Transcript Lecture #23: Distributed Systems

Lecture 23
Chapter 16/18:
Distributed Systems
Principles of Computer Operating Systems
Modified from Silberschatz, Galvin and Gagne
Chapter 16: Distributed System Structures
 Motivation
 Types of Network-Based Operating Systems
 Network Structure
 Network Topology
 Communication Structure
 Communication Protocols
 Robustness
 Design Issues
 An Example: Networking
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Chapter Objectives
 To provide a high-level overview of distributed systems and the
networks that interconnect them
 To discuss the general structure of distributed operating systems
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Motivation
 Distributed system is collection of loosely coupled processors
interconnected by a communications network
 Processors variously called nodes, computers, machines, hosts

Site is location of the processor
 Reasons for distributed systems

Resource sharing

sharing and printing files at remote sites

processing information in a distributed database

using remote specialized hardware devices

Computation speedup – load sharing

Reliability – detect and recover from site failure, function transfer,
reintegrate failed site

Communication – message passing
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A Distributed System
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Network-Operating Systems
 Users are aware of multiplicity of machines.
 Access to resources of various machines is done explicitly by:

Remote logging into the appropriate remote machine (telnet, ssh)

Remote Desktop (Microsoft Windows)

Transferring data from remote machines to local machines, via the
File Transfer Protocol (FTP) mechanism
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Distributed-Operating Systems
 Users not aware of multiplicity of machines

Access to remote resources similar to access to local resources
 Data Migration

transfer data by transferring entire file, or

transferring only those portions of the file necessary for the
immediate task
 Computation Migration

transfer the computation, rather than the data, across the system
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Distributed-Operating Systems (Cont.)
 Process Migration

execute an entire process, or parts of it, at different sites

Load balancing
–

Computation speedup
–

process execution may require specialized processor
Software preference
–

subprocesses can run concurrently on different sites
Hardware preference
–

distribute processes across network to even the workload
required software may be available at only a particular site
Data access
–
run process remotely, rather than transfer all data locally
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Network Structure
 Local-Area Network (LAN) – designed to cover small geographical area.

Multiaccess bus, ring, or star network

Speed  10 – 100 megabits/second

Broadcast is fast and cheap

Nodes:

usually workstations and/or personal computers

a few mainframes
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Network Types (Cont.)
 Wide-Area Network (WAN) – links geographically separated sites

Point-to-point connections over long-haul lines

often leased from a phone company

Speed  1.544 – 45 megbits/second

Broadcast usually requires multiple messages

Nodes:

usually a high percentage of mainframes
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Network Topology
 Sites in the system can be physically connected in a variety of ways;

They are compared with respect to the following criteria:

Installation cost
–

Communication cost
–

How expensive is it to link the various sites in the system?
How long does it take to send a message from site A to site B?
Reliability
–
If a link or a site in the system fails, can the remaining sites still
communicate with each other?
 The various topologies are depicted as graphs whose nodes correspond to
sites

An edge from node A to node B corresponds to a direct connection
between the two sites
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Network Topology
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Communication Structure
The design of a communication network must address four basic
issues:
 Naming and name resolution

How do two processes locate each other to communicate?
 Routing strategies

How are messages sent through the network?
 Connection strategies

How do two processes send a sequence of messages?
 Contention

The network is a shared resource, so how do we resolve
conflicting demands for its use?
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Naming and Name Resolution
 Name systems in the network
 Address messages with the process-id
 Identify processes on remote systems by
<host-name, identifier> pair
 Domain name service (DNS)

specifies the naming structure of the hosts, as well as name to
address resolution (Internet)
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Routing Strategies
 Fixed routing: A path from A to B is specified in advance

Path changes only if a hardware failure disables it

Since the shortest path is usually chosen, communication costs are
minimized

Fixed routing cannot adapt to load changes

Ensures that messages will be delivered in the order in which they
were sent
 Virtual circuit: A path from A to B is fixed for the duration of one session

Different sessions involving messages from A to B may have different
paths

Partial remedy to adapting to load changes

Ensures that messages will be delivered in the order in which they
were sent
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Routing Strategies (Cont.)
 Dynamic routing: The path used to send a message form site A to site
B is chosen only when a message is sent

Usually a site sends a message to another site on the link least used
at that particular time

