# Streaming Lambda function You can configure your Lambda function to stream response payloads back to clients. Response streaming can benefit latency sensitive applications by improving time to first byte (TTFB) performance. This is because you can send partial responses back to the client as they become available. Additionally, you can use response streaming to build functions that return larger payloads. Response stream payloads have a soft limit of 200 MB as compared to the 6 MB limit for buffered responses. Streaming a response also means that your function doesn’t need to fit the entire response in memory. For very large responses, this can reduce the amount of memory you need to configure for your function. Streaming responses incurs a cost. For more information, see [AWS Lambda Pricing](https://aws.amazon.com/lambda/pricing/). You can stream responses through [Lambda function URLs](https://docs.aws.amazon.com/lambda/latest/dg/urls-configuration.html), the AWS SDK, or using the Lambda [InvokeWithResponseStream](https://docs.aws.amazon.com/lambda/latest/dg/API_InvokeWithResponseStream.html) API. In this example, we create an authenticated Lambda function URL. ## Code The sample code creates a `SendNumbersWithPause` struct that conforms to the `StreamingLambdaHandler` protocol provided by the Swift AWS Lambda Runtime. The `handle(...)` method of this protocol receives incoming events as a Swift NIO `ByteBuffer` and returns the output as a `ByteBuffer`. The response is streamed through the `LambdaResponseStreamWriter`, which is passed as an argument in the `handle` function. ### Setting HTTP Status Code and Headers Before streaming the response body, you can set the HTTP status code and headers using the `writeStatusAndHeaders(_:)` method: ```swift try await responseWriter.writeStatusAndHeaders( StreamingLambdaStatusAndHeadersResponse( statusCode: 200, headers: [ "Content-Type": "text/plain", "x-my-custom-header": "streaming-example" ] ) ) ``` The `StreamingLambdaStatusAndHeadersResponse` structure allows you to specify: - **statusCode**: HTTP status code (e.g., 200, 404, 500) - **headers**: Dictionary of single-value HTTP headers (optional) ### Streaming the Response Body After setting headers, you can stream the response body by calling the `write(_:)` function of the `LambdaResponseStreamWriter` with partial data repeatedly before finally closing the response stream by calling `finish()`. Developers can also choose to return the entire output and not stream the response by calling `writeAndFinish(_:)`. ```swift // Stream data in chunks for i in 1...3 { try await responseWriter.write(ByteBuffer(string: "Number: \(i)\n")) try await Task.sleep(for: .milliseconds(1000)) } // Close the response stream try await responseWriter.finish() ``` An error is thrown if `finish()` is called multiple times or if it is called after having called `writeAndFinish(_:)`. ### Example Usage Patterns The example includes two handler implementations: 1. **SendNumbersWithPause**: Demonstrates basic streaming with headers, sending numbers with delays 2. **ConditionalStreamingHandler**: Shows how to handle different response scenarios, including error responses with appropriate status codes The `handle(...)` method is marked as `mutating` to allow handlers to be implemented with a `struct`. Once the struct is created and the `handle(...)` method is defined, the sample code creates a `LambdaRuntime` struct and initializes it with the handler just created. Then, the code calls `run()` to start the interaction with the AWS Lambda control plane. ## Build & Package To build & archive the package, type the following commands. ```bash swift package archive --allow-network-connections docker ``` If there is no error, there is a ZIP file ready to deploy. The ZIP file is located at `.build/plugins/AWSLambdaPackager/outputs/AWSLambdaPackager/StreamingNumbers/StreamingNumbers.zip` ## Test locally You can test the function locally before deploying: ```bash swift run # In another terminal, test with curl: curl -v --output response.txt \ --header "Content-Type: application/json" \ --data '"this is not used"' \ http://127.0.0.1:7000/invoke ``` ## Deploy with the AWS CLI Here is how to deploy using the `aws` command line. ### Step 1: Create the function ```bash # Replace with your AWS Account ID AWS_ACCOUNT_ID=012345678901 aws lambda create-function \ --function-name StreamingNumbers \ --zip-file fileb://.build/plugins/AWSLambdaPackager/outputs/AWSLambdaPackager/StreamingNumbers/StreamingNumbers.zip \ --runtime provided.al2 \ --handler provided \ --architectures arm64 \ --role