Workflows via Event driven architecture

From codebase to database:
Automation via workflows
A way of writing customizable logic.
patelmilanp
patelmilan-p

Agenda
● What is workflow as a concept
● Some examples of workflow
● Automate workflows using traditional coding techniques
● Problem with traditional techniques
● Design a technique using event based architecture to solve
the problem

● Use Cases
● Benefits / Advantages over traditional methods
● Overview: our implementation of this technique
● Challenges we are facing with this design

● Dictionary meaning:
The sequence of processes through which a piece of work passes from initiation
to completion.
● It is a sequence of operations, declared as work of a person or group/organization.
● From a higher-level perspective, workflow may be considered a view or
representation of real work.
● Workflows may be viewed as one fundamental building block to be combined
with other parts of an organization's structure.
● History:
One of the earliest usages of the term 'work flow' was in a railway engineering
journal from 1921.
Workflow as a concept

Sample Workflow
An example workflow for the purpose to this talk.

A simplistic workflow from medical services domain
Appointment
Patient schedules an appointment.
Arrival
Patient gets to the clinic.
Visit
Patient talks with the doctor.
Send a notification via sms to patient
& doctor.
Send a reminder sms to patient
15 mins before the appointment.

Automated workflow via
Traditional coding
technique
class Appointment < ActiveRecord::Base
belongs_to :doctor
belongs_to :patient
after_create :send_sms_to_doctor_and_patient
after_create :remind_patient_before_hand
def remind_patient_before_hand(appointment)
MailerWorker.perform_at(
(appointment.scheduled_time - 15.minutes),
appointment.patient_id)
end
end

Problems with this method:
● Suppose we try to sell this to different doctors, and found out that every doctor’s
workflow is little bit different.
● How can we achieve this kind of customizability in our application to cater for
different requirements ?
● Suppose, we came up with settings feature where a set of settings can be
enabled/disabled to handle different requirements. (but is it a feasible solution ?)
● So now we will design a solution based on event driven architecture, which is fully
customizable and can handle requirement differences easily.

Event driven architecture (EDA)
● It’s a software architecture pattern promoting the production, detection,
consumption of, and reaction to events.
● Key aspects:
1. Type of interaction: Event-driven
2. Initiator: Event
3. Participants: Open-ended
● Some principles of EDA:
1. “Real-time” events as they happen at the producer
2. One-way “ﬁre-and-forget”
3. Immediate action at the consumers
4. Informational (“someone logged in”), not commands (“audit this”)

EDA (generic block diagram)
Business
Processes
Services State
Machines
Monitors /
Probes
Business
Processes
Services State
Machines
Monitors /
Probes
Event Publishers
Event Subscribers
Event processing engine
Event Handler
Event Emitter
Event Channels
Event Flow

Design a system using EDA to solve our
problem

System comprised of following entities
System’s primary focus should be on the domain & domain logic.
● Domain events:
○ A Domain Event is something that happened that the domain expert cares about.
○ By exposing relevant Domain Events on a shared event bus we can isolate cross cutting functions to
separate system.
● Conditions:
○ Often domain logic accompanies with some conditional scenarios.
○ Through this entity, we decide whether to execute a given action after evaluating defined
conditions. Conditions may be optional.
● Actions:
○ A way of reacting to domain events in the system.
○ We can perform any activity in the system when an event is triggered.

● We will create a workflow as:
○ Triggering Event:
■ New appointment scheduled event.
○ Action:
■ Send a notification sms to doctor & patient with appointment time.
■ Send a reminder to patient 15 mins before appointment time.
● Let’s name this workflow as ‘notification_workflow’
Automate the sample workflow with this event-driven
system

● We can have a workflow model to save our
custom workflows to database.
● Instead of sending notification directly in
appointment’s callback, we now publish
appointment scheduled(created) event in it.
● Consider we have some method to subscribe a
workflow to any domain event. Using that we
can now register our notification_workflow to
appointment scheduled event.
● We can implement some sort of Event
processing engine which will find all the
workflows registered on a triggered event &
can execute them all.
● We can provide a GUI interface for creating &
editing workflows. This way every health
provider can create custom workflows
according to his needs.
class Worflow < ActiveRecord::Base
has_many :conditions
has_many :actions
def execute
# evaluate conditions & execute actions
...
end
end
class Appointment < ActiveRecord::Base
belongs_to :doctor
belongs_to :patient
after_create :new_appointment_created_event
end
register(notification_workflow,
:new_appointment_created)
# some method in Event processing engine.
def execute(worflow_id)
workflow = Workflow.find(workflow_id)
workflow.execute()
end

● Notifications module
○ Notifications can be a cross cutting concern in many systems, which makes it a perfect contestant
for using EDA.
○ For example: A system consisting of monitoring & reporting service
■ Monitoring service - want’s to send notification when some metric goes above threshold
value.
■ Reporting service - send different types of reports via notifications.
● Implement hooks within some service
○ For example: A popular code hosting platform ‘Github’ provides a webhook service.
■ It allows external services to be notified with a POST request, when certain events happen
within your repository.
Use cases for EDA

Workflows via Event driven architecture

● Fully Customizable
● Highly Scalable
● Easily extensible
● Efficiently handle crosscutting concerns by decoupled system.
Advantages of EDA over traditional methods

High level overview of ‘our’ implementation of EDA
Workflow
Branch 1 Branch 2
Trigger branch
● Binary tree structure
● Introduced notion of branches
● Branch can be action branch or
condition branch
● Workflow has one compulsory branch
called trigger branch
● Action branches are always present at
leaf nodes of the tree

Challenges we are facing in this system
● Activities performed in workflow actions can trigger other events implicitly
triggering other workflows subscribed to that event which can result into a
loop of workflows triggering each other leading to deadlock conditions.
How this problem can be solved efficiently ? (Think)
Open for discussions !

References
● Wikipedia
● http://www.slideshare.net/stnor/event-driven-architecture-3395407

Thank You
patelmilanp
patelmilan-p

Workflows via Event driven architecture

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