Composite Primary Key

In this lesson you will learn composite primary key in DynamoDB, why it matters within primary keys, and how to use it correctly with clear, copy-ready examples.

Composite Primary Key Overview

Composite Primary Key lets you structure DynamoDB work so it stays readable, testable, and easy to scale. Instead of ad-hoc code, you follow a clear pattern that other developers can recognise immediately.

The key is to keep composite primary key focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

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.

Composite Primary Key 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 Composite Primary Key example and grow it only as needed.
Keep configuration explicit so Composite Primary Key behaves the same in every environment.
Name things clearly so teammates understand your Composite Primary Key at a glance.
Add tests around Composite Primary Key early to lock in expected behaviour.

Amazon DynamoDB Cheatsheet

Handy DynamoDB (AWS SDK v3) reference related to composite primary key.

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 Composite Primary Key Works in DynamoDB

Composite Primary Key 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 Composite Primary Key

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

Key Takeaways

Composite Primary Key is a core part of working effectively with DynamoDB.
Start small and keep composite primary key focused on a single responsibility.
Apply consistent patterns so composite primary key scales across your project.
Test and document composite primary key to keep it maintainable over time.

Pro Tip

When you get stuck on composite primary key, reduce it to the smallest reproducible example first — most DynamoDB issues become obvious once the noise is gone.