你应该问自己以下问题:

(Volume, Velocity) Will you be writing and reading TONS of information , so much information that no one computer could handle the writes. (Global) Will you need this writing and reading capability around the world so that the writes in one part of the world are accessible in another part of the world? (Reliability) Do you need this database to be up and running all the time and never go down regardless of which Cloud, which country, whether it's VM , Container, or Bare metal? (Scale-ability) Do you need this database to be able to continue to grow easily and scale linearly (Consistency) Do you need TUNABLE consistency where some writes can happen asynchronously where as others need to be certified? (Skill) Are you willing to do what it takes to learn this technology and the data modeling that goes with creating a globally distributed database that can be fast for everyone, everywhere?

如果在这些问题中，你认为“可能”或“不”，你应该用别的词。如果你对所有问题的答案都是“当然”，那么你应该用卡桑德拉。

当你可以在一个盒子上做所有事情时，使用RDBMS。它可能比大多数方法都简单，任何人都可以使用它。

如果你需要一个SQL语义完全一致的数据库，Cassandra不是你的解决方案。Cassandra支持键值查找。它不支持SQL查询。Cassandra中的数据“最终是一致的”。数据的并发查找可能不一致，但最终查找是一致的。

如果你需要严格的语义，需要对SQL查询的支持，可以选择其他的解决方案，比如MySQL, PostGres，或者结合使用Cassandra和Solr。

在部署Cassandra的过程中与某人交谈，它不能很好地处理多对多。他们正在做初步测试。我和Cassandra的顾问谈过这个问题，他说如果你有这样的习题集，他就不建议你这么做。

another situation that makes the choice easier is when you want to use aggregate function like sum, min, max, etcetera and complex queries (like in the financial system mentioned above) then a relational database is probably more convenient then a nosql database since both are not possible on a nosql databse unless you use really a lot of Inverted indexes. When you do use nosql you would have to do the aggregate functions in code or store them seperatly in its own columnfamily but this makes it all quite complex and reduces the performance that you gained by using nosql.

NoSQL的一般思想是，您应该使用最适合您的应用程序的数据存储。如果您有一个财务数据表，请使用SQL。如果您的对象需要复杂/缓慢的查询才能映射到关系模式，请使用对象或键/值存储。

当然，你遇到的任何现实问题都处于这两个极端之间，没有一个解决方案是完美的。您需要考虑每个存储的功能以及使用其中一个的后果，这将非常具体于您试图解决的问题。

在评估分布式数据系统时，您必须考虑CAP定理——您可以选择以下两个:一致性、可用性和分区容差。

Cassandra是一个可用的、支持最终一致性的分区容忍系统。要了解更多信息，请参阅我写的这篇博客文章:NoSQL系统的可视化指南。