Add support for Lambda Response Streaming by normj · Pull Request #2288 · aws/aws-lambda-dotnet

normj · 2026-02-19T01:50:45Z

Issue #, if available:
#1635

Description of changes:
Integrate Lambda response streaming support into the Amazon.Lambda.RuntimeSupport.

A hello world example of using response streaming. In this case sense I wrapped the Stream returned from CreateStream in a StreamWriter the writes will be buffered in StreamWriter till the buffer is full. I call the flush method every 10 iterations to force sending data back to the client.

using Amazon.Lambda.Core;
using Amazon.Lambda.Core.ResponseStreaming;
using Amazon.Lambda.RuntimeSupport;
using Amazon.Lambda.Serialization.SystemTextJson;

#pragma warning disable CA2252

// The function handler that will be called for each Lambda event
var handler = async (string input, ILambdaContext context) =>
{
    using var responseStream = LambdaResponseStreamFactory.CreateStream();

    using var writer = new StreamWriter(responseStream);
    for (var i = 1; i <= 100; i++)
    {
        var message = $"Hello {input} - {i}";
        await writer.WriteLineAsync(message);

        if (i % 10 == 0)
        {
            await writer.FlushAsync();
            await Task.Delay(1000);
        }
    }
};

// Build the Lambda runtime client passing in the handler to call for each
// event and the JSON serializer to use for translating Lambda JSON documents
// to .NET types.
await LambdaBootstrapBuilder.Create(handler, new DefaultLambdaJsonSerializer())
        .Build()
        .RunAsync();

For a use with API Gateway or Lambda Function URL you need to create the stream with the CreateHttpStream passing the status code and response headers.

using Amazon.Lambda.APIGatewayEvents;
using Amazon.Lambda.Core;
using Amazon.Lambda.Core.ResponseStreaming;
using Amazon.Lambda.RuntimeSupport;
using Amazon.Lambda.Serialization.SystemTextJson;
using System.Net;

#pragma warning disable CA2252

// The function handler that will be called for each Lambda event
var handler = async (APIGatewayProxyRequest request, ILambdaContext context) =>
{
    var prelude = new HttpResponseStreamPrelude
    {
        StatusCode = HttpStatusCode.OK,
        Headers =
        {
            { "Content-Type", "text/plain" }
        }
    };
   
    using var responseStream = LambdaResponseStreamFactory.CreateHttpStream(prelude);

    using var writer = new StreamWriter(responseStream);
    for (var i = 1; i <= 10000000; i++)
    {
        var message = $"Hello - {i} ({responseStream.Length.ToString("N0")}) (Remaining Time: {context.RemainingTime})";
        await writer.WriteLineAsync(message);

        if (i % 100 == 0)
        {
            await writer.FlushAsync();
        }

        if (context.RemainingTime < TimeSpan.FromSeconds(5))
        {
            await writer.WriteLineAsync("Approaching Lambda timeout, stopping the stream.");
            await writer.FlushAsync();
            break;
        }
    }
};

// Build the Lambda runtime client passing in the handler to call for each
// event and the JSON serializer to use for translating Lambda JSON documents
// to .NET types.
await LambdaBootstrapBuilder.Create(handler, new DefaultLambdaJsonSerializer())
        .Build()
        .RunAsync();

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

…-lambda-dotnet into normj/response-streaming

…ream

…aders with chunk encoding which wasn't supported in SocketHandler

… of Stream so that we can potentially expose more methods in the future.

GarrettBeatty · 2026-04-06T19:38:28Z

+        {
+            var parts = hostAndPort.Split(':');
+            _host = parts[0];
+            _port = parts.Length > 1 ? int.Parse(parts[1], CultureInfo.InvariantCulture) : 80;


why default to 80 over 443? just because its http and not https?

Reality is there should always be a port defined in hostAndPort. Change the code to not have a default and throw an error if port is not set.

GarrettBeatty · 2026-04-06T19:40:19Z

+                    // Check if we've received the complete response (ends with \r\n\r\n for headers,
+                    // or we've read the content-length worth of body)
+                    var text = responseText.ToString();
+                    if (text.Contains("\r\n\r\n"))


nit: make constant since its used multiple times

GarrettBeatty · 2026-04-06T19:40:50Z

+                        }
+                    }
+
+                    if (totalRead > 16384)


whats 16384 here? can you add a comment

Added comment but the value is an arbitrary safety net value.

GarrettBeatty · 2026-04-06T19:49:29Z


-[CollectionDefinition("Integration Tests")]
-public class IntegrationTestCollection : ICollectionFixture<IntegrationTestFixture>
+[CollectionDefinition("Integration Tests", DisableParallelization = true)]


why did we have to disable parallelization?

