How do you identify the core domain and subdomains in event storming?

1 Start with a big picture

The first step in event storming is to create a big picture of the business process you want to model. You can do this by inviting the relevant stakeholders, such as domain experts, developers, users, and managers, to a workshop where they can share their perspectives and knowledge. You will need a large wall or board, sticky notes of different colors, and markers. The goal is to capture all the events that happen in the process, from the trigger to the outcome, and arrange them in a chronological order. Events are things that happen in the past and have an impact on the system, such as "order placed", "payment received", or "shipment delivered". Use orange sticky notes for events and write them in the past tense.

2 Identify the commands and actors

The next step is to identify the commands and actors that cause or react to the events. Commands are actions that request or initiate something to happen, such as "place order", "confirm payment", or "send shipment". Actors are the roles or systems that perform or receive the commands, such as "customer", "payment service", or "warehouse". Use blue sticky notes for commands and write them in the imperative form. Use yellow sticky notes for actors and write them in the noun form. Place the commands above the events they trigger, and the actors on the left or right side of the commands they perform or receive.

3 Identify the policies and boundaries

The final step is to identify the policies and boundaries that govern or constrain the events and commands. Policies are the rules or logic that determine when, how, or why something happens, such as "order is valid if payment is confirmed", "shipment is sent if order is fulfilled", or "customer is notified if shipment is delayed". Boundaries are the limits or scopes that separate different parts of the system, such as "order service", "payment service", or "warehouse service". Use pink sticky notes for policies and write them as conditional statements. Use black markers to draw boundaries around the events, commands, and actors that belong to the same part of the system.

4 Find the core domain and subdomains

Once you have a better understanding of the business process, you can start looking for the core domain and subdomains. The core domain is the part of the system that is most valuable to customers and stakeholders and requires the most innovation and expertise. Subdomains are parts of the system that support or enable the core domain, but are not critical or unique. Various criteria such as frequency, complexity, and user feedback can be used to find the core domain and subdomains. On your event storming board, you can use different colors or symbols to mark the core domain and subdomains, such as green for the core domain, purple for supporting subdomains, and gray for generic subdomains. Additionally, labels or names can be used to describe the core domain and subdomains. By identifying these elements in event storming, you can focus your attention on the most valuable and challenging part of your system while delegating or outsourcing less important or common parts. This can also be used as a basis for defining bounded contexts in DDD which separate and communicate different parts of your system.

5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?