DIPC: The Linux Way of Distributed Programming
DIPC enables the creation of clusters of PCs. Computers in the same cluster could work together to solve a problem. DIPC's clusters are logical entities, meaning they are independent of any physical network characteristics. Computers could be added or deleted from a cluster without the need to change any of the network parameters. Several clusters may exist in the same physical network, but each computer can belong to at most one of them. Computers on the same cluster can even be connected to each other by a WAN. As far as DIPC is concerned, computers in one cluster never interact with computers in other clusters.
In normal System V IPC, processes specify numerical keys to gain access to the same IPC structure (see Resources 4). They can then use these structures to communicate with each other. A key normally has a unique meaning in only one computer. DIPC makes the IPC keys globally known. Here, if the application programmer wishes, a key can have the same meaning in more than one machine. Processes on different computers can communicate with each other the same way they did in a single machine.
Information about all the IPC keys in use is kept by one of dipcd's processes called the referee. Each cluster has only one referee. In fact, it is having the same referee that places computers in the same cluster. All other processes in the cluster refer to this one to find out if a key is in use. The referee is DIPC's name server. Besides many other duties, the referee also makes sure that only one computer at a time will attempt to create an IPC structure with a given key value, hence the name. Using a central entity simplifies the design and implementation but can become a bottleneck in large configurations. Finding a remedy to this problem is left to the time when DIPC is actually running in such configurations.
Users may need to run some programs (e.g., utilities) in all the computers in the system at the same time, and these programs may need to use the same IPC keys. This could create interference. To prevent any unwanted interactions, distributed IPC structures are declared by programmers. The programmer must specify a flag to do this. The structures are local by default. The mentioned flag is the only thing the programmer should do to create a distributed program. The rest is like ordinary System V IPC programming. Other than this flag to keep DIPC compatible with older programs, the system is totally transparent to programmers.
DIPC's programming model is simple and quite similar to using ordinary System V IPC. First, a process creates and initializes the needed IPC structures. After that, other processes are started to collaborate on the job. All of them can access the same IPC structures and exchange data. These processes are usually executing in remote machines, but they could also be running on the same computer, meaning distributed programs can be written on a single machine and later run on real multi-computers.
An important point about DIPC is that no other UNIX facility is changed to work in a distributed environment. Thus, programmers cannot use system calls, such as fork, which create a process in the local computer.
The fact that DIPC programs use numerical keys to transfer data means they do not need to know where the corresponding IPC structures are located. DIPC makes sure that processes find the needed resources just by using the specified keys. The resources could be located in different computers during different runs of a distributed program. This logical addressing of resources makes the programs independent of any physical network characteristics.
Simple techniques allow the mapping from logical computing resources needed by a program to physical resources to be done with no need to remake the program. As DIPC programs do not need to use any physical network addresses, they do not need recompiling to run in new environments. Of course, this does not prevent the programmer from choosing to make his program dependent on some physical system characteristics. For example, he could hard code a computer address in his code. DIPC programmers are discouraged from doing this type of coding.
When dipcd is not running, the kernel parts of DIPC are short circuited, causing the system to behave like a normal Linux operating system. As a result, users can easily disable the distributed system. Also, normal Linux kernels are not affected by DIPC programs, meaning there is no need to change and recompile these programs when they are to be executed in single computers with no DIPC support.
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