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Xamarin Developer Guide

Amazon DynamoDB

What is Amazon DynamoDB?

Amazon DynamoDB is a fast, highly scalable non-relational database service. DynamoDB removes traditional scalability limitations on data storage while maintaining low latency and predictable performance.

Key Concepts

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

Tables

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. 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.

To learn more about working with tables, see Working with Tables in DynamoDB.

Items and Attributes

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 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 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.

To learn more about working with items, see Working with Items in DynamoDB.

Data Types

Amazon DynamoDB supports the following data types:

  • Scalar types – Number, String, Binary, Boolean, and Null.

  • Multi-valued types – String Set, Number Set, and Binary Set.

  • Document types – List and Map.

For more information about Scalar Data Types, Multi-Valued Data Types, and Document Data Types, see DynamoDB Data Types.

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 the following two types of primary keys:

  • Hash Key: The primary key is made of one attribute, a hash attribute. DynamoDB builds an unordered hash index on this primary key attribute. Each item in the table is uniquely identified by its hash key value.

  • Hash and Range Key: The primary key is made of two attributes. The first attribute is the hash attribute and the second one is the range attribute. DynamoDB builds an unordered hash index on the hash primary key attribute, and a sorted range index on the range primary key attribute. Each item in the table is uniquely identified by the combination of its hash and range key values. It is possible for two items to have the same hash key value, but those two items must have different range key values.

Secondary Indexes

When you create a table with a hash and range 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 supports two kinds of secondary indexes: local secondary indexes and global secondary indexes.

  • Local secondary index: An index that has the same hash key as the table, but a different range key.

  • Global secondary index: An index with a hash and range key that can be different from those on the table.

You can define up to 5 global secondary indexes and 5 local secondary indexes per table. For more information, see Improving Data Access with Secondary Indexes in DynamoDB in the DynamoDB Developer Guide.

Query and Scan

In addition to using primary keys to access items, Amazon DynamoDB also provides two APIs for searching the data: Query and Scan. We recommend that you read Guidelines for Query and Scan in the DynamoDB Developer Guide to familiarize yourself with some best practices.

Query

A Query operation finds items in a table or a secondary index using only primary key attribute values. You must provide a hash key attribute name and a distinct value to search for. You can optionally provide a range key attribute name and value, and use a comparison operator to refine the search results.

For sample queries, see:

For more information on Query, see Query in the DynamoDB Developer Guide.

Scan

A Scan operation reads every item in a table or a secondary index. By default, a Scan operation returns all of the data attributes for every item in the table or index. You can use the ProjectionExpression parameter so that Scan only returns some of the attributes, rather than all of them.

For sample scans, see:

For more information on Scan, see Scan in the DynamoDB Developer Guide.

Project Setup

Prerequisites

To use DynamoDB in your application, you'll need to add the SDK to your project. To do so, follow the instructions in Setting Up the AWS Mobile SDK for .NET and Xamarin.

Create a DynamoDB Table

To create a table, go to the DynamoDB console and follow these steps:

  1. Click Create Table.

  2. Enter the name of the table.

  3. Select Hash as the primary key type.

  4. Select a type and enter a value for the hash attribute name. Click Continue.

  5. On the Add Indexes page, if you plan to to use global secondary indexes, set Index Type to "Global Secondary Index" and under Index Hash Key, enter a value for the secondary index. This will allow you to query and scan using both the primary index and secondary index. Click Add Index To Table, and then click Continue. To skip using global secondary indexes, click Continue.

  6. Set the read and write capacity to your desired levels. For more information on configuring capacity, see Provisioned Throughput in Amazon DynamoDB. Click Continue.

  7. On the next screen, enter a notification email to create throughput alarms, if desired. Click Continue.

  8. On the summary page, click Create. DynamoDB will create your database.

Set Permissions for DynamoDB

To use DynamoDB in an application, you must set the correct permissions. The following IAM policy allows the user to delete, get, put, query, scan, and update items in a specific DynamoDB table, which is identified by ARN:

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{ "Statement": [ { "Effect": "Allow", "Action": [ "dynamodb:DeleteItem", "dynamodb:GetItem", "dynamodb:PutItem", "dynamodb:Query", "dynamodb:Scan", "dynamodb:UpdateItem" ], "Resource": "arn:aws:dynamodb:us-west-2:123456789012:table/MyTable" } ] }

You can modify policies in the IAM console. You should add or remove allowed actions based on the needs of your app.

To learn more about IAM policies, see Using IAM.

To learn more about DynamoDB-specific policies, see Using IAM to Control Access to DynamoDB Resources in the DynamoDB Developer Guide.

Integrating DynamoDB with your Application

The AWS Mobile SDK for .NET and Xamarin provides a high-level library for working with DynamoDB. You can also make requests directly against the low-level DynamoDB API, but for most use cases the high-level library is recommended. The AmazonDynamoDBClient is an especially useful part of the high-level library. Using this class, you can perform various create, read, update, and delete (CRUD) operations and execute queries.

The AWS Mobile SDK for .NET and Xamarin allows you to make calls using APIs from the AWS SDK for .NET to work with DynamoDB. All of the APIs are available in the AWSSDK.dll. For information about downloading the AWS SDK for .NET, see AWS SDK for .NET.

There are three ways you can interact with DynamoDB in your Xamarin application:

  • Document Model: This API provides wrapper classes around the low-level DyanmoDB API to further simplify your programming tasks. The Table and Document are the key wrapper classes. You can use the document model for the data operations such as create, retrieve, update and delete items. The API is available in the Amazon.DynamoDB.DocumentModel namespace.

  • Object Persistence Model: The Object Persistence API enables you to map your client-side classes to the DynamoDB tables. Each object instance then maps to an item in the corresponding tables. The DynamoDBContext class in this API provides methods for you to save client-side objects to a table, retrieve items as objects and perform query and scan. You can use the Object Persistence model for the data operations such as create, retrieve, update and delete items. You must first create your tables using the Service Client API and then use the object persistence model to map your classes to the tables. The API is available in the Amazon.DynamoDB.DataModel namespace.

  • Service Client API: This is the protocol-level API that maps closely to the DynamoDB API. You can use this low-level API for all table and item operations such as create, update, delete table and items. You can also query and scan your tables. This API is available in the Amazon.DynamoDB namespace.

These three models are explored in-depth in the following topics:

Topics