返回查询结果的异步流

Question

我有以下 WebApi 方法，它从 RavenDB 返回一个无限的结果流：

public IEnumerable<Foo> Get()
{
    var query = DocumentSession.Query<Foo, FooIndex>();
    using (var enumerator = DocumentSession.Advanced.Stream(query))
        while (enumerator.MoveNext())
            yield return enumerator.Current.Document;
}

现在我想让它异步。天真的方法当然行不通：

public async Task<IEnumerable<Location>> Get()
{
    var query = AsyncDocumentSession.Query<Foo, FooIndex>();
    using (var enumerator = await AsyncDocumentSession.Advanced.StreamAsync(query))
        while (await enumerator.MoveNextAsync())
            yield return enumerator.Current.Document;
}

...因为方法不能既是异步又是迭代器。

Answer 1

由于这是一种 WebAPI 操作方法，HTTP 将您限制为单个响应。如果你只返回一个IEnumerable<T>，那么 ASP.NET 会在内存中枚举它，然后发送响应。

如果你对这个内存进程没问题，那么你可以自己做同样的事情：

public async Task<List<Location>> Get()
{
  var result = new List<Location>();
  var query = AsyncDocumentSession.Query<Foo, FooIndex>();
  using (var enumerator = await AsyncDocumentSession.Advanced.StreamAsync(query))
    while (await enumerator.MoveNextAsync())
      result.Add(enumerator.Current.Document);
  return result;
}

但是，我认为使用流式响应会更好，您可以通过PushStreamContent 获得；像这样：

public HttpResponseMessage Get()
{
  var query = AsyncDocumentSession.Query<Foo, FooIndex>();
  HttpResponseMessage response = Request.CreateResponse();
  response.Content = new PushStreamContent(
      async (stream, content, context) =>
      {
        using (stream)
        using (var enumerator = await AsyncDocumentSession.Advanced.StreamAsync(query))
        {
          while (await enumerator.MoveNextAsync())
          {
            // TODO: adjust encoding as necessary.
            var serialized = JsonConvert.SerializeObject(enumerator.CurrentDocument);
            var data = UTF8Encoding.UTF8.GetBytes(serialized);
            var countPrefix = BitConverter.GetBytes(data.Length);
            await stream.WriteAsync(countPrefix, 0, countPrefix.Length);
            await stream.WriteAsync(data, 0, data.Length);
          }
        }
      });
  return response;
}

流式响应不需要您的服务器将整个响应保存在内存中；但是，您必须决定将文档写入响应流的正确方法。上面的示例代码只是将它们转换为 JSON，以 UTF8 编码，以及（二进制）长度前缀这些字符串。

Answer 2

您可以实现自己的迭代器，而不是让编译器为您生成一个。

但是，在该迭代器上调用 MoveNext 也必须是异步的 - 这意味着您无法实现 IEnumerable<T>`IEnumerator, you'd have to define your own interface, e.g.,IAsyncEnumerator`。 而且您也无法在 foreach 循环中使用该迭代器。

在我看来，最好的办法是做StreamAsync 所做的事情。创建一个自定义类型IAsyncEnumerable，它返回一个实现自定义async T MoveNextAsync() 方法的IAsyncEnumerator<T>。 enumerable 将包装您的 query 对象，并且 enumerator 将获取文档会话的文档。

internal class AsyncDocumentEnumerable : IAsyncEnumerable<Document>
{
    private readonly YourQueryType _query;
    public AsyncDocumentEnumerable(YourQueryType query)
    {
        _query = query;
    }

    IAsyncEnumerator<Document> GetEnumerator()
    {
        return new AsyncDocumentEnumerator(_query);
    }
}


internal class AsyncDocumentEnumerator : IAsyncDocumentEnumerator<Document>
{
    private readonly YourQueryType _query;
    private IAsyncEnumerator<DocumentSession> _iter;

    public AsyncDocumentEnumerator(YourQueryType query)
    {
        _query = query;
    }

    public Task<bool> async MoveNextAsync()
    {
        if(_iter == null)
            _iter = await AsyncDocumentSession.Advanced.StreamAsync(query);

        bool moved = await _iter.MoveNextAsync();

        if(moved)
            Current = _iter.Current.Document;
        return moved;
    }

    public Document Current{get; private set;}
}

Answer 3

毕竟这并不难。解决方案是一个可以异步处理枚举器并将 JSON 写入流的格式化程序：

public class CustomJsonMediaTypeFormatter : JsonMediaTypeFormatter
{
    public override async Task WriteToStreamAsync(
           Type type, object value, Stream writeStream, HttpContent content,
           TransportContext transportContext, CancellationToken cancellationToken)
    {
        if (type.IsGenericType &&
            type.GetGenericTypeDefinition() == typeof(IAsyncEnumerator<>))
        {
            var writer = new JsonTextWriter(new StreamWriter(writeStream))
                         { CloseOutput = false };
            writer.WriteStartArray();
            await Serialize((dynamic)value, writer);
            writer.WriteEndArray();
            writer.Flush();
        }
        else
            await base.WriteToStreamAsync(type, value, writeStream, content,
                                          transportContext, cancellationToken);
    }

    async Task Serialize<T>(IAsyncEnumerator<StreamResult<T>> enumerator,
                            JsonTextWriter writer)
    {
        var serializer = JsonSerializer.Create(SerializerSettings);
        while (await enumerator.MoveNextAsync())
            serializer.Serialize(writer, enumerator.Current.Document);
    }
}

现在我的 WebApi 方法比以前更短了：

public Task<IAsyncEnumerator<StreamResult<Foo>>> Get()
{
    var query = AsyncDocumentSession.Query<Foo, FooIndex>();
    return AsyncDocumentSession.Advanced.StreamAsync(query);
}

Answer 4

他们在 C#8 中引入了IAsyncEnumerable<int>

    async IAsyncEnumerable<int> GetBigResultsAsync()
    {
        await foreach (var result in GetResultsAsync())
        {
            if (result > 20) yield return result; 
        }
    }

Answer 5

您可以查看ReactiveExtensions for .Net，它们是专门为您的需求而设计的。最终结果可能如下所示：

public IObservable<Location> Get()
        {
            var locations = new Subject<Location>();

            Task.Run(() =>
                     {
                         var query = DocumentSession.Query<Foo, FooIndex>();
                         foreach (var document in DocumentSession.Advanced.Stream(query))
                         {
                             locations.OnNext(document);
                         }
                         locations.OnCompleted();
                     });

            return locations;
        }