ASP.NET Web API is an ideal platform for building RESTful applications on the .NET Framework. While I may be more partial to NodeJS these days, there is no denying that WebAPI is a well engineered framework. What follows is my investigation of how to leverage WebAPI to construct a RESTful frontend API. The Advantages of REST Methodology over SOAP Simpler API for CRUD ops Standardize Development methodology - consistent and intuitive Standards based à client interop Wide industry adoption, Ease of use à easy to add new devs Avoid service method signature blowout Smaller payloads than SOAP Stateless à no session data means multi-tenant scalability Cache-ability Testability General RESTful API Design Overview · utilize HTTP Protocol - Usage of HTTP methods for CRUD, standard HTTP response codes, common HTTP headers and Mime Types · Resources are mapped to URLs, actions are mapped to verbs and the rest goes in the headers. · keep the API semantic, resource-centric – A RESTful, resource-oriented service exposes a URI for every piece of data the client might want to operate on. A REST-RPC Hybrid exposes a URI for every operation the client might perform: one URI to fetch a piece of data, a different URI to delete that same data. utilize Uri to specify CRUD op, version, language, output format: http://api.MyApp.com/{ver}/{lang}/{resource_type}/{resource_id}.{output_format}?{key&filters} · entity CRUD operations are matched to HTTP methods: · Create - POST / PUT · Read – GET - cacheable · Update – PUT · Delete - DELETE · Use Uris to represent a hierarchies - Resources in RESTful URLs are often chained · Statelessness allows for idempotency – apply an op multiple times without changing the result. POST is non-idempotent, the rest are idempotent (if DELETE flags records instead of deleting them). · Cache indication - Leverage HTTP headers to label cacheable content and indicate the permitted duration of cache · PUT vs POST - The client uses PUT when it determines which URI (Id key) the new resource should have. The client uses POST when the server determines they key. PUT takes a second param – the id. POST creates a new resource. The server assigns the URI for the new object and returns this URI as part of the response message. Note: The PUT method replaces the entire entity. That is, the client is expected to send a complete representation of the updated product. If you want to support partial updates, the PATCH method is preferred DELETE deletes a resource at a specified URI – typically takes an id param · Leverage Common HTTP Response Codes in response headers 200 OK: Success 201 Created - Used on POST request when creating a new resource. 304 Not Modified: no new data to
return. 400 Bad Request: Invalid Request. 401 Unauthorized: Authentication. 403 Forbidden: Authorization 404 Not Found – entity does not exist. 406 Not Acceptable – bad params. 409 Conflict - For POST / PUT requests if the resource already exists. 500 Internal Server Error 503 Service Unavailable · Leverage uncommon HTTP Verbs to reduce payload sizes HEAD - retrieves just the resource meta-information. OPTIONS returns the actions supported for the specified resource. PATCH - partial modification of a resource. · When using PUT, POST or PATCH, send the data as a document in the body of the request. Don't use query parameters to alter state. · Utilize Headers for content negotiation, caching, authorization, throttling o Content Negotiation – choose representation (e.g. JSON or XML and version), language & compression. Signal via RequestHeader.Accept & ResponseHeader.Content-Type Accept: application/json;version=1.0 Accept-Language: en-US Accept-Charset: UTF-8 Accept-Encoding: gzip o Caching - ResponseHeader: Expires (absolute expiry time) or Cache-Control (relative expiry time) o Authorization - basic HTTP authentication uses the RequestHeader.Authorization to specify a base64 encoded string "username:password". can be used in combination with SSL/TLS (HTTPS) and leverage OAuth2 3rd party token-claims authorization. Authorization: Basic sQJlaTp5ZWFslylnaNZ= o Rate Limiting - Not currently part of HTTP so specify non-standard headers prefixed with X- in the ResponseHeader. X-RateLimit-Limit: 10000 X-RateLimit-Remaining: 9990 · HATEOAS Methodology - Hypermedia As The Engine Of Application State – leverage API as a state machine where resources are states and the transitions between states are links between resources and are included in their representation (hypermedia) – get API metadata signatures from the response Link header - in a truly REST based architecture any URL, except the initial URL, can be changed, even to other servers, without worrying about the client. · error responses - Do not just send back a 200 OK with every response. Response should consist of HTTP error status code (JQuery has automated support for this), A human readable message , A Link to a meaningful state transition , & the original data payload that was problematic. · the URIs will typically map to a server-side controller and a method name specified by the type of request method. Stuff all your calls into just four methods is not as crazy as it sounds. · Scoping - Path variables look like you’re traversing a hierarchy, and query variables look like you’re passing arguments into an algorithm · Mapping URIs to Controllers - have one controller for each resource is not a rule – can consolidate - route requests to the appropriate controller and action method · Keep URls Consistent - Sometimes it’s tempting to just shorten our URIs. not recommend this as this can cause confusion · Join Naming – for m-m entity relations there may be multiple hierarchy traversal paths · Routing – useful level of indirection for versioning, server backend mocking in development ASPNET WebAPI Considerations ASPNET WebAPI implements a lot (but not all) RESTful API design considerations as part of its infrastructure and via its coding convention. Overview When developing an API there are basically three main steps: 1. Plan out your URIs 2. Setup
