1. Chapter 2 ARCHITECTURES

2. software architectures system architecture
There are different ways on how to view the organization of a distributed system, but an obvious one is to make a distinction between the logical organization of the collection of software components and on the other hand the actual physical realization
software architectures
system architecture

3. Software Architectures
The organization of distributed systems is mostly about the software components that constitute the system
These software architectures tell us how the various software components are to be organized and how they should interact
logical organization software architecture

4. Component
A modular unit with well-defined required and provided interfaces that is replaceable within its environment, provided we respect its interfaces

5. Connector
It is generally described as a mechanism that mediates communication, coordination, or cooperation among components
A connector can be formed by the facilities for (remote) procedure calls, message passing, or streaming data

6. System Architecture
The final instantiation of a software architecture is also referred to as a system architecture
centralized architectures
one server implements the software components
remote clients can access that server
Decentralized architectures
machines more or less play equal roles, as well as hybrid organizations

7. The Organization of the Middleware
A middleware layer separate applications from underlying platforms
Middleware is an important architectural decision
To separate applications from underlying platforms

8. Feedback Control
Adaptability in distributed systems can also be achieved by having the system monitor its own behavior and taking appropriate measures when needed.
This 'insight has led to a class of what are now referred to as autonomic systems
These distributed systems are frequently organized in the form of feedback control loops
which form an important architectural element during a system's design

9. Contents 2.1 ARCHITECTURAL STYLES 2.2 SYSTEM ARCHITECTURES
2.3 ARCHITECTURES VERSUS MIDDLEWARE
2.4 SELF-MANAGEMENT IN DISTRIBUTED SYSTEMS
2.5 SUMMARY

10. 2.1 ARCHITECTURAL STYLES
Designing or adopting an architecture is crucial for the successful development of large systems
Architectural style
It is formulated in terms of components, the way that components are connected to each other, the data exchanged between components. and finally how these elements are jointly configured into a system

11. Architectural Styles Layered architectures Object-based architectures
Using components and connectors, we can come to various configurations, which, in turn have been classified into architectural styles
Layered architectures
Object-based architectures
Data-centered architectures
Event-based architectures

12. Layered architectures
Components are organized in a layered fashion where a component at layer L is allowed to call components at the underlying layer Li-1
Control generally flows from layer to layer: requests go down the hierarchy whereas the results flow upward

14. Object-based Architectures
Each object corresponds a component, which are connected through a (remote) procedure call mechanism

15. Data-centered architectures
Evolve around the idea that processes communicate through a common (passive or active) repository

16. A wealth of networked applications have been developed that rely on a shared distributed file system
Web-based distributed systems, are largely data-centric: proce-sses communicate through the use of shared Web-based data services.

17. Event-based Architectures
Processes publish events, the middleware ensures that only those processes that subscribed to those events will receive them
The main advantage of event-based systems is that processes are loosely coupled ( decoupled ). In principle, they need not explicitly refer to each other

19. Data space
Event-based architectures can be combined with data-centered architectures, yielding what is also known as shared data spaces
Data space
processes are now also decoupled in time
data can be accessed using a description rather than an explicit reference (events)

20. Data space
Its aim is to reduce the effort required to set up a data integration system by relying on existing matching and mapping generation techniques, and to improve the system in "pay-as-you-go" fashion as it is used. Labor-intensive aspects of data integration are postponed until they are absolutely needed

21. Various architecture styles
What makes these software architectures important for distributed systems is that they all aim at achieving (at a reasonable level) distribution transparency
no single solution will meet the requirements for all possible distributed applications, researchers have abandoned the idea that a single distributed system can be used to cover 90% of all possible cases
So we must consider various architecture styles

22. 2.2 SYSTEM ARCHITECTURES
We have briefly discussed some common architectural styles, let us take a look at how many distributed systems are actually organized by considering where software components are placed
Deciding on software components, their interaction, and their placement leads to an instance of a software architecture, also called a system architecture

23. 2.2 SYSTEM ARCHITECTURES 2.2.1 Centralized Architectures
2.2.2 Decentralized Architectures
2.2.3 Hybrid Architectures

24. 2.2.1 Centralized Architectures
thinking in terms of clients that request services from servers helps us understand and manage the complexity of distributed systems and that is a good thing.
Application Layering
Multitiered Architectures

25. General interaction between a client and a server
In the basic client-server model, processes in a distributed system are divided into two (possibly overlapping) groups. A server is a process implementing a specific service. A client is a process that requests a service from a server by sending it a request and subsequently waiting for the server's reply

