Sommerville-Software-Engineering-10ed

588 Chapter 20 ■ Systems of systems
parts of that organization. It also includes systems whose constituent systems are
owned and managed by different organizations that may, at times, compete with
each other. Maier (Maier 1998) devised a classification scheme for SoS based on
their governance and management complexity:
1. Directed systems. Directed SoS are owned by a single organization and are
developed by integrating systems that are also owned by that organization. The
system elements may be independently managed by parts of the organization.
However, there is an ultimate governing body within the organization that can
set priorities for system management. It can resolve disputes between the managers
of different elements of the system. Directed systems therefore have some
managerial complexity but no governance complexity. A military commandand-control
system that integrates information from airborne and ground-based
systems is an example of a directed SoS.
2. Collaborative systems. Collaborative SoS are systems with no central authority
to set management priorities and resolve disputes. Typically, elements of the
system are owned and governed by different organizations. However, all of the
organizations involved recognize the mutual benefits of joint governance of
the system. They therefore usually set up a voluntary governance body that
makes decisions about the system. Collaborative systems have both managerial
complexity and a limited degree of governance complexity. An integrated
public transport information system is an example of a collaborative system of
systems. Bus, rail, and air transport providers agree to link their systems to
provide passengers with up-to-date information.
3. Virtual systems. Virtual systems have no central governance, and the participants
may not agree on the overall purpose of the system. Participant systems
may enter or leave the SoS. Interoperability is not guaranteed but depends on
published interfaces that may change. These systems have a very high degree of
both managerial and governance complexity. An example of a virtual SoS is an
automated high-speed algorithmic trading system. These systems from different
companies automatically buy and sell stock from each other, with trades taking
place in fractions of a second.
Unfortunately, I think that the names that Maier has used do not really reflect the
distinctions between these different types of systems. As Maier himself says, there is
always some collaboration in the management of the system elements. So, “collaborative
systems” is not really a good name. The term directed systems implies top-down
authority. However, even within a single organization, the need to maintain good
working relationships between the people involved means that governance is agreed
to rather than imposed.
In “virtual” SoS, there may be no formal mechanisms for collaboration, but the
system has some mutual benefit for all participants. Therefore, they are likely to collaborate
informally to ensure that the system can continue to operate. Furthermore,
Maier’s use of the term virtual could be confusing because “virtual” has now come
to mean “implemented by software,” as in virtual machines and virtual reality.