If we look at traditional SOA, business transactions are modeled typically as service operations that are part of a service contract. Operation invocations in traditional SOA are not treated as first class “objects”. Operation invocations do not have own identity. Components/processes inside of a service and service clients cannot reference individual operation calls. Situation is different if we look at subject-centric and RESTFul services.
If a client of some subject-centric (or RESTFul) service needs to start a transaction, this client should create a new subject “request for a transaction” with own identity and internal state. Subject-centric service processes this request and some other subjects can be created/updated/deleted as a result of this operation. Service clients have direct access to subjects that represent transactions. Clients can check status of any initiated transaction. It is also possible to use a general query/search interface for finding various subsets of transactions.
Service invocation results can be presented as a special kind of subjects which are linked to original requests for transactions. Subject-centric services also can record “cause and effect” relationships that connect a request for a transaction and results of implementing this transaction as a network of related “events”. Subject-centric computing promotes (and helps) to build transparent services.
It is true that subject-centric services can generate many more subjects (and assertions about subjects) in comparison with modern SOA/object-oriented systems. But Subject-centric computing is in a unique position to leverage available hardware parallelism and distributed storage. Subject-centric services model changes in time differently from traditional computing systems. Subject-centric services do not do “updates”, they just add new assertions about subjects in a new time-specific context. Subject-centric services also have a built-in mechanism for merging assertions from multiple sources so new assertions can be created on different physical storage devices. Computations in subject-centric world can be described using data flow abstractions which allow perfect (natural) parallelism.