POST API by Lambda with serverless framework

Serverless is a kind of buzzword in recent years. It brings me a new concept of providing a web service without depending on the fixed amount of server machines (virtually), enabling us to build a more agile and flexible platform responding to changes faster.

Serverless Framework is one of the most notable framework implementing the concept, “serverless”. It supports a lot of major cloud service providers such as AWS, Azure. We can launch a new web-based service with minimal code writing abruptly.

I have created a web API providing a POST endpoint with serverless backed by AWS Lambda and API Gateway. But I needed a little investigation to do so. Therefore, those who are facing the requirement to provide POST API with lambda will find this useful. Here is the guide I would want to have before starting to develop an API.

serverless.yml

serverless.yml is a central place controlling all configuration of the infrastructure managed by the serverless application. It specifies the name of the provider, environment variables, and so on.

service: myservice

plugins:
  # Necessary to purge previous version
  - serverless-prune-plugin
  # Install all dependencies specified by requirements.txt
  - serverless-python-requirements

provider:
  name: aws
  runtime: python3.7
  stage: ${opt:stage, 'development'}
  region: us-east-1

custom field provides variables that likely change depending on the environment the application runs.

custom:
  stages:
    - development
    - production
  a_variable:
    development: variable_for_development
    production: variable_for_production
  pythonRequirements:
    dockerizePip: true
  prune:
    # Specify the number of retained previous versions
    automatic: true
    number: 10

Function for POST

The function definition for the POST endpoint is easy to write.

functions:
  post_endpoint:
    handler: handler.post_endpoint
    events:
      - http:
            path: myapp/post_endpoint
            method: post
    environment:
      # Set the stage specific variable
      A_VARIABLE: ${self:custom.a_variable.${self:provider.stage}}

Since the POST endpoint parses the HTTP request body, there is no need to specify the required parameters in the config.

Handler Method

We can find the POST method in the handler code as follows.

def post_endpoint(event, context):
    print("A POST endpoint")
    # Obtain the body in JSON format
    body = json.loads(event["body"])

We can extract any parameters from the body like body['key']. Note that the validation of the parameter is the responsibility of the handler. The required parameter for the app may be missing in the body. Please make sure to check the existence of the parameter beforehand.

def get_or_none(key, body):
    if key in body:
        return body[key]
    else:
        return None

get_or_none('key', body)

Thanks!