scope:

This document focuses on basic concepts of interactions between system components and therefore clarifies what the concept "service" stands for. As an initial document on this topic it facilitates the consensus finding within the Industrie 4.0 initiative.

The scope of this RM-SA DIN specification is given by the following three determinants:

1) the DIN SPEC of the Reference Architecture Model Industrie 4.0 (RAMI4.0) (subclause 1.1);

2) the purpose of a reference model dedicated to service-oriented architectures (SOA) (subclause 1.2);

3) the value chains to be considered in Industrie 4.0 (subclause 1.3).

RAMI4.0 dimensions

Looking at the RAMI4.0 and especially the three RAMI4.0 dimensions (see Figure 1), Industrie 4.0 services shall be applicable

- to all architectural aspects (RAMI4.0 Layers),

- to all hierarchy levels and, hence, all types of assets (from "product", "field device", "control device" up to the "connected world"), and

- to the whole lifecycle and value stream of both asset types and asset instances.

NOTE A "service" is considered as an asset itself, e.g. in order to enable service management in the same sense as the management of assets of other types. As a consequence, a service needs a specification of a manifest (service meta-data). This specification categorizes the structure and the mandatory information of such a manifest which enables service identification, discovery, orchestration and choreography.

SOA reference models

Following the terminology of the Organization for the Advancement of Structured Information Standards (OASIS), a reference model is an "abstract framework for understanding significant relationships among the entities of some environment. It enables the development of specific reference or concrete architectures using consistent standards or specifications supporting that environment." [2]. Reference models set the conceptual foundation of distributed systems. Applied to service-oriented architectures (SOA), SOA reference models are meant to be meta-models for architectural specifications [3]. Hence, an RM-SA is an abstract framework that encompasses, among others, the

- identification and terminological definition of basic concepts (e.g. I4.0 Service) and their relationships within the scope of the Industrie 4.0 application domain,

- rules and notations to be applied,

- viewpoints to be described when defining Industrie 4.0 (reference) architectures, and

- statements that define what has to be fulfilled when claiming compliance to the RM-SA.

The RM-SA enables the specification of architectures, and this enablement encompasses both the conceptual and the technology-oriented implementation level. This is depicted in Figure 2.

In contrast to an architectural reference model, which defines the scope and the concepts that are basically to be considered in an architectural design, architectures derive from them the basic building blocks, their function, their relationships and their interfaces. This can be investigated just on conceptual level, i.e., abstracting from existing technologies, leading to conceptual architectures (left-hand side in Figure 2), or on technology level, leading to implementation architectures (right-hand side in Figure 2).

If such architectural specifications are open, based upon standards, solve commonly agreed requirements in a given domain (possibly derived from use cases, including their non-functional requirements), and can be implemented in an efficient and cost-effective way, they can be classified as reference architectures. Reference architectures "capture the essence of existing architectures and the vision of future needs and evolution to provide guidance to assist in developing new system architectures" [4].

Both conceptual and implementation architectures are characterized by structural elements that are reusable in various instantiations and can act as blueprint specifications. In a SOA such structural elements are architectural styles.

The RM-SA defines an Interaction-based Architecture (IBA). As such it follows the idea of interactions as the basic conceptual elements that define service behavior (see below). Hence, the choice and combination of interaction policies and patterns determine the architectural style that is supported [5], e.g. messageoriented, event-driven, request/reply or resource-oriented (REpresentational State Transfer (REST) [6]). In 8.2, the RM-SA defines a list of basic interaction patterns that can be combined to define architectural styles. Its purpose is to streamline the discussions between the Industrie 4.0 working committees and support consensus finding.

Value chains

Looking from the application perspective, the scope of the RM-SA is defined by value networks comprising interconnected I4.0 abstract value chains [7], such as Product/Product Line Engineering, Process and Production Plant Engineering, Production and After Sales Services and Production Plant Construction and Operation.

Note that, having in mind that value chains will become much more flexible in the future, they do not fix any concrete value chain or exclude any other value chain options. Furthermore, value chains will be increasingly performed in a network of enterprises, which can lead to a change in the ownership of assets during the value chain. Access control mechanisms for information in the asset administration shell has to consider this.

Each value chain couples value processes along the lifetime of an asset. Note that there are additional accompanying value chains such as the edition and maintenance of standards as well as methods, tools and technologies, both in software and hardware. They are essential for the overall performance but not described here.

The role of the end-user who shall be able to influence product design, engineering and production in various processes of the value chains shall be emphasized, too. This will result in dedicated end-user services, whose needs have to be considered, too, when discussing the basic concepts of services in Industrie 4.0.