# Using DynamoDB as a Result Backend for Celery

Amazon DynamoDB is a serverless, NoSQL database service. Using it as a result backend for Celery is excellent for scalability and serverless architectures, especially when paired with SQS as the broker.

## 1. Installation

You need to install the AWS SDK for Python (`boto3`) alongside Celery.

```bash
pip install boto3
```

## 2. Configuration

Configure the `result_backend` using the `dynamodb://` scheme.

### URL Format

The format is:
`dynamodb://aws_access_key_id:aws_secret_access_key@region/table_name`

If you are using IAM Roles (EC2/ECS/Lambda) or environment variables (standard AWS setup), you can omit the credentials:

```python
# Format: dynamodb://@region/table_name
app = Celery('tasks',
             broker='sqs://',
             backend='dynamodb://@us-east-1/celery-results')
```

## 3. Table Configuration

Celery can automatically create the table for you if it doesn't exist, but in production, you should provision it via Terraform or CloudFormation to manage capacity modes (On-Demand vs Provisioned).

### Schema Requirements
*   **Partition Key (Hash Key)**: `id` (String)

### Configuration Options

You can pass specific DynamoDB settings via `result_backend_transport_options`.

```python
app.conf.result_backend_transport_options = {
    'read_capacity_units': 5,
    'write_capacity_units': 5,
}
```

## 4. Time To Live (TTL)

DynamoDB supports automatic expiration of items. This is crucial for a result backend to prevent the table from growing infinitely.

Celery does not automatically enable TTL on the table definition; you must enable it on the AWS console or via IaC.

1.  Go to the DynamoDB Table in AWS Console.
2.  Select **Time to Live (TTL)**.
3.  Enable it and set the attribute name to `ttl` (Celery uses this field name by default for expiration).

## 5. IAM Permissions

The worker needs permissions like `dynamodb:GetItem`, `dynamodb:PutItem`, and `dynamodb:DescribeTable` on the specific table resource.

[[programming/python/celery]]