26. Protocol between Client and Server
 通信协议要么是面向连接的，要么是无连接的。这依赖于信息发送方是否需要与接收方联系并通过联系来维持一个对话（面向连接的），还是没有任何预先联系就发送消息（无连接的）且希望接收方能顺序接收所有内容。这些方法揭示了网络上实现通信的两种途径。

27. Connectionless
a data transmission method used in packet switching networks by which each data unit is individually addressed and routed based on information carried in each unit, rather than in the setup information of a prearranged, fixed data channel as in connection-oriented communication

28. Connection-oriented
a network communication mode in telecommunications and computer networking, where a communication session or a semi-permanent connection is established before any useful data can be transferred, and where a stream of data is delivered in the same order as it was sent

29. connectionless protocol
Network is fairly reliable
local-area networks
Process
Request ,send ,wait , receive, send back
Problem
Client cannot detect whether the original request message was lost, or that transmission of the reply failed
Report an error
Resend
idempotent

30. reliable connection-oriented protocol
sets up a connection to the server
sending the request
Uses that same connection, to send the reply message
The connection is torn down
setting up and tearing down a connection is relatively costly, especially when the request and reply messages are smaIl.
not entirely appropriate in a local-area network due to relatively low performance, it works perfectly time in wide-area

31. Distinction client and server
There is often no clear distinction
For example, a server for a distributed database may continuously act as a client because it is forwarding requests to different file servers responsible for implementing the database tables. In such a case, the database server itself essentially does no more than process queries
Method: layering

32. Application Layering The user-interface level The processing level
terminal-oriented
handles interaction with a user. Client
The processing level
there are not many aspects common. client or server
The data level
operates on a database or file system . server

33. The user-interface level
The simplest user-interface program
nothing more than a character-based screen
used in mainframe environments
the mainframe controls all interaction
the user's terminal does some local processing
echoing typed keystrokes
supporting form-like interfaces in which a complete entry is to be edited before sending it to the main computer
Nowadays
Windows, mouse, graphics

34. The processing level There are not many aspects common Three examples
Internet search engine
A decision support system for a stock brokerage
A typical desktop package

35. Internet search engine

36. Functions for three tiers
a user types in a string of keywords and is subsequently presented with a list of titles of Web pages
The core of the search engine is a program that transforms the user's string of keywords into one or more database queries. It subsequently ranks the results into a list, and transforms that list into a series of HTML pages
A huge database of Web pages that have been prefetched and indexed

37. A decision support system
A front end
implementing the user interface
The processing level
the analysis programs
Analysis of financial data may require sophisticated methods and techniques from statistics and artificial intelligence. In some cases, the core of a financial decision support system may even need to be executed on high-performance computers in order to achieve the throughput and responsiveness
A back end
accessing a database with the financial data

38. A typical desktop package
consisting of a word processor, a spreadsheet application, communication facilities, and so on
The user-interface level
a common user interface that supports compound documents
home directory is often placed on a remote file server
The processing level
a relatively large collection of programs, each having rather simple processing capabilities
The data level
operates on files from the user's home directory

39. The data level Storing data
file systems
databases
keeping data consistent across different applications
metadata such as table descriptions, entry constraints and application-specific metadata are also stored at this level

40. An example
in the case of a bank, we may want to generate a notification when a customer's credit card debt reaches a certain value. This type of information can be maintained through a database trigger that activates a handler for that trigger at the appropriate moment

41. Data independence
The data are organized independent of the applications in such a way that changes in that organization do not affect applications, and neither do the applications affect the data organization
For example
In most business-oriented environments, the data level is organized as a relational database
In CAD system, data operations are more easily expressed in terms of object manipulations

42. 2.2.1.2 Mululti-tiered Architectures
The distinction into three logical levels as discussed so far, suggests a number of possibilities for physically distributing a client-server application across several machines

43. Only two types of machines
1. A client machine containing only the programs implementing (part of) the user-interface level
2. A server machine containing the rest, that is the programs implementing the processing and data level

44. Alternative client-server organizations

45. Alternative client-server organizations
a) terminal-dependent part of the user interface on the client machine, and give the applications remote control over the presentation of their data
c) The front end can then check the correctness and consistency of the form
e) client's local disk contains part of the data. a client can gradually build a huge cache on local disk of most recent inspected Web pages.
thin clients

46. a server acting as client
a single server is being replaced by multiple servers running on different machines.
a separate process, called the transaction processing monitor, coordinates all transactions across possibly different data servers.

