Transcript Outsourcing,subcontr..

Outsourcing, subcontracting and
COTS
Tor Stålhane
Contents
We will cover the following topics
• Testing as a confidence building activity
• Testing and outsourcing
• Testing COTS components
• Sequential testing
• Simple Bayesian methods
Responsibility
It is important to bear in mind that
• The company that brings the product to the
marketplace carries full responsibility for the
product’s quality.
• It is only possible to seek redress from the
company we outsourced to if we can show
that they did not fulfill their contract
Testing and confidence
The role of testing during:
• Development – find and remove defects.
• Acceptance – build confidence in the component
When we use testing for COTS or components
where the development has been outsourced or
developed by a subcontractor, we want to build
confidence.
A product trustworthiness pattern
System
definition
Product is
trustworthy
Trustworthiness
definition
Environment
definition
Product
related
Process
related
People
related
Means to create product trust
Based on the product trust pattern, we see that
we build trust based on
• The product itself – e.g. a COTS component
• The process – how it was developed and
tested
• People – the personnel that developed and
tested the component
A process trustworthiness pattern
Activity is
trustworthy
Argument by
considering
process
Team is
competent
Method
address
problem
Trustworthiness
definition
Process
definition
Process is
traceable
Means to create process trust
If we apply the pattern on the previous slide we
see that trust in the process stems from three
sources:
• Who does it – “Team is competent”
• How is it done – “Method addresses problem”
• We can check that the process is used
correctly – “Process is traceable”
Testing and outsourcing
If we outsource development, testing need to be
an integrated part of the development
process. Testing is thus a contract question.
If we apply the trustworthiness pattern, we
need to include requirements for
• The component - what
• The competence of the personnel – who
• The process – how
Outsourcing requirements - 1
When drawing up an outsourcing contract we
should include:
• Personnel requirements – the right persons
for the job. We need to see the CV for each
person.
• Development process – including testing. The
trust can come from
– A certificate – e.g. ISO 9001
– Our own process audits
Outsourcing requirements - 2
Last but not least, we need to see and inspect
some important artifacts:
• Project plan – when shall they do what?
• Test strategy – how will they test our
component requirements?
• Test plan – how will the tests be run?
• Test log – what were the results of the tests?
Trust in the component
The trust we have in the component will depend
on how satisfied we are with the answers to
the questions on the previous slide.
We can, however, also build our trust on earlier
experience with the company. The more we
trust the company based on earlier
experiences, the less rigor we will need in the
contract.
Testing COTS
We can test COTS by using e.g. black box testing
or domain partition testing.
Experience has shown that we will get the
greatest benefit from our effort by focusing on
tests for
• Internal robustness
• External robustness
Robustness – 1
There are several ways to categorize these two
robustness modes. We will use the following
definitions:
• Internal robustness – the ability to handle
faults in the component or its environment.
Here we will need wrappers, fault injection
etc.
• External robustness – the ability to handle
faulty input. Here we will only need the
component “as is”
Robustness – 2
The importance of the two types of robustness
will vary over component types.
• Internal robustness - components that are
only visible inside the system border
• External robustness – components that are
part of the user interface.
Internal robustness testing
Internal robustness is the ability to
• Survive all erroneous situations, e.g.
– Memory faults – both code and data
– Failing function calls, including calls to OS
functions
• Go to a defined, safe state after having given
the error message
• Continued after the erroneous situation with a
minimum loss of information.
Why do we need a wrapper
By using a wrapper, we obtain some important
effects:
• We control the component’s input, even
though the component is inserted into the
real system.
• We can collect and report input and output
from the component.
• We can manipulate the exception handling
and effect this component only.
What is a wrapper – 1
A wrapper has two essential characteristics
• An implementation that defines the functionality
that we wish to access. This may, or may not be an
object (one example of a non-object implementation
would be a DLL whose functions we need to access).
• The “wrapper” class that provides an object interface
to access the implementation and methods to
manage the implementation. The client calls a
method on the wrapper which access the
implementation as needed to fulfill the request.
What is a wrapper – 2
A wrapper provides interface for, and services to,
behavior that is defined elsewhere
Fault injection – 1
On order to test robustness, we need to be able
to modify the component’s code – usually
through fault injection.
A fault is an abnormal condition or defect which
may lead to a failure.
Fault injection involves the deliberate insertion
of faults or errors into a computer system in
order to determine its response. The goal is
not to recreate the conditions that produced
the fault
Fault injection – 2
There are two steps to Fault Injection:
• Identify the set of faults that can occur
within an application, module, class,
method. E.g. if the application does not use
the network then there’s no point in
injecting network faults
• Exercise those faults to evaluate how the
application responds. Does the application
detect the fault, is it isolated and does the
application recover?
Example
byte[] readFile() throws IOException {
...
final InputStream is = new FileInputStream(…);
...
while((offset < bytes.length) &&
(numRead = is.read(bytes,offset,(bytes.length-offset))) >=0)
offset += numRead;
...
is.close();
return bytes;
}
What could go wrong with this code?
• new FileInputStream() can throw FileNotFoundException
• InputStream.read() can throw IOException and
IndexOutOfBoundsException and can return -1 for end of file
• is.close() can throw IOException
Fault injection – 3
• Change the code
– Replace the call to InputStream.read()
with some local instrumented method
– Create our own instrumented InputStream
subclass possibly using mock objects
– Inject the subclass via IoC (requires some
framework such as PicoContainer or Spring)
• Comment out the code and replace with
throw new IOException()
Fault injection – 4
Fault injection doesn’t have to be all on or
all off. Logic can be coded around injected
faults, e.g. for InputStream.read():
• Throw IOException after n bytes are
read
• Return -1 (EOF) one byte before the
actual EOF occurs
• Sporadically mutate the read bytes
External robustness testing – 1
Error handling must be tested to show that
• Wrong input gives an error message
• The error message is understandable for the
intended users
• Continued after the error with a minimum loss
of information.
