// 16 Essential AWS Lambda Interview Questions

AWS Lambda is one of Amazon Web Service's tiers of enterprise-level compute services. It enables developers to make their serverless applications more efficient by reducing the need for an always-on service. Such a service dramatically reduces the cost and complexity of cloud services, benefitting both developers and enterprises.

This article will find interview questions that cover important aspects of AWS Lambda. Such questions will help sharpen your knowledge about the popular cloud service. Additionally, it will help you prepare for the right opportunities by showing you what enterprises look for in their cloud developers. So read on!

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Beginner Level

What do you understand about AWS Lambda?

How many languages does AWS Lambda support?

What is the role of the handler when working with Lambda functions via SDK?

Briefly describe a scenario where you can practically trigger an AWS Lambda function.

We can use the AWS CLI to trigger a Lambda function. Let us create a scenario where we start from scratch and set up a Lambda function.

You firstly create a folder called lambda and create a file with the following code.

console.log('Loading function');
exports.handler = function(event, context) {
    var date = new Date().toDateString();	
    context.succeed("Hello " + event.username +
    "! Today's date is " + date);							
};

We name this file index.js. The code above simply prints a message with the user's name passed as an event and creates a log with the current date.

Next, we need to assign the Lambda function with a minimal IAM role that AWS Lambda assumes and executes the function with. Therefore, we create a policy called demoPolicy.json containing the following content.

{
       "Version": "2012-10-17",
       "Statement": [				
         {
           "Effect": "Allow",
           "Principal": {					
             "Service": "lambda.amazonaws.com"
           },						
           "Action": "sts:AssumeRole"
         }						
] }

Let us create an IAM role through the create-role command in the code below.

aws iam create-role
     --role-name basic-lambda-role
     --assume-role-policy-document file://demoPolicy.json

We have to create a deployment package to upload to an S3 bucket. AWS Lambda will execute this as a function automatically. We use the following code to zip the file and its folder.

zip -r demoFunction.zip index.js

Next, we create our Lambda function through the create-function command.

aws lambda create-function
     --region us-west-2
     --function-name demoFunction
     --zip-file fileb://demoFunction.zip
     --role arn:aws:iam::00123456789:role/basic-lambda-role
     --handler index.handler					
     --runtime nodejs4.3
     --memory-size 128

We have everything set up now and we can test our function by triggering it through the invoke command from the CLI. The output of the invocation is saved in the output.txt file.

aws lambda invoke
     --invocation-type RequestResponse
     --function-name demoFunction
     --region us-west-2
     --log-type Tail
     --payload '{"username":"YoYo"}'
     output.txt

Note: Commonly asked AWS Lambda interview question

Is it possible to write an asynchronous function using AWS Lambda?

Logging is an essential part of any application component. While using Lambda functions, how can you monitor their state or store logs?

What is the context object in a Lambda function?

Can a Lambda function have different versions? How would you create multiple function versions?

Is there a way to test Lambda functions that doesn’t involve writing scripts?

Intermediate Level

Which permission(s) do you require to access AWS services from a Lambda function? How do you set the permissions?

Can you identify what is wrong with the following code snippet?

const https = require('https')
let url = "https://docs.aws.amazon.com/lambda/latest/dg/welcome.html"   

exports.handler = async function(event) {
  const promise = new Promise(function(resolve, reject) {
    https.get(url, (res) => {
        resolve(res.statusCode)
      }).on('error', (e) => {
        reject(Error(e))
      })
    })
}

Can you briefly tell why there is a need for a serverless.yml file to support Lambda functions?

What is the following piece of code trying to do?

const dynamodb = require('aws-sdk/clients/dynamodb');
const docClient = new dynamodb.DocumentClient();

exports.handler = async (event, context) => {
    const message = 'Hello from Lambda!';
    const tableName = process.env.DDB_TABLE;
    const logStreamName = context.logStreamName;
    var params = {
        TableName : tableName,
        Key: { id : logStreamName },
        UpdateExpression: 'set invocations = if_not_exists(invocations, :start) + :inc',
        ExpressionAttributeValues: {
            ':start': 0,
            ':inc': 1
        },
        ReturnValues: 'ALL_NEW'
    };
    await docClient.update(params).promise();

    const response = {
        body: JSON.stringify(message)
    };
    console.log(`body: ${response.body}`);
    return response;
}

How would you orchestrate multiple Lambda functions to work together in a complex workflow?

Show how the packaging and deployment of a Lambda function would look with APEX and then with Caludia.js.

Advanced Level

Can you explain what the following JSON definition is trying to achieve?

...
"Invoke": {
    "Type": "Task",
    "Resource": "arn:aws:states:::lambda:invoke",
    "Retry": [
      {
        "ErrorEquals": [ 
            "function.MaxDepthError",
            "Lambda.TooManyRequestsException",
            "Lambda.ServiceException",
            "Lambda.Unknown"
          ],
        "MaxAttempts": 5
      }
    ],
    "Catch": [
      {
        "ErrorEquals": [ "function.DoublesRolledError" ],
        "Next": "CaughtException"
      }
    ],
    "Parameters": {
      "FunctionName": "arn:aws:lambda:us-east-2:123456789012:function:my-function:1",
      ...