I was probably dealing with some test instability. I removed the flag and tests were successful form. See if the GitHub checks are successful with the flag removed.

philasmar · 2026-04-07T20:27:28Z

+
+namespace Amazon.Lambda.Core.ResponseStreaming
+{
+    /// <summary>


nit: could you give more context as to why the prelude is needed?

philasmar · 2026-04-07T20:28:21Z

+    public class HttpResponseStreamPrelude
+    {
+        /// <summary>
+        /// The Http status code to include in the response prelude.


nit: all docs for the fields of this class has "to include in the response prelude.". It feels redundant.

philasmar · 2026-04-07T20:28:59Z

+        public IDictionary<string, string> Headers { get; set; } = new Dictionary<string, string>();
+
+        /// <summary>
+        /// The response headers to include in the response prelude. This collection supports setting multiple values for the same headers.
+        /// </summary>
+        public IDictionary<string, IList<string>> MultiValueHeaders { get; set; } = new Dictionary<string, IList<string>>();


is there a reason why both are needed? why not just have 1 that supports multi value?

For API Gateway REST APIs the JSON for the prelude must use MultiValueHeaders but Lambda URLs you have to use Headers. The properties here are 1 to 1 mirror of the union of the specs for API Gateway Rest APIs and Function URLs. If API Gateway HTTP APIs ever support they would probably require the Headers collection. At the point of generating the JSON via the ToByteArray we don't have the context to say what is the event source. It is up to the user to know which one to use.

I updated the docs for the properties to call out when to use the properties.

philasmar · 2026-04-07T20:30:44Z

+        /// <exception cref="InvalidOperationException">Thrown if the stream is already completed or an error has been reported.</exception>
+        Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken = default);
+
+


nit: remove extra whitespace

philasmar · 2026-04-07T20:30:47Z

+        /// </summary>
+        long BytesWritten { get; }
+
+


nit: remove extra whitespace

philasmar · 2026-04-07T20:44:49Z

+        /// <param name="responseStream">The response stream that provides data and error state.</param>
+        /// <param name="userAgent">The User-Agent header value.</param>
+        /// <param name="cancellationToken">Cancellation token.</param>
+        public async Task SendStreamingResponseAsync(


should we be measuring the performance of this HTTP client against the default one?

The default HTTP client doesn't work for this scenario hence the reason we have to use our custom implementation.

philasmar · 2026-04-07T20:46:05Z

+
+        // The wait time is a sanity timeout to avoid waiting indefinitely if SetHttpOutputStreamAsync is not called or takes too long to call.
+        // Reality is that SetHttpOutputStreamAsync should be called very quickly after CreateStream, so this timeout is generous to avoid false positives but still protects against hanging indefinitely.
+        private readonly static TimeSpan _httpStreamWaitTimeout = TimeSpan.FromSeconds(30);


do we want to make this customizable by the user?

No, this is just a sanity check but it should never be reached unless we have a bug. Plus this is more of an implementation detail that users won't understand and by exposing a config it makes this implementation detail part of our public API contract.

* release_2026-04-08 * Update test app CloudFormation templates * Updated changelog

* Add [S3Event] annotation attribute and source generator support - S3EventAttribute with Bucket (required), ResourceName, Events, FilterPrefix, FilterSuffix, Enabled - S3EventAttributeBuilder for Roslyn AttributeData parsing - TypeFullNames constants and Events hashset registration - SyntaxReceiver secondary attribute registration - EventTypeBuilder S3 event type mapping - AttributeModelBuilder S3 branch - CloudFormationWriter ProcessS3Attribute (SAM S3 event with Ref, Events list, Filter rules) - LambdaFunctionValidator ValidateS3Events (params, return type, dependency check) - DiagnosticDescriptors InvalidS3EventAttribute (AWSLambda0133) Add S3Event annotation tests - ValidS3Events.cs.txt test source with 3 test functions - S3EventsTests.cs CloudFormation writer tests (attribute application + property sync) - S3Events project references in TestServerlessApp.csproj and test project IT test PR comments change file fixes PR comments * add header

* treat warnings as errors * Change file

* chore: improve test flakiness * update changelog

…enerator (#2324) * Phase 1: Add FunctionUrlAttribute with source generator wiring and CloudFormation FunctionUrlConfig generation - New FunctionUrlAttribute class with AuthType property (NONE/AWS_IAM) - New FunctionUrlAuthType enum - Source generator detects FunctionUrlAttribute and maps to EventType.API - Generated wrapper uses HttpApi V2 request/response types (same payload format) - CloudFormationWriter emits FunctionUrlConfig on the function resource - Dependency validation checks for Amazon.Lambda.APIGatewayEvents - SyntaxReceiver detects missing [LambdaFunction] on [FunctionUrl] methods - 6 new unit tests for CloudFormation template generation (JSON + YAML) Phase 2: Add CORS support to FunctionUrlAttribute - AllowOrigins, AllowMethods, AllowHeaders, ExposeHeaders, AllowCredentials, MaxAge properties - FunctionUrlAttributeBuilder parses all CORS properties from AttributeData - CloudFormationWriter emits Cors block under FunctionUrlConfig only when CORS properties are set - 4 new unit tests for CORS generation and no-CORS scenarios Phase 3: FunctionUrlConfig orphan cleanup and attribute switching - Remove FunctionUrlConfig from template when [FunctionUrl] attribute is removed - Clean transition when switching from [FunctionUrl] to [HttpApi] or [RestApi] - 4 new unit tests for orphan cleanup and attribute switching scenarios Phase 4: End-to-end source generator test for FunctionUrl - FunctionUrlExample.cs test source with [FunctionUrl] + [FromQuery] + IHttpResult - Generated wrapper snapshot using HttpApi V2 payload format - Serverless template snapshot with FunctionUrlConfig - Full Roslyn source generator verification test IT tests Update Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Models/Attributes/FunctionUrlAttributeBuilder.cs Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com> copilot comments change file fix cleanup add header PR comments * fix template * fix test

