Photo by Pasternak

This is a post on how to atomically increment a value in a DynamoDB item with Python and Lambda. Let's keep it short and sweet, you're only looking for the code anyways😈

Permissions

Your Lambda function's role needs to have permissions to the DynamoDB table you want to work with. These are the permissions that you need for this exact situation.

- Effect: Allow
  Action:
      - dynamodb:GetItem
      - dynamodb:PutItem
      - dynamodb:UpdateItem
  Resource: "arn of your DyanamoDB table"

Lambda

This Lambda is written in Python 3.8. Make sure to have it assume the role that contains the above IAM permissions. This is the table and the item:

TableName: my_dynamo_table_name
PrimaryKey: user1
Attribute: clicks

And here is the Lambda function's code first. I'll explain in detail down below.

import json
import boto3

# Initialize dynamodb boto3 object
dynamodb = boto3.resource('dynamodb')

def lambda_handler(event, context):

    # Set up dynamodb table object
    table = dynamodb.Table('my_dynamo_table_name')

    # Atomic update an item in table or add if doesn't exist
    ddbResponse = table.update_item(
        Key={
            "id": "user1"
        },
        UpdateExpression='ADD clicks :inc',
        ExpressionAttributeValues={
            ':inc': 1
        },
        ReturnValues="UPDATED_NEW"
    )

    # Return dynamodb response object
    print(ddbResponse)
    return ddbResponse

Breaking it down

• import json & import boto3 are for importing the necessary libraries.
• dynamodb = boto3.resource('dynamodb') initializes a DynamoDB boto3 object (outside of the function)
• table = dynamodb.Table('my_dynamo_table_name') initializes the "connection" to the table called my_dynamo_table_name and puts it into the table object.
• ddbResponse = table.update_item(... is the key here! This is the part where all communication with DyanmoDB happens.
  • Key={"id": "user1"}, is referring to the item's primary key. As you've seen above, this table's PK is id, and since this item doesn't exist yet (because I haven't created it in the table, nor will I have to), I'll just call its PK user1.
  • UpdateExpression='ADD clicks :inc', will tell DynamoDB how to update the item from above. That ADD is very important, as it could be SET or REMOVE. ADD means that instead of just setting an elready existing attribute's value, this can also create the item or the attribute if it doesn't exist. This is crucial because you don't want to manually have to create every attribute and item that's in your table. The next part is clicks :inc where clicks is the name of an/the attribute of your item.
  • ExpressionAttributeValues={ ':inc': 1 }, defines that :inc value, it can be called anything, it just hast to match with the above line. This is where you can set how much you want to increment the value every time the function runs, we'll stick with one.
  • ReturnValues="UPDATED_NEW" defines what will be returned by the update_item operation. We only care about the finished value so we'll keep it at this.

And that's it basically! After that it prints the response from the DynamoDB operation and you can see that the item and it's attribute got created and then incrementally updated in a single operation (atomically). If the item exists already, it will only increment its value and not recreate it.

Test

You can test it right in the console. Make sure that you created the DynamoDB table however (no need to create the item). Then just "Test" the Lambda in the console and you should see something like this:

Yo, let's connect

Check me out on Twitter - Chris Nagy - if you have any questions or just wanna chat😉