Event Modeling basics

EvidentSource Designer is built around Event Modeling, a lightweight method for designing information systems by sketching the events, commands, and views that make them up — before writing a line of code.

This page is a fast orientation. The Event Modeling site has the full method.

Four blocks

Event Modeling uses four block types, drawn left-to-right on a timeline:

• Events (orange) — things that happened. Immutable. Named in past tense: TodoCompleted, LoanDisbursed.
• Commands (blue) — intentions submitted to the system. Named in imperative: CompleteTodo, DisburseLoan. Produce events.
• State Views (green) — queryable projections used by UI screens or other commands. Folded from events.
• UI / Automation (yellow) — the triggers that cause commands, either user-facing screens or automated processes.

A flow reads left-to-right: a user triggers a command from a screen; the command produces events; events fold into state views; state views feed the next screen.

Why model before coding

• Non-engineers can read the model. Product owners, domain experts, and auditors can verify the system’s behavior from the whiteboard.
• The model is executable. Designer turns it into code — the decide and evolve stubs are generated from the blocks.
• It stays in sync. When the model changes, regeneration keeps the code shape aligned.

Working in Designer

Create a project. Drop blocks onto the canvas. Name and connect them. Fill in the data shapes (see Defining schemas). Hit Generate to get a buildable project — see Generating code.

Further reading

• Design your first system — guided walkthrough.
• eventmodeling.org — the method itself.