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How to continuously deploy a FastAPI to AWS Lambda with AWS SAM

My last article about FastAPI was supposed to be an article about how to deploy a FastAPI on a budget, but instead turned out to be an opinion on FastAPI and I left it at that. Let’s change that.

FastAPIs documentation is exhaustive on all accounts. It gets you started real quick, takes you by the hand if it gets more complicated and even describes features in detail when it doesn’t have to. I like it. Where it falls short, however, is when it comes to deployment. That’s probably because there are gazillions of ways to deploy an API. The documentation proposes to use Docker and while I understand that this is the way to go for most companies and most applications, I don’t see why I would want to deploy a small application with a few undemanding endpoints to a Docker Swarm or even Kubernetes cluster. Those come with a (hefty) price tag and are not interesting for the private person who just wants to get his hand dirty on FastAPI a little.

So let’s use this article to start over and learn how to set up a basic continuous deployment pipeline for a FastAPI app on a budget. We will be using AWS API Gateway, AWS Lambda the serverless computing services by AWS and Travis. Both AWS Lambda and AWS API Gateway are billed per API call and by the amount of data that you transfer. If you’re still eligible for the free tier of AWS you can use those two services completely free for the scope of this tutorial. If not refer to the pricing example of AWS API Gateway and AWS Lambda. Take a particularly detailed look at the AWS Lambda pricing as this depends on the time your application is running and the memory size that is provisioned to the AWS Lambda. Luckily AWS finally shed some transparency on that matter with a proper calculator.


Let’s assume that you already have registered an AWS account, set up a user other than root and have installed and configured AWS CLI properly. The latter is not a requirement, as you do everything we do in the AWS console, yet it’s not a bad idea to learn the AWS CLI anyways.

Setup a new role for AWS Lambda to assume

First, we set up a role that the AWS Lambda will assume within our AWS account. Roles are AWSs way to enforce the principle of least privilege when it comes to the resources an AWS Lambda can execute or have access to. Go to the IAM console, select Roles and Create. That’ll take you into the Role creation screen where you will choose an AWS service as the trusted entity and select Lambda as the service that will use the role we’re going to create. Press next and it’ll take you to the Permissions tab. Here you can either create a permission policy from scratch or select one of the existing ones. Keep in mind that the permissions vary depending on the purpose of the Lambda function. We’ll take an existing permission for now. Search for lambda and select the AWSLambdaBasicExecutionRole which only allows our Lambda to write logs to AWS Cloudwatch. Press next, give it a tag or don’t and press Review. Now you’re prompted to give it a name. Choose a meaningful name (e.g. fastapilambdarole) and continue to create the role, which takes you back to Roles where you click your newly created role and mark down the Role ARN (Amazon Resource Name).

Setup an AWS S3 bucket

For small applications that only use vanilla python without external libraries, one could quickly copy and paste the code into the AWS Lambda console. Bigger applications that use third-party libraries, however, will either be uploaded as a zip file or in our case, we will provide the location of our deployment package as an AWS S3 bucket. If you have AWS CLI properly setup you can create a bucket with:

aws s3api create-bucket \
--bucket my-travis-deployment-bucket \
--region eu-west-1 \
--create-bucket-configuration LocationConstraint=eu-west-1

Otherwise, you navigate to the S3 console and create one there.

Setup a Travis User in AWS

Next up we create a new AWS user that Travis can use to perform actions on AWS resources on our behalf. We could just use our encrypted credentials and secret, but a) spider-senses intensify b) we follow the principle of least privilege again. If Travis does not need the rights to summon the mighty p3dn.24xlarge, Travis will stick to creating AWS Lambdas. First, we will create a new policy. This time we have to be more specific because we need Travis to have access to AWS S3 for the deployment package, AWS Cloudformation to build our API stack, API Gateway, and AWS Lambda. You can do it in the AWS console or with AWS CLI (see here). Use this policy for the example:

