DynamoDB

What is DynamoDB

• Full managed.
• NoSQL, supports both key-value and document data models.
• ACID transactions

ACID stands for four key properties of a reliable database transaction:

• Atomicity – All operations in a transaction succeed or none do.
• Consistency – The database stays in a valid state before and after the transaction.
• Isolation – Transactions don’t interfere with each other; partial changes aren’t visible.
• Durability – Once committed, changes are permanent, even after failures.

Transaction means a group of read/write operations that must be processed together. In DynamoDB, a transaction ensures either all the changes succeed or none happen at all, across one or more items (even across tables).

Components of DynamoDB

Table

A table is a collection of items.

• Store data in tables using a key-value or document model.
• Data is denormalized, store in as few tables as possible, often a single table per application

Item

An item is a collection of attributes.

• Similar to a rows in relational database
• Can stroe complex, hierarchical data within a single item.

Attribute

An attribute is a fundamental data element, which does not need to be broken down any futher.

• Similar to fields or columns in relational database.
• Can be a scalar(single-valued) or nested. Strings and numbers are common examples of scalars. DynamoDB supports nested attributes up to 32 levels deep.
• Each item can have its own distinct attributes.
• Do not need to define them before storing items.

Primary Key

Each item in the table has a primary key, which is an unique identifier of each item.

• You must designate the primary key for each table.
• Select one attribute as the primary key.
• Optionally, select a second attribute as the sort key.
• Each primary key attribute must be a scalar(single-valued).

So there are two different kinds of primary keys:

• Partition key
• Partition key + sort key - Referred to as a composite primary key.

Partitions

A partition is an allocation of storage of a table, backed by SSDs, it is automatically repolicated across mutiple AZ within an AWS Region.

Partition key(hash attribute)

• DynamoDB uses partition key to determine the location in which an item is stored.
• A simple primary key consists of only a partition key.
• DynamoDB stores items with the same partition key physically store together.

Sort key(range attribute)

• A composite primary key uses the partition key and a sort key to uniquely identify items.
• Sort the order of the items with the same partition key.

Identifying items in DynamoDB

• With a simple primary key, each partition key alone uniquely identifies an item.
• With a composite primary key, partition key + sort key together uniquely identify an item.
• Access partterns based on non-key attributes is not efficient(scan the table).

Troubleshoots