arn:aws:iam::${AWS_ACCOUNT_ID}:role/lambda_basic_execution \ --timeout 15 ``` > [!IMPORTANT] > The timeout value must be bigger than the time it takes for your function to stream its output. Otherwise, the Lambda control plane will terminate the execution environment before your code has a chance to finish writing the stream. Here, the sample function stream responses during 3 seconds and we set the timeout for 5 seconds. The `--architectures` flag is only required when you build the binary on an Apple Silicon machine (Apple M1 or more recent). It defaults to `x64`. Be sure to set `AWS_ACCOUNT_ID` with your actual AWS account ID (for example: 012345678901). ### Step2: Give permission to invoke that function through an URL Anyone with a valid signature from your AWS account will have permission to invoke the function through its URL. ```bash aws lambda add-permission \ --function-name StreamingNumbers \ --action lambda:InvokeFunctionUrl \ --principal ${AWS_ACCOUNT_ID} \ --function-url-auth-type AWS_IAM \ --statement-id allowURL ``` ### Step3: Create the URL This creates [a URL with IAM authentication](https://docs.aws.amazon.com/lambda/latest/dg/urls-auth.html). Only calls with a valid signature will be authorized. ```bash aws lambda create-function-url-config \ --function-name StreamingNumbers \ --auth-type AWS_IAM \ --invoke-mode RESPONSE_STREAM ``` This calls return various information, including the URL to invoke your function. ```json { "FunctionUrl": "https://ul3nf4dogmgyr7ffl5r5rs22640fwocc.lambda-url.us-east-1.on.aws/", "FunctionArn": "arn:aws:lambda:us-east-1:012345678901:function:StreamingNumbers", "AuthType": "AWS_IAM", "CreationTime": "2024-10-22T07:57:23.112599Z", "InvokeMode": "RESPONSE_STREAM" } ``` ### Invoke your Lambda function To invoke the Lambda function, use `curl` with the AWS Sigv4 option to generate the signature. Read the [AWS Credentials and Signature](../README.md/#AWS-Credentials-and-Signature) section for more details about the AWS Sigv4 protocol and how to obtain AWS credentials. When you have the `aws` command line installed and configured, you will find the credentials in the `~/.aws/credentials` file. ```bash URL=https://ul3nf4dogmgyr7ffl5r5rs22640fwocc.lambda-url.us-east-1.on.aws/ REGION=us-east-1 # Set the AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, and AWS_SESSION_TOKEN environment variables eval $(aws configure export-credentials --format env) curl "$URL" \ --user "${AWS_ACCESS_KEY_ID}":"${AWS_SECRET_ACCESS_KEY}" \ --aws-sigv4 "aws:amz:${REGION}:lambda" \ -H "x-amz-security-token: ${AWS_SESSION_TOKEN}" \ --no-buffer ``` Note that there is no payload required for this example. This should output the following result, with a one-second delay between each numbers. ``` 1 2 3 Streaming complete! ``` ### Undeploy When done testing, you can delete the Lambda function with this command. ```bash aws lambda delete-function --function-name StreamingNumbers ``` ## Deploy with AWS SAM Alternatively, you can use [AWS SAM](https://aws.amazon.com/serverless/sam/) to deploy the Lambda function. **Prerequisites** : Install the [SAM CLI](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/install-sam-cli.html) ### SAM Template The template file is provided as part of the example in the `template.yaml` file. It defines a Lambda function based on the binary ZIP file. It creates the function url with IAM authentication and sets the function timeout to 15 seconds. ```yaml AWSTemplateFormatVersion: '2010-09-09' Transform: AWS::Serverless-2016-10-31 Description: SAM Template for StreamingLambda Example Resources: # Lambda function StreamingNumbers: Type: AWS::Serverless::Function Properties: CodeUri: .build/plugins/AWSLambdaPackager/outputs/AWSLambdaPackager/StreamingNumbers/StreamingNumbers.zip Timeout: 15 Handler: swift.bootstrap # ignored by the Swift runtime Runtime: provided.al2 MemorySize: 128 Architectures: - arm64 FunctionUrlConfig: AuthType: AWS_IAM InvokeMode: RESPONSE_STREAM Outputs: # print Lambda function URL LambdaURL: Description: Lambda URL Value: !GetAtt StreamingNumbersUrl.FunctionUrl ``` ### Deploy with SAM ```bash sam deploy \ --resolve-s3 \ --template-file template.yaml \ --stack-name StreamingNumbers \ --capabilities CAPABILITY_IAM ``` The URL of the function is provided as part of the output. ``` CloudFormation outputs from deployed stack ----------------------------------------------------------------------------------------------------------------------------- Outputs ----------------------------------------------------------------------------------------------------------------------------- Key LambdaURL Description Lambda URL Value