Adapts to load changes by avoiding routing messages on heavily
used path

Messages may arrive out of order

This problem can be remedied by appending a sequence number
to each message
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Connection Strategies
 Circuit switching: A permanent physical link is established for the duration
of the communication

i.e., telephone system
 Message switching: A temporary link is established for the duration of one
message transfer

i.e., post-office mailing system
 Packet switching: Messages of variable length are divided into fixed-
length packets which are sent to the destination


Each packet may take a different path through the network
The packets must be reassembled into messages as they arrive
 Circuit switching requires setup time, but incurs less overhead for shipping
each message, and may waste network bandwidth
 Message and packet switching require less setup time, but incur more
overhead per message
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Contention
Several sites may want to transmit information over a link
simultaneously. Techniques to avoid repeated collisions include:
 CSMA/CD - Carrier sense with multiple access (CSMA) with
collision detection (CD)

A site determines whether another message is currently being
transmitted over that link.

If two or more sites begin transmitting at exactly the same time,
then they will register a CD and will stop transmitting

When the system is very busy, many collisions may occur, and
thus performance may be degraded
 CSMA/CD is used successfully in the Ethernet system

the most common network system
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Contention (Cont.)
 Token passing: A unique message type, known as a token, continuously
circulates in the system (usually a ring structure)

A site that wants to transmit information must wait until the token
arrives

When the site completes its round of message passing, it retransmits
the token

A token-passing scheme is used by some IBM and HP/Apollo
systems
 Message slots: A number of fixed-length message slots continuously
circulate in the system (usually a ring structure)

Since a slot can contain only fixed-sized messages, a single logical
message may have to be broken down into a number of smaller
packets, each of which is sent in a separate slot

This scheme has been adopted in the experimental Cambridge Digital
Communication Ring
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Communication Via ISO Network Model
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Communication Protocol
The communication network is partitioned into the following
multiple layers:
 Physical layer: handles the mechanical and electrical details of
the physical transmission of a bit stream
 Data-link layer: handles the frames, or fixed-length parts of
packets, including any error detection and recovery that occurred in
the physical layer
 Network layer: provides connections and routes packets in the
communication network, including handling the address of outgoing
packets, decoding the address of incoming packets, and
maintaining routing information for proper response to changing
load levels
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Communication Protocol (Cont.)
 Transport layer: responsible for low-level network access and for
message transfer between clients, including partitioning messages into
packets, maintaining packet order, controlling flow, and generating
physical addresses
 Session layer: implements sessions, or process-to-process
communications protocols
 Presentation layer: resolves the differences in formats among the
various sites in the network, including character conversions, and half
duplex/full duplex (echoing)
 Application layer: interacts directly with the users’ deals with file
transfer, remote-login protocols and electronic mail, as well as schemas
for distributed databases
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The ISO Protocol Layer
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The TCP/IP Protocol Layers
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Design Issues
 Transparency

the distributed system should appear as a conventional, centralized
system to the user
 Fault tolerance

the distributed system should continue to function in the face of failure
 Scalability

as demands increase, the system should easily accept the addition of
new resources to accommodate the increased demand
 Clusters

a collection of semi-autonomous machines that acts as a single
system
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Chapter 18 Distributed Coordination
 Event Ordering
 Mutual Exclusion
 Atomicity
 Concurrency Control
 Deadlock Handling
 Election Algorithms
 Reaching Agreement
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Chapter Objectives
 To describe various methods for achieving mutual exclusion in a
distributed system
 To explain how atomic transactions can be implemented in a distributed
system
 To show how some of the concurrency-control schemes can be
modified for use in a distributed environment
 To present schemes for handling deadlock prevention, deadlock
avoidance, and deadlock detection in a distributed system
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Event Ordering
 Happened-before relation (denoted by )

If A and B are events in the same process, and A was executed before
B, then A  B

If A is the event of sending a message by one process and B is the
event of receiving that message by another process, then A  B

If A  B and B  C then A  C
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Implementation of 

Associate a timestamp with each system event


Require that for every pair of events A and B, if A  B, then the timestamp of
A is less than the timestamp of B
Within each process Pi a logical clock, LCi is associated

The logical clock can be implemented as a simple counter that is
incremented between any two successive events executed within a process

Logical clock is monotonically increasing

A process advances its logical clock when it receives a message whose
timestamp is greater than the current value of its logical clock

