Architectural styles of distributed systems and application-level peer-to-peer (P2P) topologies such as unstructure and structured.

1. Topologies of
Distributed Systems
CS4262 Distributed Systems
Dilum Bandara
Dilum.Bandara@uom.lk

2. Outline
 Architectural styles
 Layered architectures
 Object-based architectures
 Data-centered architectures
 Event-based architectures
 System architectures
 Client-server
 Peer-to-peer
 Unstructured
 Structured
2

3. Architecture
 Dictionary definitions
 Manner of construction of something & disposition of
its parts
 Design, the way components fits together
 Defines
 What are the components of the system?
 How are they connected to each other?
 How do they communicate?
3

4. Architectural Styles
 Layered architectures
 Object-based architectures
 Data-centered architectures
 Event-based architectures
 Hybrid architectures combine multiples of these
architecture styles
 Some real-world systems are like this
 e.g., P2P file transfer, networks of sensors
4

5. Layered Architectures
 Well defined layers
 Control typically flows
from layer-to-layer
 Better results through
cross-layer coordination
 Requests go down
while results go up
 e.g., OSI model, some
P2P systems
5
Application – Tier 2
File sharing, streaming, VoIP, P2P clouds
Application – Tier 1
Indexing/DHT, Caching, replication, access
control, reputation, trust
Overlay
Unstructured, structured, & hybrid
Gnutella, Chord, Kademlia, CAN
Underlay
Internet, Ethernet, Wi-Fi, Bluetooth
Request
Response

6. Object-Based Architectures
 Looser organization of objects
 Communication through Remote Procedure
Calls (RPC)
 e.g., Java RMI, Web services, REST
6
Source: http://computersciencesource.wordpress.com/2010/02/11/distributed-computing-architectures/

7. Data-Centered Architectures
 Components
communicate through a
common repository
 Can be passive or active
 e.g., distributed file
systems, producer-
consumer, web-based
data services
7
Source:
http://computersciencesource.wordpress.com/20
10/02/11/distributed-computing-architectures/

8. Event-Based Architectures
 Propagation of events
 Occasionally carry data
 Components are loosely coupled
 e.g., publisher/subscriber, ESB, akka.io
8
Source:
http://computersciencesource.wordpress.com/20
10/02/11/distributed-computing-architectures/

9. Enterprise Service Bus (ESB)
9
Source: www.fiorano.com/products/ESB-enterprise-service-bus/Fiorano-ESB-enterprise-service-bus.php

10. System-Level Architectures
 Client-server
 Peer-to-peer
 Hybrid architectures
 Some real-world systems are like this
 e.g., P2P file transfer, Google File System, Amazon
Dynamo
10

11. Client-Server
 Clients request services from a server
 Request-reply communication
 Multiple servers for resilience & load balancing
 Pros
 Easier to build & maintain
 Cons
 Less scalable
 Single point of failure
 e.g., web, NFS, MapReduce
11
Source:
www.cbsolution.net/techniques/ontarget
/mapreduce_vs_data_warehouse

12. Multi-Tiered Architecture
12
Source: http://en.kioskea.net/contents/151-networking-3-tier-client-server-architecture
 Increased reliability
 Increased scalability

13. Modern Web Applications
13
Source: www.css-cloud.com/solutions/web-application-hosting-in-the-cloud.php

14. Peer-to-Peer
 Distributed systems without any central control
 Autonomous peers
 Equivalent in functionality/privileges
 Both a client & a server
 Protocol features
 Network overlaid on top of Internet
 Protocol constructed at application layer
 Supports some type of message routing capability
 Typically peers have unique IDs
 Fairness & performance
 Self-scaling
 Peer churn
14
Internet

15. P2P Characteristics
 Tremendous scalability
 Millions of peers
 Globally distributed
 Many concurrent connections
 Bandwidth intensive
 Aggressive/unfair bandwidth utilization
 Heterogeneous
 Superpeers
 Critical for performance/functionality
15
Internet

16. P2P Overlay
 Peers directly talk to each other
 If they aren’t directly connected, uses overlay routing via other
peers
 Peers are autonomous
 Determines its own capabilities based on its resources
 Decides on its own when to join, leave
 Overlay is scalable & resilient
16
Internet

17. Terminology
 Application
 Tier 2 – Services provided to end
users
 Tier 1 – Middleware services
 Overlay
 How peers are connected
 Application layer network
 e.g., dial-up on top of telephone
network, BGP, PlanetLab, CDNs
 Underlay
 Internet, Bluetooth
 Peers implement top 3 layers
 This layering is an over
simplification 17
Application – Tier 2
File sharing, streaming, VoIP, P2P clouds
Application – Tier 1
Indexing/DHT, Caching, replication, access
control, reputation, trust
Overlay
Unstructured, structured, & hybrid
Gnutella, Chord, Kademlia, CAN
Underlay
Internet, Ethernet, Wi-Fi, Bluetooth

