In software engineering, service-oriented architecture (SOA) is an architectural style that focuses on discrete services instead of a monolithic design.
By consequence, it is also applied in the field of software design where services are provided to the other components by application components, through a communication protocol over a network.
A service is a discrete unit of functionality that can be accessed remotely and acted upon and updated independently, such as retrieving a credit card statement online.
SOA is also intended to be independent of vendors, products and technologies.
Service orientation is a way of thinking in terms of services and service-based development and the outcomes of services.
A service has four properties according to one of many definitions of SOA:
根据 SOA 的众多定义之一，服务具有四个属性：
- It logically represents a repeatable business activity with a specified outcome. 它在逻辑上表示具有指定结果的可重复业务活动。
- It is self-contained. 具有完备的特性（self-contained）
- It is a black box for its consumers, meaning the consumer does not have to be aware of the service’s inner workings. 它是消费者的黑匣子，这意味着消费者不必了解服务的内部运作。
- It may be composed of other services. 它可能由其他服务组成。 
Different services can be used in conjunction as a service mesh to provide the functionality of a large software application,
a principle SOA shares with modular programming. Service-oriented architecture integrates distributed, separately maintained and deployed software components.
It is enabled by technologies and standards that facilitate components’ communication and cooperation over a network, especially over an IP network.
它由促进组件在网络上（尤其是在 IP 网络上）的通信和协作的技术和标准实现。
SOA is related to the idea of an API (application programming interface), an interface or communication protocol between different parts of a computer program intended to simplify the implementation and maintenance of software.
An API can be thought of as the service, and the SOA the architecture that allows the service to operate.
SOA 与 API（应用程序编程接口）的概念相关，API 是计算机程序不同部分之间的接口或通信协议，旨在简化软件的实施和维护。
API 可以被认为是服务，而 SOA 是允许服务运行的架构。
In SOA, services use protocols that describe how they pass and parse messages using description metadata.
This metadata describes both the functional characteristics of the service and quality-of-service characteristics.
Service-oriented architecture aims to allow users to combine large chunks of functionality to form applications which are built purely from existing services and combining them in an ad hoc manner.
A service presents a simple interface to the requester that abstracts away the underlying complexity acting as a black box.
Further users can also access these independent services without any knowledge of their internal implementation.
Defining concepts 概念
The related buzzword service-orientation promotes loose coupling between services.
SOA separates functions into distinct units, or services, which developers make accessible over a network in order to allow users to combine and reuse them in the production of applications.
These services and their corresponding consumers communicate with each other by passing data in a well-defined, shared format, or by coordinating an activity between two or more services.
A manifesto was published for service-oriented architecture in October, 2009. This came up with six core values which are listed as follows: 2009年10月，面向服务架构发表了一份宣言。该宣言提出了六个核心价值，列举如下：
1. Business value is given more importance than technical strategy. 商业价值 比技术策略更重要。
2. Strategic goals are given more importance than project-specific benefits. 战略目标 比项目特定的收益更重要。
3. Intrinsic interoperability is given more importance than custom integration. 内在互操作性 比定制集成更重要。
4. Shared services are given more importance than specific-purpose implementations. 共享服务 比特定目的的实现更重要。
5. Flexibility is given more importance than optimization. 灵活性比优化更重要。
6. Evolutionary refinement is given more importance than pursuit of initial perfection. 渐进的改进 比追求最初的完美更重要。
SOA can be seen as part of the continuum which ranges from the older concept of distributed computing  and modular programming,
through SOA, and on to practices of mashups, SaaS, and cloud computing (which some see as the offspring of SOA).
There are no industry standards relating to the exact composition of a service-oriented architecture, although many industry sources have published their own principles. Some of these   include the following:
Standardized service contract 标准化的服务契约 
Services adhere to a standard communications agreement, as defined collectively by one or more service-description documents within a given set of services.
Service reference autonomy (an aspect of loose coupling) 服务引用自治（松耦合的一个方面）
The relationship between services is minimized to the level that they are only aware of their existence.
Service location transparency (an aspect of loose coupling) 服务位置透明性（松耦合的一个方面）
Services can be called from anywhere within the network that it is located no matter where it is present.
Service longevity 服务生命周期
Services should be designed to be long lived. Where possible services should avoid forcing consumers to change if they do not require new features, if you call a service today you should be able to call the same service tomorrow.
Service abstraction 服务抽象
The services act as black boxes, that is their inner logic is hidden from the consumers.
Service autonomy 服务自治
Services are independent and control the functionality they encapsulate, from a Design-time and a run-time perspective.
Service statelessness 服务无状态
Services are stateless, that is either return the requested value or give an exception hence minimizing resource use.
Service granularity 服务粒度
A principle to ensure services have an adequate size and scope. The functionality provided by the service to the user must be relevant.
Service normalization 服务规范化
Services are decomposed or consolidated (normalized) to minimize redundancy. In some, this may not be done.
These are the cases where performance optimization, access, and aggregation are required.
Service composability 服务组合性
Services can be used to compose other services.
Service discovery 服务发现
Services are supplemented with communicative meta data by which they can be effectively discovered and interpreted.
