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Akka HTTP: How to use Akka HTTP to build scalable REST APIs in Scala?

Updated: May 16, 2023



What is Akka?


The Akka Actor System is a powerful framework for building concurrent and distributed applications in Scala and Java. One of the many use cases for Akka is building RESTful web services that are both scalable and fault-tolerant. In this blog post, we'll explore how to use Akka Actors to build RESTful applications. In Akka, Actors are the fundamental building blocks of concurrent and distributed systems. They are lightweight, non-blocking, and thread-safe, and communicate with each other by exchanging messages. Each Actor has its own internal state and behavior, and can only be accessed by exchanging messages with other Actors. This approach makes it possible to build systems that are highly scalable, resilient, and responsive. Akka also provides a number of other features and components that are commonly used in concurrent and distributed systems, such as:


  • Akka Streams: A library for building asynchronous and back-pressure-aware stream processing systems.

  • Akka HTTP: A library for building reactive and scalable web services and APIs.

  • Akka Persistence: A module for building event-sourced and durable systems that can recover from failures and maintain consistency.

  • Akka Cluster: A set of tools and protocols for building distributed systems that can scale horizontally and handle network partitions.

Akka Streams can be used to handle requests and responses in a reactive and non-blocking way. For example, we can use Akka HTTP to define the REST API and route incoming requests to an Akka Streams flow, which can then process the request asynchronously and produce the response. This approach makes it possible to handle large volumes of requests and responses in a scalable and responsive way.


Akka Persistence, on the other hand, is a module for building event-sourced and durable systems that can recover from failures and maintain consistency. It provides a way to store and retrieve the state of Actors in a durable and persistent way, which makes it possible to recover the state even if the Actors fail or the system crashes but that is something that we will see in the next edition of this series where we look at how to use Akka clusters, persistence, and streams in order to build a restful application.


What is a RESTful Web Service?


REST (Representational State Transfer) is an architectural style for building web services. A RESTful web service is a web service that adheres to the principles of REST. It uses HTTP methods (GET, POST, PUT, DELETE, etc.) to create, read, update, and delete resources. The resources are represented by URIs (Uniform Resource Identifiers) and are manipulated using HTTP methods.


Why use Akka Actors to build RESTful applications?


Akka Actors provide a way to build concurrent and distributed applications with the Actor model. In the Actor model, everything is an Actor, and Actors communicate with each other by sending messages. This makes it easy to build scalable and fault-tolerant applications, as Actors can be distributed across multiple nodes and can handle failures gracefully.


Akka Actors also provide a way to build reactive applications, which are responsive, resilient, and elastic. Reactive applications can handle high load and concurrency can recover from failures quickly and can scale up or down as needed. There are several advantages of using Akka Actors to build RESTful applications over other approaches. Here are some of the key benefits:


1. Concurrency: Akka Actors are inherently concurrent, which means they can handle multiple requests and responses simultaneously without blocking. This makes it possible to build highly scalable and responsive RESTful applications that can handle high volumes of traffic.


2. Isolation: Each Akka Actor has its own internal state and behavior, which is isolated from other Actors. This makes it easier to reason about the behavior of the system and avoid concurrency issues such as race conditions and deadlocks.


3. Fault-tolerance: Akka Actors are designed to be resilient and fault-tolerant. They can recover from failures and restart automatically, which means the system can continue to function even in the face of errors or crashes.


4. Distributed computing: Akka Actors are well-suited for building distributed systems, where different Actors can run on different machines and communicate with each other over the network. This makes it possible to build RESTful applications that can scale horizontally and handle network partitions.


5. Reactive programming: Akka Actors are a key component of the Reactive Programming paradigm, which emphasizes the use of non-blocking and event-driven programming techniques to build responsive and resilient systems. This makes it possible to build RESTful applications that can handle high volumes of traffic and respond quickly to user requests.


Overall, using Akka Actors to build RESTful applications can provide several advantages over other approaches. It enables developers to build highly concurrent, fault-tolerant and distributed systems that are well-suited for modern web applications.


Building a RESTful application with Akka Actors


To build a RESTful application with Akka Actors, we'll use the following components:


- Akka HTTP: Akka HTTP is a library for building HTTP-based applications in Akka. It provides a high-level HTTP server and client API for building RESTful applications.