18. Overlay Connectivity
18
P2P Overlay
Unstructured
Deterministic
Napster
BitTorrent
JXTA
Nondeterministic
Gnutella
KaZaA
Structured
Sub-linear state
Chord
Kademlia
CAN
Pastry
Tapestry
Dynamo
Constant state
Viceroy
Cycloid
Hybrid
Structella
Kelip
Local minima
search

19. Bootstrapping
 How is an initial overlay is formed from a set of
nodes?
 Use some known information
 Use a well-known server to register initial set of peers
 Well-known domain name
 Dynamic DNS
 Some peer addresses are well known
 Use a local broadcast to collect nearby peers, &
merge such sets to form larger sets
19

20. How to Bootstrap
 Each peer maintains a random subset of peers
 Peers in Skype maintain a cache of superpeers
 In BitTorrent peers talk to trackers
 An incoming peer talks to 1+ known peers
 A known peer accepting an incoming peer
 Keeps track of incoming peer
 May redirect incoming peer to another peer
 Give a random set of peers to contact
 Discover more peers by random walk, gossiping,
or deterministic walk within overlay
20

21. Options for Indexing Resources
21
Centralized
O(1)
Fast lookup
Single point of failure
Unstructured
O(hopsmax)
Easy network maintenance
Not guaranteed to find resources
Distribute Hash Table (DHT)
O(log N)
Guaranteed performance
Not for dynamic systems
Superpeer
O(hopsmax)
Better scalability
Not guaranteed to find resources

22. Centralized
 Centralized database for
lookup
 Guaranteed discovery
 Low overhead
 Single point of failure
 Easy to track
 Legal issues
 e.g., Napster
 File transfer directly
between peers
22

23. Unstructured
 Fully distributed
 Random connections
 Initial entry point is
known
 Peers maintain dynamic
list of neighbors
 Connections to multiple
peers
 Highly resilient to node
failures
 e.g., Gnutella
23

24. Unstructured P2P (Cont.)
 Flooding-based search
 Guaranteed discovery
 Implosion  High overhead
 Expanding-ring flooding
 TTL-based random walk
 Discovery isn’t guaranteed
 Better performance by biasing
random walk toward nodes with
higher degree
 If response follow same path
 Anonymity
 e.g., KaZaA, BearShare,
LimeWire, McAfee
24
D
S
D
s
Flooding
Random walk

25. Superpeers
 Resource rich peers 
Superpeers
 Bandwidth, reliability, trust,
memory, CPU, etc.
 Flooding or random walk
 Only superpeers are
involved
 Lower overhead
 More scalable
 Discovery isn’t guaranteed
 Better performance when
superpeers share list of
resources/services
 e.g., Gnutella v0.6, FastTrack,
Freenet KaZaA, Skype
25
s D

26. Example – BitTorrent
 Most popular P2P file sharing
system to date
 Features
 Centralized search
 Multiple downloads
 Enforce fairness
 Rarest-first dissemination
 Incentives
 Better contribution  Better
download speeds (not always)
 Enable content delivery
networks
 Revenue through ads on search
engines 26
User
Trackers
Web-based
search engine
Content
owner
Keyword search
.torrent file
server
Download
.torrent file
Get list of
peers
Download/
upload
chunks

27. BitTorrent Protocol
 Content owner creates a
.torrent file
 File name, length, hash,
list of trackers
 Place .torrent file on a
server
 Publish URL of .torrent
file to a web site
 Torrent search engine
 .torrent file points to a
tracker(s)
 Registry of leaches &
seeds for a given file
27
User
Trackers
Web-based
search engine
Content
owner
Keyword search
.torrent file
server
Download
.torrent file
Get list of
peers
Download/
upload
chunks
1
2
3
4
1
2
3
4

28. BitTorrent Protocol (cont.)
 Tracker
 Provide a random subset
of peers sharing same file
 Peer contacts subset of
peers parallely
 Files are shared based
on chunk IDs
 Chunk – segment of file
 Periodically ask tracker
for a new set of IPs
 E.g., every 15 min
 Pick peers with highest
upload rate 28
User
Trackers
Web-based
search engine
Content
owner
Keyword search
.torrent file
server
Download
.torrent file
Get list of
peers
Download/
upload
chunks
1
2
3
4
1
2
3
4

29. Summary – Unstructured P2P
 Separate resource/service discovery & delivery
 Resource/service discovery is mostly outside of P2P overlay
 Centralized solutions
 Not scalable
 Affect resource/service delivery when failed
 Distributed solutions
 High overhead
 May not locate the resource/service
 No predictable performance
 Delay or message bounds
 Lack of QoS or QoE
29

