Introduction to Distributed Systems

Transcription

1 Introduction to Distributed Systems Minsoo Ryu Department of Computer Science and Engineering

2 2 Definition A distributed system is a collection of independent computers that appears to its users as a single coherent system Distributed Systems: Principles and Paradigms, Andrew S. Tanenbaum and Maarten Van Steen A distributed system consists of a set of processors that are connected by a communication network Distributed Computing: Principles, Algorithms, and Systems, Ajay D. Kshemkalyani and Mukesh Singhal 2

3 3 Structures 3

4 4 Why Distributed Systems? Inherently distributed computations Money transfer in banking, or reaching consensus among parties that are geographically distant Resource sharing Access to geographically remote data and resources Enhanced reliability Increased performance/cost ratio Scalability As the processors are usually connected by a wide-area network, adding more processors does not pose a direct bottleneck for the communication network Modularity and incremental expandability 4

5 5 Transparency An important goal of a distributed system is to hide the fact that its processes and resources are physically distributed across multiple computers A distributed system that is able to present itself to users and applications as if it were only a single computer system is said to be transparent 5

6 6 Characteristics of Distributed Systems No common physical clock This is an important assumption because it introduces the element of distribution in the system and gives rise to the inherent asynchrony amongst the processors No shared memory This is a key feature that requires message-passing for communication It may be noted that a distributed system may still provide the abstraction of a common address space via the distributed shared memory abstraction Autonomy and heterogeneity The processors are loosely coupled in that they have different speeds and each can be running a different operating system They are usually not part of a dedicated system, but cooperate with one another by offering services or solving a problem jointly 6

7 7 Processor Synchrony Processor synchrony indicates that all the processors execute in lock-step with their clocks synchronized As this synchrony is not attainable in a distributed system, what is more generally indicated is that for a large granularity of code, usually termed as a step, the processors are synchronized This abstraction is implemented using some form of barrier synchronization to ensure that no processor begins executing the next step of code until all the processors have completed executing the previous steps of code assigned to each of the processors 7

8 8 Synchronous Execution I. Processors are synchronized and the clock drift rate between any two processors is bounded II. Message delivery (transmission + delivery) times are such that they occur in one logical step or round III. There is a known upper bound on the time taken by a process to execute a step 8

9 9 Asynchronous Execution I. There is no processor synchrony and there is no bound on the drift rate of processor clocks, II. Message delays (transmission + propagation times) are finite but unbounded III. There is no upper bound on the time taken by a process to execute a step 9

10 10 Notes on Synchronous Execution It is easier to design and verify algorithms assuming synchronous executions because of the coordinated nature of the executions at all the processes However, it is practically difficult to build a completely synchronous system, and have the messages delivered within a bounded time Therefore, this synchrony has to be simulated under the covers, and will inevitably involve delaying or blocking some processes for some time durations Thus, synchronous execution is an abstraction that needs to be provided to the programs When implementing this abstraction, observe that the fewer the steps or synchronizations of the processors, the lower the delays and costs If processors are allowed to have an asynchronous execution for a period of time and then they synchronize, then the granularity of the synchrony is coarse This is really a virtually synchronous execution, and the abstraction is sometimes termed as virtual synchrony 10

11 11 Architectural Types Basic Client-Server Architecture Multitiered (Client-Server) Architecture Decentralized Architecture 11

12 12 Basic Client-Server Architecture A server is a process implementing a specific service A client is a process that requests a service from a server by sending it a request and subsequently waiting for the server's reply This model helps us understand and manage the complexity of distributed systems and that is a good thing 12

13 13 Multitiered (Client-Server) Architecture Divide applications into a user-interface, processing components, and a data level Many client-server applications are targeted toward supporting user access to databases 13

14 14 Multitiered (Client-Server) Architecture 14

15 15 Decentralized Architecture Vertical distribution vs. horizontal distribution Vertical distribution: organizing a client-server application as a multitiered architecture Horizontal distribution: a client or server may be physically split up into logically equivalent parts, but each part is operating on its own share of the complete data set, thus balancing the load Peer-to-peer system The processes that constitute a peer-to-peer system are all equal The functions that need to be carried out are represented by every process that constitutes the distributed system Much of the interaction between processes is symmetric: each process will act as a client and a server at the same time 15

16 16 Routing and Resource Discovery P2P networks generally implement some form of virtual overlay network on top of the physical network topology the nodes in the overlay form a subset of the nodes in the physical network Unstructured P2P networks Do not impose a particular structure on the overlay network by design, but rather are formed by nodes that randomly form connections to each other (Gnutella, Gossip, and Kazaa) Structured P2P networks The overlay is organized into a specific topology, and the protocol ensures that any node can efficiently search the network for a file/resource Hybrid P2P networks Some nodes are appointed special functions in a well-organized fashion 16

17 17 Comparison of Lookup Styles Decentralized lookup (Napster, music file sharing) Centralized lookup (Gnutella, file sharing) Superpeer style (KaZaA, file sharing) 17

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