Distributed systems are collections of independent computers that appear as a single computer to users. They have multiple computers, interconnections between computers, and shared state. Examples include data centers, cloud computing systems, and sensor networks. Distributed systems face challenges related to asynchrony, failures, heterogeneity, scale, and dynamic changes in the system. Key topics in distributed systems include classical problems like consensus and mutual exclusion, as well as modern techniques for scaling, fault tolerance, security, and transparency.

1. 1
DISTRIBUTED SYSTEMS
Introduction

2. 2
Definition of a Distributed System (1)
• A distributed system is a collection of independent
computers that appear to the users of the system
as a single computer.
[Andrew Tanenbaum]
• A distributed system is several computers doing
something together. Thus, a distributed system
has three primary characteristics: multiple
computers, interconnections, and shared state.
[Michael Schroeder]

3. Distributed System Example – A Data
center
3

4. 4
Definition of a Distributed System (2)
A distributed system organized as middleware. The middleware
layer extends over multiple machines, and offers each
application the same interface.

5. Why study distributed systems?
5
Easy to get things wrong !!

6. Why study distributed systems?
6
Easy to get things wrong !!
How can one be sure that the system does
what it is supposed to?
Under what conditions?

7. Why study distributed systems?
7
Examples:
-Scheduling a meeting over a lossy network
-Synchronizing clocks.
-Social networking.

8. The Internet – Quick Refresher
8
-Underlies many distributed systems.
-A vast interconnected collection of computer networks of
many types.
-Intranets – subnetworks operated by companies and
organizations.
-Intranets contain subnets and LANs.
-WAN – wide area networks, consists of LANs
-ISPs – companies that provide modem links and other types of
connections to users.
-Intranets (actually the ISPs’ core routers) are linked by
backbones – network links of large bandwidth, such as satellite
connections, fiber optic cables, and other high-bandwidth
circuits.

9. Distributed Systems are layered over
networks
Application
e-mail
remote terminal access
Web
file transfer
streaming multimedia
remote file server
Internet telephony
Application
layer protocol
smtp [RFC 821]
telnet [RFC 854]
http [RFC 2068]
ftp [RFC 959]
proprietary
(e.g. RealNetworks)
NFS
proprietary
(e.g., Skype)
Underlying
transport protocol
TCP
TCP
TCP
TCP
TCP or UDP
TCP or UDP
typically UDP
TCP=Transmission Control Protocol
UDP=User Datagram Protocol
Implemented via network
“sockets”. Basic primitive that
allows machines to send
messages to each other
Distributed System Protocols!
Networking Protocols

10. World Wide Web: the HTTP Standard
HTTP: hypertext transfer
protocol
• WWW’s application layer
protocol
• client/server model
– client: browser that requests,
receives, and “displays” WWW
objects
– server: WWW server, which is
storing the website, sends objects
in response to requests
• http1.0: RFC 1945
• http1.1: RFC 2068
– Leverages same connection to
download images, scripts, etc.
PC running
Explorer
Server
Running
Apache
Web
server
Mac running
Safari
http request
http request
http response
http response

11. The HTTP Protocol: More
http: TCP transport service:
• client initiates a TCP
connection (creates socket) to
server, port 80
• server accepts the TCP
connection from client
• http messages (application-
layer protocol messages)
exchanged between browser
(http client) and WWW server
(http server)
• TCP connection closed
http is “stateless”
• server maintains no
information about past
client requests
Protocols that maintain session
“state” are complex!
• past history (state) must be
maintained and updated.
• if server/client crashes, their
views of “state” may be
inconsistent, and hence must
be reconciled.
Why?

12. HTTP Example
Suppose user enters URL www.ce.ahu.edu.jo/
1a. http client initiates a TCP
connection to http server
(process) at www.ce.ahu.edu.jo.
Port 80 is default for http server.
2. http client sends a http request
message (containing URL) into
TCP connection socket
1b. http server at host
www.ce.ahu.edu.jo waiting for a
TCP connection at port 80.
“accepts” connection, notifying
client
3. http server receives request
messages, forms a response
message containing requested
object (index.html), sends
message into socket
time
(contains text,
references to 10
jpeg images)

13. HTTP Example
For fetching referenced objects, have 2 options:
• non-persistent connection: only one object fetched per TCP connection
– some browsers create multiple TCP connections simultaneously - one per object
• persistent connection: multiple objects transferred within one TCP
connection
5. http client receives a response
message containing html file,
displays html, finds 10
referenced jpeg objects
6. Steps 1-5 are then repeated for
each of 10 jpeg objects
4. http server closes the TCP
connection (if necessary).
time

14. A human as a browser (Client Side)
1. Telnet to your favorite WWW server:
Opens TCP connection to port 80
(default http server port) at www.google.com
Anything typed in sent
to port 80 at www.google.com
telnet www.google.com 80
2. Type in a GET http request:
GET /index.html
Or
GET /index.html HTTP/1.0
By typing this in (may need to hit
return twice), you send
this minimal (but complete)
GET request to http server
3. Look at response message sent by http server!

15. Distributed Systems Issues
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Asynchrony versus synchrony
Failures
Heterogeneity
- Processing speed
- Communication link bandwidth
- Communication latency
Scale
Dynamic system

16. Topics in Distributed Systems
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Distributed system examples :
Cloud Computing, Peer-to-peer systems, key-value
stores/SQL, …
Classical Problems:
Synchronization, Failure detection, Snapshots, Multicast,
Consensus, Mutual Exclusion, Election, …
Concurrency:
Concurrency Control, Replication Control, …
Security:
Faults, …
…

17. 17
Transparency in a Distributed System
Different forms of transparency in a distributed
system (ISO, 1995).

18. 18
Scalability Problems
Examples of scalability limitations.

19. 19
Scalability Problems
Characteristics of 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.

20. 20
Scaling Techniques (1)
The difference between letting (a) a server or (b) a
client check forms as they are being filled.

21. 21
Scaling Techniques (2)
An example of dividing the DNS
name space into zones.

22. 22
Pitfalls when Developing
Distributed Systems
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.

23. 23
Cluster Computing Systems
An example of a cluster computing system.

24. 24
Grid Computing Systems
A layered architecture for grid computing systems.

25. 25
Transaction Processing Systems (1)
Example primitives for transactions.

26. 26
Transaction Processing Systems (2)
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.

27. 27
Transaction Processing Systems (3)

28. 28
Transaction Processing Systems (4)
Figure 1-10. The role of a TP monitor in distributed systems.

29. 29
Enterprise Application Integration
Middleware as a communication facilitator in enterprise
application integration.

30. 30
Distributed Pervasive Systems
Requirements for pervasive systems
Encourage ad hoc composition.
Recognize sharing as the default.

31. 31
Electronic Health Care Systems (1)
Questions to be addressed for health care systems:
Where and how should monitored data be stored?
How can we prevent loss of crucial data?
What infrastructure is needed to generate and
propagate alerts?
How can physicians provide online feedback?
How can extreme robustness of the monitoring
system be realized?
What are the security issues and how can the
proper policies be enforced?

32. 32
Electronic Health Care Systems (2)
Monitoring a person in a pervasive electronic health care system,
using (a) a local hub or
(b) a continuous wireless connection.

33. 33
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?

34. 34
Sensor Networks (2)
Organizing a sensor network database, while storing and
processing data (a) only at the operator’s site or …

35. 35
Sensor Networks (3)
Organizing a sensor network database, while storing
and processing data … or (b) only at the sensors.