Adds support for ETag header in GET and HEAD requests

docs/usage/caching.md

@@ -0,0 +1,75 @@
+# Caching with ETags
+
+_since v4.2_
+
+GET requests return an [ETag](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag) HTTP header, which can be used by the client in subsequent requests to save network bandwidth.
+
+Be aware that the returned ETag represents the entire response body (a 'resource' in HTTP terminology) for a request URL that includes the query string.
+This is unrelated to JSON:API resources. Therefore, we do not use ETags for optimistic concurrency.
+
+Getting a list of resources returns an ETag:
+
+```http
+GET /articles?sort=-lastModifiedAt HTTP/1.1
+Host: localhost:5000
+```
+
+```http
+HTTP/1.1 200 OK
+Content-Type: application/vnd.api+json
+Server: Kestrel
+Transfer-Encoding: chunked
+ETag: "7FFF010786E2CE8FC901896E83870E00"
+
+{
+  "data": [ ... ]
+}
+```
+
+The request is later resent using the received ETag. The server data has not changed at this point.
+
+```http
+GET /articles?sort=-lastModifiedAt HTTP/1.1
+Host: localhost:5000
+If-None-Match: "7FFF010786E2CE8FC901896E83870E00"
+```
+
+```http
+HTTP/1.1 304 Not Modified
+Server: Kestrel
+ETag: "7FFF010786E2CE8FC901896E83870E00"
+```
+
+After some time, the server data has changed.
+
+```http
+GET /articles?sort=-lastModifiedAt HTTP/1.1
+Host: localhost:5000
+If-None-Match: "7FFF010786E2CE8FC901896E83870E00"
+```
+
+```http
+HTTP/1.1 200 OK
+Content-Type: application/vnd.api+json
+Server: Kestrel
+Transfer-Encoding: chunked
+ETag: "356075D903B8FE8D9921201A7E7CD3F9"
+
+{
+  "data": [ ... ]
+}
+```
+
+Note: To just poll for changes (without fetching them), send a HEAD request instead:
+
+```http
+HEAD /articles?sort=-lastModifiedAt HTTP/1.1
+Host: localhost:5000
+If-None-Match: "7FFF010786E2CE8FC901896E83870E00"
+```
+
+```http
+HTTP/1.1 200 OK
+Server: Kestrel
+ETag: "356075D903B8FE8D9921201A7E7CD3F9"
+```

docs/usage/toc.md

@@ -21,6 +21,7 @@
 # [Routing](routing.md)
 # [Errors](errors.md)
 # [Metadata](meta.md)
+# [Caching](caching.md)
 
 # Extensibility
 ## [Layer Overview](extensibility/layer-overview.md)

src/JsonApiDotNetCore/Configuration/JsonApiApplicationBuilder.cs

@@ -285,6 +285,7 @@ private void AddSerializationLayer()
             _services.AddScoped(typeof(AtomicOperationsResponseSerializer));
             _services.AddScoped(sp => sp.GetRequiredService<IJsonApiSerializerFactory>().GetSerializer());
             _services.AddScoped<IResourceObjectBuilder, ResponseResourceObjectBuilder>();
+            _services.AddSingleton<IETagGenerator, ETagGenerator>();
         }
 
         private void AddOperationsLayer()

src/JsonApiDotNetCore/Controllers/JsonApiController.cs

@@ -42,27 +42,31 @@ protected JsonApiController(IJsonApiOptions options, ILoggerFactory loggerFactor
 
         /// <inheritdoc />
         [HttpGet]
+        [HttpHead]
         public override async Task<IActionResult> GetAsync(CancellationToken cancellationToken)
         {
             return await base.GetAsync(cancellationToken);
         }
 
         /// <inheritdoc />
         [HttpGet("{id}")]
+        [HttpHead("{id}")]
         public override async Task<IActionResult> GetAsync(TId id, CancellationToken cancellationToken)
         {
             return await base.GetAsync(id, cancellationToken);
         }
 
