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Architectures
Software Architecture
• Describe how the various software
components are to be organized and how they
should interact.
• It describe the organization and interaction of
software components.
System Architecture
• An architecture description is a formal
description and representation of a system.
• It describe the placement of software
components on physical machines.
Architectural Styles
Important styles of architecture for
distributed systems
• Layered architectures
• Object-based architectures
• Data-centered architectures
• Event-based architectures
Architectural Styles
• Layered architectures
• Components are organized in a layered fashion
where a component at layer Li is allowed to call
components at the underlying layer Li-1.
• Requests go down the hierarchy whereas the
results flow upward.
• Layered style is used for client-server system
Figure 2-1. The layered architectural style
• Object-based architectures
• Each component are connected through a remote
procedure call mechanism
• Similar to client server architecture.
• Form the most important styles for the large
software systems
• less structured
• component = object
• connector = RPC or RMI
Architectural Styles
Figure 2-1. The object-based architectural style.
• Data-centered architectures
• Communication processes through a common
repository
• All communication takes place through files
• Event-based architectures
• Communication through the propagation of
events
• Propagation is associated with publish/subscribe
systems.
• It means that processes publish events after which
the middleware ensures that only those processes
that subscribed to those events will receive them.
• The main advantage of event-base architecture is
that processes are loosely coupled.ie they were
referentially decoupled.
• Event-based arch. supports several
communication styles:
• Publish-subscribe
• Broadcast
• Point-to-point
Figure The event-based architectural
style.
• Shared data spaces
• Access and update of data store is the main
purpose of data-centered system.
• Processes communicate/exchange info primarily
by reading and modifying data in some shared
repository (e.g database, distributed file system)
• e.g. processes of many web-based distributed
systems communicate through the use of shared
Web-based data services.
– Event based architectures can be combined with
data-centered architectures .
– Processes are decoupled in time
– Use SQL like interface
Figure The shared data-space architectural style.
System Architectures
• Centralized architecture
• Decentralized architecture
• Hybrid architecture
Centralized Architectures
• Example is client server model
• In the basic client-server model, processes in a
distributed system are divided into two groups.
• SERVER AND CLIENT
• A server is a process implementing a specific
service .
• A client is a process that requests a service by
sending it a request and subsequently waiting for
the server’s reply. This type of client-server
interaction is called request-reply behavior.
Centralized Architectures
Figure General interaction between a client and
a server.
• Idempotent operation
– When an operation can be repeated multiple
times without harm, it is said to be idempotent.
• Examples
• Web search engine Interface: type in a
keyword string
Processing level: processes to generate DB
queries, rank replies, format response
Data level: database of web pages
Application Layering
Three levels are
– The user-interface level
– The processing level
– The data level
• User interface level contains all that is necessary
to directly interface with the user, such as
display management.
• Processing level contains the applications.
• The data level manages the actual data that is
being acted on. An important property of this
level is that data is persistent.
• For example, Internet search engine
Figure: The simplified organization of an Internet
search engine into three different layers.
Multitier Architectures
The simplest organization is to have only two types
of machines:
• A client machine containing only the programs
implementing (part of) the user-interface level
• A server machine containing the rest, ie.the
programs implementing the processing and data level.
• Following figure shows a two tier architecture.
Figure Alternative client-server organizations (a)–(e).
• Fat clients and thin clients
• Having more functionality on the client
machine makes client-side software more
prone to errors.
• Following figure shows the three tired
architecture.
Figure . An example of a server acting as client.
Decentralized Architectures
• Vertical distribution refers to the distribution
of the different layers in a multitier
architectures across multiple machines.
• In principle, each layer is implemented on a
different machine.
• Traditional client-server architectures exhibit vertical
distribution. Each level serves a different purpose in
the system.
– Logically different components reside on different nodes
• Vertical distribution
– In many business environments, distributed processing
is equivalent to organizing a client server application as
a multitier architecture. this type of distribution is
known as vertical distribution.
– The characteristic feature of vertical distribution is that
it is achieved by placing logically different components
on different machines.
– This is related to vertical fragmentation in distributed
relational databases, it means that tables are split
column wise and subsequently distributed across
multiple machines.
• Horizontal distribution
– In this type of distribution, a client or server may
be physically split up into logically equivalent
parts, but each part is operating on its own share
of the complete data set, thus balancing the load.
– The system architecture that supports horizontal
distribution is known as peer- to- peer
systems(P2P systems). Here nodes act as both
client and server and the interaction is symmetric
– Horizontal distribution deals with the distribution
of a single layer across multiple machines, such as
distributing a single database.
• It has
• Better load balancing,
• more resistant to denial-of-service attacks,
• harder to manage than C/S
An example of horizontal distribution of a Web service
The replication serves for scalability and enhanced performance
• Servent
– servent is a peer-to-peer network node, which
has the functionalities of both a server and
a client.
– Each process will act as a client and a server at the
same time.
Overlay network
• A network in which the nodes are formed by the
processes and the links represent the possible
communication channels.
• Overlay networks connect nodes in the P2P
system
– Define the structure between nodes in the system
– Allow nodes to route requests to locations that
may not be known at time of request.
