Testing a Media Proxy with …

QuickCheckThomas Arts

John Hughes

Chalmers/ITU

Joakim JohanssonUlf Wiger

Ericsson

QuickCheck: Properties not Test Cases

• Write general properties of code instead of test cases

• Test in many, many randomly generated cases

prop_reverse() -> ?FORALL(Xs,list(int()), ?FORALL(Ys,list(int()), lists:reverse(Xs++Ys)== lists:reverse(Xs)++lists:reverse(Ys))).

• Simplification is extremely important—separates the signal from the noise!

QuickCheck Testing

3> eqc:quickcheck(test:prop_reverse())............Failed! After 12 tests.[-3,2][-3,1]Shrinking..........(10 times)[0][1]false

A random counter-example: Xs and Ys

NEW: Automatic simplification of failing

cases

Why Did it Fail?

• QuickCheck says: Xs=[0], Ys=[1]

• reverse([0,1])==reverse([0])++reverse([1])?• Xs, Ys the wrong way round

prop_reverse() -> ?FORALL(Xs,list(int()), ?FORALL(Ys,list(int()), lists:reverse(Xs++Ys)== lists:reverse(Xs)++lists:reverse(Ys))).

QuickCheck in a Nutshell

• Features– Properties, not test cases– Controlled random generation– Automated simplification

• Result– Compact, reusable test code– Specification checked against the code– Shortcut from property to bug

Property First Development?

• Code code code… quickcheck

• Fix it… quickcheck

• Fix it… quickcheck… quickcheck quickcheck quickcheck

• Code some more…

A Bug!

A Bug!

Ericsson’s Media Proxy

IP network, servers, etc etc

Media stream

Media Gateway Controller

How hard can it be?

• All we need to do is open and close ”media pinholes” when the controller says…

• Megaco H.248 protocol– ITU standard… 212 pages!– Ericsson Interwork Description… 183 pages!– Control software… 150KLOC

• 20-30K Megaco

Our StrategyQuickCheck

Sends random command sequences

Results make sense?

Completely random commands are all just

rejected—non-sensical!Poor test data

Generating Sensible Messages

MediaDescriptor ::= SEQUENCE{

streams CHOICE{

oneStream StreamParms,multiStream SEQUENCE OF StreamDescriptor

}}

Message Generators

• Example: A media descriptor contains a list of streams– ASN.1 from the standard (simplified):

QuickCheck Generator

mediadescriptor(Streams) when Streams=/=[]-> {mediaDescriptor, #'MediaDescriptor'{ streams = case Streams of [{Id,Mode}] -> oneof([{oneStream,streamParms(Mode)}, {multiStream,[stream(Id,Mode)]}]); _ -> {multiStream, [stream(I,M) || {I,M}<-Streams]} end}}.

stream(I,Mode) -> #'StreamDescriptor'{ streamID = I, streamParms = streamParms(Mode)}.

Message constructionLogic from the IWD

QuickCheck

Records generated by ASN.1 compiler

Conditions in the IWDAdd Request

Desc. Desc. Desc. Properties, Package M Comment

Media M

Stream M Multiple Stream descriptors can be included.

Localcontrol

O LocalControl will be included in all cases except when no media (m-line) is defined in the remote SDP.

mode O The default value of the mode property is “Inactive”, the property is not mandatory if the wanted value is “Inactive”.

StreamParms ::= SEQUENCE{

localControlDescriptor LocalControlDescriptor OPTIONAL,localDescriptor LocalRemoteDescriptor OPTIONAL,remoteDescriptor LocalRemoteDescriptor OPTIONAL,…,statisticsDescriptorStatisticsDescriptor OPTIONAL

}

Remote SDP in here

streamParms(Mode) -> ?LET(RemoteMediaDefined, bool(), if RemoteMediaDefined ->

#'StreamParms'{ localControlDescriptor = localControl(Mode),

localDescriptor = localDescriptor(RemoteMediaDefined),

remoteDescriptor = remoteDescriptor(RemoteMediaDefined)}; true -> …… end).

Two Cases: With and Without Remote Media

Included in this case

Passed on to ensure an m-line is generated

Generating Sensible Commands

Megaco Commands

ContextContext

Megaco Commands

Context

ContextContext

Termination

Add

Megaco Commands

Context

ContextContext

Termination

Termination

Add

Megaco Commands

Context

ContextContext

Termination

TerminationStream

StreamModify

Uses termination ID

Megaco Commands

Context

ContextContext

Termination

TerminationStream

Stream

Subtract

Megaco Commands

Context

ContextContext

Termination

StreamStream

Subtract

Megaco Commands

ContextContext

Sensible Command Sequences?

• Track the state of each test case– What calls in progress, which terminations in each

call…– Simple to model in Erlang

• State machine model– Preconditions say which commands are appropriate– Postconditions check command results– Next state function specifies how commands behave

• QuickCheck library– generates valid sequences, tests, and shrinks them– (developed ”just in time”)

Faults

• