[API Proposal]: Streaming HTTP POST API for the System.Net.Http.Json extensions

[IEvangelist]

Background and motivation

As a follow up to the related API proposal #87577, this aims to include Post* APIs to the HttpClientJsonExtensions class as extensions methods.

API Proposal

namespace System.Net.Http.Json;

public static partial class HttpClientJsonExtensions
{
    [RequiresUnreferencedCode(HttpContentJsonExtensions.SerializationUnreferencedCodeMessage)]
    [RequiresDynamicCode(HttpContentJsonExtensions.SerializationDynamicCodeMessage)]
    public static IAsyncEnumerable<TBodyValue?> PostAsJsonAsyncEnumerable<TBodyValue, TResponseValue>(
        this HttpClient client,
        [StringSyntax(StringSyntaxAttribute.Uri)] string? requestUri,
        TBodyValue value,
        JsonSerializerOptions? bodyOptions = null,
        JsonSerializerOptions? responseOptions = null,
        CancellationToken cancellationToken = default) { }

    [RequiresUnreferencedCode(HttpContentJsonExtensions.SerializationUnreferencedCodeMessage)]
    [RequiresDynamicCode(HttpContentJsonExtensions.SerializationDynamicCodeMessage)]
    public static IAsyncEnumerable<TBodyValue?> PostAsJsonAsyncEnumerable<TBodyValue, TResponseValue>(
        this HttpClient client,
        Uri? requestUri,
        TBodyValue value,
        JsonSerializerOptions? bodyOptions = null,
        JsonSerializerOptions? responseOptions = null,
        CancellationToken cancellationToken = default) { }

    [RequiresUnreferencedCode(HttpContentJsonExtensions.SerializationUnreferencedCodeMessage)]
    [RequiresDynamicCode(HttpContentJsonExtensions.SerializationDynamicCodeMessage)]
    public static Task<HttpResponseMessage> PostAsJsonAsyncEnumerable<TValue>(
        this HttpClient client,
        [StringSyntax(StringSyntaxAttribute.Uri)] string? requestUri,
        TBodyValue value,
        CancellationToken cancellationToken) { }

    [RequiresUnreferencedCode(HttpContentJsonExtensions.SerializationUnreferencedCodeMessage)]
    [RequiresDynamicCode(HttpContentJsonExtensions.SerializationDynamicCodeMessage)]
    public static IAsyncEnumerable<TBodyValue?> PostAsJsonAsyncEnumerable<TBodyValue, TResponseValue>(
        this HttpClient client,
        Uri? requestUri,
        TBodyValue value,
        CancellationToken cancellationToken) { }

    public static IAsyncEnumerable<TBodyValue?> PostAsJsonAsyncEnumerable<TBodyValue, TResponseValue>(
        this HttpClient client,
        [StringSyntax(StringSyntaxAttribute.Uri)] string? requestUri,
        TBodyValue value,
        JsonTypeInfo<TBodyValue> jsonBodyTypeInfo,
        JsonTypeInfo<TResponseValue> jsonResponseTypeInfo,
        CancellationToken cancellationToken = default) { }

    public static IAsyncEnumerable<TBodyValue?> PostAsJsonAsyncEnumerable<TBodyValue, TResponseValue>(
        this HttpClient client,
        Uri? requestUri,
        TBodyValue value,
        JsonTypeInfo<TBodyValue> jsonBodyTypeInfo,
        JsonTypeInfo<TResponseValue> jsonResponseTypeInfo,
        CancellationToken cancellationToken = default) { }

API Usage

var client = httpClientFactory.Create("openai");

var prompt = new Prompt();

await foreach (var response in client.PostAsJsonAsyncEnumerable<Prompt, ChatResponse>(
    requestUri: "/openai/chat",
    bodyValue: prompt,
    bodyJsonTypeInfo: GeneratedSerializerContext.Default.Prompt,
    responseJsonTypeInfo: GeneratedSerializerContext.Default.ChatResponse))
{
    // TODO: handle each response
}

public record Prompt /* Omitted for brevity */;
public record ChatResponse /* Omitted for brevity */;

[JsonSerializable(typeof(Prompt))]
[JsonSerializable(typeof(ChatResponse))]
internal sealed partial class GeneratedSerializerContext
    : JsonSerializerContext
{
}

Alternative Designs

No response

Risks

No response

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

[eiriktsarpalis]

There's no precedent in us using two generic parameters for the request and response types, the existing POST methods simply return an HttpResponseMessage. I wasn't involved in that design, but I'm guessing it was done to keep the APIs simple.

If I'm honest, I think the existing HttpContent.ReadFromJsonAsAsyncEnumerable methods are the ideally suited workaround which work for any request type:

RequestModel model = ...;
HttpResponseMessage response = await client.PostAsJsonAsync("/openai/chat", model);
await foreach (ResponseModel item in response.ReadFromJsonAsAsyncEnumerable<ResponseModel>())
{
    ...
}

[MihaZupan]

On the other hand, that sample demonstrates how easy it is to use it wrong.
The PostAsJsonAsync call will buffer the whole response, meaning you won't actually get streaming semantics.

This would preserve the goal of streaming the response, but also with the difference that you now don't enforce the timeout/response size limits.

var request = new HttpRequestMessage(HttpMethod.Post, "/openai/chat")
{
    Content = JsonContent.Create(model)
};
using HttpResponseMessage response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead);
await foreach (ResponseModel item in response.ReadFromJsonAsAsyncEnumerable<ResponseModel>())
{
    ...
}

cc: @dotnet/ncl

[JohnGalt1717]

Does this help? It's my shot at it, and it appears to not buffer and stream.

public static async IAsyncEnumerable<TResponse> PostFromJsonAsAsyncEnumerable<TRequest, TResponse>(
    this HttpClient client,
    string requestUri,
    TRequest request,
    [EnumeratorCancellation] CancellationToken cancellationToken = default) 
{
  var requestMessage = new HttpRequestMessage(HttpMethod.Post, requestUri);
  
  using var ms = new MemoryStream();
  await JsonSerializer.SerializeAsync(ms, request, JsonSerializerSettings.Options, cancellationToken: cancellationToken);
  ms.Position = 0;
  
  using var requestContent = new StreamContent(ms);
  requestMessage.Content = requestContent;
  
  requestContent.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/json");
  requestMessage.Headers.Add("Accept", "application/json");
  
  using var response = await client.SendAsync(requestMessage, HttpCompletionOption.ResponseHeadersRead, cancellationToken);
  
  response.EnsureSuccessStatusCode();

  var items = response.Content.ReadFromJsonAsAsyncEnumerable<TResponse>(cancellationToken);

  await foreach (var item in items)
  {
	if (cancellationToken.IsCancellationRequested)
		throw new TaskCanceledException();
	if (item is null)
		continue;

	yield return item;
  }
}

[eiriktsarpalis]

The PostAsJsonAsync call will buffer the whole response, meaning you won't actually get streaming semantics.

Perhaps then the answer is to expose overloads to the existing HttpClientJsonExtensions methods accepting HttpCompletionOptions?