Chapter 2 ARCHITECTURES

Transcription

1 DISTRIBUTED SYSTEMS Principles and Paradigms Second Edition ANDREW S. TANENBAUM MAARTEN VAN STEEN Chapter 2 ARCHITECTURES

2 Architectural Styles (1) DS should be composed of Components and Connectors.» How components and connectors are connected is called Arch style. Architectural styles (logical styles) of distributed systems: Layered architectures Object-based architectures Resource-centered architectures Event-based architectures

3 Layered Architec Style

4 Communication Protocols Stacks

5 Application Layering (1) Application-interface layer contains units for interfacing to users or external applications Processing layer contains the functions of an application, i.e., without specific data Data layer contains the data that a client wants to manipulate through the application components

6 Application Layering (2)

7 object-based architectural style Components are objects Connected to each other through procedure calls. Objects may be placed on different machines;

8 Resource Based Arch RESTful architectures: View a distributed system as a collection of resources, individually managed by components. Resources may be added, removed, retrieved, and modified by (remote) applications. 1 Resources are identified through a single naming scheme 2 All services offer the same interface 3 Messages sent to or from a service are fully self-described 4 After executing an operation at a service, that component forgets everything about the caller

9 Example: Amazon s Simple Storage Service Objects (i.e., files) are placed into buckets (i.e., directories). Buckets cannot be placed into buckets. Operations on ObjectName in bucket BucketName require the following identifier: All operations are carried out by sending HTTP requests: Create a bucket/object: PUT, along with the URI Listing objects: GET on a bucket name Reading an object: GET on a full URI

10 Publish-Subscribe Arch

11 Middleware Organization 1) Wrappers: A wrapper or adapter offers an interface acceptable to an application but is not acceptable to a component because the component is old. The wrapper do proper transformations 2) Interceptors: break the normal flow of control and allow other code to be executed. 3) Middleware contains solutions that are good for most applications ) you may want to adapt its behavior for specific applications.

12 Organizing wrappers

13 Interceptors Figure Using interceptors to handle remote-object invocations.

14 System Architectures Important styles of hardware architecture for distributed systems: 1.Centralized architectures 1. Basic client-server 2. Application layering 3. Multi-tiered architectures 2.Decentralized architectures 1. Structured P2P 2. Unstructured P2P 3.Hybrid architectures 1.Edge server 2.Collaborative distributed systems

15 Centralized Architectures Figure 2-3. General interaction between a client and a server.

16 Multi-tiered centralized system architectures The simplest organization is to have only two types of machines: A client machine containing only the programs implementing (part of) the userinterface level A server machine containing the rest, the programs implementing the processing and data level

17 Traditional two-tiered configurations Figure 2-5. Alternative client-server organizations (a) (e).

18 Three Tiered Architecture Figure 2-6. An example of a server acting as client.

19 Decentralized organizations Vertical distribution: Comes from dividing distributed applications into three logical layers, and running the components from each layer on a different server (machine). 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. Peer-to-peer architectures: Processes are all equal: the functions that need to be carried out are represented by every process ==> each process will act as a client and a server at the same time (i.e., acting as a servant).

20 Structured Peer-to-Peer: Hypercube 4 dimensional hypercube To build 5 dimentional hypercube, two sets of 4d hypercubes are connected to each other and so on.

21 Structured Peer-to-Peer Architectures (1) Principle Nodes are logically organized in a ring. Each node has an m-bit identifier. Each data item is hashed to an m-bit key. Data item with key k is stored at node with smallest identifier id k, called the successor of key k. The ring is extended with various shortcut links to other nodes. m=128 or 160 bits typically.

22 Unstructured Peer-to-Peer Architectures (1) Each node maintains an ad hoc list of neighbors. The resulting overlay resembles a random graph: an edge <u,v>i exists only with a certain probability P[<u,v>]. Searching Flooding: issuing node u passes request for d to all neighbors. Request is ignored when receiving node (v) had seen it before. Otherwise, v searches locally for d (recursively). May be limited by a Time-To-Live: a maximum number of hops. Random walk: issuing node u passes request for d to randomly chosen neighbor, v. If v does not have d, it forwards request to one of its randomly chosen neighbors, and so on.

23 Flooding versus random walk Assume N nodes and that each data item is replicated across r randomly chosen nodes.

24 Summary of Results If r/n = 0:001, then S 1000 With flooding and d = 10;k = 4, we contact 7290 nodes. Random walks are more communication efficient, but might take longer before they find the result. As the networks becomes bigger, locating data item becomes problematic. Solution is to use special nodes that have index to data items.

25 Super-peers Networks Figure A hierarchical organization of nodes into a superpeer network.

26 Skype s principle operation: A wants to contact B

27 Hybrid Architecture: Edge-Server Systems Viewing the Internet as consisting of a collection of edge servers.

28 Hybrid Architecture: Collaborative Distributed Systems Figure The principal working of BitTorrent [adapted with permission from Pouwelse et al. (2004)].