The Aggregate Design Canvas
The Aggregate Design Canvas is an insightful tool designed to be a part of design-level domain modelling activities. It is meant to aid in organizing and understanding how different components of a system work together, particularly through the concept of aggregates.
Understanding Aggregates
An aggregate is a concept introduced by Eric Evans, representing a collection of objects regarded as a unit in terms of data consistency and domain policies. Properly designing aggregates is crucial as they determine how different parts of a system interact and maintain consistency. Their design should enforce domain policies effectively or allow for corrective measures when needed.
How the Canvas Works
The Aggregate Design Canvas proposes a structured process for designing aggregates, encouraging thoughtful discussion on various aspects of their design. Here's a look at the steps involved:
1. Name
Choosing a suitable name for your aggregate is a fundamental first step. This can include information about the lifecycle or purpose of the aggregate to clarify its role within the system.
2. Description
You'll want to succinctly summarize what the aggregate does and why its boundaries have been defined as they are. This includes any trade-offs made during the design phase.
3. State Transitions
Aggregates often go through distinct state changes. It’s important to map these out, as excessive transitions can indicate poorly defined process boundaries, while too few might suggest insufficient logic within the aggregate.
4. Enforced Invariants & 5. Corrective Policies
Aggregates enforce key business invariants to protect core logic. Listing these allows for a clearer understanding of the aggregate's responsibilities. If boundaries are relaxed, corrective policies may need to be implemented to address inconsistencies. Documenting these helps in making design trade-offs explicit.
6. Handled Commands & 7. Created Events
Identify and connect the commands your aggregate processes and the resulting events. This ensures all building blocks are accounted for and functioning as intended.
8. Throughput
Estimating how often an aggregate will encounter concurrency conflicts helps in evaluating its interaction with concurrent clients. This can highlight potential design changes to reduce conflicts and improve user experiences.
9. Size
Over time, assessing the size of an aggregate based on the number of events can reveal its long-term viability. Bigger aggregates may slow down command processing but can be managed with appropriate design tactics like snapshots.
Additional Tools and Resources
For those seeking more in-depth understanding, several resources can provide further insight into domain-driven design and aggregate modelling.
Conclusion
The Aggregate Design Canvas is freely available for use, with contributions and feedback welcomed to refine and improve the tool. It's a valuable resource for those involved in domain-driven design, offering a structured approach to managing complex software systems.
As it operates under a Creative Commons Attribution 4.0 International License, users are free to adapt and share as needed, which encourages continuous improvement and community collaboration.