https://gaudpin2zjqizfujfnqxstnv6u0czrfu.lambda-url.us-east-1.on.aws/ ----------------------------------------------------------------------------------------------------------------------------- ``` Once the function is deployed, you can invoke it with `curl`, similarly to what you did when deploying with the AWS CLI. ```bash # Set the AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, and AWS_SESSION_TOKEN environment variables eval $(aws configure export-credentials --format env) curl "$URL" \ --user "$AWS_ACCESS_KEY_ID":"$AWS_SECRET_ACCESS_KEY" \ --aws-sigv4 "aws:amz:${REGION}:lambda" \ -H "x-amz-security-token: $AWS_SESSION_TOKEN" \ --no-buffer ``` ### Undeploy with SAM When done testing, you can delete the infrastructure with this command. ```bash sam delete ``` ## Payload decoding The content of the input `ByteBuffer` depends on how you invoke the function: - when you use [`InvokeWithResponseStream` API](https://docs.aws.amazon.com/lambda/latest/api/API_InvokeWithResponseStream.html) to invoke the function, the function incoming payload is what you pass to the API. You can decode the `ByteBuffer` with a [`JSONDecoder.decode()`](https://developer.apple.com/documentation/foundation/jsondecoder) function call. - when you invoke the function through a [Lambda function URL](https://docs.aws.amazon.com/lambda/latest/dg/urls-configuration.html), the incoming `ByteBuffer` contains a payload that gives developer access to the underlying HTTP call. The payload contains information about the HTTP verb used, the headers received, the authentication method and so on. The [AWS documentation contains the details](https://docs.aws.amazon.com/lambda/latest/dg/urls-invocation.html) of the payload. The [Swift Lambda Event library](https://github.com/awslabs/swift-aws-lambda-events) contains a [`FunctionURL` type](https://github.com/awslabs/swift-aws-lambda-events/blob/main/Sources/AWSLambdaEvents/FunctionURL.swift) ready to use in your projects. Here is an example of Lambda function URL payload: ```json { "version": "2.0", "routeKey": "$default", "rawPath": "/", "rawQueryString": "", "headers": { "x-amzn-tls-cipher-suite": "TLS_AES_128_GCM_SHA256", "x-amzn-tls-version": "TLSv1.3", "x-amzn-trace-id": "Root=1-68762f44-4f6a87d1639e7fc356aa6f96", "x-amz-date": "20250715T103651Z", "x-forwarded-proto": "https", "host": "zvnsvhpx7u5gn3l3euimg4jjou0jvbfe.lambda-url.us-east-1.on.aws", "x-forwarded-port": "443", "x-forwarded-for": "2a01:cb0c:6de:8300:a1be:8004:e31a:b9f", "accept": "*/*", "user-agent": "curl/8.7.1" }, "requestContext": { "accountId": "0123456789", "apiId": "zvnsvhpx7u5gn3l3euimg4jjou0jvbfe", "authorizer": { "iam": { "accessKey": "AKIA....", "accountId": "0123456789", "callerId": "AIDA...", "cognitoIdentity": null, "principalOrgId": "o-rlrup7z3ao", "userArn": "arn:aws:iam::0123456789:user/sst", "userId": "AIDA..." } }, "domainName": "zvnsvhpx7u5gn3l3euimg4jjou0jvbfe.lambda-url.us-east-1.on.aws", "domainPrefix": "zvnsvhpx7u5gn3l3euimg4jjou0jvbfe", "http": { "method": "GET", "path": "/", "protocol": "HTTP/1.1", "sourceIp": "2a01:...:b9f", "userAgent": "curl/8.7.1" }, "requestId": "f942509a-283f-4c4f-94f8-0d4ccc4a00f8", "routeKey": "$default", "stage": "$default", "time": "15/Jul/2025:10:36:52 +0000", "timeEpoch": 1752575812081 }, "isBase64Encoded": false } ``` ## ⚠️ Security and Reliability Notice These are example applications for demonstration purposes. When deploying such infrastructure in production environments, we strongly encourage you to follow these best practices for improved security and resiliency: - Enable access logging on API Gateway ([documentation](https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-logging.html)) - Ensure that AWS Lambda function is configured for function-level concurrent execution limit ([concurrency documentation](https://docs.aws.amazon.com/lambda/latest/dg/lambda-concurrency.html), [configuration guide](https://docs.aws.amazon.com/lambda/latest/dg/configuration-concurrency.html)) - Check encryption settings for Lambda environment variables ([documentation](https://docs.aws.amazon.com/lambda/latest/dg/configuration-envvars-encryption.html)) - Ensure that AWS Lambda function is configured for a Dead Letter Queue (DLQ) ([documentation](https://docs.aws.amazon.com/lambda/latest/dg/invocation-async-retain-records.html#invocation-dlq)) - Ensure that AWS Lambda function is configured inside a VPC when it needs to access private resources ([documentation](https://docs.aws.amazon.com/lambda/latest/dg/configuration-vpc.html), [code example](https://github.com/awslabs/swift-aws-lambda-runtime/tree/main/Examples/ServiceLifecycle%2BPostgres))