5.1 Deriving REST

5.1.1 Starting with the Null Style

Two common perspectives on the process of architectural design:

The first is that a designer starts with nothing—a blank slate, whiteboard, or drawing board—and builds-up an architecture from familiar components until it satisfies the needs of the intended system.

The second is that a designer starts with the system needs as a whole, without constraints, and then incrementally identifies and applies constraints to elements of the system in order to differentiate the design space and allow the forces that influence system behavior to flow naturally, in harmony with the system.

REST is based on the latter process.

The following eight sections depict how the applied constraints would differentiate the process view of an architecture as the incremental set of constraints is applied.

From the Null style (Figure 5-1) is simply an empty set of constraints. It is the starting point for our description of REST.

5.1.2 Client-Server

From the client-server architectural style (Figure 5-2)

Separation of concerns is the principle behind the client-server constraints. Perhaps most significant to the Web is that the separation allows the components to evolve independently, thus supporting the Internet-scale requirement of multiple organizational domains.

5.1.3 Stateless

From the client-stateless-server (CSS) style (Figure 5-3)

Each request from client to server must contain all of the information necessary to understand the request, and cannot take advantage of any stored context on the server.

Session state is kept entirely on the client.

Advantage: ease the pressure of Sever.

Disadvantage: decrease network performance by increasing the repetitive data in a series of requests in clients.

5.1.4 Cache

From the client-cache-stateless-server style (Figure 5-4).

Requiring: a response is implicitly or explicitly labeled as cacheable or noncacheable.

Advantage: eliminate some interactions.

Disadvantage: decrease reliability if state data within the cache differs significantly from the data that would have been sent directly to the server.

5.1.5 Uniform Interface

The central feature that distinguishes the REST architectural style from other network-based styles is its emphasis on a uniform interface between components (Figure 5-6).

The REST interface is designed to be efficient for large-grain hypermedia data transfer, optimizing for the common case of the Web.

REST is defined by four interface constraints: identification of resources; manipulation of resources through representations; self-descriptive messages; and, hypermedia as the engine of application state.

5.1.6 Layered System

From layered system constraints (Figure 5-7).

The layered system style allows an architecture to be composed of hierarchical layers by constraining component behavior such that each component cannot “see” beyond the immediate layer with which they are interacting.

5.1.7 Code-On-Demand

From the code-on-demand style (Figure 5-8).

REST allows client functionality to be extended by downloading and executing code in the form of applets or scripts. This simplifies clients by reducing the number of features required to be pre-implemented. Allowing features to be downloaded after deployment improves system extensibility.

5.1.8 Style Derivation Summary

Figure 5-9 depicts the derivation of REST’s constraints graphically in terms of the network-based architectural styles examined in Chapter 3.

5.2 REST Architectural Elements

5.2.1 Data Elements

The nature and state of an architecture’s data elements is a key aspect of REST.

Three fundamental options: 1) render the data where it is located and send a fixed-format image to the recipient; 2) encapsulate the data with a rendering engine and send both to the recipient; or, 3) send the raw data to the recipient along with metadata that describes the data type, so that the recipient can choose their own rendering engine.

5.2.2 Connectors

5.2.3 Components

代理和网关之间的区别是，何时使用代理是由客户端来决定的

5.3 REST Architectural Views
5.3.1 Process View
5.3.2 Connector View
5.3.3 Data View

4.1 WWW Application Domain Requirements

4.1.1 Low Entry-barrier

1. Since participation in the creation and structuring of information was voluntary, a low entry-barrier was necessary to enable sufficient adoption. This applied to all users of the Web architecture: readers, authors, and application developers.

2. Simplicity was also a goal for the sake of application developers.

4.1.2 Extensibility

Extensibility: A system intending to be as long-lived as the Web must be prepared for change.

4.1.3 Distributed Hypermedia

1. The usability of hypermedia interaction is highly sensitive to user-perceived latency: the time between selecting a link and the rendering of a usable result.

