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Event Stream - Server Side

Eduardo Julião
Software Developer | Geek since 1993 | @monsieurxuxu on twitter
・2 min read

For this post, we're going to explore a way to stream data to the client using a Web Api and C#.

Picture this requirement, you need to process an array of data, lets say of a 100 items, once each item is processed, the api should return data to the client, without SignalR.

To do this, in this example, we will have a worker that will return a IAsyncEnumerable and between each iteration, the method will wait between 5 to 10 seconds.

//Service.cs
private static readonly Random _random = new Random();

public async IAsyncEnumerable<int> GetData(int amount)
{
    foreach(var item in Enumerable.Range(0, amount))
    {
        // Wait a random amount of time to simulate some work.
        var secondsToWait = _random.Next(5,10);
        await Task.Delay(Timespan.FromSeconds(secondsToWait));
        yield return item;
    }
}
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With the simulated service in place, we can move on to the Controller. For the endpoint, we're going to have an async Task Get method that will write the result to the Response and send it for every item it was processed.

// StreamController.cs
private readonly IService _service;

public StreamController(IService service)
{
    _service = service;
}

[HttpGet("{amount:int:min(1)}")]
public async Task GetStreamData(int amount)
{
    // Set Content-Type to text/event-stream
    Response.Headers.Add("Content-Type", "text/event-stream");

    await foreach(var data in _service.GetData(amount))
    {
        // this can be anything, even just a json object
        string dataItem = $"data: {data}\n\n";

        // Convert the text to a byte array
        var utf8DataitemBytes = Encoding.UTF8.GetBytes(dataItem);

        // Write the byte array to the HttpResponse
        await Response.Body.WriteAsync(utf8DataitemBytes, 0, utf8DataitemBytes.Length);
        // Push
        await Response.Body.FlushAsync();
    }
}
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When the endpoint is called, it'll send a new "line" of data instead of waiting for all items to be processed to be sent at once.

Discussion (2)

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greenroommate profile image
Haris Secic

HTTP is using TCP/IP (for now mainly). So the please fix that part "without TCP/IP" as it's missleading. And it's not returning HttpRequest it's using it to get the response object and write to it.

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eduardojuliao profile image
Eduardo Julião Author

Thanks for the feedback!
Yes, reading through that part again it was misleading.