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|Transparent Inter-Process Communications (TIPC)|
These pages are not intended as a comprehensive tutorial in the use of TIPC services. The TIPC Programmer's Guide, http://tipc.sf.net/doc/Programmers_Guide.txt, provides assistance to developers who are creating applications that utilize TIPC services. The TIPC User's Guide, http://tipc.sf.net/doc/Users_Guide.txt, provides an administrator of a TIPC cluster with the information needed to operate one. A TIPC server loadable module, that may be used to make a host available as a TIPC enabled node, has been a part of the Linux kernel since 2.6.16. Please see: http://tipc.sf.net
In a TIPC network, a Node is comprised of a collection of lightweight threads of execution operating in the same process, or heavyweight processes operating on the same machine. A Cluster is a collection of Nodes operating on different machines, and operating indirectly by way of a local Ethernet or other networking medium. Clusters may be further aggregated into Zones, and Zones into Networks. The address space of two TIPC networks is completely disjoint. Zones on different networks may coexist on the same LAN but they may not communicate directly with one another.
TIPC provides connectionless, connection-oriented, reliable, and unreliable forwarding strategies for both stream and message oriented applications. But not all strategies can be used in every application. For example, there is no such thing as a multicast byte stream. The strategy is selected by the user for the application when the socket is instantiated.
TIPC is not TCP/IP based. Consequently, it cannot signal beyond a local network span without some kind of tunneling mechanism. TIPC is designed to facilitate deployment of distributed applications, where certain aspects of the application may be segregated, and then delegated and/or duplicated over several machines on the same LAN. The application is unaware of the topology of the network on which it is running. It could be a few threads operating in the same process, several processes operating on the same machine, or it could be dozens or even hundreds of machines operating on the same LAN, all operating as a unit. TIPC manages all of this complexity so that the programmer doesn't have to.
Unlike TCP/IP, TIPC does not assign network addresses to network interfaces; it assigns addresses (e.g. port-ids) to sockets when they are instantiated. The address is unique and persists only as long as the socket persists. A single Node therefore, may typically have many TIPC addresses active at any one time, each assigned to an active socket. TIPC also provides a means that a process can use to bind a socket to a well-known address (e.g. a service). Several peers may bind to the same well-known address, thereby enabling multi-server topologies. And server members may exist anywhere in the Zone. TIPC manages the distribution of client requests among the membership of the server group. A server instance responds to two addresses: its public well-known address that it is bound to, and that a client may use to establish a communication with a service, and its private address that the server instance may use to directly interact with a client instance.
TIPC also enables multicast and "publish and subscribe" regimes that applications may use to facilitate asynchronous exchange of datagrams with a number of anonymous sources that may come and go over time. One such regime is implemented as a naming service managed by a distributed topology server. The topology server provides surveillance on the comings and goings of publishers, with advice to interested subscribers in the form of event notifications, emitted when a publisher's status changes. For example, when a server application binds to a TIPC address , that address is automatically associated with that server instance in topology server's name table. This has the side effect of causing a "published" event to be emitted to all interested subscribers. Conversely, when the server's socket is closed or when one of its addresses is released using the "no-scope" option of tipc_bind/3, a "withdrawn" event is emitted. See tipc_service_port_monitor/2.
A client application may connect to the topology server in order to interrogate the name table to determine whether or not a service is present before actually committing to access it. See tipc_service_exists/2 and tipc_service_probe/2. Another way that the topology server can be applied is exemplified in Erlang's "worker/supervisor" behavioral pattern. A supervisor thread has no other purpose than to monitor a collection of worker threads in order to ensure that a service is available and able to serve a common goal. When a worker under the supervisor's care dies, the supervisor receives the worker's "withdrawn" event, and takes some action to instantiate a replacement. The predicate, tipc_service_port_monitor/2, is provided specifically for this purpose. Using the service is optional. It has applications in distributed, high-availability, fault-tolerant, and non-stop systems.
Adding capacity to a cluster becomes an administrative function whereby new server hardware is added to a TIPC network, then the desired application is launched on the new server. The application binds to its well-known address, thereby joining in the Cluster. TIPC will automatically begin sending work to it. An administrator has tools for gracefully removing a server from a Cluster, without effecting the traffic moving on the Cluster.
An administrator may configure a Node to have two or more network interfaces. Provided that each interface is invisible to the other, TIPC will manage them as a redundant group, thus enabling high-reliability network features such as automatic link fail-over and hot-swap.
Sometimes the socket's port-id alone is enough to establish an ad-hoc session anonymously between parent and child processes. The parent instantiates a socket, then forks into two processes. The child retrieves the port-id of the parent from the socket inherited from the parent using tipc_get_name/2, then closes the socket and instantiates a socket of its own. The child sends a message to the parent, on its own socket, using the parent's port-id as the destination address. The port-id received by the parent is unique to a specific instance of child. The handshake is complete; each side knows who the other is, and two-way communication may now proceed. A one-way communication (e.g. a message oriented pipe or mailbox) is also possible using only the socket inherited from the parent, provided that there is exactly one sender and one receiver on the socket. Both parent and child use the socket's own port-id, one side adopts the role of sender, and the other of receiver.