The Art of Defensive Programming By Ipan Ardian
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Defensive programming is a form of programming intended to ensure software continues functioning under unexpected conditions. It is important for software dealing with safety, security, or high availability. Without defensive programming, bugs can have serious consequences like falsely reporting missile attacks, delivering fatal radiation overdoses, or destroying expensive rockets. Defensive practices include writing solid code following principles like single responsibility, testing units of code, conducting code reviews, and tracking errors in real time.
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Software Construction
Defensive Programming
Textbook
Steve McConnell. Code Complete: A Practical Handbook of Software Construction. 2nd Edition. Microsoft Press, 2004. Chapter 8.
2
Outline
Protecting from Invalid Inputs
Using Assertions
Error-Handling Techniques
Using Exceptions
Barricade/Firewall Your Program
Debugging Aids
Be Defensive about Defensive Programming
3
Protecting from Invalid Inputs
Garbage in, garbage out?
Mark of sloppy nonsecure program
Garbage in, nothing/error message out
No garbage allowed in
Defensive programming
If a routine is passed bad data, it won’t be hurt, even if the bad data is another routine’s fault.
Check values of all data from external sources
Sources: file, user, network
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Check values of all routine input parameters
Decide how to handle bad inputs
4
Assertions
To document assumptions made in the code and to flush out unexpected conditions
An input/output parameter in its expected range
A file/stream is open (or closed) when a routine begins (or ends) executing
A file/stream is at the beginning
A file/stream is open for read-only, write-only, etc.
An input-only variable is not changed
A pointer is non-null
A container is empty (or full) when a routine begins (or ends) executing
Match two results
5
Assertion Mechanism
Java assertions
Assert denominator!=0 : “denominator is 0”
Define your own
#define ASSERT(condition, message) {
if (!(condition)) {
LogError(“Assertion failed: ”, #condition, message);
exit(EXIT_FAILURE);
}
}
6
Using Assertions: Guidelines
Use error-handling for conditions you expect to occur; use assertions for condition that should never occur
Error handling typically checks for bad input
Assertions check for bugs in the code
Avoid putting executable code into assertions
The compiler will eliminate the code when the assertions are turned off
Use assertions to document/verify pre/postconds
For highly robust code, assert and then handle the error anyway.
E.g. change an invalid input to the closest legal value
7
Error-Handling Techniques
Return a neutral value (known to be harmless)
Examples: 0, empty string, default color
Substitute the next place of valid data
Encounter a corrupted record when reading from a DB
Return the same answer as the previous time
Thermometer-reading code does not get a reading one time
Substitute the closest legal value
Log a warning message to a file
Return an error code
8
Error-Handling Techniques
Call an error processing routine/object
Centralize error handling in a global routine/object
Display an error message wherever the error is encountered
Handle the error in whatever way works best locally
Individual developers have the flexibility
Risky: spread UI code throughout the system
Shut down
Safety critical applications
9
Using Exceptions: Suggestions
Use exceptions to notify other parts about errors that should not be ignored
Throw an exception only for conditions that are truly ex ...
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2
Outline
Protecting from Invalid Inputs
Using Assertions
Error-Handling Techniques
Using Exceptions
Barricade/Firewall Your Program
Debugging Aids
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Assertions
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Use error-handling for conditions you expect to occur; use assertions for condition that should never occur
Error handling typically checks for bad input
Assertions check for bugs in the code
Avoid putting executable code into assertions
The compiler will eliminate the code when the assertions are turned off
Use assertions to document/verify pre/postconds
For highly robust code, assert and then handle the error anyway.
E.g. change an invalid input to the closest legal value
7
Error-Handling Techniques
Return a neutral value (known to be harmless)
Examples: 0, empty string, default color
Substitute the next place of valid data
Encounter a corrupted record when reading from a DB
Return the same answer as the previous time
Thermometer-reading code does not get a reading one time
Substitute the closest legal value
Log a warning message to a file
Return an error code
8
Error-Handling Techniques
Call an error processing routine/object
Centralize error handling in a global routine/object
Display an error message wherever the error is encountered
Handle the error in whatever way works best locally
Individual developers have the flexibility
Risky: spread UI code throughout the system
Shut down
Safety critical applications
9
Using Exceptions: Suggestions
Use exceptions to notify other parts about errors that should not be ignored
Throw an exception only for conditions that are truly ex ...
Joe armstrong erlanga_languageforprogrammingreliablesystems
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### How TDM Works
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The Art of Defensive Programming By Ipan Ardian
- 1. THE ART OF DEFENSIVE PROGRAMMING MEETUP CIREBON DEV #3 IPAN ARDIAN
- 2. <HELLO!/> Ipan Ardian Sr. Software Engineer Indosystem & Loket (GoJek Group) 9+ years’ experience http://ipanardian.com 2
- 3. WHAT’S DEFENSIVE PROGRAMMING Let’s start with the definition first 3
- 4. 4 Defensive programming is a form of defensive design intended to ensure the continuing function of a piece of software under unforeseen circumstances. Defensive programming practices are often used where high availability, safety or security is needed — Wikipedia
- 5. WHY DEFENSIVE PROGRAMMING Does it matter 5
- 6. 6 WW3, ALMOST… In 1980, NORAD reported that the US was under missile attack. The problem was caused by a faulty circuit, a possibility the reporting software hadn’t taken into account. DEADLY RADIATION THERAPY A bug in the code controlling the Therac-25 radiation therapy machine was directly responsible for at least five patient deaths in the 1980s when it administered excessive quantities of X-rays. BUGS
- 7. 7 ROCKET LAUNCH ERRORS The European Space Agency’s Ariane 5 Flight 501 was destroyed 40 seconds after takeoff (June 4, 1996). The US$1 billion prototype rocket self-destructed due to a bug in the on-board guidance software. LOST IN SPACE One of the subcontractors NASA used when building its Mars climate orbiter had used English units instead of the intended metric system, which caused the orbiter’s thrusters to work incorrectly. Due to this bug, the orbiter crashed almost immediately when it arrived at Mars in 1999. The cost of the project was $327 million.
- 8. “You can’t sleep well if you are not confident that your last commit didn’t take down the whole application” 8
- 10. 10
- 11. 11 Try to be as strict as possible. Assert that your input values are what you expect. Never Trust User Input
- 12. Don’t write STUPID code, use SOLID code!! Write SOLID Code 12
- 13. 13 From STUPID to SOLID Code! STUPID Singleton Tight Coupling Untestability Premature Optimization Indescriptive Naming Duplication SOLID Single Responsibility Principle Open/Closed Principle Liskov Substitution Principle Interface Segregation Principle Dependency Inversion Principle
- 14. Writing unit tests will help you adhering to common principles such as High Cohesion, Single Responsibility, Low Coupling and right object composition Test test test! 14
- 16. As developers shouldn’t trust others developers’ code. We shouldn’t trust our code neither. Code Review 16
- 17. Stop hoping your users will report errors. Monitor and fix crashes in real time. Iterate continuously. Boost efficiency. Improve user experience. Tracking Error 17