2. The architecture needs to minimize network interactions.

4.1.4 Internet-scale

1. The Internet is about interconnecting information networks across multiple organizational boundaries.

2. Suppliers of information services must be able to cope with the demands of anarchic scalability and the independent deployment of software components.

4.1.4.1 Anarchic Scalability

1. Anarchic scalability refers to the need for architectural elements to continue operating when they are subjected to an unanticipated load, or when given malformed or maliciously constructed data, since they may be communicating with elements outside their organizational control.

2. The anarchic scalability requirement applies to all architectural elements.

3. Security of the architectural elements, and the platforms on which they operate, also becomes a significant concern.

4.1.4.2 Independent Deployment

4.2 Problem

4.3 Approach

The first step, is to identify the constraints placed within the early Web architecture that are responsible for its desirable properties.

Hypothesis I: The design rationale behind the WWW architecture can be described by an architectural style consisting of the set of constraints applied to the elements within the Web architecture.

The next step, is to identify the properties desirable in an Internet-scale distributed hypermedia system, select additional architectural styles that induce those properties, and combine them with the early Web constraints to form a new, hybrid architectural style for the modern Web architecture.

Hypothesis II: Constraints can be added to the WWW architectural style to derive a new hybrid style that better reflects the desired properties of a modern Web architecture.

The third step, is to use the new architectural style as a guide, we can compare proposed extensions and modifications to the Web architecture against the constraints within the style.

Hypothesis III: Proposals to modify the Web architecture can be compared to the updated WWW architectural style and analyzed for conflicts prior to deployment.

This approach as a single sequence, it is actually applied in a non-sequential, iterative fashion.

4.4 Summary

My approach is to use an architectural style to define and improve the design rationale behind the Web’s architecture, to use that style as the acid test for proving proposed extensions prior to their deployment, and to deploy the revised architecture via direct involvement in the software development projects that have created the Web’s infrastructure.

3.1 Classification Methodology

In order to provide useful design guidance, a classification of architectural styles should be based on the architectural properties induced by those styles.

3.1.1 Selection of Architectural Styles for Classification

3.1.2 Style-induced Architectural Properties

3.1.3 Visualization

3.2 Data-flow Styles

3.2.1 Pipe and Filter (PF)

Description: In a pipe and filter style, each component (filter) reads streams of data on its inputs and produces streams of data on its outputs, usually while applying a transformation to the input streams and processing them incrementally so that output begins before the input is completely consumed.

Characteristic:

1. One-way data flow network;

2. Zero coupling: a filter must be completely independent of other filters.

Advantage: (by Garlan and Shaw)

1. Simplicity: PF allows the designer to understand the overall input/output of the system as a simple composition of the behaviors of the individual filters.

2. Reusability: any two filters can be hooked together, provided they agree on the data that is being transmitted between them.

3. Extensibility and evolvability: new filters can be added to existing systems (extensibility) and old filters can be replaced by improved ones (evolvability).

4. Verifiability: they permit certain kinds of specialized analysis.

5. User-perceived performance: they naturally support concurrent execution.

6. Configurability: a network of filters is typically arranged just prior to each activation, allowing the application to specify the configuration of filter components based on the task at hand and the nature of the data streams.

Disadvantage:

1. Propagation delay: propagation delay is added through long pipelines;

2. Batch sequential processing occurs if a filter cannot incrementally process its inputs, and no interactivity is allowed.

3. A filter cannot interact with its environment: because it cannot know that any particular output stream shares a controller with any particular input stream.

4. User-perceived performance: these properties decrease user-perceived performance if the problem being addressed does not fit the pattern of a data flow stream.

3.2.2 Uniform Pipe and Filter (UPF)

Characteristic: the uniform pipe and filter style adds the constraint that all filters must have the same interface.

Advantage:

Example: Unix operating system, where filter processes have an interface consisting of one input data stream of characters (stdin) and two output data streams of characters (stdout and stderr).

1. Restricting the interface allows independently developed filters to be arranged at will to form new applications.

2. It also simplifies the task of understanding how a given filter works.

Disadvantage:

It may reduce network performance if the data needs to be converted to or from its natural format.

