Core components
In DynamoDB, tables, items, and attributes are the core components that you work with. A table is a collection of items, and each item is a collection of attributes. DynamoDB uses primary keys to uniquely identify each item in a table and secondary indexes to provide more querying flexibility. You can use DynamoDB Streams to capture data modification events in DynamoDB tables.
Tables, Items, and Attributes
Tables
Similar to other database systems, DynamoDB stores data in tables. A table is a collection of data. For example, see the example table called People that you could use to store personal contact information about friends, family, or anyone else of interest. You could also have a Cars table to store information about vehicles that people drive.
Items
Each table contains zero or more items. An item is a group of attributes that is uniquely identifiable among all of the other items. In a People table, each item represents a person. For a Cars table, each item represents one vehicle. Items in DynamoDB are similar in many ways to rows, records, or tuples in other database systems. In DynamoDB, there is no limit to the number of items you can store in a table.
Attributes
Each item is composed of one or more attributes. An attribute is a fundamental data element, something that does not need to be broken down any further. For example, an item in a People table contains attributes called PersonID,LastName,FirstName, and so on. For a Department table, an item might have attributes such as DepartmentID, Name, Manager, and so on. Attributes in DynamoDB are similar in many ways to fields or columns in other database systems.
Primary Key
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 thepartition key. DynamoDB uses the partition key's value as input to an internal hash function. The output from the hash function determines the partition (physical storage internal to DynamoDB) in which the item will be stored.
In a table that has only a partition key, no two items can have the same partition key value.
The People table described in Tables, Items, and Attributes is an example of a table with a simple primary key (PersonID). You can access any item in the People table directly by providing the PersonId value for that item.
Partition key and sort key
Referred to as a composite primary key, this type of key is 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 (physical storage internal to DynamoDB) in which the item will be stored. All items with the same partition key value are stored together, in sorted order by sort key value.
In a table that has a partition key and a sort key, it's possible for two items to have the same partition key value. However, those two items must have different sort key values. Each primary key attribute must be a scalar (meaning that it can hold only a single value). The only data types allowed for primary key attributes are string, number, or binary. There are no such restrictions for other, non-key attributes.
Secondary Indexes
You can create one or more secondary indexes on a 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 doesn't require that you use indexes, but they give your applications more flexibility when querying your data. After you create a secondary index on a table, you can read data from the index in much the same way as you do from the table.
DynamoDB supports two kinds of indexes:
Global secondary index
An index with a partition key and sort key that can be different from those on the table.
Local secondary index
An index that has the same partition key as the table, but a different sort key. Each table in DynamoDB has a limit of 20 global secondary indexes (default limit) and 5 local secondary indexes per table.
https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/SecondaryIndexes.html
DynamoDB Streams
DynamoDB Streams is an optional feature that captures data modification events in DynamoDB tables. The data about these events appear in the stream in near-real time, and in the order that the events occurred.
Each event is represented by a stream record. If you enable a stream on a table, DynamoDB Streams writes a stream record whenever one of the following events occurs:
- A new item is added to the table: The stream captures an image of the entire item, including all of its attributes.
- An item is updated: The stream captures the "before" and "after" image of any attributes that were modified in the item.
- An item is deleted from the table: The stream captures an image of the entire item before it was deleted.
Each stream record also contains the name of the table, the event timestamp, and other metadata. Stream records have a lifetime of 24 hours; after that, they are automatically removed from the stream.
You can use DynamoDB Streams together with AWS Lambda to create a trigger - code that executes automatically whenever an event of interest appears in a stream. For example, consider a Customers table that contains customer information for a company. Suppose that you want to send a "welcome" email to each new customer. You could enable a stream on that table, and then associate the stream with a Lambda function. The Lambda function would execute whenever a new stream record appears, but only process new items added to the Customers table. For any item that has an EmailAddress attribute, the Lambda function would invoke Amazon Simple Email Service (Amazon SES) to send an email to that address.
DynamoDB Streams Use Cases and Design Patterns | AWS Database Blog
Core components of Amazon DynamoDB - Amazon DynamoDB
Change data capture for DynamoDB Streams - Amazon DynamoDB
What you should know about DynamoDB Streams | DeBrie Advisory
Evaluate your Streams usage - Amazon DynamoDB
Global Secondary Indexes Usage patterns
- Querying and sorting data by multiple attributes
- Data filtering
- Reduce read capacity unit (RCU) consumption for large items
- Isolating read workloads