分布式系统分类

分布式系统分类

--

分布式计算系统：集群计算系统

&

网格计算系统（硬件，操作系 统，网络都不尽相同）

--

分布式信 息系统：事务处理系统

&

企业应用集成

--

分布式普适系统

Grid computing

Provide users with seamless access to:

–

Storage capacity

–

Processing

–

Network bandwidth

Heterogeneous and geographically distributed systems

系统体系结构：

集中式

&

非集中式

&

混合模式

远程过程调用（

RPC

）

：

机器

A

上的进程调用机 器

B

上的进程时，

A

< br>上的调用进程被挂起，

B

上被调用的开始执行调用方可以 通过使用参数将信息传送给被调用方，

然后可以通过传回

的结果 得到信息。

RPC

操作分为客户部分 和服务器部分。

命名系统

Naming

Problem

Given an essentially unstructured name (e.g., an identi?er), how can

we locate its associated access point?

Simple solutions (broadcasting)

Home-based approaches

Distributed Hash Tables (structured P2P)

Hierarchical location service

Flynn’s Taxonomy

费林分类

SISD

SIMD

MISD

MIMD

单一指令流单一资料流

计算机

单一指令流多资料流

计算机

多指令流单一资料流

计算机

多指令流多资料流

计算机

Subclassifying MIMD

memory

–

shared memory systems: multiprocessors

–

no shared memory: networks of computers, multicomputers

interconnect

互联

–

bus

–

switch

delay/bandwidth

–

tightly coupled systems

–

loosely coupled systems

Transparency

透明性

(

访问

access

< br>、位置

location

、迁移

migration

、重定位

relocation

replication

、并发

、故障

failure)

–

Location transparency

:

users don’t care where resources are

–

Migration transparency

:

resources move at will

–

–

–

Replication transparency

:

users cannot tell whether there are copies of resources

Concurrency transparency

:

users share resources transparently

Parallelism transparency

:

operations take place in parallel without user’s knowledge

Design issues

Transparency

Reliability

Performance

Scalability

目标

(

可访 问、透明性、开放性、可扩展性、可靠性

)

Centralized model

集中式模型

?

No networking

?

Traditional time-sharing system

?

Direct connection of user terminals to system

?

One or several CPUs

?

Not easily scalable

?

Limiting factor: number of CPUs in system

–

Contention for same resources

Peer to peer model

?

Each machine on network has (mostly) equivalent capabilities

?

No machines are dedicated to serving others

?

E.g., collection of PCs:

–

Access other people’s files

–

Send/receive email (without server)

–

Gnutella-style content sharing

–

SETI@home computation

Processor pool model

What about idle workstations

(computing resources)?

–

Let them sit idle

–

Run jobs on them

Alternatively…

–

Collection of CPUs that can be assigned processes on demand

–

Users won’t need heavy duty workstations

?

GUI on local machine

–

Computation model of Plan 9

RPC

的步骤：

(1)

客户过程以正常的方式调用客户存根

(2)

客户存根生成一个消息，然后调用本地的操作系统

(3)

客户端操作系统将消息发送给远程操作系统

(4)

远程操作系统将消息交给服务器存根

(5)

服务器存根将参数提取出来，然后调用服务器

(6)

服务器执行要求的操作，操作完成后将结果返回给服务器存根

(7)

服务器存根讲结果打包成一个消息，然后调用本地操作系统

(8)

服务器操作系统将含有结果的消息发送回客户端操作系统

(9)

客户端操作系统将消息交给客户存根

(10)

客户存根将结果从消息中提取出来，返回给调用它的客户过程。

并发的方法

?

Two Phase locking

两阶段锁协议

?

Strict two phase locking

严格的两阶段锁

?

Read/write lock

读写锁

?

Two version locking

两版本锁

时钟

物理时钟和逻辑时钟

物理时钟：

--Real-time Clock: CMOS clock (counter) circuit driven by a quartz oscillator

--OS generally programs a timer circuit to generate an interrupt periodically

问题：

Getting two systems to agree on time

–

Two clocks hardly ever agree

–

Quartz oscillators oscillate at slightly different frequencies

Clocks tick at different rates

–

Create ever-widening gap in perceived time

–

Clock Drift

Difference between two clocks at one point in time

–

Clock Skew

时间偏移

Simplest synchronization technique

-

Issue RPC to obtain time

–

Set time

Cristian’s algorithm

Compensate for delays

–

Note times:

?

r

equest sent: T

0

?

r

eply received: T

1

–

Assume network delays are symmetric