Distributed computing

MQ series basic idea

• It is necessary to coordinate MQ series and the receiver.
• What protocol does this sound like?
• What should we require about message delay/timeout?

MQ series answers

• Need a commit protocol to get this kind of coordination.
• MQ series supports the “XA” interface (every component can do two phase commit)
• No particular requirement on messages, but there could be blocking.

ROAD MAP: TECHNIQUES FOR REAL-TIME SYSTEMS

• Fault Tolerance

• Clock
• Synchronization

• Operating System
• Scheduling

• Real-Time

SYSTMES THAT CANNOT OR SHOULD NOT WAIT

• Time-sharing operating environments: concern for throughput.
• Want to satisfy as many users as possible.
• Soft real-time systems (e.g., telemarketing): concern for statistics of response time.
• Want a few disgruntled customers.
• Firm real-time systems (e.g., obtain ticker information on Wall Street): concern to meet as many deadlines as possible.
• If you miss, you lose the deal.
• Hard real-time systems (e.g., airplane controllers): requirements to meet all deadlines.
• If you miss, then airplane may crash.

DISTINCTIVE CHARACTERISTICS OR REAL-TIME SYSTEMS

• Predictability is essential – For hard, real-time systems the time to run a routine must be known in advance.
• Implication: Much programming is done in assembly language. Changes are done by “patching” machine code.
• Fairness is considered harmful – We do not want an ambulance to wait for a taxi.
• Implication: messages must be prioritized, FIFO queues are bad.
• Preemptibility is essential – An emergency condition must be able to override a low-priority task immediately.
• Implication: Task switching must be fast, so processes must reside in memory.
• Scheduling is of major concern – The time budget of an application is as important as its monetary budget. Meeting time constraints is more than just a matter of fast hardware.
• Implication: We must look at the approaches to scheduling.