Update: The Web API team is planning to add this to v2. Please visit the Wiki and give feedback!
While there is no batching standard built into the HTTP protocol, there is a standard for MIME encoding HTTP request and response messages ("application/http" with "msgtype=request" and "msgtype=response", respectively). ASP.NET Web API has built-in support for both MIME multipart as well as encoded request and response messages, so we have all the building blocks we need to make a simple batch request handler.
All we need to make this work is an endpoint which can accept a multipart batch (an invention of our own), which then parses the requests, runs them sequentially, and returns the responses back in a multipart batch response.
Starting with a Web API project (built against the latest nightly build), I updated the Web API config to look like this:
var batchHandler = new BatchHandler(config);
config.Routes.MapHttpRoute("batch", "api/batch",
null, null, batchHandler);
config.Routes.MapHttpRoute("default", "api/{controller}/{id}",
new { id = RouteParameter.Optional });
I've inserted the handler for "api/batch" as our endpoint for batching requests, using the new "route-specific endpoint handler" feature in Web API. Note that since its URL is "api/batch", I made sure to add it before the default API route.
Using async & await in .NET 4.5 makes the implementation of BatchHandler fairly straight-forward. All we need is an in-memory HttpServer which uses our existing configuration, so that the batched requests hit the exact same endpoints as requests from the Internet:
public class BatchHandler : HttpMessageHandler
{
HttpMessageInvoker _server;
public BatchHandler(HttpConfiguration config)
{
_server = new HttpMessageInvoker(new HttpServer(config));
}
protected override async Task<HttpResponseMessage> SendAsync(
HttpRequestMessage request,
CancellationToken cancellationToken)
{
// Return 400 for the wrong MIME type
if ("multipart/batch" !=
request.Content.Headers.ContentType.MediaType)
{
return request.CreateResponse(HttpStatusCode.BadRequest);
}
// Start a multipart response
var outerContent = new MultipartContent("batch");
var outerResp = request.CreateResponse();
outerResp.Content = outerContent;
// Read the multipart request
var multipart = await request.Content.ReadAsMultipartAsync();
foreach (var httpContent in multipart.Contents)
{
HttpResponseMessage innerResp = null;
try
{
// Decode the request object
var innerReq = await
httpContent.ReadAsHttpRequestMessageAsync();
// Send the request through the pipeline
innerResp = await _server.SendAsync(
innerReq,
cancellationToken
);
}
catch (Exception)
{
// If exceptions are thrown, send back generic 400
innerResp = new HttpResponseMessage(
HttpStatusCode.BadRequest
);
}
// Wrap the response in a message content and put it
// into the multipart response
outerContent.Add(new HttpMessageContent(innerResp));
}
return outerResp;
}
}
Now we have an endpoint that we can send multipart/batch requests to, which are assumed to be HTTP request objects (anything which isn't is going to yield a 400).
On the client side, we make a multipart request and push requests into the multipart batch, one at a time:
var client = new HttpClient();
var batchRequest = new HttpRequestMessage(
HttpMethod.Post,
"http://localhost/api/batch"
);
var batchContent = new MultipartContent("batch");
batchRequest.Content = batchContent;
batchContent.Add(
new HttpMessageContent(
new HttpRequestMessage(
HttpMethod.Get,
"http://localhost/api/values"
)
)
);
batchContent.Add(
new HttpMessageContent(
new HttpRequestMessage(
HttpMethod.Get,
"http://localhost/foo/bar"
)
)
);
batchContent.Add(
new HttpMessageContent(
new HttpRequestMessage(
HttpMethod.Get,
"http://localhost/api/values/1"
)
)
);
In a console application, we can log both the request and response with code like this:
using (Stream stdout = Console.OpenStandardOutput())
{
Console.WriteLine("<<< REQUEST >>>");
Console.WriteLine();
Console.WriteLine(batchRequest);
Console.WriteLine();
batchContent.CopyToAsync(stdout).Wait();
Console.WriteLine();
var batchResponse = client.SendAsync(batchRequest).Result;
Console.WriteLine("<<< RESPONSE >>>");
Console.WriteLine();
Console.WriteLine(batchResponse);
Console.WriteLine();
batchResponse.Content.CopyToAsync(stdout).Wait();
Console.WriteLine();
Console.WriteLine();
}
When I run this console application, I see output similar to this:
<<< REQUEST >>>
Method: POST,
RequestUri: 'http://localhost/api/batch',
Version: 1.1,
Content: System.Net.Http.MultipartContent,
Headers:
{
Content-Type: multipart/batch; boundary="3bc5bd67-3517-4cd0-bcdd-9d23f3850402"
}
--3bc5bd67-3517-4cd0-bcdd-9d23f3850402
Content-Type: application/http; msgtype=request
GET /api/values HTTP/1.1
Host: localhost
--3bc5bd67-3517-4cd0-bcdd-9d23f3850402
Content-Type: application/http; msgtype=request
GET /foo/bar HTTP/1.1
Host: localhost
--3bc5bd67-3517-4cd0-bcdd-9d23f3850402
Content-Type: application/http; msgtype=request
GET /api/values/1 HTTP/1.1
Host: localhost
--3bc5bd67-3517-4cd0-bcdd-9d23f3850402--
<<< RESPONSE >>>
StatusCode: 200,
ReasonPhrase: 'OK',
Version: 1.1,
Content: System.Net.Http.StreamContent,
Headers:
{
Pragma: no-cache
Cache-Control: no-cache
Date: Thu, 21 Jun 2012 00:21:40 GMT
Server: Microsoft-IIS/8.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Content-Length: 658
Content-Type: multipart/batch
Expires: -1
}
--3d1ba137-ea6a-40d9-8e34-1b8812394baa
Content-Type: application/http; msgtype=response
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
["Hello","world!"]
--3d1ba137-ea6a-40d9-8e34-1b8812394baa
Content-Type: application/http; msgtype=response
HTTP/1.1 404 Not Found
Content-Type: application/json; charset=utf-8
{"Message":"No HTTP resource was found that matches the request URI 'http://localhost/foo/bar'."}
--3d1ba137-ea6a-40d9-8e34-1b8812394baa
Content-Type: application/http; msgtype=response
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
"world!"
--3d1ba137-ea6a-40d9-8e34-1b8812394baa--
As you can see, our batch was successfully run, and the results show what we'd expected (the two real API calls returned back 200 with their data, and the bogus request we threw in the middle returns back a 404).