* release_2026-04-14 * Update test app CloudFormation templates * Updated changelog

Add CODEOWNERS file

…ages (#2293)

normj added 11 commits February 11, 2026 16:16

Task 1

22d1469

Task 2

5e0f810

Task 3

20e5ba8

Task 4

0cdb159

Task 5

0aa892b

Task 4 (after redesign)

5e29c21

Task 5

603612d

Task 6

63224bf

Task 8

14c993b

Task 10

0b8dd52

Refactoring

414a449

normj marked this pull request as draft February 19, 2026 01:50

normj added 10 commits February 18, 2026 17:58

Cleanup

21d82d8

Remove tests

556b726

Clean up and rework Semaphore locks

4d5dee2

Merge branch 'normj/response-streaming' of https://github.com/aws/aws…

d8322ff

…-lambda-dotnet into normj/response-streaming

Start working on supporting having a prelude chuck in the response st…

645771e

…ream

Rework to support class library programming model

1f17a58

Rework encoding and add support for using API Gateway

99b83c2

Backfill tests after refactor

ab93ce9

Merge branch 'dev' of https://github.com/aws/aws-lambda-dotnet into dev

f679efa

Merge branch 'dev' into normj/response-streaming

bd0170e

normj mentioned this pull request Mar 10, 2026

Add support for Lambda Response Streaming in ASP.NET Core bridge packages #2293

Merged

normj changed the base branch from dev to feature/response-streaming March 10, 2026 23:49

Add integ tests

d60bb93

normj marked this pull request as ready for review March 11, 2026 18:25

normj marked this pull request as draft March 11, 2026 18:26

normj force-pushed the normj/response-streaming branch from 2ed63cc to d60bb93 Compare March 11, 2026 20:10

Improve test error message

9b308ae

normj force-pushed the normj/response-streaming branch from 96dc138 to 0bd63a9 Compare March 12, 2026 03:03

normj added 3 commits March 17, 2026 16:59

Address PR comments

c637ef1

Rework to have my own HttpClient implementation to handle trailing he…

42d3212

…aders with chunk encoding which wasn't supported in SocketHandler

Change response stream factory to return LambdaResponseStream instead…

b7b51bd

… of Stream so that we can potentially expose more methods in the future.

normj deleted the branch feature/response-streaming April 3, 2026 21:05

normj closed this Apr 3, 2026

normj reopened this Apr 3, 2026

normj added 2 commits April 3, 2026 15:29

Clean up

2eb2eb4

Ensure prelude with delimiter are written as a single chunk

1a86609

normj force-pushed the normj/response-streaming branch from d3dbe0c to 1a86609 Compare April 3, 2026 23:21

Bug fixes

98d2d25

normj marked this pull request as ready for review April 5, 2026 01:11

GarrettBeatty reviewed Apr 6, 2026

View reviewed changes

philasmar reviewed Apr 7, 2026

View reviewed changes

GarrettBeatty and others added 14 commits April 8, 2026 13:10

Application Load Balancer (ALB) (#2318)

d86f1af

Release 2026-04-08 (#2326)

dcfacf2

* release_2026-04-08 * Update test app CloudFormation templates * Updated changelog

Add CODEOWNERS file

88ba85a

treat warnings as errors (#2329)

78afc7a

* treat warnings as errors * Change file

chore: improve test flakiness (#2325)

98a1f28

* chore: improve test flakiness * update changelog

Release 2026-04-13 #2 (#2330)

49d8839

Release 2026-04-14 (#2332)

ee1a65e

* release_2026-04-14 * Update test app CloudFormation templates * Updated changelog

Merge pull request #2327 from aws/adaines/add-codeowners

8c960cc

Add CODEOWNERS file

Merge branch 'dev' of https://github.com/aws/aws-lambda-dotnet into dev

e41aea2

Merge branch 'dev' into normj/response-streaming

4a5e000

Address PR comments

2e70f8d

Remove obsolete unit test

62a9ad7

GarrettBeatty approved these changes Apr 15, 2026

View reviewed changes

philasmar approved these changes Apr 15, 2026

View reviewed changes

Add support for Lambda Response Streaming in ASP.NET Core bridge pack…

6d5d887

…ages (#2293)

		/// <exception cref="InvalidOperationException">Thrown if the stream is already completed or an error has been reported.</exception>
		Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken = default);

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normj commented Feb 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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normj commented Feb 19, 2026 •

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