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16: Distributed Structures 1

OPERATING SYSTEMS

Distributed System Structures

16: Distributed Structures 2

DISTRIBUTED STRUCTURES

This chapter sets the foundation for our discussion about networks and distributed OS.

VOCABULARY

Tightly coupled systems Same clock, usually shared memory. Multiprocessors. Communication is via this shared memory.

Loosely coupled systems Different clock, use communication links. Distributed systems.

sites = nodes = computers = machines = hosts

Local The resources on your "home" host.

Remote The resources NOT on your "home" host.

Server A host at a site that has a resource used by a Client.

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NETWORK STRUCTURES

Clusters The hardware on which distributed systems run. A

current buzzword. It allows more compute power, compared to a mainframe, by

running on many inexpensive small machines.

Vocabulary

Chapter 17 talks in great deal about distributed systems as a whole; meanwhile we'll discuss the components of these systems.

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NETWORK STRUCTURES

Advantages of distributed systems:

Resource Sharing Items such as printers, specialized processors, disk farms,

files can be shared among various sites.

Computation Speedup Load balancing - dividing up all the work evenly between

sites. Making use of parallelism.

Reliability Redundancy. With proper configuration, when one site

goes down, the others can continue. But this doesn't

happen automatically.

Communications Messaging can be accomplished very efficiently.

Messages between nodes are akin to IPCs within a

UniProcessor.

Easier to talk/mail between users.

Why

Distributed

OS?

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NETWORK STRUCTURES

Advantages of distributed systems:

Why

Distributed

OS?

16: Distributed Structures 8

NETWORK STRUCTURES

Methods of connecting sites together can be evaluated as follows:

Basic cost: This is the price of wiring, which is proportional to the number of connections.

Communication cost: The time required to send a message. This is proportional to the amount of wire and the number of nodes traversed.

Reliability: If one site fails, can others continue to communicate.

Let's look at a number of connection mechanisms using these criteria:

Topology

FULLY CONNECTED

• All sites are connected to all other sites.
• Expensive( proportional to N squared ), fast communication, reliable.

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NETWORK STRUCTURES

RING

• Uni or bi-directional, single, double link.
• Cost is linear with number of sites; communication cost is high; failure of any site partitions ring.

MULTIACCESS BUS

• Nodes hang off a ring rather than being part of it.
• Cost is linear; communication cost is low; site failure doesn't affect partitioning.

Topology

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NETWORK STRUCTURES

LOCAL AREA NETWORKS (LAN):

• Designed to cover small geographical area.
• Multiaccess bus, ring or star network.
• Speed around 1 gigabit / second or higher.
• Broadcast is fast and cheap.
• usually workstations or personal computers with few mainframes.

WIDE AREA NETWORK (WAN):

• Links geographically separated sites.
• Point to point connections over long-haul lines (often leased from a phone company.)
• Speed around 1 megabits / second. (T1 is 1.544 megabits/second.)
• Broadcast usually requires multiple messages.
• Nodes usually contain a high percentage of mainframes.

Network

Types

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NETWORK STRUCTURES

NAMING AND NAME RESOLUTION

• Naming systems in the network.
• Address messages with the process-id.
• Identify processes on remote systems by < hostname, identifier > pair.
• Domain name service -- specifies the naming structure of the hosts, as well as name to address resolution ( internet ).

Name

Resolution

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NETWORK STRUCTURES

FIXED ROUTING

• A path from A to B is specified in advance and does not change unless a hardware failure disables this path.
• 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.
• A partial remedy to adapting to load changes.
• Ensures that messages will be delivered in the order in which they were sent.

DYNAMIC ROUTING

• The path used to send a message from 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.

Routing

Strategies

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NETWORK STRUCTURES

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) 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.

Contention

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NETWORK STRUCTURES

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.

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 smaller packets, each of which is sent in a separate slot.

Contention

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NETWORK STRUCTURES

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 routing of packets in the communication network. Includes handling the address of outgoing packets, decoding the address of incoming packets, and maintaining routing information for proper response to changing load levels.

Transport layer Responsible for low-level network access and for message transfer between clients. Includes partitioning messages into packets, maintaining packet order, controlling flow, and generating physical addresses.

Design

Structure

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NETWORK STRUCTURES

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.

Design

Structure