Declare One Process Type per Role

A ToolTalk protocol is expressed in terms of the roles that each tool plays (that is, the kinds of tasks each tool is assigned to perform). A ToolTalk ptype essentially instructs the ToolTalk service how to handle any messages in which a tool is interested that are sent when that tool is not running. To design your protocol to be open, you want to declare one ptype for each role in your protocol. When you declare only one ptype per role in your protocol, you provide users with the flexibility to interchange tools as their needs require. For example, a user may want a sophisticated sound-authoring tool for recording but also prefers a simple audio tool to perform the playback.

Thus, you will sometimes want to include only one message signature per ptype. When you include more than one message signature in the same ptype, you are requiring that any program that can handle one message can handle the other messages. For example, a ptype “UWriteIt” can include the two message signatures “Display“ and “Edit” because it is expected that any tool that understands the UWriteIt document format can perform both of these operations.