There are a number of additional constraints one can apply on SOA in order to improve its scalability, performance and, reliability. Below are kind of web service:

Stateless Service

Each message that a consumer sends to a provider must contain all necessary information for the provider to process it. This constraint makes a service provider more scalable because the provider does not have to store state information between requests. This is effectively "service in mass production" since each request can be treated as generic. It is also claimed that this constraint improves visibility because any monitoring software can inspect one single request and figure out its intention. There are no intermediate states to worry about, so recovery from partial failure is also relatively easy. This makes a service more reliable.

Stateful Service

Stateful service is difficult to avoid in a number of situations. One situation is to establish a session between a consumer and a provider. A session is typically established for efficiency reasons. For example, sending a security certificate with each request is a serious burden for both any consumer and provider. It is much quicker to replace the certificate with a token shared just between the consumer and provider. Another situation is to provide customized service.

Stateful services require both the consumer and the provider to share the same consumer-specific context, which is either included in or referenced by messages exchanged between the provider and the consumer. The drawback of this constraint is that it may reduce the overall scalability of the service provider because it may need to remember the shared context for each consumer. It also increases the coupling between a service provider and a consumer and makes switching service providers more difficult.

Idempotent Request

Duplicate requests received by a software agent have the same effects as a unique request. This constraint allows providers and consumers to improve the overall service reliability by simply repeating the request if faults are encountered.

Deriving Web Services from SOA

Everyone knows roughly what a "web service" is, but there is no universally accepted definition. The definition of web service has always been under hot debate within the W3C Web Services Architecture Working Group. Despite the difficulty of defining web services, it is generally accepted that a web service is a SOA with at least the following additional constraints:

1. Interfaces must be based on Internet protocols such as HTTP, FTP, and SMTP.

2. Except for binary data attachment, messages must be in XML.

There are two main styles of Web services: SOAP web services and REST web services.

SOAP Web services

A SOAP web service introduces the following constraints:

1. Except for binary data attachment, messages must be carried by SOAP.

2. The description of a service must be in WSDL.

A SOAP web service is the most common and marketed form of web service in the industry. Some people simply collapse "web service" into SOAP and WSDL services. SOAP provides "a message construct that can be exchanged over a variety of underlying protocols" according to the SOAP 1.2 Primer. In other words, SOAP acts like an envelope that carries its contents. One advantage of SOAP is that it allows rich message exchange patterns ranging from traditional request-and-response to broadcasting and sophisticated message correlations. There are two flavors of SOAP web services, SOAP RPC and document-centric SOAP web service. SOAP RPC web services are not SOA; document-centric SOAP web services are SOA.

SOAP RPC Web Services

A SOAP RPC web service breaks the second constraint required by an SOA. A SOAP RPC Web service encodes RPC (remote procedure calls) in SOAP messages. In other words, SOAP RPC "tunnels" new application-specific RPC interfaces though an underlying generic interface. Effectively, it prescribes both system behaviors and application semantics. Because system behaviors are very difficult to prescribe in a distributed environment, applications created with SOAP RPC are not interoperable by nature. Many real life implementations have confirmed this.

Faced with this difficulty, both WS-I basic profile and SOAP 1.2 have made the support of RPC optional. RPC also tends to be instructive rather than descriptive, which is against the spirit of SOA. Ironically, SOAP was originally designed just for RPC. It won't be long before someone claims that "SOAP" actually stands for "SOA Protocol".

REST Web Services

The term REST was first introduced by Roy Fielding to describe the web architecture. A REST web service is an SOA based on the concept of "resource". A resource is anything that has a URI. A resource may have zero or more representations. Usually, people say that a resource does not exist if no representation is available for that resource. A REST web service requires the following additional constraints:

1. Interfaces are limited to HTTP. The following semantics are defined:

   • HTTP GET is used for obtaining a representation of a resource. A consumer uses it to retrieve a representation from a URI. Services provided through this interface must not incur any obligation from consumers.
   • HTTP DELETE is used for removing representations of a resource.
   • HTTP POST is used for updating or creating the representations of a resource.
   • HTTP PUT is used for creating representations of a resource.
2. Most messages are in XML, confined by a schema written in a schema language such as XML Schema from W3C or RELAX NG.
3. Simple messages can be encoded with URL encoding.
4. Service and service providers must be resources while a consumer can be a resource.

REST web services require little infrastructure support apart from standard HTTP and XML processing technologies, which are now well supported by most programming languages and platforms. REST web services are simple and effective because HTTP is the most widely available interface, and it is good enough for most applications. In many cases, the simplicity of HTTP simply outweighs the complexity of introducing an additional transport layer.