3.3 Replication Styles

3.3.1 Replicated Repository (RR)

Example: (1) distributed filesystems, such as XMS, and, (2) remote versioning systems, like CVS [www.cyclic.com].

Advantage: Improved user-perceived performance, both by reducing the latency of normal requests and enabling disconnected operation in the face of primary server failure or intentional roaming off the network(线下漫游).

Concern: Maintaining consistency is the primary concern.

3.3.2 Cache ($)

This form of replication is most often found in cases (1) where the potential data set far exceeds the capacity of any one client, as in the WWW, or (2) where complete access to the repository is unnecessary.

Advantage:caching is much easier to implement, doesn’t require as much processing and storage, and is more efficient because data is transmitted only when it is requested.

A typical network-based architectural style: client-stateless-server style (rest rationale).

3.4 Hierarchical Styles

3.4.1 Client-Server (CS)

Description:

1. A server component: offering a set of services, listens for requests upon those services.

A client component: desiring that a service be performed, sends a request to the server via a connector.

2. by Andrews:

A client is a triggering process;

A server is a reactive process.

Clients make requests that trigger reactions from servers.

A server is usually a non-terminating process and often provides service to more than one client.

Principle:Separation of concerns.

Simplify the server component in order to improve scalability.

Method: moving all of the user interface functionality into the client component.

The partition between C and S components: It is often referred to by the mechanisms used for the connector implementation, such as remote procedure call or message-oriented middleware.

3.4.2 Layered System (LS) and Layered-Client-Server (LCS)

Layered System:

1. A layered system is organized hierarchically, each layer providing services to the layer above it and using services of the layer below it.

2.Layered systems reduce coupling across multiple layers by hiding the inner layers from all except the adjacent outer layer. In all, in multiple layers, every layer only couples the adjacent outer layer, thus improving evolvability and reusability (e.g., TCP/IP and OSI protocol stacks, and hardware interface libraries).

3. Disadvantage:layered systems add overhead and latency to the processing of data, reducing user-perceived performance.

Layered-Client-Server:

Layered-client-server adds proxy and gateway components to the client-server style.

Client -> Proxy -> Gateway -> Server

A proxy acts as a shared server for one or more client components, taking requests and forwarding them, with possible translation, to server components.

A gateway component appears to be a normal server to clients or proxies that request its services, but is in fact forwarding those requests, with possible translation, to its “inner-layer” servers.

3.4.3 Client-Stateless-Server (CSS)

Characteristic: no session stateis allowed on the server component.

Each request from client to server must contain all of the information necessary to understand the request, and cannot take advantage of any stored context on the server. Session state is kept entirely on the client.

Advantage: visibility, reliability, and scalability.

Disadvantage:it may decrease network performance by increasing the repetitive data (per-interaction overhead) sent in a series of requests, since that data cannot be left on the server in a shared context.

3.4.4 Client-Cache-Stateless-Server (C$SS)

Characteristic: the client-cache-stateless-server style derives from the client-stateless-server and cache styles via the addition of cache components.

Description: A cache acts as a mediator between client and server in which the responses to prior requests can, if they are considered cacheable, be reused in response to later requests that are equivalent.

Example: an example system that makes effective use of this style is Sun Microsystems’ NFS.

Advantage: cache components have the potential to partially or completely eliminate some interactions, improving efficiency and user-perceived performance.

3.4.5 Layered-Client-Cache-Stateless-Server (LC$SS)

Characteristic: the layered-client-cache-stateless-server style derives from both layered-client-server and client-cache-stateless-server through the addition of proxy and/or gateway components.

Example: an example system that uses an LC$SS style is the Internet domain name system (DNS).

3.4.6 Remote Session (RS)

Characteristic: minimize the complexity, or maximize the reuse of the client components,andthe application state on server.

Description: each client initiates a session on the server and then invokes a series of services on the server, finally exiting the session. Application state is kept entirely on the server.

Example: when it is desired to access a remote service using a generic client (e.g., TELNET) or via an interface that mimics a generic client (e.g., FTP).

