Building a Serverless Visitor Counter with AWS

As part of the Cloud Resume Challenge, I needed to implement a visitor counter that would track and display the number of visitors to my resume website. Here's how I built it using AWS serverless services.

The Architecture

The visitor counter consists of three main AWS services working together: API Gateway receives HTTP requests from the frontend, Lambda processes the business logic, and DynamoDB stores the counter value persistently.

Why Serverless?

I chose a serverless architecture for several reasons:

• Cost efficiency: Pay only for what you use—perfect for a low-traffic portfolio site
• Zero server management: No patching, scaling, or maintenance required
• Automatic scaling: Handles traffic spikes without configuration
• High availability: Built-in redundancy across availability zones

The Lambda Function

The core logic lives in a Python Lambda function. The key challenge was implementing an atomic counter increment to avoid race conditions when multiple visitors hit the site simultaneously.

response = dynamodb.update_item(
    TableName=table_name,
    Key={'visitor_count': {'S': 'main'}},
    UpdateExpression='ADD #count :inc',
    ExpressionAttributeNames={'#count': 'count'},
    ExpressionAttributeValues={':inc': {'N': '1'}},
    ReturnValues='ALL_NEW'
)

Using DynamoDB's ADD operation ensures atomicity—the increment happens in a single operation at the database level, eliminating race conditions.

Handling CORS

One of the trickier aspects was configuring CORS correctly. The frontend runs on a different domain than the API, so proper headers are essential:

'headers': {
    'Access-Control-Allow-Origin': '*',
    'Access-Control-Allow-Headers': 'Content-Type',
    'Access-Control-Allow-Methods': 'GET,OPTIONS'
}

Lessons Learned

Building this seemingly simple feature taught me several valuable lessons:

• DynamoDB has reserved words (like "count") that require ExpressionAttributeNames
• The boto3 client API requires explicit type descriptors for all values
• API Gateway OPTIONS method is crucial for CORS preflight requests
• Always test your Lambda functions locally before deploying

What's Next

In my next post, I'll cover how I automated the deployment of this infrastructure using Terraform and GitHub Actions for continuous delivery.