我将开始研究一个巨大的应用程序。在此之前，我想为它做好准备。
这是一个将在全球范围内使用的应用程序，并且将有大量用户同时使用它。
所以我认为，一台数据库服务器上会有很大的负载，在很短的时间范围内会有大量的数据库请求/查询。所以这会影响系统的性能，对吧？
如果数据库服务器出现故障，网站所有者将付出巨大的代价。
所以我该怎么做？有什么技术或我可以使用的东西吗？如果我可以使用多个数据库，那么如何？所有数据库都包含相同的信息吗？我可以在差异之间加入表格（使用任何技术或工具）吗？数据库？或者我必须在每个数据库中进行多个查询？
应用程序将使用：jboss、jsp、servlet、mysql
It is always best if requirements can be given with some precision. For instance, we expect no more than 1,000 users to be hitting the database simultaneously is a clear design requirement. "There will be a big number of users" is much less helpful. Of course, we can not always predict how much traffic a web application will get and requirements often change over time.
If a database will be experiencing far more reads than writes, then caching data from the database can improve performance. This was the topic of a recent podcast that you might want to listen to: http://javaposse.com/java-posse-399-roundup-12-distributed-caching
Spreading a database among many servers is a complicated task because you need to deal with synchronization issues. Your best bet is to find a service or API that takes care of this issue for you. You can use MySQL, (see this question) but another database designed for this sort of scaling is probably a better choice. Take a look at hadoop.