SOA has multiple definitions that vary with the vendors that provide platforms to host SOA services. One of the commonly accepted SOA definitions was coined by Don Box of Microsoft. His definition is essentially a set of design guidelines which a service-oriented system should adhere to.
Boundaries are Explicit Services are Autonomous Services share Schema and Contract, not Class Compatibility is based upon Policy – Don Box, Microsoft
Although this definition was originally explained in relation to Microsoft Indigo (now WCF), the tenets still hold true for other SOA platforms as well. An understanding of this principle is that all the services should be available in the network. This tenet dictates that no modules, routines, or procedures can be considered as participants in SOA. Let's take a look at the original tenets in a little detail. The first tenet says that a service should implement a domain functionality and should be discoverable by the other services making up the system. The discovery of service is generally done by registering each service in a directory. The clients of the services can discover each service at runtime. The second tenet explains that the services should be independent of the other services that make up the system. Since the services are independent of each other, they may also enjoy independence of platform and programming language. The third tenet advices that each service should expose an interface through which the rest of the services can communicate with it. The knowledge of this contract should be sufficient to operate with the service. The fourth tenet dictates that the services define the boundaries in which they would work. An example of such a boundary can be a range of integers within which a service that performs arithmetic operations would operate. Such policies should be mentioned in the form of policy expressions and should be machine readable. In WCF, the policies are implemented by the Web Services Policy (WS-Policy) framework.
Although none of the original tenets dictate the size of individual services built using SOA architecture, to obtain independence from other services in the system, an individual service in SOA needs to be coarse-grained. To minimize interaction between services, each service should implement functionalities that work together.
In essence, both the Microservices architecture and the SOA architecture try to solve the problems of monolithic design by modularizing the components. In fact, a system already designed using the SOA architecture is a step in the right direction to realize Microservices architecture.
An inherent problem in the SOA architecture is that it tries to mimic the communication levels in an enterprise. SOA principles take a holistic look at the various communication channels in an enterprise and try to normalize them. To understand this problem in a better manner, let us take a look at a real-world SOA implementation done for an organization.
The following is the architecture of a real-life SOA-based application of a car rental company. The architecture diagram presented below has intentionally been simplified to ease understanding:
SOA architecture
This model is a classic example of an SOA-based system. The various participants in this SOA landscape are as follows:
- The corporate office services: These services provides data pertaining to fleet management, finances, data warehouse, and so on.
- The reservation services: These services help manage bookings and cancellations.
- Backend services: These services interface the systems that supply rules to the system and that supply reservation data to the system. There might be additional systems involved in the application, but we will consider only two of them at the moment.
- Integration platform: The various services of the system need to interact with each other. The integration platform is responsible for orchestrating the communication between the various services. This system understands the data that it receives from the various systems and responds to the various commands that it receives from the portal.
- The Point of Sale portal: The portal is responsible for providing an interface for the users to interact with the services. The technology to realize the frontend of the application is not important. The frontend might be a web portal, a rich client, or a mobile application.
The various systems involved in the application may be developed by different teams. In the preceding example, there can be a team responsible for the backend systems, one for the reservation center, one for the corporate office, one for the portal, and one for the integration services. Any change in the hierarchy of communication may lead to a change in the architecture of the system and thus drive up the costs. For instance, if the organization decides to externalize its finance systems and offload some of the information to another system, then the existing orchestrations would need to be modified. This would lead to increased testing efforts and also redeployment of the entire application.
Another aspect worth noting here is that the integration system forms the backbone of SOA. This concept is generally wrongly interpreted and Enterprise Service Bus (ESB) is used to hook up multiple monoliths which may communicate over complicated, inefficient, and inflexible protocols. This not only adds the overhead of complex transformations to the system but also makes the system resilient to change. Any change in contract would lead to composing of new transformations.
Typical SOA implementations also impede agility. Implementing a change in application is slow because multiple teams need to coordinate with each other. For example, in the preceding scenario, if the application needs to accept a new means of payment, then the portal team would need to make changes in the user interface, the payment team would need to make changes in their service, the backend team would need to add new fields in the database to capture the payment details, and the orchestration team would need to make changes to tie the communication together.
The participant services in SOA also face versioning issues. If any of the services modify their contract, then the orchestrator systems would need to undergo changes as well. In case the changes are too expensive to make, the new version of service would need to maintain backward compatibility with the old contract, which may not always be possible. The deployment of modified services requires more coordination as the modified service needs to be deployed before the affected services get deployed, leading to the formation of deployment monoliths.
The orchestration and integration system runs the risk of becoming a monolith itself. As most of the business logic is concentrated in the orchestration system, the services might just be administering data whereas the orchestration system contains all the business logics of the entire application. Even in a domain-driven design setting, any change in an entity that leads to a change in the user interface would require redeployment of many services. This makes SOA lose its flexibility.