Atomic Counters - DynamoDB CRUD Operations

Syntax

ddb.update_item(
    TableName='table_name',
    Key={
        'key': 'value'
    },
    UpdateExpression='SET counter = counter + :value',
    ExpressionAttributeValues={
        ':value': {'N': '1'}
    },
    ReturnValues='UPDATED_NEW'
)

Example

Suppose we have a DynamoDB table named visitors with the following schema:

{
    "PK": "visitor_id",
    "SK": "visit_timestamp",
    "first_name",
    "last_name",
    "page_views"
}

We want to increment the number of page_views for a specific visitor identified by their visitor_id. We can use atomic counters in DynamoDB to achieve this with the following code:

import boto3

client = boto3.client('dynamodb')

response = client.update_item(
    TableName='visitors',
    Key={
        'PK': {'S': 'user_123'},
        'SK': {'S': '20220414213245'},
    },
    UpdateExpression="ADD page_views :value",
    ExpressionAttributeValues={
        ':value': {'N': '1'}
    },
    ReturnValues="UPDATED_NEW"
)

print(response)

Output

The above code will increment the page_views counter for the specified visitor by 1 and return the updated item as follows:

{
    "Attributes": {
        "PK": {
            "S": "user_123"
        },
        "SK": {
            "S": "20220414213245"
        },
        "first_name": {
            "S": "John"
        },
        "last_name": {
            "S": "Doe"
        },
        "page_views": {
            "N": "5"
        }
    }
}

Explanation

In the example code above, we execute a update_item operation to increment the page_views counter for a specific visitor identified by their visitor_id.

We specify the UpdateExpression to use the ADD operation to increment the page_views attribute by the value of :value which is specified in the ExpressionAttributeValues as 1.

Lastly, we specify ReturnValues to return the updated item with the page_views counter incremented by 1.

Use

Atomic counters in DynamoDB are useful when we want to increment/decrement a numeric attribute without risking the chance of concurrent writes causing conflicts.

For example, we can use atomic counters to keep track of views, likes, follows, and other such metrics.

Important Points

• Atomic counters in DynamoDB are designed to solve the problem of concurrent writes to a single numeric attribute.
• Atomic counters in DynamoDB use the ADD operation to increment/decrement a numeric attribute.
• Atomic counters in DynamoDB provides strongly consistent reads, which means that reads will reflect the latest updates to the counter.

Summary

Atomic counters in DynamoDB provide an easy way to increment/decrement a numeric attribute without risking the chance of concurrent writes causing conflicts. They use the ADD operation to increment/decrement a numeric attribute and provide strongly consistent reads, ensuring that reads reflect the latest updates to the counter.