If the timestamps of two events A and B are the same, then the events are
concurrent

We may use the process identity numbers to break ties and to create a total
ordering
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Distributed Mutual Exclusion (DME)
 Assumptions

The system consists of n processes;

Each process Pi resides at a different processor

Each process has a critical section that requires mutual exclusion
 Requirement

If Pi is executing in its critical section, then no other process Pj is
executing in its critical section
 We present two algorithms to ensure the mutual exclusion execution of
processes in their critical sections
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DME: Centralized Approach
 One of the processes in the system is chosen to coordinate the entry to
the critical section
 A process that wants to enter its critical section sends a request
message to the coordinator
 The coordinator decides which process can enter the critical section
next, and its sends that process a reply message
 When the process receives a reply message from the coordinator,

it enters its critical section
 After exiting its critical section,

the process sends a release message to the coordinator and
proceeds with its execution
 This scheme requires three messages per critical-section entry:

request

reply

release
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DME: Fully Distributed Approach
 When process Pi wants to enter its critical section, it generates a new
timestamp, TS, and sends the message request (Pi, TS) to all other
processes in the system
 When process Pj receives a request message, it may reply immediately or it
may defer sending a reply back
 When process Pi receives a reply message from all other processes in the
system, it can enter its critical section
 After exiting its critical section, the process sends reply messages to all its
deferred requests
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DME: Fully Distributed Approach (Cont)
 The decision whether process Pj replies immediately to a request(Pi, TS)
message or defers its reply is based on three factors:

If Pj is in its critical section, then it defers its reply to Pi

If Pj does not want to enter its critical section, then it sends a reply
immediately to Pi

If Pj wants to enter its critical section but has not yet entered it, then it
compares its own request timestamp with the timestamp TS

If its own request timestamp is greater than TS, then it sends a reply
immediately to Pi (Pi asked first)

Otherwise, the reply is deferred
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Desirable Behavior of Fully Distributed Approach
 Freedom from Deadlock is ensured
 Freedom from starvation is ensured, since entry to the critical section is
scheduled according to the timestamp ordering

The timestamp ordering ensures that processes are served in a firstcome, first served order
 The number of messages per critical-section entry is
2 x (n – 1)
This is the minimum number of required messages per critical-section entry
when processes act independently and concurrently
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Three Undesirable Consequences
 The processes need to know the identity of all other processes in the
system, which makes the dynamic addition and removal of processes more
complex
 If one of the processes fails, then the entire scheme collapses

This can be dealt with by continuously monitoring the state of all the
processes in the system
 Processes that have not entered their critical section must pause frequently
to assure other processes that they intend to enter the critical section

This protocol is therefore suited for small, stable sets of cooperating
processes
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Token-Passing Approach
 Circulate a token among processes in system

Token is special type of message

Possession of token entitles holder to enter critical section
 Processes logically organized in a ring structure
 Unidirectional ring guarantees freedom from starvation
 Two types of failures

Lost token – election must be called

Failed processes – new logical ring established
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Atomicity
 Either all the operations associated with a program unit are executed to
completion, or none are performed
 Ensuring atomicity in a distributed system requires a transaction
coordinator, which is responsible for the following:

Starting the execution of the transaction

Breaking the transaction into a number of subtransactions, and
distribution these subtransactions to the appropriate sites for
execution

Coordinating the termination of the transaction, which may result in
the transaction being committed at all sites or aborted at all sites
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Election Algorithms
 Determine where a new copy of the coordinator should be restarted
 Assume that a unique priority number is associated with each active
process in the system, and assume that the priority number of process
Pi is i
 Assume a one-to-one correspondence between processes and sites
 The coordinator is always the process with the largest priority number.