Service reusability 服务复用
Logic is divided into various services, to promote reuse of code.
Service encapsulation 服务封装
Many services which were not initially planned under SOA, may get encapsulated or become a part of SOA.
Each SOA building block can play any of the three roles: 每个面向服务架构构建块可以具有以下三种角色之一：
Service provider 服务提供者
It creates a web service and provides its information to the service registry.
Each provider debates upon a lot of hows and whys like which service to expose, which to give more importance:
security or easy availability, what price to offer the service for and many more.
The provider also has to decide what category the service should be listed in for a given broker service and what sort of trading partner agreements are required to use the service.
Service broker, service registry or service repository 服务代理、服务注册中心或服务仓库
Its main functionality is to make the information regarding the web service available to any potential requester. Whoever implements the broker decides the scope of the broker.
Public brokers are available anywhere and everywhere but private brokers are only available to a limited amount of public.
UDDI was an early, no longer actively supported attempt to provide Web services discovery.
Service requester/consumer 服务请求者/消费者
It locates entries in the broker registry using various find operations and then binds to the service provider in order to invoke one of its web services.
Whichever service the service-consumers need, they have to take it into the brokers, bind it with respective service and then use it. They can access multiple services if the service provides multiple services.
The service consumer–provider relationship is governed by a standardized service contract, which has a business part, a functional part and a technical part.
Service composition patterns have two broad, high-level architectural styles: choreography and orchestration.
Lower level enterprise integration patterns that are not bound to a particular architectural style continue to be relevant and eligible in SOA design.  
Implementation approaches 实现方法
Service-oriented architecture can be implemented with web services or Microservices.
This is done to make the functional building-blocks accessible over standard Internet protocols that are independent of platforms and programming languages.
These services can represent either new applications or just wrappers around existing legacy systems to make them network-enabled.23
Implementers commonly build SOAs using web services standards.
One example is SOAP, which has gained broad industry acceptance after recommendation of Version 1.2 from the W3C (World Wide Web Consortium) in 2003.
These standards (also referred to as web service specifications) also provide greater interoperability and some protection from lock-in to proprietary vendor software.
One can, however, also implement SOA using any other service-based technology, such as Jini, CORBA, Internet Communications Engine, REST, or gRPC.
然而，也可以使用任何其他基于服务的技术来实现SOA，例如 Jini、CORBA、Internet Communications Engine、REST或gRPC。
Architectures can operate independently of specific technologies and can therefore be implemented using a wide range of technologies, including:
– Messaging, e.g., with ActiveMQ, JMS, RabbitMQ 消息传递、MQ，例如ActiveMQ、JMS、RabbitMQ
– RESTful HTTP, with Representational state transfer (REST) constituting its own constraints-based architectural style
RESTful HTTP，其中采用了基于约束的REST（Representational State Transfer）作为其自身的架构风格
– Web services based on WSDL and SOAP 基于WSDL和SOAP的Web服务
– Internet Communications Engine Internet通信引擎
– WCF (Microsoft’s implementation of Web services, forming a part of WCF) WCF（微软的Web服务实现，是WCF的一部分）
– Apache Thrift
Implementations can use one or more of these protocols and, for example, might use a file-system mechanism to communicate data following a defined interface specification between processes conforming to the SOA concept.
The key is independent services with defined interfaces that can be called to perform their tasks in a standard way,
without a service having foreknowledge of the calling application,
and without the application having or needing knowledge of how the service actually performs its tasks.
SOA enables the development of applications that are built by combining loosely coupled and interoperable services.
实现可以使用这些协议中的一个或多个，例如，可以使用文件系统机制按照定义的接口规范在符合 SOA 概念的进程之间传递数据。
High-level programming languages such as BPEL and specifications such as WS-CDL and WS-Coordination extend the service concept by providing a method of defining and supporting orchestration of fine-grained services into more coarse-grained business services,
which architects can in turn incorporate into workflows and business processes implemented in composite applications or portals.
BPEL 等高级编程语言和 WS-CDL 和 WS-Coordination 等规范通过提供一种方法来定义和支持将细粒度服务编排为更粗粒度的业务服务，进而扩展服务概念，
Service-oriented modeling is an SOA framework that identifies the various disciplines that guide SOA practitioners to conceptualize, analyze, design, and architect their service-oriented assets.
The Service-oriented modeling framework (SOMF) offers a modeling language and a work structure or “map” depicting the various components that contribute to a successful service-oriented modeling approach.
It illustrates the major elements that identify the “what to do” aspects of a service development scheme.
The model enables practitioners to craft a project plan and to identify the milestones of a service-oriented initiative.
SOMF also provides a common modeling notation to address alignment between business and IT organizations.
面向服务建模是一个 SOA 框架，它确定了指导 SOA 实践者概念化、分析、设计和架构其面向服务的资产的各种规程。
面向服务建模框架 (SOMF) 提供了一种建模语言和一种工作结构或“地图”，描述了有助于成功实现面向服务的建模方法的各种组件。
SOMF 还提供了一个通用的建模符号来解决业务和 IT 组织之间的一致性问题。