         /// <inheritdoc />
         [HttpGet("{id}/{relationshipName}")]
+        [HttpHead("{id}/{relationshipName}")]
         public override async Task<IActionResult> GetSecondaryAsync(TId id, string relationshipName, CancellationToken cancellationToken)
         {
             return await base.GetSecondaryAsync(id, relationshipName, cancellationToken);
         }
 
         /// <inheritdoc />
         [HttpGet("{id}/relationships/{relationshipName}")]
+        [HttpHead("{id}/relationships/{relationshipName}")]
         public override async Task<IActionResult> GetRelationshipAsync(TId id, string relationshipName, CancellationToken cancellationToken)
         {
             return await base.GetRelationshipAsync(id, relationshipName, cancellationToken);

src/JsonApiDotNetCore/Middleware/JsonApiMiddleware.cs

@@ -4,7 +4,6 @@
 using System.Linq;
 using System.Net;
 using System.Net.Http;
-using System.Net.Http.Headers;
 using System.Text;
 using System.Threading.Tasks;
 using JetBrains.Annotations;
@@ -16,6 +15,7 @@
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.Controllers;
 using Microsoft.AspNetCore.Routing;
+using Microsoft.Net.Http.Headers;
 using Newtonsoft.Json;
 
 namespace JsonApiDotNetCore.Middleware
@@ -45,8 +45,12 @@ public async Task InvokeAsync(HttpContext httpContext, IControllerResourceMappin
             ArgumentGuard.NotNull(request, nameof(request));
             ArgumentGuard.NotNull(resourceContextProvider, nameof(resourceContextProvider));
 
-            RouteValueDictionary routeValues = httpContext.GetRouteData().Values;
+            if (!await ValidateIfMatchHeaderAsync(httpContext, options.SerializerSettings))
+            {
+                return;
+            }
 
+            RouteValueDictionary routeValues = httpContext.GetRouteData().Values;
             ResourceContext primaryResourceContext = CreatePrimaryResourceContext(httpContext, controllerResourceMapping, resourceContextProvider);
 
             if (primaryResourceContext != null)
@@ -77,6 +81,21 @@ public async Task InvokeAsync(HttpContext httpContext, IControllerResourceMappin
             await _next(httpContext);
         }
 
+        private async Task<bool> ValidateIfMatchHeaderAsync(HttpContext httpContext, JsonSerializerSettings serializerSettings)
+        {
+            if (httpContext.Request.Headers.ContainsKey(HeaderNames.IfMatch))
+            {
+                await FlushResponseAsync(httpContext.Response, serializerSettings, new Error(HttpStatusCode.PreconditionFailed)
+                {
+                    Title = "Detection of mid-air edit collisions using ETags is not supported."
+                });
+
+                return false;
+            }
+
+            return true;
+        }
+
         private static ResourceContext CreatePrimaryResourceContext(HttpContext httpContext, IControllerResourceMapping controllerResourceMapping,
             IResourceContextProvider resourceContextProvider)
         {
@@ -130,7 +149,7 @@ private static async Task<bool> ValidateAcceptHeaderAsync(MediaTypeHeaderValue a
 
             foreach (string acceptHeader in acceptHeaders)
             {
-                if (MediaTypeWithQualityHeaderValue.TryParse(acceptHeader, out MediaTypeWithQualityHeaderValue headerValue))
+                if (MediaTypeHeaderValue.TryParse(acceptHeader, out MediaTypeHeaderValue headerValue))
                 {
                     headerValue.Quality = null;
 
@@ -189,7 +208,7 @@ private static async Task FlushResponseAsync(HttpResponse httpResponse, JsonSeri
         private static void SetupResourceRequest(JsonApiRequest request, ResourceContext primaryResourceContext, RouteValueDictionary routeValues,
             IJsonApiOptions options, IResourceContextProvider resourceContextProvider, HttpRequest httpRequest)
         {
-            request.IsReadOnly = httpRequest.Method == HttpMethod.Get.Method;
+            request.IsReadOnly = httpRequest.Method == HttpMethod.Get.Method || httpRequest.Method == HttpMethod.Head.Method;
             request.Kind = EndpointKind.Primary;
             request.PrimaryResource = primaryResourceContext;
             request.PrimaryId = GetPrimaryRequestId(routeValues);