 "Version": "2012-10-17",
 "Statement": [
    "Sid": "AllowListBucket",
    "Effect": "Allow",
    "Action": [
 "Version": "2012-10-17",
"Statement": [
        "Sid": "AllowListBucket",
        "Effect": "Allow",
        "Action": [
        "Resource": [
        "Sid": "AllowPassLambdaRole",
        "Effect": "Allow",
        "Action": [
        "Resource": [
        "Sid": "AllowS3Actions",
        "Effect": "Allow",
        "Action": [
        "Resource": "arn:aws:s3:::my-travis-deployment-bucket/*"
        "Sid": "AllowLambda",
        "Effect": "Allow",
        "Action": [
        "Resource": "*"
        "Sid": "AllowListPolicies",
        "Effect": "Allow",
        "Action": [
        "Resource": "*"
        "Sid": "AllowApiGateway",
        "Effect": "Allow",
        "Action": [
        "Resource": "*"
        "Sid": "AllowCloudFormation",
        "Effect": "Allow",
        "Action": [
        "Resource": "*"

Continuing, you go to the IAM console, but this time you will create a user with programmatic access and a meaningful name such as travisdeploymentuser or the likes. Now we attach the policy we just created to the Travis user. We get prompted with the AWS-ACCESS-KEY-ID and the AWS-SECRET-ACCESS-KEY of the user. Note those down for now. Done.

The example application

For the sake of this tutorial, I created a Github repository with an example application that you can use as a first step and to/or built on top.

Application structure

Inspired by the FastAPI-realworld-example-app, I neatly separated the pydantic models, the configuration, the endpoints, and the routers.

├── Dockerfile
├── README.md
├── example_app
│ ├── __init__.py
│ ├── api
│ │ ├── __init__.py
│ │ └── api_v1
│ │ ├── __init__.py
│ │ ├── api.py
│ │ └── endpoints
│ │ ├── __init__.py
│ │ └── example.py
│ ├── core
│ │ ├── __init__.py
│ │ ├── config.py
│ │ └── models
│ │ ├── input.py
│ │ └── output.py
│ └── main.py
├── requirements.txt
├── scripts
│ └── example.ipynb
├── setup.py
├── template.yml
├── .travis.yml
├── .pre-commit-config.yaml
├── tests
│ ├── __init__.py
│ ├── test_example_endpoint.py
│ └── test_ping.py

For simplicities sake, we have exactly two endpoints. One is /ping in main.py and the other is /api/v1/example that takes two integer values and returns their product. If you want you can expand this functionality with your pedantic models and additional routes. I also already included a .pre-commit-configuration.yaml for you to start using pre-commit right away. It comes with pre-configured hook for black.

Test the application locally

To test the example application locally we have a couple of options. One is by cloning the repository and starting it locally with uvicorn. The other is to build a docker image from the Dockerfile in the repository and expose the app from within a container.

git clone https://github.com/iwpnd/FastAPI-aws-lambda-example
cd FastAPI-aws-lambda-example
# create and activate a virtual environment
pip install -e .
pip install uvicorn # or anything else that can handle ASGI
pytest . -v
uvicorn example_app.main:app --host --port 8080 --reload


docker build -t example_app_image .
docker run -p 8080:8080 -name example-app-container example_app_image

No matter what you choose you can now go to your browser and check the documentation of the application via http://localhost:8080/docs and test the API through the Swagger UI right there.

Wrap the application with Mangum

For this application to run with AWS Lambda & AWS API Gateway, we have to wrap it with Mangum. Mangum works as an adapter for ASGI applications like the ones you can create with FastAPI so that they can send and receive information from API Gateway to Lambda and vice versa.

from FastAPI import FastAPI
from example_app.api.api_v1.api import router as api_router
from example_app.core.config import API_V1_STR, PROJECT_NAME
from mangum import Mangum

app = FastAPI(title=PROJECT_NAME)
app.include_router(api_router, prefix=API_V1_STR)

def pong():
    Sanity check.

    This will let the user know that the service is operational.