47. 2.2.2 Decentralized Architectures
Multi-tiered client-server architectures are a direct consequence of dividing applications into a user-interface, processing components, and a data level
The different tiers correspond directly with the logical organization of applications

48. Vertical Distribution
Distributed processing is equivalent to organizing a client-server application as a multi-tiered architecture
achieved by placing logically different components on different machines
each machine is tailored to a specific group of functions

49. Horizontal 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
system architectures that support horizontal distribution, known as peer-to-peer systems

50. peer-to-peer architectures
each process will act as a client and a server at the same time
symmetric behavior
Given this symmetric behavior, peer-to-peer architectures evolve around the how to organize the processes in an overlay network
a network in which the nodes are formed by the processes and the links represent the possible communication channels

51. Overlay Network
An overlay network is a computer network which is built on the top of another network
Nodes in the overlay can be thought of as being connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network
For example,distributed systems such as cloud computing, peer-to-peer networks, and client-server applications are overlay networks because their nodes run on top of the Internet
The Internet was originally built as an overlay upon the telephone network while today the telephone network is increasingly turning into an overlay network built on top of the Internet

52. Overlay network
An overlay network is a virtual network of nodes and logical links that is built on top of an existing network with the purpose to implement a network service that is not available in the existing network.

53. Structured Peer-to-Peer Architectures
In a structured peer-to-peer architecture, the overlay network is constructed using a deterministic procedure
The most-used procedure is to organize the processes through a distributed hash table (DHT).

54. Structured Peer-to-Peer Architectures
distributed hash table (DHT)
Data items or nodes are assigned a random key from a large identifier space, such as a 128-bit or 160-bit identifier
The crux: uniquely maps the key of a data item to the identifier of a node based on some distance metric
Routing
when looking up a data item, the network address of the node responsible for that data item is returned

55. The mapping of data items onto nodes in Chord

56. Lookup a data item membership management succ(k) LOOKUP(k)
how nodes organize themselves into an overlay network
succ(k)
a data item with key k is mapped to the node with the smallest identifier id >= k. This node is referred to as the successor of key k and denoted as succ(k)
LOOKUP(k)
return the network address of succ(k)
lookups can generally be done in O(log (N) number of steps

57. A node joins or leaves the system
joining
generating a random identifier id
the node can simply do a lookup on id
insert itself in the ring
each node also stores information on its predecessor
each data item whose key is now associated with node id, is transferred from succ(id)
Leaving
node id informs its departure to its predecessor and successor
transfers its data items to succ(id)

58. CAN(Content Addressable Network)
Each node has an associated region
Every data item in CAN will be assigned a unique point in this space
A node is responsible for the data in its region

59. CAN: join or leave node P joins node P leaves picks an arbitrary point
looks up the node Q in whose region that point falls.
Q splits, and one half is assigned to the node P.
Nodes keep track of their neighbors
the data items for which node P is from node Q.
node P leaves
a bit more problematic

60. Unstructured Peer-to- Peer Architectures
Unstructured peer-to-peer systems largely rely on randomized algorithms for constructing an overlay network
main idea
each node maintains a list of neighbors
this list is constructed in a more or less random way
data items are assumed to be randomly placed on nodes
when a node needs to locate a specific data item, the only thing it can effectively do is flood the network with a search query

61. overlay construction
One of the goals of many unstructured peer-to-peer systems is to construct an overlay network that resembles a random graph.
Each node maintains a list of c neighbors
each of these neighbors represents a randomly chosen live node
a partial view of a node: a list of neighbors
nodes regularly exchange entries from their partial view
Each entry identifies another node in the network, and has an associated age that indicates how old the reference to that node is

62. active thread and passive thread
The active thread takes the initiative to communicate with another node. It selects that node from its current partial view.
passive thread shown in Fig. 2-9(b), whose activities strongly resemble that of the active thread

65. observations A groups of nodes may become isolated
leaving the network turns out to be a very simple operation
when a node P selects node Q, and discovers that Q no longer responds, it simply removes it

66. Topology Management of Overlay Networks
Although it would seem that structured and unstructured peer-to-peer systems form strict independent classes, this 'need actually not be case
One key observation is that by carefully exchanging and selecting entries from partial views, it is possible to construct and maintain specific topologies of overlay networks

