Code Evaluation With AWS Lambda and API Gateway

Create Function

Steps…

1. Select blueprint: Click “Author from scratch” to start with a blank function:

   

2. Configure Triggers: We’ll set up the API Gateway integration later, so simply click “Next” to skip this part.

3. Configure function: Name the function execute_python_code, and add a basic description - Execute user-supplied Python code. Select “Python 3.6” in the “Runtime” drop-down.

   

4. Within the inline code editor, update the lambda_handler function definition with:

   Python

   import sys
   from io import StringIO
   
   
   def lambda_handler(event, context):
       # Get code from payload
       code = event['answer']
       test_code = code + '\nprint(sum(1,1))'
       # Capture stdout
       buffer = StringIO()
       sys.stdout = buffer
       # Execute code
       try:
           exec(test_code)
       except:
           return False
       # Return stdout
       sys.stdout = sys.stdout
       # Check
       if int(buffer.getvalue()) == 2:
           return True
       return False
   

   Here, within lambda_handler, which is the default entry point for Lambda, we parse the JSON request body, passing the supplied code along with some test code - sum(1,1) - to the exec function - which executes the string as Python code. Then, we simply ensure the actual results are the same as what’s expected - e.g., 2 - and return the appropriate response.

   

   Under “Lambda function handler and role”, leave the default handler and then select “Create a new Role from template(s)” from the drop-down. Enter a “Role name”, like api_gateway_access, and select ” Simple Microservice permissions” for the “Policy templates”, which provides access to API Gateway.

   

   Click “Next”.

5. Review: Create the function after a quick review.

Test

Next click on the “Test” button to execute the newly created Lambda:

Using the “Hello World” event template, replace the sample with:

{
  "answer": "def sum(x,y):\n    return x+y"
}

Click the “Save and test” button at the bottom of the modal to run the test. Once done, you should see something similar to:

With that, we can move on to configuring the API Gateway to trigger the Lambda from user-submitted POST requests…

Create the API

1. To start, from the API Gateway page, click the “Get Started” button to create a new API:

   

2. Select “New API”, and then provide a descriptive name, like code_execute_api:

   

   Then, create the API.

3. Select “Create Resource” from the “Actions” drop-down.

   

4. Name the resource execute, and then click “Create Resource”.

   

5. With the resource highlighted, select “Create Method” from the “Actions” drop-down.

   

6. Choose “POST” from the method drop-down. Click the checkmark next to it.

   

7. In the “Setup” step, select “Lambda Function” as the “Integration type”, select the “us-east-1” region in the drop-down, and enter the name of the Lambda function that you just created.

   

8. Click “Save”, and then click “OK” to give permission to the API Gateway to run your Lambda function.

Test It Manually

To test, click on the lightning bolt that says “Test”.

Scroll down to the “Request Body” input and add the same JSON code we used with the Lambda function:

{
  "answer": "def sum(x,y):\n    return x+y"
}

Click “Test”. You should see something similar to:

Enable CORS

Next, we need to enable CORS so that we can POST to the API endpoint from another domain.

With the resource highlighted, select “Enable CORS” from the “Actions” drop-down:

Just keep the defaults for now since we’re still testing the API. Click the “Enable CORS and replace existing CORS headers” button.

Deploy the API

Finally, to deploy, select “Deploy API” from the “Actions” drop-down:

Create a new “Deployment stage” called ‘v1’:

API gateway will generate a random subdomain for the API endpoint URL, and the stage name will be added to the end of the URL. You should now be able to make POST requests to a similar URL:

https://c0rue3ifh4.execute-api.us-east-1.amazonaws.com/v1/execute

Test via cURL

$ curl -H "Content-Type: application/json" -X POST \
  -d '{"answer":"def sum(x,y):\n    return x+y"}' \
  https://c0rue3ifh4.execute-api.us-east-1.amazonaws.com/v1/execute