External robustness testing – 2
External robustness is the ability to
• Survive the input of faulty data – no crash
• Give an easy-to-understand error message
that helps the user to correct the error in the
input
• Go to a defined state
• Continue after the erroneous situation with a
minimum loss of information.
Easy-to-understand message – 1
While all the other characteristics of the
external robustness are easy too test, the
error message requirement can only be tested
by involving the users.
We need to know which info the user needs in
order to:
• Correct the faulty input
• Carry on with his work from the component’s
current state
Easy-to-understand message – 2
The simple way to test the error messages is to
have a user to
• Start working on a real task
• Insert an error in the input at some point
during this task
We can then observe how the user tries to get
out of the situation and how satisfied he is
with the assistance he get from the
component.
Sequential testing
In order to use sequential testing we need:
• Target failure rate p1
• Unacceptable failure rate p2 and p2 > p1
• The acceptable probability of doing a type I or
type II decision error – a and b. These two
values are used to compute a and b, given as
b
a  ln
1a
b  ln
1 b
a
Background - 1
We will assume that the probability of failure is
Binomially distributed. We have:
 n x
n x
f ( x, p, n)    p (1  p)
 x
The probability of and the probability of
observing the number-of-defects sequence x1,
x2,…xn can be written as
N
 xi
f ( x1 , x2 ,...xN , p, n)  Cp i1 (1  p)
Nn 
N
 xi
i 1
Background - 2
We will base our test on the log likelihood ratio,
which is defined as:
 f ( xi , p1, n)
N
p1 
 1  p1
ln   ln
  xi ln   Nn   xi  ln
 f ( xi , p2 , n) i1 p2  i1  1  p2
N
For the sake of simplicity, we introduce
p1
1  p1
u ln , v  ln
p2
1  p2
The test statistics
Using the notation from the previous slide, we
find that
n
b  ln   a  b  Nnv  (u  v) xi  a  Nnv
i 1
N
b  Mv
a  Mv
  xi 
u v
u v
i 1
We have p1, p2 << 1 and can thus use the
approximations ln(1-p) = -p, v = (p2 – p1) and
further that (u – v) = u
Sequential test – example
We will use a = 0.05 and b = 0.20. This will give
us a = -1.6 and b = 2.8.
We want a failure rate p1 = 10-3 and will not
accept a component with a failure rate p2
higher than 2*10-3. Thus we have u = - 0.7 and
v = 10-3.
The lines for the “no decision” area are
• Sxi(reject) = - 4.0 + M*10-3
• Sxi(accept) = 2.3 + M*10-3
Sequential test – example
Sx
2.3
M
4*103
-4.0
Accept
Sequential testing - summary
• Testing software – e.g. p < 10-3:
The method needs a large number of tests. It
should thus only be used for testing robustness
based on automatically generated random
input.
• Inspecting documents – e.g. p < 10-1:
The method will give useful results even when
inspecting a reasonable number of documents
Simple Bayesian methods
Instead of building our trust on only test results,
contractual obligations or past experience, we
can combine these three factors.
The easy way to do this is to use Bayesian
statistics.
We will give a short intro to Bayesian statistics
and show one example of how it can be
applied to software testing
Bayes theorem
In a simplified version., Bayes’ theorem says that
P( B | A)  P( A | B) P( B)
When we want to estimate B, we will use the
likelihood of our observations as our P(B|A)
and use P(B) to model our prior knowledge.
A Bayes model for reliability
For reliability it is common to use a Beta
distribution for the reliability and a Binomial
distribution for the number of observed
failures. This gives us the following results:
P( X | obs)  p (1  p)
x
P( X | obs)  p
n x
x a 1
a 1
p (1  p)
(1  p)
n x  b 1
b 1
Estimates
A priori we have that
^
R
a
a b
If x is the number of successes and n is the total
number of tests, we have posteriori, that
ax
R
a b n
^
Some Beta probabilities
Testing for reliability
We will use a Beta distribution to model our
prior knowledge. The knowledge is related to
the company that developed the component
or system, e.g.
• How competent are the developers
• How good is their process, e.g.
– Are they ISO 9001 certified
– Have we done a quality audit
• What is our earlier experience with this
company
Modeling our confidence
Several handbooks on Bayesian analysis contain
tables where we specify two out of three
values:
• R1: our mean expected reliability
• R2: our upper 5% limit. P(R > R2) = 0.05
• R3: our lower 5% limit. P(R < R3) = 0.05
When we know our R-values, we can read the
two parameters n0 and x0 out of a table.
The result
We an now find the two parameters for the
prior Beta distribution as:
• a = x0
• b = n0 – x0
if we run N tests and observe x successes then
the Bayesian estimate for the reliability is:
R = (x + x0) / (N + n0)
Sequential test with Bayes
We can combine the info supplied by the
Bayesian model with a standard sequential
test chart by starting at (n0 - x0, n0) instead of
starting at origo as shown in the example on
the next slide. Note - we need to use n0 - x0,
since we are counting failures
We have the same number of tests necessary,
but n0 of them are virtual and stems from our
confidence in the company.
Sequential test with Bayes – example
Sx
2.3
n0
-4.0
M
4*103
Accept