67. A specific topologies of overlay networks

68. The lowest layer
The lowest layer constitutes an unstructured peer-to-peer system in which nodes periodically exchange entries of their partial views with the aim to maintain an accurate random graph.
Accuracy in this case refers to the fact that the partial view should be filled with entries referring to randomly selected live nodes

69. The nearest neighbors
The lowest layer passes its partial view to the higher layer
Use a ranking function, nodes are ordered according to some criterion relative to a given node
node P will gradually build up a list of its nearest neighbors, provided the lowest layer continues to pass randomly selected nodes

70. Generating a specific overlay network using a two-layered unstructured peer-to-peer system

71. Super-peers
locating relevant data items can become problematic as the network grows
there is no deterministic way of routing a lookup request to a specific data item
the only technique a node can resort to is flooding the request
There are various ways in which flooding can be dammed
There are other situations in which abandoning the symmetric nature of peer to peer systems is sensible

72. Super-peers Super-peers
Nodes such as those maintaining an index or acting as a broker
Super-peers are often also organized in a peer-to-peer network, leading to a hierarchical organization
All communication from and to a regular peer proceeds through that peer's associated superpeer

73. An example
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
In this case a node P may need to seek for resources in a specific part of the network
Making use of a 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

74. Super-peers

75. Super-peers
A regular peer joins the network, it attaches to one of the superpeers
Superpeers are long-lived processes with a high availability
To compensate for potential unstable behavior of a superpeer, backup schemes can be deployed
requiring clients to attach to both
The client-superpeer relation can change as clients discover better superpeers to associate with
How to select the nodes that are eligible to become superpeer

76. 2.2.3 Hybrid Architectures Edge-Server
Collaborative Distributed Systems

77. Edge-Server Systems servers are placed "at the edge" of the network
This edge is formed by the boundary between enterprise networks and the actual Internet
For example, Internet Service Provider (ISP)
end users at home connect to the Internet through their ISP
the ISP can be considered as residing at the edge of the Internet

78. Edge-Server Systems

79. Edge-Server
An edge server, in a system administration context, is any server that resides on the "edge" between two networks, typically a private network and the Internet. Edge servers can serve different purposes depending on the context of the functionality in question.

80. BitTorrent BitTorrent is a peer-to-peer file downloading system
The basic idea
an end user is looking for a file, he downloads chunks of the file from other users until the downloaded chunks can be assembled together yielding the complete file.

81. Collaborative Distributed Systems

82. Collaborative Distributed Systems(cont.)
A .torrent file contains the information that is needed to download a specific file. In particular, it refers to what is known as a tracker
tracker
a server that is keeping an accurate account of active nodes that have the requested file
Once the nodes have been identified from where chunks can be downloaded, the downloading node effectively becomes active. At that point, it will be forced to help others
BitTorrent combines centralized with decentralized solutions. As it turns out, the bottleneck of the system is, not surprisingly, formed by the trackers.

83. Collaborative Distributed Systems(cont.)
Once the nodes have been identified from where chunks can be downloaded, the downloading node effectively becomes active. At that point, it will be forced to help others
BitTorrent combines centralized with decentralized solutions. the bottleneck of the system is formed by the trackers
This enforcement comes from a very simple rule
if node P notices that node Q is downloading more than it is uploading, P can decide to decrease the rate at which it sends data to Q

84. 2.3 ARCHITECTURES VERSUS MIDDLEWARE
where middleware fits in
middleware systems actually follow a specific architectural style
May no longer be optimal
adding new features to middlewares can easily lead to bloated middleware solutions
e.g. messaging
specific solutions of Middleware should be adaptable to application requirements
make several versions of a middleware system, where each version is tailored to a specific class of applications

85. 2.3.1 Interceptors
an interceptor is a software construct that will break the usual flow of control and allow other (application specific) code to be executed.

86. A calls a method of B
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.
The call by A is transformed into a generic object invocation, made possible through a general object-invocation interface offered by the middleware at the machine where A resides.
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.

