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Add FastAPI response streaming example (#211)
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‎examples/fastapi-response-streaming/.gitignore‎

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‎examples/fastapi-response-streaming/README.md‎

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# FastAPI Response Streaming
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This example shows how to use Lambda Web Adapter to run a FastAPI application with response streaming via a Function URL.
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### How does it work?
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We add Lambda Web Adapter layer to the function and configure wrapper script.
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1. attach Lambda Adapter layer to your function. This layer containers Lambda Adapter binary and a wrapper script.
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1. x86_64: `arn:aws:lambda:${AWS::Region}:753240598075:layer:LambdaAdapterLayerX86:16`
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2. arm64: `arn:aws:lambda:${AWS::Region}:753240598075:layer:LambdaAdapterLayerArm64:16`
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2. configure Lambda environment variable `AWS_LAMBDA_EXEC_WRAPPER` to `/opt/bootstrap`. This is a wrapper script included in the layer.
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3. set function handler to a startup command: `run.sh`. The wrapper script will execute this command to boot up your application.
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To get more information of Wrapper script, please read Lambda documentation [here](https://docs.aws.amazon.com/lambda/latest/dg/runtimes-modify.html#runtime-wrapper).
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This is the resource for Lambda function. The function urls's invoke mode is configured as "RESPONSE_STREAM", and Lambda environment variable "AWS_LWA_INVOKE_MODE" is set to "response_stream".
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```yaml
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FastAPIFunction:
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Type: AWS::Serverless::Function
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Properties:
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CodeUri: app/
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Handler: run.sh
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Runtime: python3.9
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MemorySize: 256
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Environment:
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Variables:
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AWS_LAMBDA_EXEC_WRAPPER: /opt/bootstrap
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AWS_LWA_INVOKE_MODE: response_stream
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PORT: 8000
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Layers:
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- !Sub arn:aws:lambda:${AWS::Region}:753240598075:layer:LambdaAdapterLayerX86:16
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FunctionUrlConfig:
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AuthType: NONE
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InvokeMode: RESPONSE_STREAM
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```
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### Build and Deploy
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Run the following commands to build and deploy the application to lambda.
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```bash
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sam build --use-container
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sam deploy --guided
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```
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When the deployment completes, take note of FastAPI's Value. It is the API Gateway endpoint URL.
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### Verify it works
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Open FastAPI's URL in a browser, you should see "This is streaming from Lambda" streams back 10 times.
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‎examples/fastapi-response-streaming/__init__.py‎

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‎examples/fastapi-response-streaming/app/__init__.py‎

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‎examples/fastapi-response-streaming/app/main.py‎

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from fastapi import FastAPI
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from fastapi.responses import StreamingResponse
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import asyncio
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app = FastAPI()
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async def streamer():
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for i in range(10):
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await asyncio.sleep(1)
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yield b"This is streaming from Lambda \n"
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@app.get("/")
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async def index():
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return StreamingResponse(streamer(), media_type="text/plain; charset=utf-8")

‎examples/fastapi-response-streaming/app/requirements.txt‎

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anyio==3.6.2
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click==8.1.3
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fastapi==0.92.0
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h11==0.14.0
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idna==3.4
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importlib-metadata==6.0.0
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pydantic==1.10.5
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sniffio==1.3.0
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starlette==0.25.0
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typing_extensions==4.5.0
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uvicorn==0.20.0
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zipp==3.13.0

‎examples/fastapi-response-streaming/app/run.sh‎

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#!/bin/bash
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PATH=$PATH:$LAMBDA_TASK_ROOT/bin PYTHONPATH=$LAMBDA_TASK_ROOT exec python -m uvicorn --port=$PORT main:app

‎examples/fastapi-response-streaming/template.yaml‎

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AWSTemplateFormatVersion: '2010-09-09'
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Transform: AWS::Serverless-2016-10-31
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Description: >
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fastapi response streaming
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# More info about Globals: https://github.com/awslabs/serverless-application-model/blob/master/docs/globals.rst
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Globals:
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Function:
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Timeout: 60
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Resources:
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FastAPIFunction:
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Type: AWS::Serverless::Function
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Properties:
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CodeUri: app/
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Handler: run.sh
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Runtime: python3.9
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MemorySize: 256
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Environment:
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Variables:
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AWS_LAMBDA_EXEC_WRAPPER: /opt/bootstrap
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AWS_LWA_INVOKE_MODE: response_stream
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PORT: 8000
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Layers:
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- !Sub arn:aws:lambda:${AWS::Region}:753240598075:layer:LambdaAdapterLayerX86:16
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FunctionUrlConfig:
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AuthType: NONE
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InvokeMode: RESPONSE_STREAM
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Outputs:
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FastAPIFunctionUrl:
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Description: "Function URL for FastAPI function"
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Value: !GetAtt FastAPIFunctionUrl.FunctionUrl
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FastAPIFunction:
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Description: "FastAPI Lambda Function ARN"
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Value: !GetAtt FastAPIFunction.Arn

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