• Are virtual networks, built on top of one or
more other networks.
• A link between two nodes in the overlay may
consist of several physical links.
Circles represent nodes in the
network. Blue nodes are also part of
the overlay network. Dotted lines
represent virtual links. Actual
routing is based on TCP/IP protocols
Overlay Network Example
Overlay Networks
• Each node in a P2P system knows how to
contact several other nodes.
• The overlay network may be two types.
– structured P2P
• nodes and content are connected according to some
design that simplifies later lookups
– unstructured P2P
• content is assigned to nodes without regard to the
network topology.
Structured Peer-to-Peer Architectures
• In a structured peer-to-peer architecture, the
overlap network is constructed using a deterministic
procedure
• Organize the process through a distributed hash
table (DHT).
• In a DHT based system, data items are assigned a
random key from a large identifier space, such as a
128 bit or 160- bit identifier.
Structured P2P Architectures
• A common approach is to use a distributed
hash table (DHT) to organize the nodes
• Traditional hash functions convert a key to a
hash value, which can be used as an index into
a hash table.
– Keys are unique – each represents an object to
store in the table; e.g., at UAH, your A-number
– The hash function value is used to insert an object
in the hash table and to retrieve it.
Structured P2P Architectures
• In a DHT, data objects and nodes are each
assigned a key which hashes to a random
number from a very large identifier space (to
ensure uniqueness)
• A mapping function assigns objects to nodes,
based on the hash function value.
• A lookup, also based on hash function value,
returns the network address of the node that
stores the requested object.
Characteristics of DHT
• Scalable – to thousands, even millions of
network nodes
– Search time increases more slowly than size;
usually Ο(log(N))
• Fault tolerant – able to re-organize itself when
nodes fail
• Decentralized – no central coordinator
(example of decentralized algorithms)
Structured Peer-to-Peer Architectures
• Accomplished by routing a request for a data item
to the responsible node
• The Chord system
• Ring
• A data item with key k is mapped to the node
with the smallest identifier id >=k
• The successor of the key k and denoted as succ(k)
• LOOKUP(k) – return the network address of
succ(k)
Chord Routing Algorithm
Structured P2P
• Nodes are logically arranged in a circle
• Nodes and data items have m-bit identifiers
(keys) from a 2m namespace.
– e.g., a node’s key could be a hash of its IP address
and a file’s key might be the hash of its name or of
its content.
– Can’t do key-word searches with this approach.
Structured Peer-to-Peer Architectures
Figure 2-7. The
mapping of data
items onto nodes
in Chord.
Structured Peer-to-Peer Architectures
• Content addressable network (CAN)
• When a node p wants to join a CAN
• Pick up an arbitrary point from the coordinate
space
• Look up the node Q in whose region that point
falls
• Node Q then splits its region into two halves
Structured Peer-to-Peer Architectures
Figure :The mapping of
data items onto
nodes in CAN.
Structured Peer-to-Peer Architectures
Figure Splitting a
region when a
node joins.
Unstructured Peer-to-Peer Architectures
• Rely on randomized algorithms for
constructing an overlay network
• The list of neighbors is also referred to as a
partial view
• Assuming that entries need to be pushed to
the selected peer
• If the node is in pull mode, it will waits for a
response from the selected peer
Unstructured P2P
• Unstructured P2P organizes the overlay
network as a random graph.
• Each node knows about a subset of nodes, its
“neighbors”.
– Neighbors are chosen in different ways: physically
close nodes, nodes that joined at about the same
time, etc. -
• Data items are randomly mapped to some node
in the system & lookup is random, unlike the
structured lookup in Chord.
Comparison
• Structured networks typically guarantee that if an
object is in the network it will be located in a
bounded amount of time – usually O(log(N))
• Unstructured networks offer no guarantees.
– For example, some will only forward search requests a
specific number of hops
– Random graph approach means there may be loops
– Graph may become disconnected
Superpeers
• In a collaborative content delivery
network(CDN) nodes may offer storage for
hosting copies of web pages allowing web
clients to access pages nearby, and thus to
access them quickly.
• The use of broker that collects resource usage
for a number of nodes that are in each other’s
proximity will allow to quickly select a node
with sufficient resources.
• Nodes such as those maintaining an index or
acting as a broker are generally referred to as
superpeers.
• They are organized in a peer-to-peer network,
leading into a hierarchical organization.
Figure . A hierarchical organization of nodes into a
superpeer network.
• In this hierarchical organization, every regular
peer is connected as a client to a superpeer.
• All communication from and to a regular peer
proceeds through that peer’s associated
superpeer.
• The client – superpeer relation is fixed.
Whenever a regular peer joins the network, it
attaches to one of the superpeers and remains
attached until it leaves the network.
Hybrid Architectures
• Combine client-server and P2P architectures
– Edge-server systems; e.g. ISPs, which act as servers
to their clients, but cooperate with other edge
servers to host shared content.
– Collaborative distributed systems; e.g., BitTorrent,
which supports parallel downloading and uploading
of chunks of a file. First, interact with C/S system,
then operate in decentralized manner.
