Redis是一个在内存中的数据存储，它可以将它的状态持久化到磁盘(以便重启后恢复)。但是，作为内存中的数据存储，意味着数据存储的大小(单个节点上)不能超过系统上的总内存空间(物理RAM +交换空间)。实际上，Redis会与系统上的许多其他进程共享这个空间，如果它耗尽了系统内存空间，它很可能会被操作系统杀死。

Mongo是一个基于磁盘的数据存储，当它的工作集适合物理RAM(像所有软件一样)时，它是最有效的。作为基于磁盘的数据意味着对Mongo数据库的大小没有内在的限制，但是配置选项、可用的磁盘空间和其他问题可能意味着超过一定限制的数据库大小可能变得不切实际或效率低下。

Redis和Mongo都可以通过集群来实现高可用性、备份和增加数据存储的整体大小。

也许这个资源可以帮助你在两者之间做出选择。 它还讨论了其他几个NoSQL数据库，并提供了一个简短的特征列表，以及对每个数据库的“我将使用它做什么”的解释。

http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-vs-redis

如果你有足够的RAM，你应该使用这两种方法。Redis和MongoDB达到了通用工具的价格。这会带来很多开销。

有一种说法是Redis比Mongo快10倍。这可能不再是真的了。MongoDB(如果我没记错的话)声称只要内存配置相同，就可以在存储和缓存文档方面击败memcache。

不管怎样。Redis不错，MongoDB也不错。如果你关心子结构并且需要聚合，那么就选择MongoDB。如果存储键和值是你主要关心的，它都是关于Redis。(或任何其他键值存储)。

我只是注意到这个问题已经很老了。不过，我认为以下方面值得补充:

Use MongoDB if you don't know yet how you're going to query your data. MongoDB is suited for Hackathons, startups or every time you don't know how you'll query the data you inserted. MongoDB does not make any assumptions on your underlying schema. While MongoDB is schemaless and non-relational, this does not mean that there is no schema at all. It simply means that your schema needs to be defined in your app (e.g. using Mongoose). Besides that, MongoDB is great for prototyping or trying things out. Its performance is not that great and can't be compared to Redis. Use Redis in order to speed up your existing application. Redis can be easily integrated as a LRU cache. It is very uncommon to use Redis as a standalone database system (some people prefer referring to it as a "key-value"-store). Websites like Craigslist use Redis next to their primary database. Antirez (developer of Redis) demonstrated using Lamernews that it is indeed possible to use Redis as a stand alone database system. Redis does not make any assumptions based on your data. Redis provides a bunch of useful data structures (e.g. Sets, Hashes, Lists), but you have to explicitly define how you want to store you data. To put it in a nutshell, Redis and MongoDB can be used in order to achieve similar things. Redis is simply faster, but not suited for prototyping. That's one use case where you would typically prefer MongoDB. Besides that, Redis is really flexible. The underlying data structures it provides are the building blocks of high-performance DB systems.

什么时候使用Redis?

Caching Caching using MongoDB simply doesn't make a lot of sense. It would be too slow. If you have enough time to think about your DB design. You can't simply throw in your documents into Redis. You have to think of the way you in which you want to store and organize your data. One example are hashes in Redis. They are quite different from "traditional", nested objects, which means you'll have to rethink the way you store nested documents. One solution would be to store a reference inside the hash to another hash (something like key: [id of second hash]). Another idea would be to store it as JSON, which seems counter-intuitive to most people with a *SQL-background. If you need really high performance. Beating the performance Redis provides is nearly impossible. Imagine you database being as fast as your cache. That's what it feels like using Redis as a real database. If you don't care that much about scaling. Scaling Redis is not as hard as it used to be. For instance, you could use a kind of proxy server in order to distribute the data among multiple Redis instances. Master-slave replication is not that complicated, but distributing you keys among multiple Redis-instances needs to be done on the application site (e.g. using a hash-function, Modulo etc.). Scaling MongoDB by comparison is much simpler.

何时使用MongoDB

Prototyping, Startups, Hackathons MongoDB is perfectly suited for rapid prototyping. Nevertheless, performance isn't that good. Also keep in mind that you'll most likely have to define some sort of schema in your application. When you need to change your schema quickly. Because there is no schema! Altering tables in traditional, relational DBMS is painfully expensive and slow. MongoDB solves this problem by not making a lot of assumptions on your underlying data. Nevertheless, it tries to optimize as far as possible without requiring you to define a schema.

博士TL; -如果性能是重要的，你愿意花时间优化和组织你的数据使用Redis。 -使用MongoDB，如果你需要建立一个原型，而不用担心你的DB太多。

进一步阅读:

当使用Redis作为主要数据存储时，需要考虑一些有趣的方面

Redis和MongoDB都是非关系数据库，但它们属于不同的类别。

Redis是一个键/值数据库，它使用内存存储，这使得它非常快。它是缓存和临时数据存储(在内存中)的一个很好的候选人，因为大多数云平台(如Azure,AWS)都支持它，它的内存使用是可扩展的。但是如果您要在资源有限的机器上使用它，请考虑它的内存使用量。

另一方面，MongoDB是一个文档数据库。对于保存大型文本、图像、视频等以及几乎所有与数据库相关的内容(除了事务)来说，这是一个很好的选择。例如，如果你想开发一个博客或社交网络，MongoDB是一个合适的选择。它具有向外扩展的策略。它使用磁盘作为存储介质，因此数据将被持久化。

如果你的项目预算允许你有足够的RAM内存在你的环境-答案是Redis。尤其是考虑到新的Redis 3.2的集群功能。