lunes, 30 de abril de 2012

Communication in distributed systems

The most important difference between a distributed system and a single processor systemis the communication between processes. In a single-processor communication implicitly assumes the existence of shared memory:
• Ex: problem of producers and consumers, where a process writes to a shared buffer and anotherprocess reads from it.
In a distributed system there is no shared memoryand thus the whole nature of communicationbetween processes must be rethought. Processes to communicate, they must adhere to rules known as protocols. For distributed systems over a wide area, these protocols often take the form of several layers and each layer has its own goals and rules. Messages are exchanged in various ways, thereare many design options in this regard, an important option is the "remote procedure call." It is also important to consider the possibilities of communication between groups of processes, not only between two processes. In distributed systems, the absence of physical connection between the different memories of the teams, communication is performed by message transfer.

The standard ISO OSI
To send messages using the standard ISO OSI (Open Systems Interconnection), a layered model for communication of open systems. The layers provide multiple interfaces with different levels of detail, the last being the most general. The OSI standard defines the following seven layers: physical, data link, network, transport, session, presentation and application. The OSI model distinguishes two types of protocols, connection oriented and connectionless protocols. In the first, before any data transmission requires a virtual connection, which must end after sending. Connectionless protocols do not require this step, and messages are sent as datagrams.

Asynchronous transmission mode ATMAsynchronous transfer mode or ATM provides a quick means of transmission. Higher speeds are achieved regardless of the information flow control and error control in the intermediate nodes of the transmission. ATM uses connection-oriented mode, and allows transmission of different types of information such as voice, video, data, etc..
The client-server model of communication based on a simplification of the OSI model. The seven layers that provides produce a waste of the transfer speed of the network, which will be used only three layers: physical, data link and request / reply. Transfers are based on the protocol request / response and eliminates the need for connection.
Group communication
Communication systems that support groups can be divided into two categories: closed groups andopen groups. In the first only allowed communication between group members,while in the latter processes that do not belong to the group can interact with it. Closed groups are often used, in general, parallel processing, where each member has its own objective and does not interact with the `` outside world''. Instead, open groups are intended for application developmentclient / server, where the processes interact with the group to apply for services.

We can also categorize groups of processestaking into account the role played by the membersthat compose them: peer groups and hierarchical groups. If in a group there is nodistinction between the processes that formed anddecisions are taken collectively, is said to be apeer group. However, if relations are establishedhierarchy in a group, where some processes have a greater say than others, we speak of hierarchical groups. Each of these organizations group has its advantages and disadvantages. In hierarchical groups, if they fail the processes that make decisions, can produce a mismatch of the other members for as long as this situation remains. Inthe peer group does not produce this effect, if it fails one of its members can continue working (witha smaller group).


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