    And this path operation will:
    * show a life sign

    return {"ping": "pong!"}

handler = Mangum(app, enable_lifespan=False)

The AWS Lambda handler

The handler is necessary for AWS Lambda for it is the function that AWS Lambda can invoke when the service executes your code. It follows a simple syntax:

from your_module import Yourclass
from your_database import database

db = database.connect()

def handler(event, context):
    msg = Yourclass(
    return msg.transformed

You can import your libraries, like your_module or your_database and you can create variables or database connections. Everything outside of the handler function will execute when the AWS Lambda is provisioned. After that, you can use it within the handler that AWS Lambda will use on consecutive calls. The event is what AWS Lambda uses to pass in event data to the handler. The context on the other hand provides information about the invocation, function, and execution environment (see docs for more details).

Mangum as the handler for event and context

A FastAPI application does not have a handler, so that’s what Mangum is for. It wraps the app, therefore it will receive event and context in an AWS Lambda execution environment and will pass those on to the app itself. For this to work we have to setup AWS API Gateway proxy integration to pass the raw request to the AWS Lambda, and let the app decide on how to process the information and what to return, including 404s, etc. This is what allows this setup in the first place.

Deploy with AWS SAM

To deploy the AWS Lambda function we have now built, we will use the AWS Serverless Application Model (AWS SAM, an open-source framework to build serverless applications. As an extension to AWS Cloudformation it integrates nicely with all the other AWS services we need and lets us build our infrastructure from code - the template.yml in the repository.

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >
    FastAPI aws lambda example
        Type: AWS::Serverless::Function
                            Ref: FastapiExampleGateway
                        Path: /{proxy+}
                        Method: ANY
                    Type: Api
            FunctionName: FastAPI-lambda-example
            CodeUri: ./
            Handler: example_app.main.handler
            Runtime: python3.7
            Timeout: 300 # timeout of your lambda function
            MemorySize: 128 # memory size of your lambda function
            Description: FastAPI aws lambda example
            # other options, see ->
            # https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-specification-template-anatomy-globals.html#sam-specification-template-anatomy-globals-supported-resources-and-properties
            Role: !Sub arn:aws:iam::${AWS::AccountId}:role/fastapilambdarole

        Type: AWS::Serverless::Api
            StageName: prod
            OpenApiVersion: '3.0.0'

There are some things we have to unpack here. What we do is, we tell AWS Cloudformation to provision resources on our behalf and to deploy them in a stack. In the Resources section you see the API Gateway first, then the Lambda function we want to build from the code in the CodeUri with the handler in Handler. We define the Runtime of the Lambda, as well as MemorySize and Timeout. You need to attach the proper role in the Role section, that we have created earlier. In the Events section we tell AWS Cloudformation to use FastapiExampleGateway as the API Gateway with {proxy+} integration, because as you recall that’s what makes this setup work in the first place. Check out the official Template Anatomy to get a better understanding of other options available.

Once set up, you can now deploy the FastAPI-aws-lambda-example application from your local machine, through the template that tells AWS Cloudformation to build a stack and provision the resources necessary.

1. Stage: Validate the SAM template

The first thing we do is to validate the SAM template to check if the YAML we provide is valid.

sam validate
2020-01-21 10:28:39 Found credentials in environment variables.
/path/to/FastAPI-aws-lambda-example/template.yml is a valid SAM Template

2. Stage: Build the deployment package

Next up, we build the deployment package. If your application depends on packages that have natively compiled programs you pass --use-container and SAM will attempt to build the application in a Docker container using based on LambCI. Optionally you can see what’s happening in the container if you also pass the --debug flag.

sam build --use-container --debug

This will build the deployment package and store it in .aws-sam/build along with a new template.yaml that now also contains the values we !Sub ‘ed or substituted like so ${AWS::AccountId}, in the initial template.

