Transcript Understanding Publish/Subscribe Communication Systems

Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
www.dis.uniroma1.it/~midlab
Distributed Systems
Corso di Laurea Specialistica in Ingegneria
Informatica
AA 2006/2007
Prof. Roberto Baldoni
Ing. Alessia Milani
Ing. Leonardo Querzoni
Ing. Silvia Bonomi
Introduction
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
A definition
• A distributed system is a set of spatially
separate entities, each of these with a certain
computational power that are able to
communicate and to coordinate among
themselves for reaching a common goal
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Primary Goal:
sharing data/resources
Problems
• Synchronization
• Coordination
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Coordination has to be implemented taking
into account the following conditions that
deviates from centralized systems:
1.
2.
3.
4.
Temporal and spatial concurrency
No global Clock
Failures
Unpredictable latencies
These limitations restrict, for example, the
set of coordination problems we can be
solve in a distributed setting
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Distributed Systems Examples
But also.....
Service Oriented Architectures
Overlay Networks
Grid
P2P
Pervasive Systems&Ubiquitous Computing
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Pervasive Systems
Internet everywhere
One-persons Many computers
Mobility
………..
How can I keep consistent my mailbox???
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Characteristics..and Challenges
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Heterogeneity
Openess
Security
Scalability
Fault Tolerance
Concurrency
Transparency
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Heterogeneity
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Networks
Hardware
Operating Systems
Programming Languages
Implementations from different Developers
Soluzioni
• Middleware (from RPC to Service oriented Architectures)
• Mobile code and Virtual Machine
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Openess
• Capability of a system to be extended and re-implemented
• Necessary condition, set of documents with software interfaces
• Interface Definition Language (it describes the syntax and the semantic of
a service/component, available functions/services, input parameters,
exceptions, etc)
• Problem: semantic description of a service.
• A specification of a service/componet is well-formed if it is :
– Complete. A specification is complete if everything related to the
implementation has been specified. If something has not been specified, the
designer needs to add implementation dependent details.
– Neutral. A specification is neutral is it does not offer any detail on a possible
implementation
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Openess (ii)
• Interoperability. The capability of two systems to cooperate by using
services/components specified by a common standard
• Portability. The capability if a service/component implemented on a
distributed system A to work on a system B without doing any
modification
• Flexibility. The capacity of a system to configure/ orchestrate
components developed by various programmers
• Add-on Features. The capacity of a distributed system of adding
components/services and be integrating in a running system
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Openess (iii)
• Other recent capabilities:
– Evolvability. The capacity of a system to evolve in time for example
leaving active two different version of the same service.
– Self-* (self organization, self management, self healing etc). The
capacity of a system to reconfigure, to manage itself without uman
intervention
• The number of independent software developers make very
complex the development of a distributed platform
• Examples:
– RFC for internet
– JBoss for J2EE platform
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Security
• Confidenziality (protection against the interception of data from
unauthorized users)
• Integrity (protection against data alteration)
• Availability (protection against the interference in the access to a
resource)
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Scalability
• A system is scalable if it remains running with adequate
performance even if the number of “users” grow up of orders of
magnitude
• Centralization is against scalability :
– Service (single service for all users)
– Data (single data structure for all processes)
Computers connected to the internet
Date
1979, Dec.
1989, July
1999, July
Web servers
Computers
188
0
130,000
56,218,000
0
5,560,866
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Scalability (ii)
• It becomes necessary using:
– Service Replication
• Coordination Problems
– Data Replication
• Consistency Problems
– Distributed Algorithms
• No node has the current state of the whole system
• Nodes base their decisions on data they own
• A failure of a node should not compromize the goal of the algorithm
• Geographical Scalability
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
What is a large-scale distributed system?
Internet-scale
Applications
Scalable Consistencybased Applications
Scalable QoS based Applications
Enterprise
Data Centers
Cooperative Information Systems
eGov
envelo
p
First Open Workshop Budapest 21-3-2007
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Failure Management
• Failure detection
– Example: Checksum detects a corrupted packet
• Failure masking
– Example: message retransmission
• Tolerating Failures
– Example: intrusion tolerant systems
• Failure Recovery
– Example: completing long running computation
• Redoundancy
– Example: DNS
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Concurrency
• Multiple access to shared resources
– If clients invokes concurrently read and write methods on a shared
variable
– Which value returns each read?
• Coordination
• Synchronization
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Transparency
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Access: allow to access remote and local resources with the same operations
Location: allows to access resources without knowing their physical location
Concurrency: allows a set of processes to run concurrently on shared resources
without interfering among themselves
Failures: allow to mask failures in order that users can complete remaining
requested operations
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Mobility: allows to move resources and users without influencing operation issued by
users
Performance: allow system reconfiguration changing the load
Performance of a solution based on a distributed system not always improve with
respect to a solution based on a centralized system.
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Layering hw and sw
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Interaction Models
client/server
peer-to-peer
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Interaction Models
• Interaction models impact on scalability, availability, cost, security
performances
• Eg. client/server with replicated services:
availability, scalability
performance: replication
imposes an extra work for
maintaining consistency
despite replica crashes
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Dipartimento di Informatica e Sistemistica
Web proxy server
Web
s erver
Client
Proxy
s erver
Client
Web
s erver
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Web applets
a) c lient reques t res ults in the downl oadi ng of applet c ode
Client
Applet c ode
Web
s erver
b) c lient interac ts with the appl et
Client
Applet
Web
s erver
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Thin clients and compute servers
Compute server
Network computer or PC
Thin
Client
network
Application
Process
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Spontaneous networking in a hotel
Music
service
gateway
Alarm
service
Internet
Hotel wireless
network
Discovery
service
Camera
TV/PC
Laptop
PDA
Guests
devices
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Real-time ordering of events
s end
X
receive
1
m1
2
Y
receive
4
s end
3
m2
receive
Physic al
ti me
receive
s end
Z
receive
receive
m3
A
t1
t2
m1
m2
receive receive receive
t3
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Processes and channels
proc es s p
proc es s q
send m
receive
Communic ation c hannel
Outgoing mess age buffer
Incoming mess age buffer
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Middleware: problems to face
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Heterogeneity: OS, clock speeds, data representation, memory, architecture HW
Local Asynchrony: load on a noad, diffeent OW, Interrupts
Lack of global knowledge: knowledge propagates through messages whose
messages whose propagation times will be much slower than time taken by
the execution of an internal event
Network Asyncrony: propagation times of message can be
unpredictable.
Failures of nodes or network partitions.
Lack of a global order of events
This limits the set of problems that can be solved through
deterministic algorithms on some distributed systems