Lambda function with DynamoDB - Step-by-Step Execution

Concept Flow - Lambda function with DynamoDB

Event triggers Lambda

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Lambda starts execution

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Lambda reads event data

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Lambda calls DynamoDB API

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DynamoDB processes request

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DynamoDB returns response

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Lambda processes response

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Lambda returns result or error

This flow shows how a Lambda function is triggered, interacts with DynamoDB, and returns a result.

Execution Sample

DynamoDB

exports.handler = async (event) => {
  const AWS = require('aws-sdk');
  const db = new AWS.DynamoDB.DocumentClient();
  const params = { TableName: 'Users', Key: { userId: event.userId } };
  const data = await db.get(params).promise();
  return data.Item;
};

This Lambda function fetches a user item from DynamoDB based on userId from the event.

Execution Table

Step	Action	Input/Condition	DynamoDB Call	Response	Output
1	Lambda triggered	event.userId = '123'	No	No	Start execution
2	Create DynamoDB client	AWS SDK loaded	No	No	Client ready
3	Prepare params	TableName='Users', Key={userId:'123'}	No	No	Params ready
4	Call db.get(params).promise()	params as above	GetItem on 'Users' with Key userId='123'	Returns item {userId:'123', name:'Alice'}	Data received
5	Return data.Item	Item exists	No	No	Returns {userId:'123', name:'Alice'}
6	Lambda execution ends	No	No	No	Response sent

💡 Lambda finishes after returning the DynamoDB item or error if any

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
event.userId	'123'	'123'	'123'	'123'	'123'
db	undefined	AWS.DynamoDB.DocumentClient instance	AWS.DynamoDB.DocumentClient instance	AWS.DynamoDB.DocumentClient instance	AWS.DynamoDB.DocumentClient instance
params	undefined	undefined	{ TableName: 'Users', Key: { userId: '123' } }	{ TableName: 'Users', Key: { userId: '123' } }	{ TableName: 'Users', Key: { userId: '123' } }
data	undefined	undefined	undefined	{ Item: { userId: '123', name: 'Alice' } }	{ Item: { userId: '123', name: 'Alice' } }
return value	undefined	undefined	undefined	undefined	{ userId: '123', name: 'Alice' }

Key Moments - 3 Insights

Why do we use 'await' before the DynamoDB call?

Because the DynamoDB call returns a promise, 'await' pauses Lambda execution until the data is returned, as shown in step 4 of the execution_table.

What happens if the item with userId '123' does not exist?

The 'data.Item' will be undefined, so the Lambda will return undefined or null. This is implied in step 5 where the item is returned if it exists.

Why do we create a new DynamoDB client inside the Lambda handler?

To interact with DynamoDB, the Lambda needs a client instance, created at step 2, which allows calling DynamoDB APIs.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'params' after step 3?

A undefined

B { TableName: 'Users', Key: { userId: '123' } }

C { userId: '123' }

D AWS.DynamoDB.DocumentClient instance

💡 Hint

Check the 'params' variable in variable_tracker after step 3

At which step does the Lambda function receive the data from DynamoDB?

A Step 4

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the 'Response' column in execution_table where DynamoDB returns the item

If the event.userId was missing, what would change in the execution?

A Lambda would return a default item

B Lambda would skip DynamoDB call

C DynamoDB call would fail or return no item

D Lambda would create a new user

💡 Hint

Consider how 'params' depends on event.userId in variable_tracker and execution_table

Concept Snapshot

Lambda function with DynamoDB:
- Lambda triggered by event
- Create DynamoDB client inside Lambda
- Prepare params with TableName and Key
- Use await db.get(params).promise() to fetch item
- Return data.Item as Lambda response
- Handle missing items gracefully

Full Transcript

This visual execution shows how a Lambda function interacts with DynamoDB. When the Lambda is triggered, it reads the event data, creates a DynamoDB client, prepares parameters for the query, calls DynamoDB to get an item, waits for the response, and returns the item. Variables like event.userId, params, and data change step-by-step. Key moments include understanding the use of await, handling missing items, and why the client is created inside Lambda. The quiz questions help reinforce these steps by referencing the execution table and variable states.