work shop eventstorming
TRANSCRIPT
Event Storming PROCESS, STICKES & EVENTS
Francesco Garavaglia
01/2016
Agenda The problem
The solution: Event storming
Summary
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The Problem The Problem
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1,2
1,00 2,00 3,00 4,00 5,00 6,00 7,00 8,00 9,00 10,00
Co
mp
lexity
Time elapsed
Changing Cost
Green Field illusion
Maintainence
I cannot do this
Refactoring from
scratch
How to gather
requirements from
many different
departments/speci
alists?
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Department C
Department E
Department A
Department D
Department B
Sequential data
gathering?
Time
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Or design by committee? The Problem
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Event Storming
Event Storming is a fun way of bringing developers and business experts together and drive your
analysis from the outside and quickly explore complex business domains in hours instead of
days/weeks.
Event Storming (or Model storming) is a way of
starting your analysis from the outside and quickly
explore complex business domains.
Invented by Alberto Brandolini
See
http://ziobrando.blogspot.dk/2013/11/introducing-
event-storming.html
The Problem
Much
requirements
gathering still
follow in the steps
of structural or
classic OO design
WE WRITE USE CASES/STORIES
AND AFTERWARDS LOOK
FOR NOUNS AND VERBS
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Usually we start with a simple model The Problem
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And as time goes by… The Problem
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It gets more complicated The Problem
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And finally we drown The Problem
We end up with
One Domain Model
to Rule them All
CAN’T WE AVOID
CEREMONIAL MODELING?
SOMETHING EASIER, FASTER
AND BETTER?
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Event Storming
All togher into the same room
Unlimited modelling surface
Invite the right People
Those who bring the questions
Those who know the answers (not all of them)
Value is in the interaction between people: miss one and you’ll miss n-1 interactions
Refine the model step-by-step
The Solution: ES
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Hack the space
There is no perfect room for a event storming
Hacking the space is up to you
Unlimited modelling space: can’t tell a problem size before exploring it
Table creates the wrong phisical space
position are fixed, no emerging alliances
posture is far from optimal
too easy to switch on laptops or other distractions
No body language to observe
The Solution: ES
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Needed stuff The Solution: ES
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How to run EventStorming session
Explore the domain starting from Domain/Business
Simple semantic and notation
Represent the state transitions
Gives us precision
Easy to grasp by everybody in the room
Place them along a timeline to visualize the process
An event might be the predecessor or the follower of another one
The Solution: ES
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An Event message is non-prescriptive of what should happen in other services. It leaves this open to the recipients, so that they themselves determine what to do based on occurrence of the event.
Events always carry a name in its past-tense form: OrderWasAccepted, OrderHasShipped, CustomerWasReimbursed
Other qualities
Immutable, i.e. content cannot be changed
Always carries the ID of the Business Object it relates to
An event can and will typically will be published to multiple consumers.
The publisher of the event does not know who the recipients are
And it doesn’t know what the recipients intend to do with the event
“An Event describes something that HAS happened”
What is an Event? The Solution: ES
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User Action model it as a Command
External system
Time
Other Domain Events
Documents
External
Time
Command Role
Event Event
Document
What does it cause Domain Events? The Solution: ES
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Define a language…
A physical model provides a safe
support for spotting critical areas in
your business process
It’s not like
that
The Solution: ES
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..Along the time line
Enforce narratives: People like telling Stories, not just filling diagrams or template
Encourage experts to tell a story
Look for concrete examples
look for edge cases, they fit perfectly in a narrative and they should fit into your model too
Eventually turn them into tests
The Solution: ES
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Or reverse narrative
Use Time line in a reverse mode:
What was the
path that led us
here?
What needs to
happen before,
so that this event
can happen
too?
The Solution: ES
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A Command message is prescriptive of what should happen. This is a stronger form of coupling than Event and Document messages.
