Handling Commands

Command Handler

Background

A Command Handler handles commands. The command handler is responsible for executing the appropriate methods on Aggregate Roots and Domain Services to fulfill the intent of the Command.

Requirements

A Command Handler must implement the IHandleCommands interface. This is a simple marker interface with no methods to implement.

To handle a Command, you create a method called Handle with a single parameter that is the Command you wish to handle. A Command Handler does not return anything. It is not permitted to return a value from a Command. When a Command is executed, you will receive a CommandResult which will indicate whether the Command succeeded and, in the case of failure, why the Command failed.

No entities or representations that were created or updated as a result of the Command are present on the CommandResult, as it is not meant to return such state. To inspect state (other than a result indicating success or failure) after sending a Command you can use the read-side with its queries and read-models.

public void Handle(AddRecommendationToCart cmd)

There MUST be a single handler for a Command. Dolittle will throw an exception if there is no handler for a command or if there is more than one handler for a command. There can only be a single handler for a command as there must be a clear and unambiguous indication of the result of the execution of the command. Multiple command handlers for a single command would require co-ordination of multiple results with associated rollback of successful handlers after unsuccessful ones. Where you wish multiple things to happen on receipt of a Command, you must implement and handle this yourself in your Command Handler.

Responsibilities

The sole responsibility of the Command Handler is to execute the Command.

A Command Handler should not implement any validation, security checks, or such like. These should be handled earlier in the command pipeline by validators. When we invoke a Command Handler, it is with the expectation that the action will succeed. We should have validated the command, checked business rule conditions, checked the permissions and therefore the command should succeed. It is for this reason that within a Command Handler, the only way to indicate that the Command did not succeed is via an Exception. This is because, due to our previous checks, it is truly an exceptional condition for the Command to fail.

Design

It is REQUIRED that a Command Handler invoke a single method on a single Aggregate Root or none at all. The Command is intended to be transactional, either succeeding or failing totally. Think of the Command, it’s Command Handler and the single method on the Aggregate Root as one transaction that must either succeed or fail. It can be useful to think of a Command as a serialized call on a method on an Aggregate Root. If you could call multiple Aggregate Roots you would need to handle one of them failing and rolling back the transaction across multiple Aggregate Roots, which is likely impossible.

Calling multiple Aggregate Roots from the same command handler for the same commands would indicate that you need to create a new Aggregate Root that captures this business transaction as one thing. In our experience it can be fruitful to consider modelling the verb, not the noun (i.e. the adding of a product to the cart, not the cart with an add -method).

As nothing can be returned from a Command Handler, we discourage reliance on database-generated identities or keys. The invoker of the Command would not be able to receive the Id generated by the database. We RECOMMENDED using meaningless but unique keys such as Guids on a Command which can be provided by the invoker of the Command and Command Handler. Where a database generated key is required, it is recommended to include another candidate key that can be provided by the caller.