电子工程代写|计算机系统原理代写Principles of Computer Systems代考|CSC110

电子工程代写|计算机系统原理代写Principles of Computer Systems代考|Dispatch and Reception

There are various notational forms to initiate and carry out communication, in languages that offer mechanisms for concurrent and distributed programming. Their syntax and semantics concern not only message transmission itself, but a number of some accompanying properties. It conforms to general concept and structure of a language supporting a certain programming paradigm: imperative, object, functional, in logic, event-directed, etc. The detail presentation of such mechanisms exceeds the subject of this chapter and may be found in the respective writings as e.g. Hoare (1985), Milner (1980), Ben-Ari (1990), OCCAM (1984), Szałas and Warpechowska (1991), Brinch Hansen (1975), Arnold et al. (2005), Stevens (1997), [Tanenbaum and Wetherall 2011]. The main task of the mechanisms is to deliver a message to a certain site of reception. This takes place in effect of executing an operation, differently named in various programming languages and operation systems. Let us name them send and receive. In distributed systems, such operations play a similar part like input/output operations in a stand-alone, autonomous computer with external devices, where suitable protocols are also applied and message delivery sites (ports, sockets) specified. Some low-level (machine) operations of communication: !, ?, S, R, were shown in Sects. $1.4$ and $1.5$ of Chap. 1. Their parameters were name of receiver’s process in the sending process and name of sender’s process in the receiving process, as well as respective addresses where the message is being sent from, and delivered to. These low-level operations represent maximally simplified synchronous and asynchronous mechanism, neglecting identifiers of ports, sockets, channels and neglecting reaction to exceptions like damage or loss of messages and delivery receipt, time-out, etc. Some programming and operating systems, offer means for specifying properties of message transmission or include means for such specification in send/receive operations as parameters. Here, we confine ourselves to parameters specifying messages, processes and threads, sites of dispatch and reception like ports or sockets and to media of transmission like channels.

电子工程代写|计算机系统原理代写Principles of Computer Systems代考|Synchronous communication

A model-example where synchronous transmission is the basic communication mode is CSP (Hoare 1978, 1985) and its realization (slightly modified by introducing the channels) as a programming language OCCAM (OCCAM 1984). An example of extensive high-level programming language with explicit commands for such mode of communication is ADA (Barnes 2005). In the synchronous communication, the sending process, starting a send operation is suspended until the message has been received by a receiver and the acknowledgment of delivery arrives. The process resumes the main activity, when the dispatch and reception of data (in general-communication session) is fully completed. This is called a synchronous (or blocking) send operation. The receiving process, starting a receive operation, is suspended until an expected message arrives and the acknowledgment of delivery is dispatched to the sender. Then the process resumes activity. This is called a synchronous (or blocking) receive operation.

Metaphorically one may say that a meeting (“randez vous”) of the sender with receiver takes place in the period of communication session. A created transmission channel contains, at that time, one message at most. The channel disappears on completion of the session. Exceeding of predetermined time of waiting for the interlocutor (the timeout) may interrupt the session and, possibly, retransmission of the message by the sender. The synchronous transmission between processes $p 1$ and $p 2$ is depicted in Fig. 5.2a, b, where events and actions marked black represent sending, marked grey represent reception and white are other events. The wavy arrows denote message flow between processes. Their slant illustrates elapsed time from dispatch to reaching destination by elementary signal, e.g. a single bit. An implementation detail is, which components of the system realize the meeting: the process directly, as in Fig. 5.2a, or the operating system on request from the process, as in Fig. 5.2b. Here, process $p 2$ waits for arrival of the send operation in process $p 1$ and during the next meeting, $p 1$ waits for arrival of the receive operation in $p 2$.

电子工程代写|计算机系统原理代写Principles of Computer Systems代考|Synchronous communication

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