Building blocks are layered on top of channels. Most of them do not even need a channel, all they need is a class that implements interface Transport (channels do). This enables them to work on any type of group transport that obeys this interface. Building blocks can be used instead of channels whenever a higher-level interface is required. Whereas channels are simple socket-like constructs, building blocks may offer a far more sophisticated interface. In some cases, building blocks offer access to the underlying channel, so that - if the building block at hand does not offer a certain functionality - the channel can be accessed directly. Building blocks are located in the org.javagroups.blocks package. Only the ones that are relevant for application programmers are discussed below. For a more detailed description and for architecture and implementation details refer to the Programmer's Guide.