Chapter 1
Introduction
DISTRIBUTED SYSTEMS (dDist)
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Plan
•  Definition
•  Characteristics of distributed systems
•  Main types of distributed systems
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
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.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
An Example
•  Electronic Healthcare Record
–  Medication, planning, integration
–  Critical system
–  A large, distributed system
•  System architecture
–
–
–
–
–
Clients
Servers
Legacy systems
Peripherals
…
A distributed system is a collection of independent
computers that appears to its users as a single
coherent system.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Definition of a Distributed System (2)
A distributed system is a collection of independent
computers that appears to its users as a single coherent
system.
•  Other examples of distributed systems?
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Definition of a Distributed System (3)
Figure 1-1. A distributed system organized as middleware. The
middleware layer extends over multiple machines, and offers
each application the same interface.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Definition of a Distributed System (4)
•  Leslie Lamport
–  “A distributed system is a system where I
can’t get my work done because a computer
has failed that I’ve never even heard of.”
•  Why are such failures central and specific
to distributed systems?
•  Can we not just build better systems?
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Motivation and Goals
•  Making resources available
–  Systems should allow for efficient and controlled
access to and sharing of distributed/remote resources
•  Hiding distribution
–  Systems should look as if only a single computer
system is running
•  Openness
–  Systems should offer services according to standard
rules that describe the syntax and semantics of those
services
•  Scalability
–  Systems should scale with respect to size,
geography, and administrative boundaries
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Transparency in a Distributed System
Figure 1-2. Different forms of transparency in a
distributed system (ISO, 1995).
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Scalability?
Figure 1-3. Examples of scalability limitations.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Scalability Problems
•  Using decentralized algorithms:
–  No machine has complete information about
the system state.
–  Machines make decisions based only on local
information.
–  Failure of one machine does not ruin the
algorithm.
–  There is no implicit assumption that a global
clock exists.
•  Why the last requirement?
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Scaling Techniques (1)
•
Synchronous communication
–  Ttotal > T1 + T2
•
Asynchronous communication
–  Possible that Ttotal <= T1 + T2
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Scaling Techniques (2)
Figure 1-4. The difference between letting (a) a server
or (b) a client check forms as they are being filled.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Scaling Techniques (3)
Figure 1-5. An example of dividing the DNS
name space into zones.
•  Can distribution and replication give problems?
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Pitfalls when Developing
Distributed Systems
•  Transparency cannot be a reality
•  False assumptions made by first time developer:
–  The network is reliable.
–  The network is secure.
–  The network is homogeneous.
–  The topology does not change.
–  Latency is zero.
–  Bandwidth is infinite.
–  Transport cost is zero.
–  There is one administrator.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Main Distributed System Types
•  Distributed computing systems
–  Focus on computation
•  Distributed information systems
–  Focus on interoperability
•  Distributed pervasive systems
–  Focus on mobile, embedded, communicating
systems
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Cluster Computing Systems
Figure 1-6. An example of a cluster computing system.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
NFIT Spam Cluster
•  University of Aarhus cluster
–  2 * 2 redundant front-ends (64-bit Opteron servers) at two sites
–  2 redundant back-ends at each site (64-bit Opteron servers)
–  It is possible to add new back-ends
•  Use
–  Spam filtering for postAU, NF, SAMF, HUM, ADM, HIH
–  25k-40k
users
–  Now around
500k spams
a day
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Grid Computing Systems
•  Figure 1-7. A layered architecture for grid
computing systems.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
NorduGrid
•
Grid research and development
collaboration
–  Denmark, Finland, Sweden,
Norway
–  Supporting researchers
–  Originally in particular for Large
Hadron Collider experiments
•
Advanced Resource Connector
(ARC) middleware
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Transaction Processing Systems
•  Figure 1-8. Example primitives for transactions.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Example
BEGIN_TRANSACTION
salary1 = doctor1.getSalary()
doctor1.setSalary(salary1 + bonus)
salary2 = doctor2.getSalary()
doctor2.setSalary(salary2 - bonus)
END_TRANSACTION
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Characteristic properties of
transactions
•  Atomic
–  To the outside world, the transaction happens
indivisibly.
•  Consistent
–  The transaction does not violate system invariants.
•  Isolated
–  Concurrent transactions do not interfere with each
other.
•  Durable
–  Once a transaction commits, the changes are
permanent.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Isolation/Serializability
•  T1 and T2 executed concurrently/interleaved,
effects will be as
–  T1 executed, then T2 executed, or
–  T2 executed, then T1 executed
•  Simple, but slow approach:
–  Run transactions serially, i.e., no interleaving...
•  The point here is that database server may
optimize based on interleaved scheduling of
transcation steps!
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Lost Update
•
T1
•
T2
–  salary1 = doctor1.getSalary()
–  salary1 = doctor1.getSalary()
–  doctor1.setSalary(salary1 +
bonus)
–  doctor1.setSalary(salary1 +
bonus)
–  salary2 = doctor2.getSalary()
–  doctor2.setSalary(salary2 - bonus)
–  salary3 = doctor3.getSalary()
–  doctor3.setSalary(salary3 - bonus)
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
A Serializable Transaction
Interleaving
•
T1
–  salary1 = doctor1.getSalary()
–  doctor1.setSalary(salary1 +
bonus)
–  salary2 = doctor2.getSalary()
–  doctor2.setSalary(salary2 - bonus)
•
T2
–  salary1 = doctor1.getSalary()
–  doctor1.setSalary(salary1 +
bonus)
–  salary3 = doctor3.getSalary()
–  doctor3.setSalary(salary3 - bonus)
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Inconsistent Retrievals
•  T1
•  T2
–  ward1.discharge(patient)
–  sum += ward1.getNoPatients()
–  sum += ward2.getNoPatients()
–  ward2.admit(patient)
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Transaction Processing Systems (3)
•  Figure 1-9. A nested transaction.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Transaction Processing Systems (4)
•  Figure 1-10. The role of a TP monitor in
distributed systems.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Enterprise Application Integration
•  Figure 1-11. Middleware as a communication facilitator
in enterprise application integration.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Distributed Pervasive Systems
!
!
!
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Distributed Pervasive Systems
•  Requirements for pervasive systems
–  Embrace contextual changes.
–  Encourage ad hoc composition.
–  Recognize sharing as the default.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Electronic Health Care Systems
•  Figure 1-12. Monitoring a person in a pervasive electronic
health care system, using
–  (a) a local hub or
–  (b) a continuous wireless connection.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Network Centric Warfare
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Sensor Networks (1)
•  Questions concerning sensor networks:
–  How do we (dynamically) set up an efficient
tree in a sensor network?
–  How does aggregation of results take place?
Can it be controlled?
–  What happens when network links fail?
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Sensor Networks (2)
•  Figure 1-13. Organizing a sensor network database, while
storing and processing data (a) only at the operator’s site or …
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Sensor Networks (3)
•  Figure 1-13. Organizing a sensor network database, while
storing and processing data … or (b) only at the sensors.
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5
Summary
A distributed system is a collection of independent
computers that appears to its users as a single
coherent system.
•  Goals and problems include
–
–
–
–
Access to resources
Transparency
Openness
Scalability
•  Types of systems
–  Distributed Computing Systems
–  Distributed Information Systems
–  Distributed Pervasive Systems
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5