Dependable Systems I, Reliability-Software Engineering-Lecture Slides, Slides of Software Engineering

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CS 501: Software Engineering

Fall 2000

Lecture 21

Dependable Systems I Reliability

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Administration

Assignment 3

• Report due tomorrow at 5 p.m. group design with individual parts
• Presentations Wednesday through Friday every group member must present during the semester

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Reliability Metrics

• Probability of failure on demand
• Rate of failure occurrence (failure intensity)
• Mean time between failures
• Availability (up time)
• Mean time to repair
• Distribution of failures

Hypothetical example: Cars are safer than airplane in accidents (failures) per hour, but less safe in failures per mile.

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Reliability Metrics for Distributed Systems

Traditional metrics are hard to apply in multi-component systems:

• In a big network, at a given moment something will be giving trouble, but very few users will see it.
• A system that has excellent average reliability may give terrible service to certain users.
• There are so many components that system administrators rely on automatic reporting systems to identify problem areas.

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Cost of Improved Reliability

Up time 99% 100% Will you spend your money on new functionality or improved reliability? docsity.com

Specification of System Reliability

Example: ATM card reader

Failure class Example Metric Permanent System fails to operate 1 per 1,000 days non-corrupting with any card -- reboot Transient System can not read 1 in 1,000 transactions non-corrupting an undamaged card Corrupting A pattern of Never transactions corrupts database docsity.com

Principles for Dependable Systems

High-quality has to be built-in => Each stage of development must be done well => Testing and correction does not lead to quality => Changes should be incorporated into the structure

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Quality Management Processes

Assumption: Good processes lead to good software The importance of routine: Standard terminology (requirements, specification, design , etc.) Software standards (naming conventions, etc.) Internal and external documentation Reporting procedures docsity.com

Design and Code Reviews

• Colleagues review each other's work: can be applied to any stage of software development can be formal or informal
• The developer provides colleagues with: documentation (e.g., specification or design), or code listing talks through the work while answering questions
• Most effective when developer and reviewers prepare well

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Benefits of Design and Code Reviews

Benefits:

• Extra eyes spot mistakes, suggest improvements
• Colleagues share expertise; helps with training
• An occasion to tidy loose ends
• Incompatibilities between modules can be identified
• Helps scheduling and management control Fundamental requirements:
• Senior team members must show leadership
• Must be helpful, not threatening docsity.com

Statistical Testing

• Determine the operational profile of the software
• Select or generate a profile of test data
• Apply test data to system, record failure patterns
• Compute statistical values of metrics under test conditions

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Statistical Testing

Advantages:

• Can test with very large numbers of transactions
• Can test with extreme cases (high loads, restarts, disruptions)
• Can repeat after system modifications Disadvantages:
• Uncertainty in operational profile (unlikely inputs)
• Expensive
• Can never prove high reliability

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Example: Dartmouth Time Sharing (1980)

Step 2. Analyze the data.

• Weekly, monthly, and annual statistics Number of failures and interruptions Mean time to repair
• Graphs of trends by component, e.g., Failure rates of disk drives Hardware failures after power failures Crashes caused by software bugs in each module docsity.com

Example: Dartmouth Time Sharing (1980)

Step 3. Invest resources where benefit will be maximum, e.g.,

• Orderly shut down after power failure
• Priority order for software improvements
• Changed procedures for operators
• Replacement hardware

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