Amazon DynamoDB Data Model

The Amazon DynamoDB data model concepts include tables, items and attributes.

In Amazon DynamoDB, a database is a collection of tables. 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. Amazon DynamoDB is a NoSQL database: Except for the required primary key, an Amazon DynamoDB table is schema-less. Individual items in an Amazon DynamoDB table can have any number of attributes, although there is a limit of 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 single-valued or multi-valued set. For example, a book item can have title and authors attributes. Each book has one title but can have many authors. The multi-valued attribute is a set; duplicate values are not allowed.

For example, consider storing a catalog of products in Amazon DynamoDB. You can create a table, ProductCatalog, with the Id attribute as its primary key.

ProductCatalog ( Id, ... )

You can store various kinds of product items in the table. The following table shows sample items.

{ 
   Id = 101                                       
   ProductName = "Book 101 Title"
   ISBN = "111-1111111111"
   Authors = [ "Author 1", "Author 2" ]
   Price = -2
   Dimensions = "8.5 x 11.0 x 0.5"
   PageCount = 500
   InPublication = 1
   ProductCategory = "Book" 
}                                    

{
   Id = 201 
   ProductName = "18-Bicycle 201"
   Description = "201 description"
   BicycleType = "Road"
   Brand = "Brand-Company A"
   Price = 100
   Gender = "M"
   Color = [ "Red", "Black" ]
   ProductCategory = "Bike"
}

{
   Id = 202 
   ProductName = "21-Bicycle 202"
   Description = "202 description"
   BicycleType = "Road"
   Brand = "Brand-Company A"
   Price = 200
   Gender = "M"
   Color = [ "Green", "Black" ]
   ProductCategory = "Bike"
}

In the example, the ProductCatalog table has one book item and two bicycle items. Item 101 is a book with many attributes including the Authors multi-valued attribute. Item 201 and 202 are bikes, and these items have a Color multi-valued attribute. The Id is the only required attribute. Note that attribute values are shown using JSON-like syntax for illustration purposes.

Amazon DynamoDB does not allow null or empty string attribute values.

When you create a table, in addition to the table name, you must specify the primary key of the table. Amazon DynamoDB supports the following two types of primary keys:

When you create a table with a hash-and-range key, you can optionally define one or more local secondary indexes on that table. A local secondary index lets you query the data in the table using an alternate range key, in addition to queries against the primary key.

With the Reply table, you can query data items by Id (hash) or by Id and ReplyDateTime (hash and range). Now suppose you had an attribute in the table—PostedBy—with the user ID of the person who posted each reply. With a local secondary index on PostedBy, you could query the data by Id (hash) and PostedBy (range). Such a query would let you retrieve all the replies posted by a particular user in a thread, with maximum efficiency and without having to access any other items.

You can define up to five local secondary indexes per table. For more information, see Local Secondary Indexes.

Amazon DynamoDB supports the following data types:

For example, in the ProductCatalog table, the Id is a number type attribute and Authors is a String Set type attribute. Note that primary key attributes can be any scalar types, but not multi-valued types.