A Command’s primary goal is to capture USER INTENT
A Command supports a single usecase and targets a single Business Object
Commands always carry a name in its imperative form: CreateOrder, ShipOrder, CancelOrder, ReimburseCustomer, etc.
Other qualities
Immutable, i.e. content cannot be changed
Always carries the ID of the Business Object that it relates to
A command can only be handled by ONE service
The Operations that consume Commands don’t return values – but can throw exceptions
Commands mutate Business Object state which results in one or more Events being published
“A command describes a Task that you want someone else to carry out for you and the recipient can
reject the Command”
Events are often the side effect of Commands The Solution: ES
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Commands & Events
Receives a
Command
Asynchronousl
y
Publishes multiple
Events
asynchronously
The Solution: ES
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Entering the process manager
Process managers are the home of reactive behaviour in the systems «Whanaver» is the most
common word here
Whenever we receive an
invoice we schedule a
payment
Whenever we receive a Bs.we
try to reconcile it
Suppler
invoice
receiver
Bank
Statement
Received
Schedule
Payment
Reconcile
Operation
The Solution: ES
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Using Business Events to drive Business Processes
Sales Service
Shipping
Billing
Sales
Customers
Message C
hannel
Online Ordering System
Web Shop
(Composite UI)
Billing Service
Shipping Service Order
Accepted
Event
AcceptOrde
r
Command
The sales
fulfillment
processing
can now
begin…
The Solution: ES
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Not all the results are visible
The model on the paper roll is huge and tangible but more value is created along the way
Event
Storming
More often than not, old
problems find a solution
during the workshop
The Solution: ES
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Not all the results are visible
Conflicts and potential overlappings are highlighted
Can finally see the Big Picture
Conversations can finally Happen
And ideas are popping out
The Solution: ES
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Explore Aggregates Outside In
An Aggregate receives Commands and decides to execute them or reject
them
If the Aggregate executes a Command it will produce one or more Domain
Events as a result
Exploring….. The Solution: ES
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Invoice
InvoiceLine
*
Account *
What:
• Cluster coherent Entities and Value
Objects, with complex associations into
Aggregates with well defined boundaries.
• Choose one entity to be root and control
access to objects inside the boundary
through the root.
• External objects hold references to the
root
• Aggregates only refer to other aggregates
by identity (their id)
Motivation:
Control invariants and consistency through the aggregate root.
Enables: Loading schemes, coarse grained locking and…
Ensuring consistency & transactional boundaries for Distributed scenarios
Root
….Aggregates The Solution: ES
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Explore Sub domains
Different areas of responsibility map pretty well to different subdomains or portions of the pork
Explore Bounded Contexts
During the discussion, some conflict areas with regards to different interpretations of terms might emerge
Draw boundaries between the multiple consistent models that will coexist in your domain
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… And sub-domains The Solution: ES
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Conceptual CQRS
Users decide actions
according to the real world
experience (things they know
which are not in the system) and read Model
Humans decide according
to different sets of info (ex:
faces, pictures, surnames,
ecc)
The model of anything: CQRS The Solution: ES
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Summary: Event Storming
Legend:
User Action modeled as a Command
External system
Time
Domain Events
Documents
IT Operations (Gateway)
Aggregate
Read Model
External
Time
Comma
nd Role
Event Event
Docume
nt
IT Ops
Read
Model
Aggregate
Summary
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Event Storming is
Powerful
come up with a comprehensive model of a complete business flow in hours instead of weeks.
Engaging:
bring people with the questions and people who know the answer in the same room and to build a
model together
Efficient
the resulting model is perfectly aligned with Event Sourcing and EDA, and allows for a quick
determination of Bounded Context and Aggregate boundaries and thereby also Service boundaries
Easy
the notation is ultra-simple. No complex UML
Fun
Eventstorming workshops and fun, people are energized and deliver more than they expected. The right
questions arise, and the atmosphere is the right one.
Summary
Thanks [email protected]