Getting Started With Server-Side Swift and Amazon Smoke

Defining Errors

To add an operation, you need to define the errors your route will handle. Add the following below ModelOperations:

// 1
public struct ErrorTypes: OptionSet {
  public let rawValue: Int
  
  public init(rawValue: Int) {
    self.rawValue = rawValue
  }
  
  // 2
  public static let serverError = ErrorTypes(rawValue: 1)
}

// 3
extension ErrorTypes: ErrorIdentifiableByDescription {
  public var description: String {
    switch rawValue {
    case 1:
      return "serverError"
    default:
      return ""
    }
  }
}

Here’s what you’re doing in the code above:

1. You define a struct called ErrorTypes to represent any errors that occur in your REST API.
2. This makes it easy to create an ErrorType of type serverError.
3. Errors must conform to ErrorIdentifiableByDescription, which is a typealias for a Swift.Error and CustomStringConvertible. This defines your error description.

You’re using an OptionSet here instead of an enum because even though there’s only one ErrorType defined, you can imagine that multiple internal errors may happen in your application and you only want to expose a single ErrorType to a consumer. For example, if you had two errors, invalidUsername and invalidPassword, you would only want to expose a generic error like invalidCredentials to the client from your application.

Specifying Operations

Once you create your routes, you need to specify which operation to use for which routes.

Do this by adding the following below ErrorTypes, which you just created:

// 1
public func addOperations<SelectorType: SmokeHTTP1HandlerSelector>(
  selector: inout SelectorType
) where SelectorType.ContextType == ApplicationContext,
        SelectorType.OperationIdentifer == ModelOperations {
    // 2
    selector.addHandlerForOperation(.topicGet,
                                    httpMethod: .GET,
                                    operation: topicsGetOperation,
                                    allowedErrors: [(ErrorTypes.serverError, 500)])
}

Here’s a walk-through:

• This takes in your .topicGet route.
• Since this is a GET method, you use the .GET HTTP method.
• You pass in topicsGetOperation as the operation that handles this request.
• Lastly, you pass the error types this route can throw.

1. This function specifies which operation executes for each individual route. SmokeHTTP1HandlerSelector restricts the handler to an operation type using the HTTP1 protocol. It also has an associated type requirements for its ContextType and OperationIdentifier, which you set to your ApplicationContext and ModelOperations.
2. Using the provided selector, you invoke addHandlerForOperation(_:httpMethod:operation: allowedErrors).

Conforming to Smoke Protocols

At this point, your build isn’t passing again. Smoke leverages Swift protocols to ensure that your types satisfy the type requirements it expects.

In this case, TopicsGetRequest and TopicsGetResponse need to conform to OperationHTTP1InputProtocol and OperationHTTP1OutputProtocol respectively. You’ll fix that next.

Add the following code at the bottom of TopicsGetRequest.swift:

extension TopicsGetRequest: OperationHTTP1InputProtocol {
  public static func compose(queryDecodableProvider: () throws -> TopicsGetRequest, 
                              pathDecodableProvider: () throws -> TopicsGetRequest, 
                              bodyDecodableProvider: () throws -> TopicsGetRequest, 
                           headersDecodableProvider: () throws -> TopicsGetRequest) 
                                                        throws -> TopicsGetRequest {
    try queryDecodableProvider()
  }
}

OperationHTTP1InputProtocol is a protocol that represents the input to an operation for an HTTP request. It defines associatedtype requirements for how the operation will be used and decoded. In your case, your TopicsGetRequest is Codable and it’s the type that is to be decoded.

Similarly, add the following protocol conformance to TopicsGetResponse.swift:

// 1
extension TopicsGetResponse: OperationHTTP1OutputProtocol {
  // 2
  public var bodyEncodable: TopicsGetResponse? { self }
  // 3
  public var additionalHeadersEncodable: TopicsGetResponse? { nil }
}

Here’s the breakdown of the above code:

1. OperationHTTP1OutputProtocol is a protocol that represents the output from an operation for an HTTP response.
2. You define what you’ll encode in the body of the response, which is TopicsGetResponse.
3. Since you don’t need any additional headers, you return nil for additionalHeadersEncodable.

Success! Build once again, and you’ll see everything builds with no compilation errors.

You’re almost ready to finally run your server. The last thing you need to do is to configure your application server.

Configuring the Application Server

Time to set up the server so that when you finally get to build and run, the server knows what to do with any requests that it receives.

In main.swift, add the following:

import SmokeOperationsHTTP1
import SmokeOperationsHTTP1Server
import AsyncHTTPClient
import NIO
import SmokeHTTP1

// 1
struct LevelUpInvocationContextInitializer: SmokeServerPerInvocationContextInitializer {
  // 2
  typealias SelectorType =
    StandardSmokeHTTP1HandlerSelector<ApplicationContext,
                                      MyOperationDelegate,
                                      ModelOperations>

  let handlerSelector: SelectorType

  // 3
  let applicationContext = ApplicationContext()
}

// 4
typealias MyOperationDelegate =
  JSONPayloadHTTP1OperationDelegate<SmokeInvocationTraceContext>

Looking at the code above:

1. To start the HTTP1 server, Smoke requires a type that conforms to SmokeServerPerInvocationContextInitializer.
2. You then define a SelectorType type alias to pick a handler based on the URI and HTTP method of the incoming request. Notice it uses both ApplicationContext and ModelOperations, which you created earlier, as well as a new type that you’ll learn about in a moment.
3. You create an ApplicationContext that will be passed to each of the operations.
4. Smoke has a convenience type alias to delegate any special encoding and decoding to an operation delegate. The framework also provides a default JSONPayloadHTTP1OperationDelegate that expects JSON requests and responses, which your REST API uses. The SmokeInvocationTraceContext provides some basic tracing of your request and response headers.

Now, add the following to the end of LevelUpInvocationContextInitializer:

// 1
init(eventLoop: EventLoop) throws {
    var selector = SelectorType(
      defaultOperationDelegate: JSONPayloadHTTP1OperationDelegate()
    )
    addOperations(selector: &selector)

    self.handlerSelector = selector
}

// 2
public func getInvocationContext(
  invocationReporting: SmokeServerInvocationReporting<SmokeInvocationTraceContext>)
  -> ApplicationContext {
  applicationContext
}

// 3
func onShutdown() throws {}

Here’s what this code does:

1. When initializing this class, you use SwiftNIO’s EventLoop to create an instance of SelectorType and add the operations to your server.
2. Smoke uses getInvocationContext(invocationReporting:) when a new request comes in. From it, you return the same instance of applicationContext. If you created a new instance here, you’d never persist any of your awesome topics!
3. If you had anything to do on server shutdown, like cleaning up resources, you’d do so here. For this tutorial, leave this blank.

Finally, add at the bottom of the file:

SmokeHTTP1Server.runAsOperationServer(LevelUpInvocationContextInitializer.init)

This will start your server on port 8080.

Give it a go! Build and run.

The Xcode console will show an entry like this to tell you that the server is up and running on port 8080.

2020-07-18T10:01:33+1000 info: SmokeHTTP1Server started on port 8080.

Running Your Server

Now that your server is running, use your favorite REST client to test it out. This tutorial uses the free version of Insomnia.

Set up the request as follows:

• URL: http://localhost:8080/topics
• Method: GET

You’ll see an empty list of topics returned, which isn’t too interesting.
You still need a way to add topics before this can help you level up your skills. It’s time to level up this API and add the ability to add new topics.