global data space in RTI's implementation of DDS

Question

Can anyone explain briefly about the global data space in DDS? Is it like a virtual memory?? When does a global data space is required?? I know that in Unicast of UDPv4, for every dataWriter/dataReader pair one port is assigned through which the communications take place between dataWriter and dataReader ,then from where does the concept of global data space come ?

Answer 1

The global dataspace is an abstract concept. It is not a physical existence.

UDP and shmem are simply transports.

Where the shared dataspace might be embodied (shared memory, malloc'd memory, in buffers spread out across drivers, wherever) is a function of the implementation details, and something you can start out by ignoring, while keeping in mind that the concept is something you can use to understand where the data is maintained while it is motion.

As the system integrator, or system implementer, you only need to understand that the DDS implementation provides, to your applications, a global dataspace for the data to exist in as necessary.

The DDS implementation takes your published data and pushes it to the global dataspace. The DDS implementation takes data out of the global dataspace and hands it to your applications, based on its announced subscriptions.

How the DDS implementation chooses to present the global, shared dataspace to your application will depend on the implementation details, and will be discussed in the higher-level documentation. How it implements the "dataspace" will be in the lower-level documentation which you may never need to refer to.

rw