src/JsonApiDotNetCore/Serialization/ETagGenerator.cs

@@ -0,0 +1,46 @@
+using System.Linq;
+using System.Security.Cryptography;
+using System.Text;
+using Microsoft.Net.Http.Headers;
+
+namespace JsonApiDotNetCore.Serialization
+{
+    /// <inheritdoc />
+    internal sealed class ETagGenerator : IETagGenerator
+    {
+        private static readonly uint[] LookupTable = Enumerable.Range(0, 256).Select(ToLookupEntry).ToArray();
+
+        private static uint ToLookupEntry(int index)
+        {
+            string hex = index.ToString("X2");
+            return hex[0] + ((uint)hex[1] << 16);
+        }
+
+        /// <inheritdoc />
+        public EntityTagHeaderValue Generate(string requestUrl, string responseBody)
+        {
+            byte[] buffer = Encoding.UTF8.GetBytes(requestUrl + "|" + responseBody);
+
+            using HashAlgorithm hashAlgorithm = MD5.Create();
+            byte[] hash = hashAlgorithm.ComputeHash(buffer);
+
+            string eTagValue = "\"" + ByteArrayToHex(hash) + "\"";
+            return EntityTagHeaderValue.Parse(eTagValue);
+        }
+
+        // https://stackoverflow.com/questions/311165/how-do-you-convert-a-byte-array-to-a-hexadecimal-string-and-vice-versa
+        private static string ByteArrayToHex(byte[] bytes)
+        {
+            char[] buffer = new char[bytes.Length * 2];
+
+            for (int index = 0; index < bytes.Length; index++)
+            {
+                uint value = LookupTable[bytes[index]];
+                buffer[2 * index] = (char)value;
+                buffer[2 * index + 1] = (char)(value >> 16);
+            }
+
+            return new string(buffer);
+        }
+    }
+}

src/JsonApiDotNetCore/Serialization/IETagGenerator.cs

@@ -0,0 +1,24 @@
+using Microsoft.Net.Http.Headers;
+
+namespace JsonApiDotNetCore.Serialization
+{
+    /// <summary>
+    /// Provides generation of an ETag HTTP response header.
+    /// </summary>
+    public interface IETagGenerator
+    {
+        /// <summary>
+        /// Generates an ETag HTTP response header value for the response to an incoming request.
+        /// </summary>
+        /// <param name="requestUrl">
+        /// The incoming request URL, including query string.
+        /// </param>
+        /// <param name="responseBody">
+        /// The produced response body.
+        /// </param>
+        /// <returns>
+        /// The ETag, or <c>null</c> to disable saving bandwidth.
+        /// </returns>
+        public EntityTagHeaderValue Generate(string requestUrl, string responseBody);
+    }
+}

src/JsonApiDotNetCore/Serialization/JsonApiWriter.cs

@@ -14,6 +14,7 @@
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.Formatters;
 using Microsoft.Extensions.Logging;
+using Microsoft.Net.Http.Headers;
 
 namespace JsonApiDotNetCore.Serialization
 {
@@ -26,25 +27,28 @@ public class JsonApiWriter : IJsonApiWriter
     {
         private readonly IJsonApiSerializer _serializer;
         private readonly IExceptionHandler _exceptionHandler;
+        private readonly IETagGenerator _eTagGenerator;
         private readonly TraceLogWriter<JsonApiWriter> _traceWriter;
 
-        public JsonApiWriter(IJsonApiSerializer serializer, IExceptionHandler exceptionHandler, ILoggerFactory loggerFactory)
+        public JsonApiWriter(IJsonApiSerializer serializer, IExceptionHandler exceptionHandler, IETagGenerator eTagGenerator, ILoggerFactory loggerFactory)
         {
             ArgumentGuard.NotNull(serializer, nameof(serializer));
             ArgumentGuard.NotNull(exceptionHandler, nameof(exceptionHandler));
+            ArgumentGuard.NotNull(eTagGenerator, nameof(eTagGenerator));
             ArgumentGuard.NotNull(loggerFactory, nameof(loggerFactory));
 
