Distributed Systems - Chapter 2: Communication - Thoai Nam

Nội dung text: Distributed Systems - Chapter 2: Communication - Thoai Nam

1. Distributed Systems Thoai Nam Faculty of Computer Science and Engineering HCMC University of Technology
2. Communication Protocols  Protocols are agreements/rules on communication  Protocols could be connection-oriented or connectionless Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
3. Client-Server TCP a) Normal2-4 operation of TCP. b) Transactional TCP. Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
4. Client-Server Communication Model  Structure: group of servers offering service to clients  Based on a request/response paradigm  Techniques: – Socket, remote procedure calls (RPC), Remote Method Invocation (RMI) client file process terminal server server server kernel kernel kernel kernel Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
5. Addressing Issues  Question: how is the server user server located?  Hard-wired address – Machine address and process address are known a priori user server  Broadcast-based – Server chooses address from a sparse address space – Client broadcasts request NS user server – Can cache response for future  Locate address via name server Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
6. Buffering Issues  Unbuffered communication – Server must call receive before clientuser server can call send  Buffered communication – Client send to a mailbox – Server receives from a mailbox user server Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
7. Remote Procedure Calls  Goal: Make distributed computing look like centralized computing  Allow remote services to be called as procedures – Transparency with regard to location, implementation, language  Issues – How to pass parameters – Bindings – Semantics in face of errors  Two classes: integrated into prog, language and separate Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
8. Parameter Passing  Local procedure parameter passing – Call-by-value – Call-by-reference: arrays, complex data structures  Remote procedure calls simulate this through: – Stubs – proxies – Flattening – marshalling  Related issue: global variables are not allowed in RPCs Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
9. Stubs  Client makes procedure call (just like a local procedure call) to the client stub  Server is written as a standard procedure  Stubs take care of packaging arguments and sending messages  Packaging parameters is called marshalling  Stub compiler generates stub automatically from specs in an Interface Definition Language (IDL) – Simplifies programmer task Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
10. Example of an RPC 2-8 Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
11. Binding  Problem: how does a client locate a server? – Use Bindings  Server – Export server interface during initialization – Send name, version no, unique identifier, handle (address) to binder  Client – First RPC: send message to binder to import server interface – Binder: check to see if server has exported interface » Return handle and unique identifier to client Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
12. Binder: Port Mapper  Server start-up: create port  Server stub calls svc_register to register prog. #, version # with local port mapper  Port mapper stores prog #, version #, and port  Client start-up: call clnt_create to locate server port  Upon return, client can call procedures at the server Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
13. Lightweight RPCs  Many RPCs occur between client and server on same machine – Need to optimize RPCs for this special case => use a lightweight RPC mechanism (LRPC)  Server S exports interface to remote procedures  Client C on same machine imports interface  OS kernel creates data structures including an argument stack shared between S and C Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
14. Doors  Which RPC to use? - run-time bit allows stub to choose between LRPC and RPC Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
15. Asynchronous RPC 2-12 a) The interconnection between client and server in a traditional RPC b) The interaction using asynchronous RPC Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
16. Remote Method Invocation (RMI)  RPCs applied to objects, i.e., instances of a class – Class: object-oriented abstraction; module with data and operations – Separation between interface and implementation – Interface resides on one machine, implementation on another  RMIs support system-wide object references – Parameters can be object references Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
17. Proxies and Skeletons  Proxy: client stub – Maintains server ID, endpoint, object ID – Sets up and tears down connection with the server – [Java:] does serialization of local object parameters – In practice, can be downloaded/constructed on the fly (why can’t this be done for RPCs in general?)  Skeleton: server stub – Does deserialization and passes parameters to server and sends result to proxy Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM
18. Message-oriented Transient Communication  Many distributed systems built on top of simple message-oriented model – Example: Berkeley sockets Khoa Coâng Ngheä Thoâng Tin – Ñaïi Hoïc Baùch Khoa Tp.HCM