Advantage: (1) easier to centrally maintain the interface at the server, (2) reducing concerns about inconsistencies in deployed clients, and (3) improves efficiency if the interactions make use of extended session context on the server.

Disadvantage:(1) reduces scalability of the server, due to the stored application state, and, (2) reduces visibility of interactions, since a monitor would have to know the complete state of the server.

3.4.7 Remote Data Access (RDA)

Characteristic: the remote data access style is a variant of client-server that spreads the application state across both client and server.

Description: a client sends a database query in a standard format, such as SQL, to a remote server. The server allocates a workspace and performs the query,which may result in a very large data set. The client must know about the data structure of the service to build structure-dependent queries.

Advantage:

Eficiency: a large data set can be iteratively reduced on the server side without transmitting it across the network.

Visibility: is improved by using a standard query language.

Disadvantage:

Lacking simplicity: the client needs to understand the same database manipulation concepts as the server implementation

Decreases scalability: storing application context on the server decreases scalability.

Suffered reliability: partial failure can leave the workspace in an unknown state.

3.5 Mobile Code Styles

3.6 Peer-to-Peer Styles

3.7 Limitations

3.8 Related Work

2.1 Scope

Network-based Application Architectures

2.1.1 Network-based vs. Distributed

The primary distinction between network-based architectures and software architectures in general is that communication between components is restricted to message passing, or the equivalent of message passing if a more efficient mechanism can be selected at runtime based on the location of components.

The disctinction between distributed systems and network-based systems: a distributed system is one that looks in users’ opinion like an ordinary centralized system, but runs on multiple, independent CPUs. In contrast, network-based systems are those capable of operation across a network, but not necessarily in a fashion that is transparent to the user.

2.1.2 Application Software vs. Networking Software

Application software architecture is an abstraction level of an overall system, in which the goals of a user action are representable as functional architectural properties. The goal of networking abstraction (behavioral architectural properties) is to move bits from one location to another without regard towhy those bits are being moved.

2.2 Evaluating the Design of Application Architectures

An architecture is the realization of an architectural design and not the design

itself.

The first level of evaluating the design: the application’s functional requirements.

The second level of evaluating it: regard the architectural style asa derivation tree based on a set of architectural constraints because the architectural constraints are in line with the architectural style.

The third level: build such a derivation tree of architectural constraints for a given application domain, and then use the tree to evaluate many different architectural designs for applications within that domain. Thus, building a derivation tree provides a mechanism for architectural design guidance.

The fourth level: ensuring consistency is to make the tree domain-specific.

The fifth level: rationally evaluate the design between trade-offs.

2.3 Architectural Properties of Key Interest

2.3.1 Performance

The performance of a network-based application is bound first by the application

requirements, then by the chosen interaction style, followed by the realized architecture, and finally by the implementation of each component.

2.3.1.1 Network Performance

Network performance is measured by some attributes of communication: throughput, overhead, bandwidth, (useable bandwidth).

Styles impact network performance by their influence on the number of interactions

per user action and the granularity of data elements.

2.3.1.2 User-perceived Performance

The primary measures for user-perceived performance are latency and completion time.

1. Latency occurs:

1) the time needed for the application to recognize the event that initiated the action; 2) the time required to setup the interactions between components; 3) the time required to transmit each interaction to the components; 4) the time required to process each interaction on those components; and, 5) the time required to complete sufficient transfer and processing of the result of the interactions before the application is able to begin rendering a usable result. It is important to notethat, (i) only (3) and (5) represent actual network communication, (ii) all five points can be impacted by the architectural style.

2. Completion is the amount of time taken to complete an application action.

It is important to note that design considerations for optimizing latency will often have the side-effect of degrading completion time, and vice versa.

2.3.1.3 Network Efficiency

1.The most efficient architectural styles for a network-based application are those that can effectively minimize use of the network when it is possible to do so.

2.Method: reuse of prior interactions (caching), reduction of the frequency of network interactions in relation to user actions (replicated data and disconnected operation), or by removing the need for some interactions by moving the processing of data closer to the source of the data (mobile code).

