Chapter 2 System Models Architectural Models Fundamental Models

Chapter 2: System Models §Architectural Models § Fundamental Models

Models z Architectural Models ya high-level view of the distribution of functionality between components and the relationships between them. z Fundamental Models y. These are vertical views or slices, representing some key aspects of distributed systems. y. Each fundamental model represents a set of issues that must be addressed in the design of distributed systems y. Interaction model, failure model, security model

Figure 2. 1 Software and hardware service layers in distributed systems

Figure 2. 2 Clients invoke individual servers

Figure 2. 3 A distributed application based on peer processes

Figure 2. 4 A service provided by multiple servers

Figure 2. 5 Web proxy server

Figure 2. 6 Web applets

Figure 2. 7 Thin clients and compute servers Compute server Network computer or PC Thin Client network Application Process

Interaction Models z Performance of Communication Channels y. Delay, Loss z Unsynchronized Clocks z Synchronous Model y. Execution time bounded y. Communication delay bounded y. Clock drift bounded y. Wait the response z Asynchronous Model

Figure 2. 8 Real-time ordering of events

Figure 2. 9 Processes and channels

Figure 2. 10 Omission and arbitrary failures Class of failure Affects Fail-stop Process Description Process halts and remains halted. Other processes may detect this state. Crash Process halts and remains halted. Other processes may not be able to detect this state. Omission Channel A message inserted in an outgoing message buffer never arrives at the other end’s incoming message buffer. Send-omission Process A process completes send, a but the message is not put in its outgoing message buffer. Receive-omission. Process A message is put in a process’s incoming message buffer, but that process does not receive it. Arbitrary Process or. Process/channel exhibits arbitrary behaviour: it may (Byzantine) channel send/transmit arbitrary messages at arbitrary times, commit omissions; a process may stop or take an incorrect step.

Figure 2. 11 Timing failures Class of Failure Affects Clock Process Performance Channel Description Process’s local clock exceeds the bounds on its rate of drift from real time. Process exceeds the bounds on the interval between two steps. A message’s transmission takes longer than the stated bound.

Failures z Processes and communication channels can fail. z The classification of their failures is useful for the analysis of failures of protocols. z Components that exhibit Byzantine or arbitrary failures may do anything at any time. z Timing failures occur only in synchronous systems. z Most failures in distributed systems are benign (e. g. omission but not Byzantine failures). z A service may mask the failures of the components from which it is constructed, for example, reliable one-to-one communication may be built by masking omission failures.

Figure 2. 12 Objects and principals

Figure 2. 13 The enemy Copy ofm The enemy Processp m’ m Processq Communication channel

Figure 2. 14 Secure channels Principal. B Principal. A Processp Secure channel Processq