return values and response codes for your URIs 3. Implement a framework for your API. Design · Leverage Models MVC folder · Repositories – support IoC for tests, abstraction · Create DTO classes – a level of indirection decouples & allows swap out · Self links can be generated using the UrlHelper · Use IQueryable to support projections across the wire · Models can support restful navigation properties – ICollection<T> · async mechanism for long running ops -
return a response with a ticket – the client can then poll or be pushed the final result later. · Design for testability - Test using HttpClient , JQuery ( $.getJSON , $.each) , fiddler, browser debug. Leverage IDependencyResolver – IoC wrapper for mocking · Easy debugging - IE F12 developer tools: Network tab, Request Headers tab Routing · HTTP request method is matched to the method name. (This rule applies only to GET, POST, PUT, and DELETE requests.) · {id}, if present, is matched to a method parameter named id. · Query parameters are matched to parameter names when possible · Done in config via Routes.MapHttpRoute – similar to MVC routing · Can alternatively: o decorate controller action methods with HttpDelete, HttpGet, HttpHead,HttpOptions, HttpPatch, HttpPost, or HttpPut., + the ActionAttribute o use AcceptVerbsAttribute to support other HTTP verbs: e.g. PATCH, HEAD o use NonActionAttribute to prevent a method from getting invoked as an action · route table Uris can support placeholders (via curly braces{}) – these can support default values and constraints, and optional values · The framework selects the first route in the route table that matches the URI. Response customization · Response code: By default, the Web API framework sets the response status code to 200 (OK). But according to the HTTP/1.1 protocol, when a POST request results in the creation of a resource, the server should reply with status 201 (Created). Non Get methods should
return HttpResponseMessage · Location: When the server creates a resource, it should include the URI of the new resource in the Location header of the response. public HttpResponseMessage PostProduct(Product item) { item = repository.Add(item); var response = Request.CreateResponse<Product>(HttpStatusCode.Created, item); string uri = Url.Link("DefaultApi", new { id = item.Id }); response.Headers.Location = new Uri(uri);
return response; } Validation · Decorate Models / DTOs with System.ComponentModel.DataAnnotations properties RequiredAttribute, RangeAttribute. · Check payloads using ModelState.IsValid · Under posting – leave out values in JSON payload à JSON formatter assigns a default value. Use with RequiredAttribute · Over-posting - if model has RO properties à use DTO instead of model · Can hook into pipeline by deriving from ActionFilterAttribute & overriding OnActionExecuting Config · Done in App_Start folder > WebApiConfig.cs – static Register method: HttpConfiguration param: The HttpConfiguration object contains the following members. Member Description DependencyResolver Enables dependency injection for controllers. Filters Action filters – e.g. exception filters. Formatters Media-type formatters. by default contains JsonFormatter, XmlFormatter IncludeErrorDetailPolicy Specifies whether the server should include error details, such as exception messages and stack traces, in HTTP response messages. Initializer A function that performs final initialization of the HttpConfiguration. MessageHandlers HTTP message handlers - plug into pipeline ParameterBindingRules A collection of rules for binding parameters on controller actions. Properties A generic property bag. Routes The collection of routes. Services The collection of services. · Configure JsonFormatter for circular references to support links: PreserveReferencesHandling.Objects Documentation generation · create a help page for a web API, by using the ApiExplorer class. · The ApiExplorer class provides descriptive information about the APIs exposed by a web API as an ApiDescription collection · create the help page as an MVC view public ILookup<string, ApiDescription> GetApis() {
return _explorer.ApiDescriptions.ToLookup( api => api.ActionDescriptor.ControllerDescriptor.ControllerName); · provide documentation for your APIs by implementing the IDocumentationProvider interface. Documentation strings can come from any source that you like – e.g. extract XML comments or define custom attributes to apply to the controller [ApiDoc("Gets a product by ID.")] [ApiParameterDoc("id", "The ID of the product.")] public HttpResponseMessage Get(int id) · GlobalConfiguration.Configuration.Services – add the documentation Provider · To hide an API from the ApiExplorer, add the ApiExplorerSettingsAttribute Plugging into the Message Handler pipeline · Plug into request / response pipeline – derive from DelegatingHandler and override theSendAsync method – e.g. for logging error codes, adding a custom response header · Can be applied globally or to a specific route Exception Handling · Throw HttpResponseException on method failures – specify HttpStatusCode enum value – examine this enum, as its values map well to typical op problems · Exception filters – derive from ExceptionFilterAttribute & override OnException. Apply on Controller or action methods, or add to global HttpConfiguration.Filters collection · HttpError object provides a consistent way to
