I am doing a system architecture and my knowledge from college doesn't help me when it comes to understand the subtle differences between centralized, distributed and service oriented architecture/application.
If I take a typical client/server architecture, the client sends requests to a server, the server then sends responses to the client. That is a centralized architecture.
An application that handles both server and client sides will be a distributed application (because working on different platforms), but that is still a centralized architecture.
Therefore, a distributed architecture must involve a distributed application.
Questions: am I right? What does all that become when it comes to service oriented architectures / applications?
Distributed: The whole process involved in a computation task is divided into pieces and assigned to multiple computational nodes. Each node when doing its part of the processing does not have access to the whole information of the system that is necessary to achieve a globally optimized result. The aggregate of the results from multiple nodes will converge towards a global optimal result through usually multiple iterations of computations distributed across multiple nodes.
A good example is a router system in which each router has only the information it exchanges with its neighbours. At the start the neighbours known only part of the whole network system. Once a router gets more information from its neighbours it incorporates the new information into its view of the whole system, then spreads its view to its neighbours. Through multiple iteration of these steps, each separately computed by individual routers, all routers would settle on a consistent global view of the whole network system.
Another example could be a web ordering system where the browser initially gets a list of commonly order goods. The browser may have logic to track user viewing behaviour and make a decision to fetch from the server a different category of goods list, but does not send all the user behaviour parameters to the server. In this imaged example, the browser knows something the serer does not know, and the server does too. Thus the whole application would be a distributed system. In addition to this part of distributed nature, the user authentication could be done on one of the servers, the inventory is done on another server, and reservation on yet another one. Each of the servers involved would not have the whole information of the specific user browsing and ordering instance. But the aggregate work from all these nodes will fulfill the business need to sell more goods and satisfy more customers.
Opposite to the distributed, is the centralized, in that the computation logic would be always able to get the information of the whole picture.
Given this view, a client-server application can be a viewed as a distributed system if you think the client side involves non-trivial decision making. Or can be a centralized system if you think the client is dumb.
The service-oriented term is more about how the functional processing power is integrated into the system. In a service-oriented system, new capability may be introduced into the system at runtime by new API functionality discovery, or new logic capability discovery behind an unchanged API. Think about it, you could build an application that initially has no much built-in capability, then it expands its capability by discovering and incorporating new capability on the service providers. In contrast a traditional system needs to be built at build time, typically as a consequence of human-involving discussion-design-documentation activity. A service-oriented design is a good fit to a distributed system.