In Amazon DynamoDB, a table is a collection of items and each item is a collection of attributes. In a relational database, a table has a predefined schema such as the table name, primary key, list of its column names and their data types. All records stored in the table must have the same set of columns. In contrast, DynamoDB only requires that a table has a primary key, but does not require you to define all of the attribute names and data types in advance. Individual items in a DynamoDB table can have any number of attributes, although there is a limit of 400 KB on the item size. An item size is the sum of lengths of its attribute names and values (binary and UTF-8 lengths).
Each attribute in an item is a name-value pair. An attribute can be a scalar (single-valued), a JSON document, or a set. For example, consider storing a catalog of products in DynamoDB.You can create a table, ProductCatalog, with the Id attribute as its primary key. The primary key uniquely identifies each item, so that no two products in the table can have the same Id.
When you create a table, in addition to the table name, you must specify the primary key of the table. The primary key uniquely identifies each item in the table, so that no two items can have the same key. DynamoDB supports two different kinds of primary keys:
• Partition Key – A simple primary key, composed of one attribute known as the partition key. DynamoDB uses the partition key's value as input to an internal hash function; the output from the hash function determines the partition where the item will be stored. No two items in a table can have the same partition key value.
• Partition Key and Sort Key – A composite primary key, composed of two attributes. The first attribute is the partition key, and the second attribute is the sort key. DynamoDB uses the partition key value as input to an internal hash function; the output from the hash function determines the partition where the item will be stored. All items with the same partition key are stored together, in sorted order by sort key value. It is possible for two items to have the same partition key value, but those two items must have different sort key values.
When you create a table with a composite primary key (partition key and sort key), you can optionally define one or more secondary indexes on that table. A secondary index lets you query the data in the table using an alternate key, in addition to queries against the primary key.
DynamoDB Data Types
Amazon DynamoDB supports the following data types:
• Scalar types – Number, String, Binary, Boolean, and Null.
• Document types – List and Map.
• Set types – String Set, Number Set, and Binary Set.
DynamoDB stores data in partitions. A partition is an allocation of storage for a table, backed by solid state drives (SSDs) and automatically replicated across three facilities within an AWS region. Partition management is handled entirely by DynamoDB—customers never need to manage partitions themselves. If your storage requirements exceed a partition's capacity, DynamoDB allocates additional partitions automatically.