return error information in the HttpResponseException response body. · For model validation, you can pass the model state to CreateErrorResponse, to include the validation errors in the response public HttpResponseMessage PostProduct(Product item) { if (!ModelState.IsValid) {
return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState); Cookie Management · Cookie header in request and Set-Cookie headers in a response - Collection of CookieState objects · Specify Expiry, max-age resp.Headers.AddCookies(new CookieHeaderValue[] { cookie }); Internet Media Types, formatters and serialization · Defaults to application/json · Request Accept header and response Content-Type header · determines how Web API serializes and deserializes the HTTP message body. There is built-in support for XML, JSON, and form-urlencoded data · customizable formatters can be inserted into the pipeline · POCO serialization is opt out via JsonIgnoreAttribute, or use DataMemberAttribute for optin · JSON serializer leverages NewtonSoft Json.NET · loosely structured JSON objects are serialzed as JObject which derives from Dynamic · to handle circular references in json: json.SerializerSettings.PreserveReferencesHandling = PreserveReferencesHandling.All à {"$ref":"1"}. · To preserve object references in XML [DataContract(IsReference=true)] · Content negotiation Accept: Which media types are acceptable for the response, such as “application/json,” “application/xml,” or a custom media type such as "application/vnd.example+xml" Accept-Charset: Which character sets are acceptable, such as UTF-8 or ISO 8859-1. Accept-Encoding: Which content encodings are acceptable, such as gzip. Accept-Language: The preferred natural language, such as “en-us”. o Web API uses the Accept and Accept-Charset headers. (At this time, there is no built-in support for Accept-Encoding or Accept-Language.) · Controller methods can take JSON representations of DTOs as params – auto-deserialization · Typical JQuery GET request: function find() { var id = $('#prodId').val(); $.getJSON("api/products/" + id, function (data) { var str = data.Name + ': $' + data.Price; $('#product').text(str); }) .fail( function (jqXHR, textStatus, err) { $('#product').text('Error: ' + err); }); } · Typical GET response: HTTP/1.1 200 OK Server: ASP.NET Development Server/10.0.0.0 Date: Mon, 18 Jun 2012 04:30:33 GMT X-AspNet-Version: 4.0.30319 Cache-Control: no-cache Pragma: no-cache Expires: -1 Content-Type: application/json; charset=utf-8 Content-Length: 175 Connection: Close [{"Id":1,"Name":"TomatoSoup","Price":1.39,"ActualCost":0.99},{"Id":2,"Name":"Hammer", "Price":16.99,"ActualCost":10.00},{"Id":3,"Name":"Yo yo","Price":6.99,"ActualCost": 2.05}] True OData support · Leverage Query Options $filter, $orderby, $top and $skip to shape the results of controller actions annotated with the [Queryable]attribute. [Queryable] public IQueryable<Supplier> GetSuppliers() · Query: ~/Suppliers?$filter=Name eq ‘Microsoft’ · Applies the following selection filter on the server: GetSuppliers().Where(s => s.Name == “Microsoft”) · Will pass the result to the formatter. · true support for the OData format is still limited - no support for creates, updates, deletes, $metadata and code generation etc · vnext: ability to configure how EditLinks, SelfLinks and Ids are generated Self Hosting no dependency on ASPNET or IIS: using (var server = new HttpSelfHostServer(config)) { server.OpenAsync().Wait(); Tracing · tracability tools, metrics – e.g. send to nagios · use your choice of tracing/logging library, whether that is ETW,NLog, log4net, or simply System.Diagnostics.Trace. · To collect traces, implement the ITraceWriter interface public class SimpleTracer : ITraceWriter { public void Trace(HttpRequestMessage request, string category, TraceLevel level, Action<TraceRecord> traceAction) { TraceRecord rec = new TraceRecord(request, category, level); traceAction(rec); WriteTrace(rec); · register the service with config · programmatically trace – has helper extension methods: Configuration.Services.GetTraceWriter().Info( · Performance tracing - pipeline writes traces at the beginning and end of an operation - TraceRecord class includes aTimeStamp property, Kind property set to TraceKind.Begin / End Security · Roles class methods: RoleExists, AddUserToRole · WebSecurity class methods: UserExists, .CreateUserAndAccount · Request.IsAuthenticated · Leverage HTTP 401 (Unauthorized) response · [AuthorizeAttribute(Roles="Administrator")] – can be applied to Controller or its action methods · See section in WebApi document on "Claim-based-security for ASP.NET Web APIs using DotNetOpenAuth" – adapt this to STS.--> Web API Host exposes secured Web APIs which can only be accessed by presenting a valid token issued by the trusted issuer. http://zamd.net/2012/05/04/claim-based-security-for-asp-net-web-apis-using-dotnetopenauth/ · Use MVC membership provider infrastructure and add a DelegatingHandler child class to the WebAPI pipeline - http://stackoverflow.com/questions/11535075/asp-net-mvc-4-web-api-authentication-with-membership-provider - this will perform the login actions · Then use AuthorizeAttribute on controllers and methods for role mapping- http://sixgun.wordpress.com/2012/02/29/asp-net-web-api-basic-authentication/ · Alternate option here is to rely on MVC App : http://forums.asp.net/t/1831767.aspx/1