30. Terminology
 Hash function
 Converts a large amount of data into
a small datum
 Hash table
 Data structure that uses hashing to
index content
 Distributed Hash Table (DHT)
 A hash table that is distributed
 Types of hashing
 Consistent or random
 Locality preserving
30
f()
f()
f() g()
g()
g()

31. Structured P2P
 Deterministic approach to locate resources, services, &
peers
 Resources/services expressed as a (key, value) pair
 Unique key
 Hash of file name, metadata, or actual content
 128-bit or higher
 Peers also have a key
 Random bit string or IP address
 Index keys on a Distributed Hash Table (DHT)
 Distributed address space [0, 2m – 1]
 Locate peer(s) responsible for a given key
 Deterministic overlay to publish & locate content
 Bounded performance under standard conditions, typically O(log n)
31

32. Structured P2P – Example
 2 operations
 store(key, value)
 locate(key)
32
Ring – 16 addresses
Song.mp3
Cars.mpeg
f()
f()
Find Cars.mpeg
n + 2i – 1, 1  i  m
Successor
11 Song.mp3
6 Cars.mpeg
O(log N) hops

33. Chord
 Key space arranged as a ring
 Peers responsible for segment of
the ring
 Called successor of a key
 1st peer in clockwise direction
 Routing table
 Keep a pointer (finger) to m peers
 Keep a finger to (2i – 1)-th peer, 1 ≤ i ≤ m
 Key resolution
 Go to peer with the closest key
 Recursively continue until key is find
 Can be located within O(log n) hops
33
m =3-bit key ring
Stoica et al., "Chord: A scalable peer-to-peer lookup service for internet
applications," ACM SIGCOMM Computer Communication Review, 31(4), 149-160, 2001.

34. Chord (Cont.)
 New peer entering overlay
 Takes keys from the successor
 Peer leaving overlay
 Give keys to the successor
 Fingers are updated as peers join & leave
 Peer failure or churn makes finger table entries stale 34
New peer with key 6 joins the overlay Peer with key 1 leave the overlay
Stoica et al., "Chord: A scalable peer-to-peer
lookup service for internet applications," ACM
SIGCOMM Computer Communication Review,
31(4), 149-160, 2001.

35. Chord Performance
 Path length
 Worst case O(log N)
 Average ½log2N
 Updates O(log2 N)
 Fingers O(log N)
 Alternative paths (log N)!
 Balanced distribution of
keys
 Under uniform distribution
 N(log N) virtual nodes
provides best load
distribution
35
Stoica et al., "Chord: A scalable peer-to-peer lookup service
for internet applications," ACM SIGCOMM Computer
Communication Review, 31(4), 149-160, 2001.

36. Structured P2P – Other Solutions
 Kademlia
 Used in BitTorrent, eMule, aMule, & AZUREUS
 Distance between 2 keys is determined by XOR
 Routing in the ring is bidirectional
 dist(a  b) = dist(b  a)
 Enable nodes to learn about new nodes from received messages
 Content-Addressable Network (CAN)
 Based on a d-Torus
 Pastry
 Based on a Hypercube
 Cycloid
 Based on a cube connected cycle 36

37. Other Well-Known Solutions
37

38. Summary – Structured P2P
 Resource/service discovery is within P2P overlay
 Deterministic performance
 Chord
 Unidirectional routing
 Recursive routing
 Peer churn & failure is an issue
 Issues
 MySong.mp3 is not same as mysong.mp3
 High churn
 Unbalanced distribution of keys & load
38

39. Structured vs. Unstructured
39
Unstructured P2P Structured P2P
Overlay
construction
High flexibility Low flexibility
Resources Indexed locally Indexed remotely on a distributed
hash table
Query messages Broadcast or random walk Unicast
Content location Best effort Guaranteed
Performance Unpredictable Predictable bounds
Overhead High Relatively low
Object types Mutable, with many complex
attributes
Immutable, with few simple
attributes
Peer churn &
failure
Supports high failure rates Supports moderate failure rates
Applicable
environments
Small-scale or highly dynamic, e.g.,
mobile P2P
Large-scale & relatively stable,
e.g., desktop file sharing
Examples Gnutella, LimeWire, KaZaA,
BitTorrent
Chord, CAN, Pastry, eMule,
BitTorrent

40. Example – Amazon Dynamo
 Highly-available key-value system
 Many large datasets/objects that only require primary
key access
 Shopping carts, best seller lists, customer preferences,
product catalogs, etc.
 Relational databases aren’t required, too slow, or
bulky
 Fast reads, high availability for writes
 Always failing servers, disks, switches
40

41. Amazon Dynamo (Cont.)
 Objects are replicated in successors
 All peers know about each other using gossiping
 Can read/write to any replica
 Mechanisms to deal with different versions of objects
41

42. Amazon Dynamo (Cont.)
42
G. DeCandia et al., "Dynamo: amazon's highly
available key-value store," In ACM SIGOPS
operating systems review, Vol. 41, No. 6, Oct. 2007.