Dynamodb 简明教程

DynamoDB - Overview

DynamoDB 允许用户创建能够存储和检索任意数据量并服务于任意流量的数据库。它在服务器上自动分配数据和流量，以动态管理每个客户的请求，并且还保持了快速性能。

DynamoDB allows users to create databases capable of storing and retrieving any amount of data, and serving any amount of traffic. It automatically distributes data and traffic over servers to dynamically manage each customer’s requests, and also maintains fast performance.

DynamoDB vs. RDBMS

DynamoDB 使用 NoSQL 模型，这意味着它使用非关系系统。下表重点介绍了 DynamoDB 和 RDBMS 之间的差异 −

DynamoDB uses a NoSQL model, which means it uses a non-relational system. The following table highlights the differences between DynamoDB and RDBMS −

Common Tasks

RDBMS

DynamoDB

Connect to the Source

It uses a persistent connection and SQL commands.

It uses HTTP requests and API operations

Create a Table

Its fundamental structures are tables, and must be defined.

It only uses primary keys, and no schema on creation. It uses various data sources.

Get Table Info

All table info remains accessible

Only primary keys are revealed.

Load Table Data

It uses rows made of columns.

In tables, it uses items made of attributes

Read Table Data

It uses SELECT statements and filtering statements.

It uses GetItem, Query, and Scan.

Manage Indexes

It uses standard indexes created through SQL statements. Modifications to it occur automatically on table changes.

It uses a secondary index to achieve the same function. It requires specifications (partition key and sort key).

Modify Table Data

It uses an UPDATE statement.

It uses an UpdateItem operation.

Delete Table Data

It uses a DELETE statement.

It uses a DeleteItem operation.

Delete a Table

It uses a DROP TABLE statement.

It uses a DeleteTable operation.

Advantages

DynamoDB 的两个主要优点是可伸缩性与灵活性。它不要求使用特定数据源与结构，这让用户能在几乎所有内容上操作，但以统一的方式。

The two main advantages of DynamoDB are scalability and flexibility. It does not force the use of a particular data source and structure, allowing users to work with virtually anything, but in a uniform way.

其设计也支持从较轻任务与操作 到 要求苛刻的企业功能 的广泛用途。它还允许使用多种语言：Ruby、Java、Python、C#、Erlang、PHP 和 Perl。

Its design also supports a wide range of use from lighter tasks and operations to demanding enterprise functionality. It also allows simple use of multiple languages: Ruby, Java, Python, C#, Erlang, PHP, and Perl.

Limitations

但是，DynamoDB 确实存在某些限制，不过这些限制不会一定造成大问题或妨碍稳固的开发。

DynamoDB does suffer from certain limitations, however, these limitations do not necessarily create huge problems or hinder solid development.

您可以从以下观点对其进行审阅 −

You can review them from the following points −

  1. Capacity Unit Sizes − A read capacity unit is a single consistent read per second for items no larger than 4KB. A write capacity unit is a single write per second for items no bigger than 1KB.

  2. Provisioned Throughput Min/Max − All tables and global secondary indices have a minimum of one read and one write capacity unit. Maximums depend on region. In the US, 40K read and write remains the cap per table (80K per account), and other regions have a cap of 10K per table with a 20K account cap.

  3. Provisioned Throughput Increase and Decrease − You can increase this as often as needed, but decreases remain limited to no more than four times daily per table.

  4. Table Size and Quantity Per Account − Table sizes have no limits, but accounts have a 256 table limit unless you request a higher cap.

  5. Secondary Indexes Per Table − Five local and five global are permitted.

  6. Projected Secondary Index Attributes Per Table − DynamoDB allows 20 attributes.

  7. Partition Key Length and Values − Their minimum length sits at 1 byte, and maximum at 2048 bytes, however, DynamoDB places no limit on values.

  8. Sort Key Length and Values − Its minimum length stands at 1 byte, and maximum at 1024 bytes, with no limit for values unless its table uses a local secondary index.

  9. Table and Secondary Index Names − Names must conform to a minimum of 3 characters in length, and a maximum of 255. They use the following characters: AZ, a-z, 0-9, “_”, “-”, and “.”.

  10. Attribute Names − One character remains the minimum, and 64KB the maximum, with exceptions for keys and certain attributes.

  11. Reserved Words − DynamoDB does not prevent the use of reserved words as names.

  12. Expression Length − Expression strings have a 4KB limit. Attribute expressions have a 255-byte limit. Substitution variables of an expression have a 2MB limit.