The Art of Immutable Architecture by Michael L. Perry

Author:Michael L. Perry
Language: eng
Format: epub
ISBN: 9781484259559
Publisher: Apress

Handling the Next Action

State machine–based patterns show us one of the reasons that we want to know the state of an entity: to understand how to respond to the next action. A message handler usually follows a predictable series of steps:Look up the entity (by correlation ID or some other property of the message).

Determine the state of the entity.

Validate the message.

Operate on the entity.

Update the state.

The state determines how to perform the subsequent operations. We could run the declarative function to determine the current state and then from that state determine the strategy for handling a message. Or we could skip a step and simply determine the strategy directly from history. In many domains, deciding how to respond to an action is much simpler than determining state.

For example, in the order fulfillment system, the strategy for responding to a cancellation request depends upon the financial state of the order. If the order is Paid, then we issue a refund. If not, then we simply cancel the order. Determining the state of the order is somewhat complicated, as we saw earlier. And yet determining whether an order is paid is much simpler:paid (order) =

there exists Invoice

such that there exists Payment

such that there does not exist Refund

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