isaac Mao's Network

面向文档的数据库中根本不存在表、行、列或关系。
这并不意味着不能从 CouchDB 数据库获取一组关系数据。一个称为视图的特性允许您为没有在数据库中定义的文档创建一种任意关系。这意味着您能够获得典型的 SQL 联合查询的所有好处，但又不需要在数据库层预定义它们的关系。
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- CouchDB 的目的并不是替换关系数据库，而是为那些更适合使用面向文档模型（而不是传统的关系数据模型）的项目提供一种选择，比如 wikis、博客和文档管理系统。
- B-树储存引擎
- 它依赖于使用视图创建文档之间的任意关系，以及提供聚合和报告特性。使用 Map/Reduce 计算这些视图的结果
- CouchDB 中的 Map/Reduce 特性生成键/值对，CouchDB 将它们插入到 B-树引擎中并根据它们的键进行排序。这就能通过键进行高效查找，并且提高 B-树中的操作的性能。
- 在 CouchDB 中没有锁机制，它使用的是多版本并发性控制（Multiversion concurrency controlMVCC）— 它向每个客户机提供数据库的最新版本的快照。这意味着在提交事务之前，其他用户不能看到更改。许多现代数据库开始从锁机制前移到 MVCC，包括 Oracle（V7 之后）和 Microsoft® SQL Server 2005 及更新版本。
- 包含元数据（关于数据的数据），比如惟一的文档 ID 和修改号。文档字段可以包含各种类型的数据，比如文本字符串、数字和布尔值（true/false）等。
- 当更改 CouchDB 文档时，这些更改实际上并不是附加在原来的文档之上，而是创建整个文档的一个新版本，即修订。
- 不过，如果一个客户机保存了所有更改时，另一个客户机在尝试保存更改时将收到一个编辑冲突。可以通过加载更新版本的文档来解决该冲突，然后重新进行编辑并再次保存。
- 想一下关系数据库，对于每个应用程序，严格定义的表之间的关系对于为数据赋予意义至关重要。不过，当要求实现更高的性能时，则需要创建物化视图来反规范化（de-normalize）数据。在很多情况下，面向文档数据库采用相反的方法处理事情。它将数据储存在一个平面地址空间中，这就像一个完全反规范化的数据仓库。然后它提供一个视图模型为数据添加结构，因此能够聚合数据得出有用的含义。
- CouchDB 在 Web 应用程序、iPhone 应用程序和 Facebook 应用程序中越来越流行。

Datastore Python API Overview - Google App Engine - Google Code

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The App Engine datastore is strongly consistent, but it's not a relational database. While the datastore interface has many of the same features of traditional databases, the datastore's unique characteristics imply a different way of designing and managing data to take advantage of the ability to scale automatically.
Each entity also has a key that uniquely identifies the entity. The simplest key has a kind and a unique numeric ID provided by the datastore. The ID can also be a string provided by the application.

An application can fetch an entity from the datastore by using its key, or by performing a query that matches the entity's properties.
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- Unlike relational databases, the App Engine datastore does not require that all entities of a given kind have the same properties.
- Every datastore query uses an index, a table that contains the results for the query in the desired order.
- As the application makes changes to datastore entities, the datastore updates the indexes with the correct results. When the application executes a query, the datastore fetches the results directly from the corresponding index.
- You can make changes to multiple entities within a single transaction. To support this, App Engine needs to know in advance which entities will be updated together, so it knows to store them in a way that supports transactions. You must declare that an entity belongs to the same entity group as another entity when you create the entity. All entities fetched, created, updated or deleted in a transaction must be in the same entity group.
- Each entity has a path of parent-child relationships from a root entity to itself (the shortest path being no parent). This path is an essential part of the entity's complete key.
- The datastore uses optimistic concurrency to manage transactions.

2009.11京都‧底片系列 - a set on Flickr

Flickr is almost certainly the best online photo management and sharing application in the world. Show off your favorite photos and videos to the world, securely and privately show content to your friends and family, or blog the photos and videos you take with a cameraphone.

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Datastore Python API Overview - Google App Engine - Google Code

2009.11京都‧底片系列 - a set on Flickr

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