Starting Build inside a container
Building resource 'FastapiExampleLambda'

Fetching lambci/lambda:build-python3.7 Docker container image......
Mounting /path/to/FastAPI-aws-lambda-example as /tmp/samcli/source:ro,delegated inside runtime container

Build Succeeded

Built Artifacts : .aws-sam/build
Built Template : .aws-sam/build/template.yaml

Commands you can use next
[*] Invoke Function: sam local invoke
[*] Package: sam package --s3-bucket <yourbucket>

Running PythonPipBuilder:ResolveDependencies
Running PythonPipBuilder:CopySource

3. Stage: Package the application

Up next, packaging. As stated in the beginning, when an application in an AWS Lambda exceeds a certain size or has dependencies, we have to package the application and either upload it in the AWS console or prepare an intermediate AWS S3 bucket and let AWS Lambda get the application package from there. AWS SAM requires you to do the latter, or better, does it for you if you provision a bucket and pass it with --s3-bucket my-travis-deployment-bucket.

sam package --s3-bucket my-travis-deployment-bucket --output-template-file out.yml --region eu-west-1

Which returns:

2020-01-21 10:48:12 Found credentials in environment variables.
Uploading to 2adfa5ddb62b541b7cf323cda43ee394 8523862 / 8523862.0 (100.00%) 
Successfully packaged artifacts and wrote output template to file out.yml.
Execute the following command to deploy the packaged template
sam deploy --template-file /path/to/FastAPI-aws-lambda-example/out.yml --stack-name <YOUR STACK NAME>

Now the application is packaged and a final template has been created that will now be used to tell AWS Cloudformation where the package is.

4. Stage: Deploy the application

sam deploy --template-file out.yml --stack-name example-stack-name --region eu-west-1 --no-fail-on-empty-changeset --capabilities CAPABILITY_IAM
Waiting for stack create/update to complete
Successfully created/updated stack - example-stack-name

This last step will finally deploy the application --stack-name to a --region. Important to note here is the option --no-fail-on-empty-changeset. Deploying a new version of your application code does not change the stack itself. So running the command without this option will fail. With this, you can, however, push consecutive updates of your codes to the same stack.

If you now go to the API Gateway Console you will see your API deployed to the prod stage at https://xxxxxxxxxx.execute-api.eu-west-1.amazonaws.com/prod.

Continuous deployment with Travis

Now for the last part, the continuous deployment through Github and Travis. The idea is that any code commit that passes an automated testing phase is automatically released into the production environment, and is accessible by the user. This means that the stages we laid out above, will no longer be executed manually by you, but instead in a Travis CI pipeline. Aight, let’se go.

  1. Go to https://travis-ci.com/ and sign up with your GitHub account.
  2. Accept the Authorization of Travis CI and you’ll be redirected to GitHub.
  3. Click on your profile picture in the top right of your Travis Dashboard, click the green Activate button, and select the repositories you want to use with Travis CI.
  4. Select the repository of your application
  5. Encrypt your AWS-ACCESS-KEY-ID and your AWS-SECRET-ACCESS-KEY of the travisdeploymentuser we created at the beginning like this, and put those in the .travis.yml file
  6. Commit the .travis.yml file to your repository and check your build process in the Travis dashboard.
language: python
cache: pip
- '3.7'
- pip install awscli
- pip install aws-sam-cli
    - stage: test
        - pip install pytest
        - pip install -e .
        - pytest . -v
    - stage: deploy
        - sam validate
        - sam build --debug
        - sam package --s3-bucket my-travis-deployment-bucket --output-template-file out.yml --region eu-west-1
        - sam deploy --template-file out.yml --stack-name example-stack-name --region eu-west-1 --no-fail-on-empty-changeset --capabilities CAPABILITY_IAM
      skip_cleanup: true
      if: branch = master
    on_failure: always
  - AWS_DEFAULT_REGION=eu-west-1
  - secure: your-encrypted-aws-access-key-id
  - secure: your-encrypted-aws-secret-access-key

From now on every time you commit changes to your repository master branch, it will be immediately be deployed to AWS Lambda.


  1. We learned how to create a new user and policies in AWS IAM
  2. We now have a basic application we can build upon
  3. We learned about AWS API Gateway and AWS Lambda
  4. We learned how Mangum works
  5. We learned about AWS SAM and how to deploy an application from your machine
  6. We learned how to use Travis for continuous deployment of the said application instead of doing it manually

If you have any questions feel free to reach out to me in the example repository or via mail.

· python, FastAPI, aws, aws lambda, aws sam