Orca Terminology

When talking about how components connected to each other it's helpful to define some terminology.

• Platform. Platform is a logical unit, a robot would typically be a platform. We use the term platform so not to exclude simple devices like cameras or PDA's. Calling them robots would be a stretch. Platforms are the highest level in our hierarchy, so we don't define 'teams of platforms' for example. Each platform can have one or several hosts
• Host. Hosts are physical computers. Each host may execute one or several components.
• Component. Components are software modules which can be independently distributed, deployed, and configured. Each component may have zero or more interfaces.
• Interface. Interfaces describe contractual obligations between components. Some interfaces are provided -- they provide services, other are required -- they connect to provided interfaces.

These four terms are often used in describing a distributed system but one has to be careful to keep in mind that they refer to different aspects of the system. Platforms describe a logical grouping of hardware and software. This grouping is useful because it reflects physical proximity; cheap/fast/reliable communication, etc. Hosts are units of computing hardware. Components and interfaces describe software organization. Assigning software (components) to hardware (hosts) is called deployment.

In this illustration platform X has one computer host (1) which executes one component (4). Platform Y has two hosts. One would say that Component 3 is deployed on Host 2 which is located on Platform Y.

Component 1 requires Interface A and it is connected to the provided Interfaces A and B of Component 2. Interface connections can be intra-host (Interface C), intra-platform (Interfaces A and B of Component 2), or inter-platform (Interfaces A and B of Component 3).