Serverless - Deploying Function

Navigate to a new folder wherein you want to create your first project to be deployed to serverless. Within that folder, run the following command −
sls create --template aws-python3

This command will create the boilerplate code for deploying lambda functions using serverless and python runtime.
Note that you can use other runtimes as well. Run sls create --help to get a list of all templates.
Once the boilerplate code is created, you will see two files in your folder: handler.py and serverless.yml. handler.py is the file containing the lambda function code. serverless.yml is the file that tells AWS how to create your lambda functions. It is the configuration file or the settings file that is going to be the focus of several chapters of this tutorial. Let us go through the handler.py file first.
import json
def hello(event, context):
   body = {
      "message": "Go Serverless v1.0! Your function executed successfully!", "input": event
   }
   response = {
      "statusCode": 200, "body": json.dumps(body)
   }
   return response
   # Use this code if you don't use the http event with the LAMBDA-PROXY
   # integration
   """
   return {
      "message": "Go Serverless v1.0! Your function executed successfully!", "event": event
   }
   """
It contains one function hello. This function takes in two arguments: event and context. Both of these are required arguments for any AWS Lambda function. Whenever the lambda function is invoked, the lambda runtime passes two arguments to the function − event and context.
The event argument contains the data for the lambda function to process. For instance, if you trigger your lambda function through a REST API, whatever data you send in the path parameters or the body of the API, are sent to the lambda function in the event parameter. More on that in later chapters. The important thing to note is that the event is usually of the python dict type, although can also be of str, float, int, list, or NoneType type.
The context object is another argument passed on to your lambda function at runtime. It is not used very often. The official AWS Documentation states that this object provides methods and properties that provide information about the invocation, function, and runtime environment. You can read more about the event and context objects here.
The function is pretty straightforward. It simply returns a message with status code 200. There is a comment at the bottom that should be used if we don't use the HTTP event with the LAMBDA-PROXY setting.More on that in the API-triggered lambda chapter.
Now, let us look at the serverless.yml file. It is a heavily commented file. The comments are extremely useful for someone starting off with serverless. You are encouraged to go through the comments thoroughly. We will be looking at a lot of concepts related to serverless.yml in the upcoming chapters. Let us just skim through the basic concepts here.
If you look at the serverless.yml file after removing the comments, this is how it will look like −
service: aws-serverless
frameworkVersion: '2'

provider:
   name: aws
   runtime: python3.8
   lambdaHashingVersion: 20201221
functions:
   hello:
      handler: handler.hello
The service field determines the name of the CloudFormation stack within which your lambda function and all the required resources will be created. Think of the service as your project. Everything required for the AWS Lambda function to execute will be created within that service. You can set a service name of your choice.
The framework version refers to the version of the serverless framework. It is an optional field, ususally kept to ensure that the same version number is used by people with whom you share your code. If ther frameworkVersion mentioned in serverless.yml is different than the version of serverless installed in your machine, you will receive an error during deployment. You can also specify a range for frameworkVersion like frameworkVersion − >=2.1.0 && <3.0.0. You can read more about frameworkVersions here.
The next section, provider, can be considered as a set of global settings. We will be discussing additional parameters covered under provider in later chapters. Here, we will focus on the parameters available. The name field determines the name of your platform environment, which is aws in this case. The runtime is python3.8 because we used the python3 template. The lambdaHashingVersion refers to the name of the hashing algorithm that should be used by the framework.
Please note that if you've added a custom profile in the config credentials step in the previous chapter, you will need to add the profile parameter in provide. For instance, I set my profile name to yash-sanghvi. Therefore, my provider looks like −
provider:
   name: aws
   runtime: python3.8
   lambdaHashingVersion: 20201221
   profile: yash-sanghvi
Finally, the functions block defines all the lambda functions. We have just one function here, in the handler file. The name of the function is hello. The path of the function is mentioned in the handler field.
To deploy the function you need to open the Command Prompt, navigate to the folder containing your serverless.yml, and enter the following command −
sls deploy -v

The -v is an optional argument that indicates verbose output. It helps you understand the background processes better. Once your function is deployed, you should be able to see it on the AWS Console in the us-east-1 region (which is the default). You can invoke it from the console, using the 'Test' feature (you can keep the same default event since our lambda function is anyway not using the event input). You can also test it using the Command Prompt using −
sls invoke --function hello

Please note that you cannot always test your function locally if it interfaces with other AWS Services like S3 or dynamoDB.Only the very basic functions can be tested locally.
If you want to deploy an existing project to AWS, modify the existing function to take in only the event and context as arguments. Next, add a serverless.yml file in the folder, and define your function within serverless.yml. Then open the command prompt, navigate to the folder, and hit sls deploy -v. That way, your existing function can also be deployed to AWS Lambda.

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