Robust Software – Robust Software – Dotting the I’s and Dotting the I’s and

Crossing the T’sCrossing the T’sChris OldwoodChris Oldwood

ACCU Conference 2013ACCU Conference 2013

@chrisoldwood / @chrisoldwood / gort@cix.co.ukgort@cix.co.uk

The I’s & T’sThe I’s & T’s

RobustnessRobustness Handling ErrorsHandling Errors Safely Ignoring ErrorsSafely Ignoring Errors TimeoutsTimeouts Unit Testing FailuresUnit Testing Failures Flexible ConfigurationFlexible Configuration Monitoring ClarityMonitoring Clarity

RobustnessRobustness

Stable in the face of Stable in the face of unexpected unexpected behaviourbehaviour

Pop Quiz – Exit Code?Pop Quiz – Exit Code?

int main(int argc, char* argv[]){ throw UnhandledException();}

Exit Code ConventionExit Code Convention

program.exe

if %errorlevel% neq 0 ( echo ERROR: Program failed exit /b 1)

Big Outer Try BlockBig Outer Try Blockint main(int argc, char* argv[]){ try { return DoUsefulWork(argc, argv); } catch (const std::exception& e) { /* Report failure */ } catch (…) { /* Report failure */ }

return EXIT_FAILURE;}

Module BoundariesModule BoundariesHRESULT DoSomething(...){ try { return Impl::DoSomething(...); } catch (const std::bad_alloc& e) { return E_OUTOFMEMORY; } catch (const std::exception& e) { return E_FAIL; } catch (...) { return E_UNEXPECTED; }}

Exception Safety Exception Safety GuaranteesGuarantees

NoneNone BasicBasic StrongStrong No ThrowNo Throw

Exception Unsafe CodeException Unsafe CodeIServicePtr AcquireService(){ if (!m_service) { m_service = new Service(); m_service.CreateInstance(); }

return m_service;}

IServicePtr m_service;

Exception Safe CodeException Safe CodeIServicePtr AcquireService(){ if (!m_service) { ServicePtr service = new Service(); service.CreateInstance();

m_service.swap(service); }

return m_service;}

IServicePtr m_service;

Forever is a Really Long Forever is a Really Long TimeTime

Handle completed = BeginAsyncOperation();. . .Wait(completed, INFINITE);

Cancellable OperationsCancellable Operations

Handle completed = BeginAsyncOperation();Handle aborted = GetAbortHandle();Handle waitables[] = { aborted, completed };. . .Handle signalled = Wait(waitables, timeout);

if (signalled == aborted){

Retries: immediate then Retries: immediate then queuedqueued

Unit Testing FailuresUnit Testing Failures

Testing Write+Rename Testing Write+Rename IdiomIdiom

[Test]public Void OriginalFilePreservedOnException(){ var fakeIo = new FakeIo();

fakeIo.Write = (file, buffer) => { throw new IoException(); }

var writer = new WriterService(fakeIo); var filename = “original.txt”;

Assert.Throws(() => writer.WriteFile(filename)); Assert.True(fakeIo.FileExists(filename)); Assert.That(. . .);}

Flexible ConfigurationFlexible Configuration

Monitoring ClarityMonitoring Clarity

Release It!Release It!

Questions?Questions?

Blog:Blog:http://chrisoldwood.blogspot.comhttp://chrisoldwood.blogspot.com

@chrisoldwood / @chrisoldwood / gort@cix.co.ukgort@cix.co.uk