Canonical Protocol is a

design pattern A design pattern is the re-usable form of a solution to a design problem. The idea was introduced by the architect Christopher Alexander and has been adapted for various other disciplines, particularly software engineering. The "Gang of Four" boo ...

, applied within the

service-orientation Service-orientation is a design paradigm for computer software in the form of services. The principles of service-oriented design stress the separation of concerns in the software. Applying service-orientation results in units of software partiti ...

design paradigm A design is a plan or specification for the construction of an object or system or for the implementation of an activity or process or the result of that plan or specification in the form of a prototype, product, or process. The verb ''to design' ...

, which attempts to make services, within a service inventory,Matthew Daile
Software Architecture Design Service Oriented Architectures (Part II)
nlineDate accessed: 25 April 2010.

Rationale

Services developed by different project teams could be based on different communication mechanisms. As a result, a service inventory may end up having different sets of services, each conforming to a different set of protocols. When it comes to reusing services having different communication protocols, some sort of communication bridging mechanism is required. For example, services developed using JMS messaging protocol are incompatible with services using .NET Remoting, so in order to make use of these two types of services, some

middleware Middleware is a type of computer software that provides services to software applications beyond those available from the operating system. It can be described as "software glue". Middleware makes it easier for software developers to implement ...

technology needs to be in place that bridges the communication protocol disparity. Apart from incurring extra cost, the use of such a bridging technology adds latency and communication overhead. This makes the service less of a reusable and a recomposableService Composition
resource and goes against the guidelines of the Service Composability design principle. In order to design a service inventory where all services are interoperable with each other so that they can be composed into different solutions, the application of the Canonical Protocol pattern dictates standardizing the communication protocols used by the services. When all services are using the same communication protocol, the requirement for a bridging technology is eliminated and the communication between services is more streamlined.Mauro. et al
Service Oriented Device Integration - An Analysis of SOA Design Patterns.
nline pp.1-10, 2010 43rd Hawaii International Conference on System Sciences, 2010. Date accessed: 30 April 2010.

Usage

The application of this design pattern requires choosing a technology architecture that provides a common communication framework so that all services in an inventory can communicate with each other using the same communication protocol. This depends upon how the services within a service inventory are going to be used. If the services are only going to be part of service compositions that always use a particular communication protocol (because of efficiency and security reasons), then all the services within that service inventory can be built upon such a communication protocol even if it is not the most widely used protocol. The Canonical Protocol pattern by

Thomas Erl Thomas Erl (born 1967) is a Canadian author, and public speaker known for major contributions to the field of service-oriented architecture. Author of eight books on Service Orientation, Erl defined eight widely accepted principles of service ...

answers the question: "How can services be designed to avoid protocol bridging?"SOA Patterns - Canonical Protocol
The problem is that services that support different communication technologies compromise interoperability, limit the quantity of potential consumers, and introduce the need for undesirable protocol bridging measures. The solution is for the architecture to establish a single communications technology as the sole or primary medium by which services can interact. Therefore, the communication protocols (including protocol versions) used within a service inventory boundary are standardized for all services (see diagram). One of the most mature and widely used communication mechanisms is provided by the Web services framework. Further to choosing a communication framework, the actual message protocols also need to be standardized upon. For example, whether web services are built using

SOAP Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are us ...

over

HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...

or by simply using RESTful services. Similarly, when standardizing on SOAP based web services, the specific version of SOAP protocol needs to be agreed upon as well i.e. SOAP v 1.1 or SOAP v 1.2.

Considerations

In order to standardize on a communication protocol, the features of the protocol need to be compared against the service interaction requirements including security, efficiency and transaction support. In case of web services, for example, if a service composition requires explicit transaction support, then SOAP over HTTP would be a better choice than using RESTful services. In some cases, depending upon the technology used to build the service, it may be possible to support two different set of protocols in order to make the service accessible to different types of service consumers (Dual Protocols design pattern