On-Demand Capacity

On-Demand Capacity is an important part of building production-ready DynamoDB systems. This lesson explains what on-demand capacity means, how it works, and how to apply it with practical examples you can reuse.

On-Demand Capacity Overview

On-Demand Capacity is a building block you will reach for often in DynamoDB. It keeps related logic together and makes your intent obvious to reviewers and future maintainers.

When you learn on-demand capacity properly, you avoid the guesswork that leads to bugs and rework. The example below shows the shape you will use in most real DynamoDB projects.

import { DynamoDBClient } from '@aws-sdk/client-dynamodb';
import { DynamoDBDocumentClient, GetCommand, PutCommand } from '@aws-sdk/lib-dynamodb';

const client = new DynamoDBClient({});
const docClient = DynamoDBDocumentClient.from(client);

// reuse docClient across the module for efficient, typed access
await docClient.send(new PutCommand({ TableName: 'Orders', Item: { pk: '1' } }));

The DynamoDBDocumentClient maps plain JavaScript objects to DynamoDB item format for you.

On-Demand Capacity Example

import { DynamoDBClient } from '@aws-sdk/client-dynamodb';
import { DynamoDBDocumentClient } from '@aws-sdk/lib-dynamodb';

const docClient = DynamoDBDocumentClient.from(new DynamoDBClient({}));
// docClient.send(new PutCommand(...)) etc.

Start from a minimal On-Demand Capacity example and grow it only as needed.
Keep configuration explicit so On-Demand Capacity behaves the same in every environment.
Name things clearly so teammates understand your On-Demand Capacity at a glance.
Add tests around On-Demand Capacity early to lock in expected behaviour.

Amazon DynamoDB Cheatsheet

Handy DynamoDB (AWS SDK v3) reference related to on-demand capacity.

Operation	Command	Purpose
Create/replace	`PutCommand`	Write an item
Read one	`GetCommand`	Fetch by primary key
Update	`UpdateCommand`	Modify attributes
Delete	`DeleteCommand`	Remove an item
Query	`QueryCommand`	Efficient key-based read
Scan	`ScanCommand`	Full-table read (avoid)
Transaction	`TransactWriteCommand`	Atomic multi-item writes

How On-Demand Capacity Works in DynamoDB

On-Demand Capacity builds on DynamoDB's key-value and document model, where every item lives in a partition chosen by its partition key and is optionally ordered by a sort key.

The DynamoDBDocumentClient maps plain JavaScript objects to DynamoDB item format for you.

Design access patterns first, then model keys around them.
Prefer Query over Scan for predictable performance.
Use expressions to read and write only what you need.
Keep items small and avoid hot partitions.

Practical Guidance for On-Demand Capacity

In production, on-demand capacity should be cost-aware and resilient. Right-size capacity, handle throttling with retries, and lean on indexes to support your query patterns.

Concern	Recommendation
Performance	Query by key; avoid table scans
Cost	Use on-demand or right-sized provisioned capacity
Modeling	Design for known access patterns
Reliability	Retry throttled requests with backoff

Common Mistakes

Copying on-demand capacity snippets without understanding what each line does.
Skipping error handling and edge cases when wiring up on-demand capacity.
Leaving on-demand capacity untested, so regressions slip into production.
Over-engineering on-demand capacity before you actually need the extra flexibility.

Key Takeaways

On-Demand Capacity is a core part of working effectively with DynamoDB.
Start small and keep on-demand capacity focused on a single responsibility.
Apply consistent patterns so on-demand capacity scales across your project.
Test and document on-demand capacity to keep it maintainable over time.

Pro Tip

Pair on-demand capacity with automated tests from day one. It is far cheaper to catch DynamoDB regressions in CI than in production.