- Akka Actors: We'll use Akka Actors to implement the business logic of our application. Actors provide a way to handle concurrency and can communicate with each other using messages.


- Akka Persistence: We'll use Akka Persistence to store and retrieve data in our application. Persistence provides a way to store and retrieve state even if Actors fail or are restarted.


The following steps outline the process of building a RESTful application with Akka Actors:


1. Define the data model: The first step in building a RESTful application is to define the data model. This includes defining the resources, their properties, and their relationships.


For example, let's say we're building a simple Todo list application. We might define the following data model:


```

Todo

- id: String

- title: String

- description: String

- completed: Boolean

```


2. Define the Actors: Once we have the data model, we can define the Actors that will handle the business logic of our application. In our Todo list example, we might define the following Actors:


- TodoActor: This Actor handles CRUD (Create, Read, Update, Delete) operations on the Todo resources.


- TodoListActor: This Actor handles operations on a collection of Todo resources.


3. Define the REST API: Once we have the Actors, we can define the REST API for our application. This includes defining the routes, their methods, and their handlers.


For example, we might define the following routes for our Todo list application:


- GET /todos - Returns a list of all Todo resources.

- GET /todos/:id - Returns a single Todo resource by ID.

- POST /todos - Creates a new Todo resource.

- PUT /todos/:id - Updates an existing Todo resource by ID.

- DELETE /todos/:id - Deletes an existing Todo resource by ID.


4. Implement the Actors: With the data model and REST API defined, we can now implement the Actors that will handle the business logic of our application To implement the Actors, we'll need to define their behavior and message protocol.


For example, we might define the behavior of the TodoActor as follows:


- Create: When receiving a Create message with a new Todo resource, the TodoActor creates a new Todo with a unique ID and adds it to its internal state. It then responds with a message indicating success or failure.

- Read: When receiving a Read message with a Todo ID, the TodoActor retrieves the Todo from its internal state and responds with the Todo resource.

- Update: When receiving an Update message with a Todo ID and new data, the TodoActor updates the Todo with the new data and responds with a message indicating success or failure.

- Delete: When receiving a Delete message with a Todo ID, the TodoActor deletes the Todo with the specified ID from its internal state and responds with a message indicating success or failure.


We might define the message protocol for the TodoActor as follows:


```

case class Create(todo: Todo)

case class Read(id: String)

case class Update(id: String, todo: Todo)

case class Delete(id: String)

```


5. Implement the REST API: With the Actors implemented, we can now implement the REST API by defining the routes and their handlers.


For example, we might define the route for getting all Todos as follows:


```

path("todos") {

get {

val todosFuture = (todoListActor ? GetTodos).mapTo[Todos]

onSuccess(todosFuture) { todos =>

complete(todos)

}

}

}

```


In this example, we define a GET route for "/todos". When this route is requested, we send a GetTodos message to the TodoListActor using the ask pattern (represented by "?") and wait for a response of type Todos. Once we receive the Todos, we complete the request with the Todos as the response.


6. Configure the server: With the REST API implemented, we can now configure the Akka HTTP server to handle requests and responses.


For example, we might configure the server as follows:


```

val routes = todoRoutes(todoListActor)


val httpConfig = ConfigFactory.load().getConfig("http")

val host = httpConfig.getString("host")

val port = httpConfig.getInt("port")


Http().newServerAt(host, port).bind(routes)

```


In this example, we define the routes using the todoRoutes function, which takes the TodoListActor as a parameter. We then load the HTTP configuration from the application.conf file, and bind the server to the specified host and port.


Conclusion


In this blog post, we've explored how to use Akka Actors to build RESTful applications. We've seen how Actors can provide a way to handle concurrency, and how Akka Persistence can provide a way to store and retrieve data even if Actors fail or are restarted. We've also seen how Akka HTTP can be used to define the REST API and handle requests and responses.


While building RESTful applications with Akka Actors may require some additional complexity compared to traditional web frameworks, the benefits of scalability, fault-tolerance, and reactivity can make it a powerful choice for building high-performance and resilient web services.


If you need help with your Software engineering requirements, Please contact 'Hello@fusionpact.com'


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