             _serializer = serializer;
             _exceptionHandler = exceptionHandler;
+            _eTagGenerator = eTagGenerator;
             _traceWriter = new TraceLogWriter<JsonApiWriter>(loggerFactory);
         }
 
         public async Task WriteAsync(OutputFormatterWriteContext context)
         {
             ArgumentGuard.NotNull(context, nameof(context));
 
+            HttpRequest request = context.HttpContext.Request;
             HttpResponse response = context.HttpContext.Response;
-            response.ContentType = _serializer.ContentType;
 
             await using TextWriter writer = context.WriterFactory(response.Body, Encoding.UTF8);
             string responseContent;
@@ -63,8 +67,27 @@ public async Task WriteAsync(OutputFormatterWriteContext context)
                 response.StatusCode = (int)errorDocument.GetErrorStatusCode();
             }
 
-            string url = context.HttpContext.Request.GetEncodedUrl();
-            _traceWriter.LogMessage(() => $"Sending {response.StatusCode} response for request at '{url}' with body: <<{responseContent}>>");
+            bool hasMatchingETag = SetETagResponseHeader(request, response, responseContent);
+
+            if (hasMatchingETag)
+            {
+                response.StatusCode = (int)HttpStatusCode.NotModified;
+                responseContent = string.Empty;
+            }
+
+            if (request.Method == HttpMethod.Head.Method)
+            {
+                responseContent = string.Empty;
+            }
+
+            string url = request.GetEncodedUrl();
+
+            if (!string.IsNullOrEmpty(responseContent))
+            {
+                response.ContentType = _serializer.ContentType;
+            }
+
+            _traceWriter.LogMessage(() => $"Sending {response.StatusCode} response for {request.Method} request at '{url}' with body: <<{responseContent}>>");
 
             await writer.WriteAsync(responseContent);
             await writer.FlushAsync();
@@ -96,6 +119,11 @@ private string SerializeResponse(object contextObject, HttpStatusCode statusCode
             return _serializer.Serialize(contextObjectWrapped);
         }
 
+        private bool IsSuccessStatusCode(HttpStatusCode statusCode)
+        {
+            return new HttpResponseMessage(statusCode).IsSuccessStatusCode;
+        }
+
         private static object WrapErrors(object contextObject)
         {
             if (contextObject is IEnumerable<Error> errors)
@@ -111,9 +139,41 @@ private static object WrapErrors(object contextObject)
             return contextObject;
         }
 
-        private bool IsSuccessStatusCode(HttpStatusCode statusCode)
+        private bool SetETagResponseHeader(HttpRequest request, HttpResponse response, string responseContent)
         {
-            return new HttpResponseMessage(statusCode).IsSuccessStatusCode;
+            bool isReadOnly = request.Method == HttpMethod.Get.Method || request.Method == HttpMethod.Head.Method;
+
+            if (isReadOnly && response.StatusCode == (int)HttpStatusCode.OK)
+            {
+                string url = request.GetEncodedUrl();
+                EntityTagHeaderValue responseETag = _eTagGenerator.Generate(url, responseContent);
+
+                if (responseETag != null)
+                {
+                    response.Headers.Add(HeaderNames.ETag, responseETag.ToString());
+
+                    return RequestContainsMatchingETag(request.Headers, responseETag);
+                }
+            }
+
+            return false;
+        }
+
+        private static bool RequestContainsMatchingETag(IHeaderDictionary requestHeaders, EntityTagHeaderValue responseETag)
+        {
+            if (requestHeaders.Keys.Contains(HeaderNames.IfNoneMatch) &&
+                EntityTagHeaderValue.TryParseList(requestHeaders[HeaderNames.IfNoneMatch], out IList<EntityTagHeaderValue> requestETags))
+            {
+                foreach (EntityTagHeaderValue requestETag in requestETags)
+                {
+                    if (responseETag.Equals(requestETag))
+                    {
+                        return true;
+                    }
+                }
+            }
+
+            return false;
         }
     }
 }