POSA2 Notes (1)

Inherent and accidental complexities:

• Inherent complexities
• (POSA2) Inherent complexities arise from fundamental domain challenges, such as dealing with partial failures, distributed deadlock, and end-to-end quality of service(QoS) requirements.
• Accidental complexities
• Accidental complexity - Wikipedia, the free encyclopedia
• Accidental (or incidental) complexity is complexity that arises in computer programs or their development process which is non-essential to the problem to be solved. 
• (POSA2) Accidental complexities arise from limitations with software tools and development techniques, such as non-portable programming APIs and poor distributed debuggers.
• Ironically, many accidental complexities stem from deliberate choices made by developers who favor low-level languages and tools that scale up poorly when applied to complex concurrent and networked software.

Topics (POSA2)

• Connection establishment and service initialization.
• Event demultiplexing and event handler dispatching.
• Interprocess comminication (IPC) and network protocols.
• Primary and secondary storage management and caching.
• Static and dynamic component configuration.
• Concurrency and synchronization.

Challenges of Concurrent and Networked Software (POSA2)

• Service Access and Configuration
• Accidental complexities for using low-level operating system APIs. Key design challengs for infrastructure networking or system programs center on minimizing the accidental complexities without sacrificing performance:
• Excessive low-level details.
• Continuous rediscovery and reinvention of incompatible higher-level programming abstractions
• High potential for errors
• Lack of portability
• Steep learning curve
• Inability to scale up to handle increasing complexity.
• A key design challenge at higher-level distributed object computing applications centers on ensuring that clients do not access invalid or unauthorized component service roles.
• Supporting the static and dynamic evolution of services and applications is another key challenge in networked software system.
• Interfaces to and connnectivity between component service roles can change, often at run-time, and new service roles can be implemented and installed into existing components.
• Distributed system performance can be improved by reconfiguring service load to harness the processing power of multiple hosts.
• Another design challenge is to ensure that an entire system need not be shutdown, recompiled, relinked, and restarted simply because a particular service role in a component is reconfigured or its load is redistrubted.
• It is even more challenging to determine how to access services that are configured into a system 'on-demand' and whose implementations are unknown when the system was designed originally.
• Event Handling
• Concurrency
• Synchronization
• Others