Event Storming
Event Storming is a Collaborative exercise, objective of which is to identify the Events, Producers & Consumers in a given scope, to create a shared understanding of the domain.
Event Storming is a facilitated workshop that assists in create a shared understanding of the cause and effect of domain events.
Focus is on gathering knowledge to create and disperse shared understanding of the domain
- Objective is NOT to design the system
- Objective is NOT to be able to answer all questions
- Objective is NOT to produce DDD models
Workshop
Conducted by an experienced facilitator with prior experience with Event Storming
- Decide on the scope
- Domain level
- Process level
- OR somewhere in-between
- Number of participants
- 4 to 8 for process level
- Duration depends on scope
- Couple of hours/days
- In person or online
- In person : use stickies on wall
- Online : use collab tools
Color coded stickies are used to represent the knowledge.
Process
All events are considered *Domain Events. Start by identifying a Domain Actor and their actions.
Look for statements like:
Command
Represents the cause of the event. It is used by a domain Actor; it leads to an Event. Use a Blue sticky to represent a command.
- Represents the intent of an actor
- That MUST be carried out by the domain
Command processor a.k.a. Aggregate
A Command Processor is an Aggregate that processes the command, and that leads to an Event. It is represented on a Yellow sticky. For example it could be a software system, manual process, etc
Event
Event is represented on an Orange sticky. Name the event is always in Past tense.
Policy a.k.a. Reaction a.k.a. Side effect
It is the reaction to the event. In other words it represents the effect. Policy is placed on a Purple sticky note.
External sourcse
An external system can also be a source of event. Such events are represented on a Pink sticky note.
Event data a.k.a. Query model
Event is directly or indirectly associated with state change . A Green sticky note is used to depict key information on the event that is of value to one or more stakeholders.
User interface
The Event data is projected on a user interface, that is depicted on a White sticky.
User Interface is sometimes not considered as part of Event Storming
Relationships to keep in mind
Note: Inspired by Alberto Brandolini’s talk/book
ACME travels workshop
This is an example from the case study used in the course.
Benefits
- Proven technique
- Establishes common knowledge of domain
- Identifies knowledge gaps
- Accelerates development
- Focussed