Icebergs, the Interface Segregation Principle and microservices
An essential characteristic of the microservice architecture is loose design-time coupling. In a loosely coupled architecture, changes to a service rarely require other services to be changed in lockstep. As a result, it’s easier to make changes. What’s more, teams need to spend much less time coordinating their work.
If you neglect design-time coupling, you risk creating a distributed monolith, which combines the complexity of the microservice architecture with the friction of a monolith. While such an architectural disaster might result in conference talk about why microservices are a bad idea, it could create an existential crisis for your business and is best avoided.
Services should be like icebergs
Ensuring that your services are loosely coupled requires careful design. One very helpful idea is the Iceberg principle.
A service should be like an iceberg, mostly below the waterline. Its API, which consists of operations and published events, is the visible part of the service. It should be much smaller than the service’s (hidden) implementation. That’s because what’s hidden can be easily changed. A service’s API should meet the clients’ needs while hiding much of the implementation.
When a service doesn’t look like an iceberg
Sometimes, however, a service doesn’t look like an iceberg. Consider, the takeout Burrito example from my 2021 QCON presentation on design-time coupling.
Takeout burritos version 1
There are two versions of the food ordering application. The first version looks like this:
Restaurant Service has an API that implements
<crud>Restaurant() operations and publishes
Restaurant<crud> events, which contain the restaurant’s menu.
Order Service uses the
Restaurant<crud> events to maintain a CQRS replica of restaurant menus, which it uses to validate and price an orders.
This architecture seems simple.
But one drawback, however, is that the
Order Service is coupled to the menu structure.
Enhancing the application to support customized burritos requires the
Restaurant Service and the
Order Service to change in lock step.
Let’s look at an architecture alternative that has less coupling between the
Order Service and
Takeout burritos version 2
The second version of the architecture reduces coupling by encapsulating all knowledge of the menu structure within the
Responsibility for storing, validating the line items and calculating the order subtotal is moved from the
Order Service to the
Order Service is primarily responsible for calculating fees, taxes, yet more fees and the order total.
In this version of the architecture,
Restaurant Service’s API still defines
But, instead of publishing
Restaurant<crud> events, it publishes simpler
RestaurantOrder<crud> events, which contain the outcome of validating and pricing the order.
Order Service uses the
subtotal from the
RestaurantOrderCreated event to calculate the
On the one hand, the
Order Service is significantly less coupled to the
It has no knowledge of a menu’s structure.
Order Service now only consumes a very simple event that contains an outcome and a
But on the other hand, the complexity of
Restaurant Service’s API has barely changed.
It still doesn’t look like an iceberg.
Applying the interface segregation principle
An important OO design principle is Interface Segregation Principle (ISP). It states that a client should not be exposed to methods in an interface that it does not use. In other words, a class should have multiple smaller, specialized interfaces rather than one large interface. Perhaps, the same is true of services albeit for different reasons.
With the ISP in mind, another way of looking at the
Restaurant Service is that it has two APIs.
The first API is
Restaurant Management, which defines the
It’s used by the
Restaurant Management UI to manage restaurants.
In fact, you can think of this API as internal to the
Restaurant Management capability.
Consequently, the Iceberg principle does not need to apply.
The second API implemented by the
Restaurant Service is
Restaurant Order Management.
This API publishes events such as
It’s used by services outside of
Restaurant Management, such as the
Restaurant Order Management API should and, in fact, does conform to the Iceberg principle.
It’s a simple API that encapsulates the implementation details of the