workflow patterns support
➔ Decide what task should be executed next
➔ Easy to add logic by extending class or
providing callable

10. Activation
@classmethod
def activate(cls, flow_task, prev_activation, token):
def initialize(self, flow_task, task):
def prepare(self, **data)
def assign(self, user)
def start(self)
➔ Weaves declarative Flow node
with Process and Task instance
def execute(self) def error(self, exc)
def done(self)
def activate_next(self)

11. Parallel task execution and Joins
Task Token
➔ Split Node adds suffix
➔ Join Node waits all nodes
with common prefix
Short pessimistic locks
➔ @flow_view
➔ @flow_job
Optimistic lock with django-fsm on Task
OR ➔ If your are ok with fails
Concurrent updates are rare, but:

12. Error handling
View Jobs
➔ Any exception in subsequent task activation
would rollback whole transaction.
➔ View task still available for user second try
➔ Job results committed immediately as job
completes.
➔ Any exception in subsequent tasks activations
would save subsequent task in error state.
Could be customized by providing custom Activation class

13. Viewsite
Completely optional, but you will like it
➔ Admin like speed for interface development
➔ Works with your Views
➔ Ready for end user
➔ Forms layouts, dynamic formsets, bootstrap
More in next videos...

14. Thank you
$ pip install django-viewflow
http://viewflow.io
http://github.com/kmmbvnr/django-viewflow/
kmmbvnr@gmail.com