Edge – Server Systems
• These systems are deployed in the Internet
where servers are placed “at the edge” of the
network.
• Eg: Internet Service Provider(ISP)
Edge-Server Systems
Figure . Viewing the Internet as consisting of a
collection of edge servers.
Collaborative Distributed Systems
Components of Globule collaborative content
distribution network:
• A component that can redirect client requests to
other servers.
• A component for analyzing access patterns.
• A component for managing the replication of Web
pages.
Collaborative Distributed Systems
BitTorrent
• Clients contact a global directory (Web server)
to locate a .torrent file with the information
needed to locate a tracker; a server that can
supply a list of active nodes that have chunks
of the desired file.
• Using information from the tracker, clients can
download the file in chunks from multiple
sites in the network. Clients must also provide
file chunks to other users.
Collaborative Distributed Systems
Figure . The principal working of BitTorrent
ARCHITECTURES VERSUS MIDDLEWARE
• middleware forms a layer between applications and
distributed platforms.
• to a certain extent hiding the distribution of-data,
processing,and control from applications.
• many middleware solutions have adopted
an object-based architectural style, such as CORBA
• Others,like TIB/Rendezvous (TIBCO, 2005) provide
middleware that follows the event-based
architectural style
• An approach that is generally considered better is to
make middleware systems such that they are easy to
configure, adapt, and customize as needed by
anapplication.
• commonly followed approaches.
• Interceptors -an interceptor is nothing but a software
construct that willbreak the usual flow of control and
allow other (application specific) code to be
executed.
interception as supported in many object based
distributed systems.
• The basic idea is simple: an object A can call a method that belongs
to an object B, while the latter resides on a different machine than
A.
• a remote-object invocation is carried as a three-step approach:
• 1. Object A is offered a local interface that is exactly the same as the
interface offered by object B. A simply calls the method available in'
• that interface.
• 2. The call by A is transformed into a generic object invocation,
made possible through a general object-invocation interface offered
by themiddleware at the machine where A resides.
• 3. Finally, the generic object invocation is transformed into a
message that is sent through the transport-level network interface
as offered by A's local operating system.
General Approaches to Adaptive
Software
• What interceptors actually offer is a means to adapt
the middleware.
• The need for adaptation comes from the fact that the
environment in which distributed applications are
executed changes continuously.
• Changes include those resulting from mobility, a strong
variance in the quality-of-service of networks, failing
hardware, and battery drainage, amongst others.
• Rather than making applications responsible for
reacting to changes, this task is placed in the
middleware.
• These strong influences from the environment
have brought many designers of middleware
to consider the construction of adaptive
software.
• three basic techniques to come to software
adaptation:
1. Separation of concerns
2. Computational reflection
3. Component-based design
Separating concerns - way of modularizing systems
• separate the parts that implement functionality from
those that take care of other things (known as extra
functionalities) such as reliability, performance,
security,etc.
• main problem is that we cannot easily separate these
extra functionalities by means of modularization.
• For example, simply putting security into a
separatemodule is not going to work
• it is hard to imagine how fault tolerance can be isolated
into a separate box and sold as an independent service
Computational reflection
• ability of a program to inspect itself and, if
necessary, adapt its behavior.
• Reflection has been built into programming
languages, including Java, and offers a
powerful facility for runtime modifications.
• In addition, some middleware systems
provide the means to apply reflective
techniques.
component-based design
• adaptation through composition
• A system may either be configured statically at
design time, or dynamically at runtime.
• The latter requires support for late binding, a
technique that has been successfully applied
in programming language environments
SELF-MANAGEMENT IN DISTRIBUTED SYSTEMS
• distributed systems should be adaptive, but
notably when it comes to adapting their
execution behavior and not the software
components they comprise.
• weneed to organize the components of a
distributed system such that monitoring and
adjustments can be done
• while on the other hand we need to decide
where the processes are to be executed that
handle the adaptation.
self-managing systems
• autonomic computing or self..star systems or
self-managing systems- organizing distributed
systems as high-level feedback-control
systems allowing automatic adaptations to
changes.
• the variety by which automatic adaptations
are being captured: self-managing, selfhealing, self-configuring, self-optimizing, and
so on.
The Feedback Control Model
• Assumption that adaptations take place by
means of one or more feedback control loops.
• Accordingly, systems that are organized by
means of such loops are referred to as
feedback control systems.
• Feedback control has since long been applied
in various engineering fields, and its
mathematical foundations are gradually also
finding their way in computing systems
• The core of a feedback control system is
formed by the components that need to be
managed.
• These components are assumed to be driven
through controllable input parameters, but
their behavior may be influenced by all kinds
of uncontrollable input, also known as
disturbance or noise input.
Three elements that form the feedback control
loop.
• First,the system itself needs to be monitored, which
requires that various aspects of the system need to be
measured.
• Another part of the feedback control loop analyzes the
measurements and compares these to reference
values. This feedback analysis component forms the
heart of the control loop, as it will contain the
algorithms that decide on possible adaptations.
• The last group of components consist of various
mechanisms to directly influence the behavior of the
system.(placing replicas, changing scheduling priorities
etc)