I think there is something missing in how we think about API’s and CQRS. It came to my mind again when I was reading the InfoQ Virtual Panel: Microservices Interaction and Governance Model - Orchestration v Choreography.
The article mentions CQRS a few times, and Chris Richardson gives a concise description : One simple way to think about microservice interaction is in terms of commands and queries. This has become a the common way to think about an API: “An API has queries and commands”. I think it makes sense to add subscriptions on the same level.
Subscriptions are also known as webhooks or event listeners. I believe that is also a fundamental concept in an API.
We describe API’s too much in combination with the underlying technology like eg a REST API or messaging. I think it helps to consider the conceptual level of an API first and subscriptions should be a part of it. In my opinion, any API consists conceptually of
These concepts are to be considered apart from the underlying technology which are used. Queries, commands and subscriptions can be implemented as REST over HTTP, over messages, GraphQL and many more concrete combinations.
Let’s zoom in on the subscriptions for a minute. Event Driven Architecture means to me the realization that you can move a large part of the logic of your application into subscriptions. If this event happens, do this action. Sounds familiar?
I think that IFTTT, Zapier and their clones show the value of subscriptions for consumers. That is exactly one of the use cases that RockScript envisions, but than for developers. An event triggers a script. The script calls out to a number of commands in other services. API Commands should be mapped to RockScript activities. Either by yourself or by using an existing serviceFunction worker. And we also envision the addition of a trigger as a reusable, configurable subscription that starts a RockScript.