如何将业务层公开给表示层?

I think I do understand n-tier concept. I however have problems figuring out how would I communicate with Business tier from presentation tier?

What do companies use to expose their business layer? I've looked at apache Thrift - is that what's usually done when building multi-tier apps?

P.S> Business tier and presentation tier are physically separated - on different servers.

There are several ways:
1. Web services
2. RMI
3. Message Queues ( although this is more of a business layer to business layer integration)