When a coordinator fails, the algorithm must elect that active
process with the largest priority number
 Two algorithms, the bully algorithm and a ring algorithm, can be used to
elect a new coordinator in case of failures
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Bully Algorithm
 Applicable to systems where every process can send a message to every
other process in the system
 If process Pi sends a request that is not answered by the coordinator
within a time interval T, assume that the coordinator has failed;

Pi tries to elect itself as the new coordinator
 Pi sends an election message to every process with a higher priority
number, Pi then waits for any of these processes to answer within T
 If no response within T, assume that all processes with numbers greater
than i have failed; Pi elects itself the new coordinator
 If answer is received, Pi begins time interval T´, waiting to receive a
message that a process with a higher priority number has been elected

If no message is sent within T´, assume the process with a higher
number has failed; Pi should restart the algorithm
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Bully Algorithm (Cont)
 If Pi is not the coordinator, then, at any time during execution, Pi may
receive one of the following two messages from process Pj

Pj is the new coordinator (j > i). Pi, in turn, records this information

Pj started an election (j > i). Pi, sends a response to Pj and begins its
own election algorithm, provided that Pi has not already initiated such
an election
 After a failed process recovers, it immediately begins execution of the
same algorithm
 If there are no active processes with higher numbers, the recovered
process forces all processes with lower number to let it become the
coordinator process, even if there is a currently active coordinator with a
lower number
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Ring Algorithm
 Applicable to systems organized as a ring (logically or physically)
 Assumes that the links are unidirectional, and that processes send
their messages to their right neighbors
 Each process maintains an active list, consisting of all the priority
numbers of all active processes in the system when the algorithm
ends
 If process Pi detects a coordinator failure, it creates a new active list
that is initially empty.

It then sends a message elect(i) to its right neighbor, and adds
the number i to its active list
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Ring Algorithm (Cont)

If Pi receives a message elect(j) from the process on the left, it must
respond in one of three ways:
1.
If this is the first elect message it has seen or sent, Pi creates a new
active list with the numbers i and j

2.
If i  j, then the active list for Pi now contains the numbers of all the
active processes in the system

3.
It then sends the message elect(i), followed by the message
elect(j)
Pi can now determine the largest number in the active list to
identify the new coordinator process
If i = j, then Pi receives the message elect(i)

The active list for Pi contains all the active processes in the system

Pi can now determine the new coordinator process.
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Reaching Agreement
 There are applications where a set of processes wish to agree on a
common “value”
 Such agreement may not take place due to:

Faulty communication medium

Faulty processes

Processes may send garbled or incorrect messages to other
processes

A subset of the processes may collaborate with each other in an
attempt to defeat the scheme
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Faulty Communications
 Process Pi at site A, has sent a message to process Pj at site B;

to proceed, Pi needs to know if Pj has received the message
 Detect failures using a time-out scheme

When Pi sends out a message, it also specifies a time interval during
which it is willing to wait for an acknowledgment message form Pj

When Pj receives the message,


If Pi receives the acknowledgment message within the specified time
interval,


it immediately sends an acknowledgment to Pi
it concludes that Pj has received its message
If a time-out occurs, Pj needs to retransmit its message and wait for an
acknowledgment
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Faulty Communications (Cont)
 Continue until Pi either receives an acknowledgment, or is notified by the
system that B is down
 Suppose that Pj also needs to know that Pi has received its
acknowledgment message, in order to decide on how to proceed

In the presence of failure,


it is not possible to accomplish this task
It is not possible in a distributed environment for processes Pi and Pj to
agree completely on their respective states
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Faulty Processes
(Byzantine Generals Problem)
 Communication medium is reliable, but processes can fail in
unpredictable ways
 Consider a system of n processes, of which no more than m are faulty

Suppose that each process Pi has some private value of Vi
 Devise an algorithm that allows each non-faulty Pi to construct a vector
Xi = (Ai,1, Ai,2, …, Ai,n) such that

If Pj is a nonfaulty process, then Aij = Vj.

If Pi and Pj are both non-faulty processes, then Xi = Xj.
 Solutions share the following properties

A correct algorithm can be devised only if n  3 x m + 1

The worst-case delay for reaching agreement is proportionate to
m + 1 message-passing delays
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Faulty Processes (Cont)
 An algorithm for the case where m = 1 and n = 4 requires two rounds
of information exchange:

Each process sends its private value to the other 3 processes

Each process sends the information it has obtained in the first
round to all other processes
 If a faulty process refuses to send messages,

a non-faulty process can choose an arbitrary value and pretend
that that value was sent by that process
 After the two rounds are completed, a non-faulty process Pi can
construct its vector Xi = (Ai,1, Ai,2, Ai,3, Ai,4) as follows:

Ai,j = Vi

For j  i, if at least two of the three values reported for process Pj
agree, then the majority value is used to set the value of Aij

Otherwise, a default value (nil) is used
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