SST can be used at two major levels of abstraction, remote procedure call and remote object invocation. Using the procedure call approach, you send remote messages and interact directly with the messaging infrastructure. Some of the machines in the system may not be running Smalltalk or the data exchanged may not be objects. This model is typically found in strictly client-server systems.

The object invocation approach presents you with an implicit model of remote objects to which you send messages. Using proxies and remote references, remote objects are largely indistinguishable from local objects. Polymorphism can be fully exploited and the same application code is used to manipulate local and remote objects. SST extends this to the point where Smalltalk process semantics can be extended across multiple machines.

The SST framework is layered such that the object invocation model uses the infrastructure which implements the procedure call model. To integrate the two and fully utilize Smalltalk itself, SST uses the following approach: wherever non-object entities (local or remote) are encountered, they are converted to objects as soon as possible. Once they look like objects, their origins are transparent.

In the following sections, you explore a number of concepts SST supplies in support of this approach. The emphasis here is on using these concepts rather than the details of how they work. Depending on the kind of distributed application you are developing, some of these concepts may be exactly what you require. If some of the concepts do not appear relevant to your application, skip to the next one.