DynamoDB Conditional Put - Ensure Data Integrity - dynamodb Cheatsheets

This section demonstrates how to perform a conditional PUT operation in AWS DynamoDB using Python and the Boto3 SDK. A conditional PUT ensures that an item is only written to the table if a specified condition is met. This is crucial for maintaining data integrity and preventing unintended data overwrites.

Understanding DynamoDB Conditional Writes

Conditional writes in DynamoDB allow you to execute operations like PutItem, UpdateItem, or DeleteItem only if certain conditions on the item's attributes are satisfied. The ConditionExpression parameter is used to define these conditions. If the condition evaluates to false, the operation fails, and DynamoDB returns a ConditionalCheckFailedException.

Python Example: Conditional PutItem

The following Python code snippet illustrates a conditional PutItem operation. The condition specified is attribute_not_exists(gamerid), which means the item will only be put if the gamerid attribute does not already exist in the table. This is a common pattern to ensure that a new record is created and not accidentally duplicated or overwritten.

Code Implementation

import boto3
from botocore import exceptions
import time

# Initialize DynamoDB client
# Replace with your actual AWS credentials and endpoint if not using default configuration
client = boto3.Session(region_name='eu-west-1').client('dynamodb', aws_access_key_id='', aws_secret_access_key='', endpoint_url='http://localhost:4567')

try:
    # Attempt to put an item with a condition
    response = client.put_item(
        TableName='gamescores',
        Item={
            'event': {'S': 'gaming_nationals_zaf'},
            'timestamp': {'S': '2019-02-08T14:53'},
            'score': {'N': '11885'},
            'name': {'S': 'will'},
            'gamerid': {'S': 'wilson9335'},
            'game': {'S': 'counter strike'},
            'age': {'N': '27'},
            'rank': {'S': 'professional'},
            'location': {'S': 'sweden'}
        },
        # Condition: Only put the item if 'gamerid' attribute does not exist
        ConditionExpression='attribute_not_exists(gamerid)'
    )
    print("Item successfully put with conditional check.")
    print(response)

except exceptions.ClientError as e:
    # Handle the case where the condition fails
    if e.response['Error']['Code'] == 'ConditionalCheckFailedException':
        print('ConditionalCheckFailedException: The gamerid already exists. Item was not put.')
    else:
        print(f"An unexpected error occurred: {e}")

Key Concepts and Best Practices

`ConditionExpression`

The ConditionExpression is a powerful feature for implementing optimistic locking and ensuring data consistency. You can use various operators and functions to define complex conditions based on attribute values, existence, and comparisons.

`attribute_not_exists()`

This function within ConditionExpression checks if a specified attribute exists. It's commonly used to ensure that you are creating a new record rather than updating an existing one, or to prevent duplicate entries based on a unique identifier.

Error Handling

Always wrap your DynamoDB operations in a try-except block to catch potential ClientError exceptions, especially the ConditionalCheckFailedException. This allows your application to gracefully handle situations where the condition is not met.

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DynamoDB Conditional Put Example