Stadium example #

Domain-driven design has the concept of a bounded context (BC). Have a look at how Martin Fowler defines this. In that definition he also refers to what he calls multiple canonical models.

To make the concept of a bounded context more clear we could take seats in a stadium as an example. A stadium contains seats but a seat represents different things to different people.

Financial Management #

In most scenarios one would have to keep an asset register and it is typically owned by the financial folks since they are the system of record for any assets.

The finance department would be interested in data such as:

• asset number
• asset type
• date of purchase
• cost
• method of depreciation
• date commissioned
• date decommissioned

They would also be interested in the following behaviour:

• Commission()
• Depreciate()
• Decommission()

Maintenance Management #

The maintenance department would be interested in data such as:

• maintenance required
• maintenance schedules

The behaviour for them would be along the lines of:

• ScheduleMaintenance()
• RegisterMaintenanceSchedule()

Event Management #

The event booking folks would be have data like this:

• event name
• event date
• relevant seat number(s)

And the relevant behaviour would include:

• RegisterEvent()
• SellTicket()

Putting it all together #

So, as we can see, even though a physical seat is one thing it can mean very different things to different people. This is where a Bounded Context comes in. You may even find that a similar concept is called something else in each context. You may find that an Employee in the HR BC is called a User in the Identity & Access Control BC, or an Author in the Collaboration BC.

As behaviour is invoked on various objects within each BC the other BCs may need to be informed. To accomplish this one would need some communication mechanism based on an Event Driven Architecture. That is, of course, where a service bus like Shuttle.Esb would come in handy.

The Financial Management BC could publish an AssetRegistered and the other BCs would subscribe to that event and then determine if they need to register the asset number as an item they are interested in.

When the Maintenance Management BC removes a seat for maintenance it would publish a ItemRemovedForMaintenance that the Booking Management BC would subscribe to in order to exclude the seat as a bookable item for any events until the item is made available again.

Canonical Models #

All this is quite different and contrasting to what many enterprise-level architects refer to as a canonical data model. Stefan Tilkov has something to say about these too.

For our scenario a canonical model would need to include all the data from the various BCs. This is quite cumbersome and really does not add any real value.