ASP.NET Core SignalR supports streaming from client to server and from server to client. This is useful for scenarios where fragments of data arrive over time. When streaming, each fragment is sent to the client or server as soon as it becomes available, rather than waiting for all of the data to become available.
[View or download sample code](https://github.com/dotnet/AspNetCore.Docs/tree/live/aspnetcore/signalr/streaming/samples/) ([how to download](xref:index#how-to-download-a-sample))
A hub method automatically becomes a streaming hub method when it returns <xref:System.Collections.Generic.IAsyncEnumerable`1>, <xref:System.Threading.Channels.ChannelReader%601>, `Task<IAsyncEnumerable<T>>`, or `Task<ChannelReader<T>>`.
Streaming hub methods can return `IAsyncEnumerable<T>` in addition to `ChannelReader<T>`. The simplest way to return `IAsyncEnumerable<T>` is by making the hub method an async iterator method as the following sample demonstrates. Hub async iterator methods can accept a `CancellationToken` parameter that's triggered when the client unsubscribes from the stream. Async iterator methods avoid problems common with Channels, such as not returning the `ChannelReader` early enough or exiting the method without completing the <xref:System.Threading.Channels.ChannelWriter`1>.
The following sample shows the basics of streaming data to the client using Channels. Whenever an object is written to the <xref:System.Threading.Channels.ChannelWriter%601>, the object is immediately sent to the client. At the end, the `ChannelWriter` is completed to tell the client the stream is closed.
> Write to the `ChannelWriter<T>` on a background thread and return the `ChannelReader` as soon as possible. Other hub invocations are blocked until a `ChannelReader` is returned.
> Wrap logic in a [`try ... catch` statement](/dotnet/csharp/language-reference/keywords/try-catch). Complete the `Channel` in a [`finally` block](/dotnet/csharp/language-reference/keywords/try-catch-finally). If you want to flow an error, capture it inside the `catch` block and write it in the `finally` block.
Server-to-client streaming hub methods can accept a `CancellationToken` parameter that's triggered when the client unsubscribes from the stream. Use this token to stop the server operation and release any resources if the client disconnects before the end of the stream.
A hub method automatically becomes a client-to-server streaming hub method when it accepts one or more objects of type <xref:System.Threading.Channels.ChannelReader%601> or <xref:System.Collections.Generic.IAsyncEnumerable%601>. The following sample shows the basics of reading streaming data sent from the client. Whenever the client writes to the <xref:System.Threading.Channels.ChannelWriter%601>, the data is written into the `ChannelReader` on the server from which the hub method is reading.
The `StreamAsync` and `StreamAsChannelAsync` methods on `HubConnection` are used to invoke server-to-client streaming methods. Pass the hub method name and arguments defined in the hub method to `StreamAsync` or `StreamAsChannelAsync`. The generic parameter on `StreamAsync<T>` and `StreamAsChannelAsync<T>` specifies the type of objects returned by the streaming method. An object of type `IAsyncEnumerable<T>` or `ChannelReader<T>` is returned from the stream invocation and represents the stream on the client.
A `StreamAsync` example that returns `IAsyncEnumerable<int>`:
```csharp
// Call "Cancel" on this CancellationTokenSource to send a cancellation message to
// the server, which will trigger the corresponding token in the hub method.
var cancellationTokenSource = new CancellationTokenSource();
* The `StreamAsChannelAsync` method on `HubConnection` is used to invoke a server-to-client streaming method. Pass the hub method name and arguments defined in the hub method to `StreamAsChannelAsync`.
* The generic parameter on `StreamAsChannelAsync<T>` specifies the type of objects returned by the streaming method.
* A `ChannelReader<T>` is returned from the stream invocation and represents the stream on the client.
There are two ways to invoke a client-to-server streaming hub method from the .NET client. You can either pass in an `IAsyncEnumerable<T>` or a `ChannelReader` as an argument to `SendAsync`, `InvokeAsync`, or `StreamAsChannelAsync`, depending on the hub method invoked.
Whenever data is written to the `IAsyncEnumerable` or `ChannelWriter` object, the hub method on the server receives a new item with the data from the client.
If using an `IAsyncEnumerable` object, the stream ends after the method returning stream items exits.
[!INCLUDE[](~/includes/csharp-8-required.md)]
```csharp
async IAsyncEnumerable<string> clientStreamData()
{
for (var i = 0; i <5;i++)
{
var data = await FetchSomeData();
yield return data;
}
//After the for loop has completed and the local function exits the stream completion will be sent.
* Arguments defined in the hub method. In the following example, the arguments are a count for the number of stream items to receive and the delay between stream items.
`connection.stream` returns an `IStreamResult`, which contains a `subscribe` method. Pass an `IStreamSubscriber` to `subscribe` and set the `next`, `error`, and `complete` callbacks to receive notifications from the `stream` invocation.
To end the stream from the client, call the `dispose` method on the `ISubscription` that's returned from the `subscribe` method. Calling this method causes cancellation of the `CancellationToken` parameter of the Hub method, if you provided one.
JavaScript clients call client-to-server streaming methods on hubs by passing in a `Subject` as an argument to `send`, `invoke`, or `stream`, depending on the hub method invoked. The `Subject` is a class that looks like a `Subject`. For example in RxJS, you can use the [Subject](https://rxjs-dev.firebaseapp.com/api/index/class/Subject) class from that library.
The `stream` method on `HubConnection` returns an Observable of the stream item type. The Observable type's `subscribe` method is where `onNext`, `onError` and `onCompleted` handlers are defined.
The SignalR Java client can call client-to-server streaming methods on hubs by passing in an [Observable](https://rxjs-dev.firebaseapp.com/api/index/class/Observable) as an argument to `send`, `invoke`, or `stream`, depending on the hub method invoked.