87. Using interceptors to handle remote-object invocations

88. object B is replicated
Now imagine that object B is replicated. In that case, each replica should actually be invoked
What the request-level interceptor will do is simply call invoke(B, &do_something, value) for each of the replicas
the object A and middleware need not be aware of the replication of B, only request-level interceptor

89. message-level interceptor
a message-level interceptor may assist in transferring the invocation to the target object
A fragmentation may improve performance or reliability.
the middleware need not be aware of this fragmentation

90. 2.3.2 General Approaches to Adaptive Software
The environment in which distributed applications are executed changes continuously
Rather than making applications responsible for reacting to changes, this task is placed in the middleware.
What interceptors actually offer is a means to adapt the middleware

91. adaptive software an important aspect of modern distributed systems
Has not been as successful as anticipated
three basic techniques of software adaptation
Separation of concerns
Computational reflection
Component-based design

92. Separating concerns Modularizing main problem
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
cannot easily separate these extra functionalities
Example: how fault tolerance can be isolated into a separate box and sold as an independent service

93. Computational reflection
the 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
applying reflection to a broad domain of applications is yet to be done.
reflection is the ability of a computer program to examine (see type introspection) and modify the structure and behavior (specifically the values, meta-data, properties and functions) of an object at runtime

94. When a reflective program operates, it does so in the same manner as a person. It takes variables, such as its own conditions, and contextual information into account. As an analogy, think of the operations involved in getting from your car to your house. If you see an obstacle in your path, you take in that information and adapt to it by either stepping around or over the object, or picking it up. When you get to your door, if you find it locked, usually you don't stop and stand there, continue to turn the knob, or turn around and walk away; usually you take out your key and unlock the door. In the same way, a reflective program has the ability to think about what is happening and to alter itself to address the circumstances

95. component-based design
A system may either be configured statically at design time, or dynamically at runtime
Language: DLL
Operating systems
LKM: modules can be loaded and unloaded at will
allow automatically selection of the best implementation of a component during runtime
It remains complex for distributed systems

96. 2.3.3 Discussion bulky and complex
From the transparency needs, 80-20
Applications’ specific extra-functional requirements conflict with transparency middleware
Goals:
Transparency
Openness
Flexibility
simplicity

97. 2.4 SELF-MANAGEMENT IN DISTRIBUTED SYSTEMS
reason, distributed systems should be adaptive
full distribution transparency is not what most applications actually want
autonomic computing
organizing distributed systems as high-level feedback-control systems allowing automatic adaptations to changes
self-managing
self-healing,
self-configuring
self-optimizing

98. system architectures and software architectures
adaptation needs to be done automatically
a strong interplay between system architectures and software architectures
On the one hand, we need to organize the components of a distributed system such that monitoring and adjustments can be done
on the other hand we need to decide where the processes are to be executed that handle the adaptation

99. 2.4.1 The Feedback Control Model
self-managing systems
Adaptations take place by means of one or more feedback control loops
Systems that are organized by means of such loops are referred to as feedback control systems.
The core of a feedback control system is formed by the components that need to be managed

100. The logical organization of a feedback control system

101. the feedback control loop
three elements
Measuring
Analyzes
adjustment

102. 2.4.2 Systems Monitoring：Astrolabe
General tool for observing systems behavior
Orgnization of Astrolable
The lowest-level zones consist of just a single host, which are subsequently grouped into zones of increasing size. Astrolabe organizes a large collection of hosts into a hierarchy of zones.
The top-level zone covers all hosts.
Every host runs an Astrolabe process, called an agent, that collects information on the zones
The agent also communicates each other.
Each host maintains a set of attributes for collecting local information.
collect the average number of processes for the zone

103. Data collection and information aggregation in Astrolabe

104. 2.5 SUMMARY
Distributed systems can be organized in many different ways.
software architecture vs system architecture.
The latter considers where the components are placed across the various machines.
The former is more concerned about the logical organization of the software: how do components interact, what ways can they be structured, how can they be made independent, and so on

105. architectures
A style reflects the basic principle that is followed in organizing the interaction between the software components comprising a distributed system
Important styles
Layering
object orientation
event orientation
data-space orientation

106. client-server traditional way of modularizing software
By placing different components on different machines, we obtain a natural physical distribution
Client-server architectures are often highly centralized.

107. peer-to-peer systems
The processes are organized into an overlay network, which is a logical network in which every process has a local list of other peers that it can communicate with. The overlay network can be structured, in which case deterministic schemes can be deployed for routing messages between processes.

108. self-managing distributed systems
These systems, to an extent, merge ideas from system and software architectures. Self-managing systems can be generally organized as feedback-control loops. Such loops contain a monitoring component by the behavior of the distributed system is measured, an analysis component to see whether anything needs to be adjusted, and a collection of various instruments for changing the behavior. Feedback -control loops can be integrated into distributed systems at numerous places. Much research is still needed before a common understanding how such loops such be developed and deployedis reached.