[swift-server-dev] HTTP API Sketch v2

[George Leontiev]

I’ve fleshed out my earlier suggestion.

For responses coming back from a server or requests going to a server I like what Helge referred to as a “parser delegate” (naming suggestions welcome) because it allows us to parse the request or response exactly as it comes of the wire, and make no assumptions about how the user wants that data. Also, this API can be used with both an async API (built on top of dispatch) but could also be leveraged to create a synchronous API. 

In terms of utility, I like that clients can provide their own HTTPResponseDecoders which can decode to the specific type they are expecting from that endpoint. Servers, on the other hand, can perform their routing logic while processing that path and use the results of that to more appropriately decode the rest of the request. 

As I and Helge mentioned, there is overhead to using a protocol cross-module, but the benefit is that we no longer have to have a catch-all type to represent a decoded request or response; this allows users who care about performance to encode their concrete types as efficiently as they can. We can also have a catch-all type provided by the standard library which leaves all the fields as strings (indeed, it can be backed by the actual request data and just point to ranges) and would benefit from full-module optimization. Even better, we can make an opinionated concrete implementation (which can use enums for stata and headers and enforce things like the ContentType header value should always be a MediaType, etc.) but allow specialized applications which need more exotic semantics to fall back to implementing their own type (instead of having `.custom(…)` enum cases)

Note: I specifically omitted the body implementation and we may need something like Johannes’  *Writer API to support sending chunked things.

- George

Code:
// MARK: - Stubs

public protocol HTTPBody {
    // API omitted.
    // There are decisions about how to handle streaming bodies which are outside the scope of this example.
}

// Definition omitted
public typealias HTTPHeader = Void

public struct PathIterator: IteratorProtocol {
    
    func next() -> String {
        fatalError()
    }
    
}

// MARK: - Body Decoder

public protocol HTTPBodyDecoder {
    
    mutating func decodeTrailingHeader(_ key: String, value: String) throws
    
    mutating func decodeBodyChunk(_ data: Data) throws
    
}

// MARK: - Server API

// A type which can be created from an HTTP Request as it is coming off the wire. Suggestions on naming are welcome!
public protocol HTTPRequestDecoder: HTTPBodyDecoder {
    
    associatedtype Request
    
    // Thrown exceptions terminate decoding
    
    // Called before any other methods
    mutating func decodeMethod(_ value: String) throws
    
    // Called after `decodeMethod` and before any other methods. Types conforming to `HTTPRequestDecoder` can use this method to walk the requested path and prepare to handle the rest of the request appropriately. Implementation does not need to exhaust the iterator.
    mutating func decodePath(_ iterator: inout PathIterator) throws
    
    // Called after all of the above methods, and before any methods below.
    mutating func decodeVersion(_ value: String) throws
    
    // Called after all of the above methods and before any methods below. May be called multiple times
    mutating func decodeHeader(_ key: String, value: String) throws
    
    // HTTPBodyDecoder methods conceptually appear here
    
    // Returns an object to be used to represent the request
    func decoded() throws -> Request
    
}

public extension HTTPRequestDecoder {
    
    mutating func decodeVersion(_ value: String) throws {
        // Implementing decodeVersion is not necessary
    }
    
}

/**
 A type which can be represented as an HTTP Response
 */
public protocol HTTPResponse {
    var statusCode: UInt { get }
    var headers: [HTTPHeader] { get }
    var body: HTTPBody { get }
}

// MARK: - Client API

/**
 A type which can be represented as an HTTP Request
 */
public protocol HTTPRequest {
    var method: String { get }
    
    // Note that the target host a part of an HTTPRequest
    var path: String { get }
    var headers: [HTTPHeader] { get }
    var body: HTTPBody { get }
}

public protocol HTTPResponseDecoder: HTTPBodyDecoder {
    
    associatedtype Response
    
    // Thrown exceptions terminate decoding
    
    // Called before any other methods
    mutating func decodeVersion(_ value: String) throws
    
    // Called after `decodeVersion` and before any other methods
    mutating func decodeStatus(_ value: UInt) throws
    
    // Called after `decodeVersion` and before any methods below
    mutating func decodeStatusText(_ value: String) throws
    
    // Called after all of the above methods and before any methods below. May be called multiple times
    mutating func decodeHeader(_ key: String, value: String) throws
    
    // HTTPBodyDecoder methods conceptually appear here
    
    // Returns an object to be used to represent the response
    func decoded() throws -> Response
    
}

// MARK: - Implementing concrete types.

// We can provide come concrete implementations to handle *most* cases. For instance, a concrete type could only support IANA registered stata (as an enum) while a separate concrete type can allow for custom stata.

// Here is an example for a very simple server

public enum GeorgeMethod: String {
    case get, post
}

public struct GeorgeHTTPRequest {
    
    let status: GeorgeStatus
    
    // Implementation omitted
    
}

// We can provide convenience types/protocols for generating decoders from types like my simple enum type above
public struct GeorgeHTTPRequestDecoder: HTTPRequestDecoder {
    
    // Implementation omitted
    
    func decode() throws -> GeorgeHTTPRequest {
        fatalError()
    }
    
}


> On Apr 9, 2017, at 4:50 AM, Helge Heß via swift-server-dev <swift-server-dev at swift.org> wrote:
> 
> Hi,
> 
> On 9. Apr 2017, at 04:12, George Leontiev via swift-server-dev <swift-server-dev at swift.org> wrote:
>> Earlier in this thread, I think someone dismissed protocols/generics because “at some point, we need to implement a concrete type”.
> 
> I don’t think anything was ‘dismissed’. The sole meaning was that there will be a Swift Server module and that this should/will come with a concrete type (so that people can have a minimal running http server out of the box by just using that module). You can still have protocols or whatever you chose.
> 
> 
>> I think we can leverage the Swift type system to have something much better.
>> 
>> I recommend having protocols for HTTP{Request,Response}{Encodable,Decoder} types
> 
> <snip> protocols <snip>
> 
> Your `HTTPRequestEncodable` is in a way the reverse of Johannes’ HTTPResponseWriter. The advantage of the latter is that it also doesn’t enforce a specific response type and neither requires a type at all. W/ your approach there always has to be a type implementing the protocol.
> 
> I’m not entirely sure why you would call it `HTTPRequestEncodable`. I would just call it `HTTPRequest`? To me an `Encodable` has a method which provides another value (the encoded representation). Like this:
> 
>  protocol HTTPRequestEncodable {
>    func encodeAsHTTPRequest() -> HTTPRequest
>  }
> 
> which may still be useful. But extra indirections :-)
> 
> 
> Your `HTTPRequestDecoder` is like a parser delegate, kinda like the peer to the `HTTPResponseWriter`. Could be done like this.
> Though if you provide an “Encodable” I suppose the assumption would be that you also provide a “Decodable” too ...
> 
> 
>> I can flesh out the APIs more if people are interested, but I think this is sufficient to communicate the idea, and I’d love to hear more feedback. 
> 
> A complete proposal like Johannes’ would be more useful. Or maybe even better a revision of Johannes’ proposal with your ideas in it.
> 
> 
>> There is a performance overhead to using protocols/generics, but I believe these can be effectively mediated by the compiler.
> 
> I don’t see how you would eliminate the protocol overhead cross-module. But I think it should be small enough (after all the proposal also uses Strings ;-).
> 
> hh
> 
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