Definition of a Distributed System (1) A distributed system is: A collection of independent...
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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)
A distributed system organized as middleware.Note that the middleware layer extends over multiple machines.
1.1?
users
Standard OS
Goals
• Access and share remote resources
• Interoperability
• Portability
• Flexibility
• Transparency
• Scalability
Transparency in a Distributed System
Different forms of transparency in a distributed system.
Transparency Description
AccessHide 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
RelocationHide that a resource may be moved to another location while in use
Replication Hide that a resource may be replicated
ConcurrencyHide that a resource may be shared by several competitive users
Failure Hide the failure and recovery of a resource
PersistenceHide whether a (software) resource is in memory or on disk
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 algorithmsDoing routing based on complete information
Scalability •Size•Geography•Administrative organizations
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 ConceptsOperating systems for distributed computers
An overview between • DOS (Distributed Operating Systems)• NOS (Network Operating Systems)• Middleware
System Description Main Goal
DOSTightly-coupled operating system for multi-processors and homogeneous multicomputers
Hide and manage hardware resources
NOSLoosely-coupled operating system for heterogeneous multicomputers (LAN and WAN)
Offer local services to remote clients
MiddlewareAdditional layer atop of NOS implementing general-purpose services
Provide distribution transparency
Uniprocessor Operating Systems
Separating applications from operating system code through a microkernel.
1.11
Multicomputer Operating Systems (1)
General structure of a multicomputer operating system
Communication Message passing
1.14
Multicomputer Operating Systems (2)
Alternatives for blocking and buffering in message passing
• Buffering: sender and receiver side
• 4 possible synchronization points
Reliable communication?
1.15
Multicomputer Operating Systems (3)
Relation between blocking, buffering, and reliable communications.
Synchronization point Send bufferReliable 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
Network Operating System (1)
General structure of a network (not distributed) operating system.Users see the structure
1-19
Network Operating System (2)Evolution : file server
Two clients and a file server in a network operating system.
File servers generally maintain hierarchical file system.
1-20
Network Operating System (3)
Different clients may mount the servers in different places.
Different clients may have different views of the file systems
1.21
With a basic network operating system we loose in transparency
•Explicit remote login
•Independent accounting
•Independent access permission
•Low protection
Positioning Middleware
General structure of a distributed system as middleware.
Middleware does not manage an individual node
1-22
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
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.
1.23
Interfaces definition has to be complete. Incompleteness lead to bad portability and bad interoperability
Comparison between Systems
ItemDistributed OS
Network OS
Middleware-based OSMultipro
c.Multicomp
.
Degree of transparency
Very High High Low High
Same OS on all nodes
Yes Yes No No
Number of copies of OS
1 N N N
Basis for communication
Shared memory
MessagesMessages/
FilesModel
specific
Resource management
Global, central
Global, distributed
Per node Per node
Scalability No Moderately Yes Varies
Openness Closed Closed Open Open…
In green open problems are emphasized
Clients and Servers
General interaction between a client and a server.
Request-reply behavior What kind of protocol for communication?
1.25
Client-server model is widely used to understand the complexity of distributed systems processes
Clients and Servers
• Connectionless protocol
– Efficient
– Not reliable
• Retransmission on “failure” can be dangerous
• Connection oriented protocol
– Lower performance
– reliable
The three Levelsfor client server model
The general organization of an Internet search engine into three different layers
(we can have this organization on two or more physically distributed machines)
1-28
How can we make a distinction between client and server ?
Multitiered Architectures (1)
Alternative client-server organizations (two-tiered architecture)
1-29
Multitiered Architectures (2)
An example of a server acting as a client( vertical distribution). Three-tiered architecture
1-30
Modern Architectures
An example of horizontal distribution of a Web service.
1-31
Often, clients and servers distribution counts more then different logical components
Request from client