Modular Open Systems Approach (MOSA) is an integrated business and technical strategy for the assessment and implementation of open systems in the DoD. An open system is a system that employs modular design tenets, uses widely supported and consensus-based standards for its key Interfaces, and is subject to Validation and Verification, including Test and Evaluation, to ensure the openness of its key interfaces.
Definition: A Modular Open Systems Approach (MOSA) is an acquisition, modular design, and technical strategy that utilizes open standards for designing an affordable and adaptable system.
An open systems design is a design approach for developing an affordable and adaptable open system. It derives inputs from both the technical management processes and technical processes undertaken within the systems engineering and other life-cycle processes, and in turn impacts these processes. The open systems design strategy should be implemented as part of the program’s overall technical approach and becomes an integral part of the program’s Systems Engineering Plan (SEP) and a summary in their Acquisition Strategy.
Benefits of Modular Open System Approach (MOSA)
The main benefits of using a MOSA as stated by :
- Significant cost-saving or avoidance
- Schedule reduction and rapidly deploy new technology
- Opportunities for technical upgrades and refresh
- Interoperability, including the system of systems interoperability and mission integration
Modular Open Systems Approach (MOSA) Standards
A key to the design of open systems is the use of open standards. The DoD Information Technology Standards Registry (DITSR) mandates the minimum set of standards and guidelines for the acquisition of all DoD systems that produce, use, or exchange information.
The standards use for MOSA are widely accepted, supported, and consensus-based standards set by recognized standards organizations or the marketplace. These standards support interoperability, portability, and scalability and are equally available to the public under fair and reasonable license terms. The Office of the Secretary of Defense (OSD) has determined continued implementation and further development of MOSA enabling standards are essential to the ability to rapidly share information across domains, with quick and affordable updates or improvements to both hardware and software components. 
Modular Open Systems Approach (MOSA) Requirement
Title 10 U.S.C. 2446a.(b), Sec 805 states that MOSA is the preferred method for the implementation of open systems, and it is required by United States law. A major defense acquisition program that receives Milestone A or Milestone B approval after January 1, 2019, shall be designed and developed, to the maximum extent practicable, with a modular open system approach to enable incremental development and enhance competition, innovation, and interoperability. 
5 Modular Open Systems Approach (MOSA) Principles
Programs should design their system based on adherence to the following five (5) MOSA principles:
- Establish an Enabling Environment: The Program Manager (PM) establishes supportive requirements, business practices, and technology development, acquisition, test and evaluation, and product support strategies needed for the effective development of open systems.
- Employ Modular Design: Effective modular design is contingent upon adherence to four major modular design tenets:
- Cohesive (contain well-focused and well-defined functionality)
- Encapsulated (hide the internal workings of a module’s behavior and its data)
- Self-contained (do not constrain other modules)
- Highly binned (use broad modular definitions to enable commonality and reuse)
- Designate Key Interfaces: Interfaces should be group into key and non-key interfaces. Such distinction enables designers and configuration managers to distinguish among interfaces that exist between technologically stable and volatile modules, between highly reliable and more frequently failing modules, between modules that are essential for net-centricity and those that do not perform net-centric functions, and between modules that pass vital interoperability information and those with least interoperability impact.
- Use Open Standards: Interface standards should be well defined, mature, widely used, and readily available.
- Certify Conformance: The openness of systems is verified, validated, and ensured through rigorous and well-established assessment mechanisms, well-defined interface control and management, and proactive conformance testing. The program manager, in coordination with the user, should prepare validation and verification mechanisms such as conformance certification and test plans to ensure that the system and its component modules conform to the external and internal open interface standards allowing plug-and-play of modules, net-centric information exchange, and re-configuration of mission capability in response to new threats and evolving technologies.
Open systems characteristics and principles address:
- Design requirements (e.g., mandated open standards and protocols)
- Derived requirements (e.g., need for open interfaces to enable interoperability)
- Design constraints (e.g., need to adhere to open interface specifications as system components are designed)
- Architectural attributes (e.g., need for an adaptable, upgradeable, and reconfigurable system architecture)
- Design considerations (e.g., taking into considerations modular and open systems design benefits and concerns)
- Business strategies to gain access to competitive sources of supply and effectively manage technological obsolescence.
AcqLinks and Reference:
-  Wesbite: Title 10 U.S.C. 2446a.(b), Sec 805 Requirement for modular open system approach in major defense acquisition programs; definitions
- Open System Architecture (OSA) Contract Guidebook for Program Managers – June 13
- Open Systems Architecture (OSA) Contract Guidebook for Program Manager – Dec 11
- Program Managers Guide to Open Systems – Sept 2004
- MOSA Program Assessment and Review Tool (PART)