3.The impact of the various performance issues is often related to the scope of

distribution for the application.(LAN or WAN, a single process or multi-processes, etc)

2.3.2 Scalability

Scalability refers to the ability of the architecture to support large numbers of components, or interactions among components, within an active configuration.

2.3.3 Simplicity

1. principle of separation of concerns(分离关注点原则)

2. principle of generality(通用性原则), it generates middleware.

3. properties: complexity, understandability, and verifiability

2.3.4 Modifiability

Modifiability can be explained as evolvability, extensibility, customizability, configurability, and reusability, as described below.

2.3.4.1 Evolvability(可进化性)

1. Static evolution: depends on how well the architectural abstraction is enforced by the implementation, thus is not unique to any particular architectural style.

2. Dynamic evolution: can be influenced by the style if it includes constraints on the maintenance and location of application state.

2.3.4.2 Extensibility

1. Dynamic extensibility implies that functionality can be added to a deployed system without impacting the rest of the system.

2. Extensibility is induced within an architectural style by reducing the coupling between components.

2.3.4.3 Customizability

1. Customizability refers to the ability to temporarily specialize the behavior of an architectural element, such that it can then perform an unusual service.

2. Styles that support customization may also improve simplicity and scalability, since service components can be reduced in size and complexity by directly implementing only the most frequent services and allowing infrequent services to be defined by the client.

2.3.4.4 Configurability

Two examples: the pipe-and-filter and code-on-demand styles (管道-过滤器风格和按需代码风格).

2.3.4.5 Reusability

The primary mechanisms for inducing reusability within architectural styles is reduction of (1) coupling (knowledge of identity) between components and (2) constraining the generality of component interfaces.

2.3.5 Visibility

2.3.6 Portability

2.3.7 Reliability

Styles can improve reliability by avoiding single points of failure, enabling redundancy, allowing monitoring, or reducing the scope of failure to a recoverable action.

1.1 run-time abstraction

A software architecture is an abstraction of the run-time elements of a software system during some phase of its operation. A system may be composed of many levels of abstraction and many phases of operation, each with its own software architecture.

At the heart of software architecture is the principle of abstraction: hiding some of the

details of a system through encapsulation in order to better identify and sustain its

properties.

The differences between software architecture and software structure: the former is an abstraction of the run-time behavior of a software system, whereas the latter is a property of the static software source code.

1.2 Elements

A software architecture is defined by a configuration of architectural elements — components, connectors, and data — constrained in their relationships in order to achieve a desired set of architectural properties.

Components are those that perform transformations on data, data elements are those that contain the information that is used and transformed, and connectors are the glue that holds the different pieces of the architecturetogether.

Software architecture includes architectural properties. Rationale explicates those properties, but the rationale itself is not part of the architecture.

Rather than defining the software’s architecture as existing within the software, it is defining a description of the software’s architecture as if that were the architecture.

1.2.1 Components

A component is an abstract unit of software instructions and internal state that provides a transformation of data via its interface.

1.2.2 Connectors

A connector is an abstract mechanism that mediates communication, coordination, or cooperation among components.

1.2.3 Data

A datum is an element of information that is transferred from a component, or received by a component, via a connector.

1.3 Configurations

A configuration is the structure of architectural relationships among components, connectors, and data during a period of system run-time.

1.4 Properties

1.5 Styles

An architectural style is a coordinated set of architectural constraints that restricts the roles/features of architectural elements and the allowed relationships among those elements within any architecture that conforms to that style.

1.6 Patterns and Pattern Languages

A design pattern is defined as an important and recurring system construct.

The pattern is, in short, at the same time a thing, which happens in the

world, and the rule which tells us how to create that thing, and when we must

create it. It is both a process and a thing; both a description of a thing which is

alive, and a description of the process which will generate that thing.

1.7 Views

three important views in software architecture: processing, data, and connection views. A process view emphasizes the data flow through the components and some aspects of the connections among the components with respect to the data. A data view emphasizes the processing flow, with less emphasis on the connectors. A connection view emphasizes the relationship between components and the state of   communication.