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Definition of a Distributed System (1)
A distributed system is:
A collection of independent
computers that appears to its users
as a single coherent system.
Definition of a Distributed System (2)
users
?
1.1
Standard OS
A distributed system organized as middleware.
Note that the middleware layer extends over multiple machines.
Goals
•
•
•
•
•
•
Access and share remote resources
Interoperability
Portability
Flexibility
Transparency
Scalability
Transparency in a Distributed System
Transparency
Description
Access
Hide differences in data representation and how a
resource is accessed
Location
Hide where a resource is located
Migration
Hide that a resource may move to another location
Relocation
Hide that a resource may be moved to another
location while in use
Replication
Hide that a resource may be replicated
Concurrency
Hide that a resource may be shared by several
competitive users
Failure
Hide the failure and recovery of a resource
Persistence
Hide whether a (software) resource is in memory or
on disk
Different forms of transparency in a distributed system.
Scalability
•Size
•Geography
•Administrative organizations
Scalability problems in a distributed system
Concept
Example
Centralized services
A single server for all users
Centralized data
A single on-line telephone book
Centralized algorithms
Doing routing based on complete information
Decentralized algorithms characteristics
•None has complete information about the system state
•Machines take decisions on local info
•Failure of one machine doesn’t affect the algorithm
•There is no assumption about a global clock
Scalability
How to solve scalability problems?
a)
Hiding communication latencies
•
Asynchronous communications (but not only, not always)
b) Distribution
c)
Replication (with care for consistency)
Scaling Techniques (1)
1.4
Moving part of the computation to the client…(hiding comm. latency)
The difference between letting:
a)
a server or
b)
a client
check forms as they are being filled
Scaling Techniques (2)
1.5
Distribution… Replication…
DNS name space divided into zones, with possibility of slave server
Software Concepts
Operating systems for distributed computers
System
Description
Main Goal
DOS
Tightly-coupled operating system for multiprocessors and homogeneous
multicomputers
Hide and manage
hardware
resources
NOS
Loosely-coupled operating system for
heterogeneous multicomputers (LAN and
WAN)
Offer local
services to remote
clients
Middleware
Additional layer atop of NOS implementing
general-purpose services
Provide
distribution
transparency
An overview between
• DOS (Distributed Operating Systems)
• NOS (Network Operating Systems)
• Middleware
Uniprocessor Operating Systems
1.11
Separating applications from operating system code through a microkernel.
Multicomputer Operating Systems (1)
1.14
General structure of a multicomputer operating system
Communication
Message passing
Multicomputer Operating Systems (2)
1.15
Alternatives for blocking and buffering in message passing
• Buffering: sender and receiver side
• 4 possible synchronization points
Reliable communication?
Multicomputer Operating Systems (3)
Synchronization point
Send buffer
Reliable comm.
guaranteed?
Block sender until buffer not full (S1)
Yes
Not necessary
Block sender until message sent (S2)
No
Not necessary
Block sender until message received (S3)
No
Necessary
Block sender until message delivered (S4)
No
Necessary
Relation between blocking, buffering, and reliable communications.
Network Operating System (1)
1-19
General structure of a network (not distributed) operating system.
Users see the structure
Network Operating System (2)
Evolution : file server
1-20
Two clients and a file server in a network operating system.
File servers generally maintain hierarchical file system.
Network Operating System (3)
1.21
Different clients may mount the servers in different places.
Different clients may have different views of the file systems
With a basic network operating system we loose in
transparency
•Explicit remote login
•Independent accounting
•Independent access permission
•Low protection
Positioning Middleware
1-22
General structure of a distributed system as middleware.
Middleware does not manage an individual node
Middleware Models
•Plan 9 – file oriented paradigm (Unix like)
•Distributed file system – transparency supported only for
traditional files
•Models based on RPCs – hide network communications
•Distributed objects – each object can be located on a single
machine, each object interface hides internal details including
communications
•Distributed documents - WWW
Middleware Services
•Naming allows entities to be shared and looked up.
•Persistence for storage, i.e. databases or facilities to connect to
databases
•Distributed transactions allow multiple read and write
operations to occur atomically.
•Security
•Access transparency by communication facilities hiding
message passing through network
Middleware and Openness
Interfaces definition has to be complete. Incompleteness lead to
bad portability and bad interoperability
1.23
In an open middleware-based distributed system, the protocols
used by each middleware layer should be the same, as well as
the interfaces they offer to applications.
Comparison between Systems
Distributed OS
Item
Network
OS
Middlewarebased OS
Multiproc.
Multicomp.
Very High
High
Low
High
Yes
Yes
No
No
1
N
N
N
Basis for
communication
Shared
memory
Messages
Messages/Fi
les
Model specific
Resource management
Global,
central
Global,
distributed
Per node
Per node
Scalability
No
Moderately
Yes
Varies
Openness
Closed
Closed
Open
Open…
Degree of transparency
Same OS on all nodes
Number of copies of OS
In green open problems are emphasized
Clients and Servers
Client-server model is widely used to understand the complexity of distributed
systems processes
1.25
General interaction between a client and a server.
Request-reply behavior
What kind of protocol for communication?
Clients and Servers
• Connectionless protocol
– Efficient
– Not reliable
• Retransmission on “failure” can be dangerous
• Connection oriented protocol
– Lower performance
– reliable
The three Levels
for client server model
How can we make a distinction between client and server ?
1-28
The general organization of an Internet search engine into three different layers
(we can have this organization on two or more physically distributed machines)
Multitiered Architectures (1)
1-29
Alternative client-server organizations (two-tiered architecture)
Multitiered Architectures (2)
1-30
An example of a server acting as a client( vertical distribution).
Three-tiered architecture
Modern Architectures
Often, clients and servers distribution counts more then different logical components
Request
from
client
1-31
An example of horizontal distribution of a Web service.