Architecture model
Akka simplifies application development by allowing developers to focus on their domain models and APIs without worrying about how the data is stored. The architectural model is the key to making this work.
Architecture
Akka applications are built with an Onion Architecture, where services are organized in concentric layers. The core of the application is at the center, with additional layers surrounding it. Each layer has a specific role.
Layers in the architecture
The layers of the Akka programming architecture are outlined below.
Domain
At the center is the Domain Model which encapsulates your business logic. The domain should be mostly pure Java: that is not Akka components. In Akka projects and samples, this should be a package called domain.
Application
The middle layer is the Application layer. This layer uses the domain model and coordinates domain objects. This layer is where your Akka components will live. In this sense, an Akka Component acts as the glue between your domain model and the Akka runtime. This package should be called application and only contains Akka components like Entities and Views.
Mechanics of each layer
Like most onion architectures, each layer only calls to the layers directly inside of it. This means an endpoint (API) will not directly call a domain object. It also means the outside world should never directly call domain objects or application components. If you want to call an Akka component like an entity or view, do so through an endpoint (HttpEndpoint). This separation is meant to contain the inner implementation and makes it easier to evolve applications over time.
These layers should be implemented as the three packages outlined above: domain, application, api. The mechanics of these layers are as follows.
Domain
Domain objects are independent of the other layers. As these are pure Java objects they are the place for implementing business logic. Examples of business logic include checking credit limits for a loan or return policy enforcement. You should write unit tests for this code that tests the business logic.
Much of the inner loop of the developer experience will be spent here. Using Java’s record type declaration simplifies the amount of ceremony involved in creating domain objects that are understood and used by Akka. This keeps your domain model clean and free of any dependencies - including Akka.
Application
The application layer is where Akka becomes a first class participant as the glue between your domain code and the runtime. Most classes in this layer will extend an Akka component class (e.g., within the akka.javasdk package), making them event-based and actor-based behind the scenes, while abstracting those details from you.
These components include Event Sourced Entities, Key Value Entities, Views, Workflows, Timers and Consumers. Each component type provides specific functionalities to handle state, events, and interactions efficiently.
API
The outermost layer is the API layer which is how the outside world interacts with your application or service. In this layer, you define endpoints that expose your application. Each endpoint is marked with an @HttpEndpoint annotation, which allows the runtime to build the appropriate endpoint URLs for uniquely identifying the components. Each public method on the endpoint that is annotated with method @Get, @Post, @Put, @Patch or @Delete serves those respective HTTP methods.
Having received requests, the api layer interacts with the application layer through the ComponentClient which makes calls in a type safe way. This is the layer boundary that keeps the isolation necessary to remain resilient that is core to an Akka application.
Additionally this layer is the place for a public event model that a service exposes, often via Kafka or other messaging capabilities. This allows the event driven nature of Akka to be easily integrated into the rest of your information space. In Akka you don’t reach into the database to get state, you use the event stream itself.
The API layer also uses other annotations to control access. For more information on endpoints see Designing HTTP Endpoints.
Akka components
You use Akka Components to build your application. These Components are crucial for ensuring responsiveness. Here is a brief overview of each. With the exception of endpoints, Akka components will live in your application package.
The list of components are:
Akka components are marked with a @ComponentId or @HttpEndpoint annotation to identify them to the runtime.
Entities
Entities are the core components of Akka and provide persistence and state management. They map to your domain aggregates. If you have a "Customer" domain aggregate, you almost certainly will have a CustomerEntity component to expose and manipulate it. This separation of concerns allows the domain object to remain purely business logic focused while the Entity handles runtime mechanics. Additionally, you may have other domain objects that are leafs of the domain aggregate. These do not need their own entity if they are just a leaf of the aggregate. An address is a good example.
There are two types of entities in Akka. Their difference lies in how they internally function and are persisted.
Key Value Entities
Key Value Entities are, as the name implies, an object that is stored and retrieved based on a key - an identifier of some sort. The value is the entire state of the object. Every write to a Key Value Entity persists the entire state of the object. Key Value Entities are similar in some ways to database records. They write and effectively lock the whole row. They still use an underlying event-based architecture so other components can subscribe to the stream of their updates. For more information see Key Value Entities.
Event Sourced Entities
Event Sourced Entities persist events instead of state in the event journal . The current state of the entity is derived from these events. Readers can access the event journal independently of the active entity instance to create read models, known as Views, or to perform business actions based on the events via Consumers. For more information, see Event Sourced Entities.
Views
Views provide a way to materialize read only state from multiple entities based on a query. You can create views from Key Value Entities, Event Sourced Entities, and by subscribing to a topic. For more information see Views.
Consumers
Consumers listen for and process events or messages from various sources, such as Event Sourced Entities, Key Value Entities and external messaging systems. They can also produce messages to topics, facilitating communication and data flow between different services within an application. For more information see Consuming and producing.
Workflows
Workflows enable the developer to implement long-running, multi-step business processes while focusing exclusively on domain and business logic. Technical concerns such as delivery guarantees, scaling, error handling and recovery are managed by Akka. For more information see Workflows.
Timed actions
Timed Actions allow for scheduling calls in the future. For example, to verify that some process have been completed or not. For more information see Timed actions.
Endpoints
Endpoints are defined points of interaction for services that allow external clients to communicate via standard HTTP. They facilitate the integration and communication between the other types of internal Akka components. Unlike other Akka components, endpoints will live in your api package. For more information on endpoints see HTTP Endpoints.
Akka Services
A Service is the base deployment unit in Akka. It contains the layers and packages described above. A service deploys to a Project. A project can contain more than one service and is tied to one or more regions. To learn more about deployment see Akka Deployment Model.
Next steps
Now that you understand the overall architecture of Akka you are ready to learn more about the Akka Deployment Model.
The following topics may also be of interest.
Start building your own Akka Service using the Akka SDK.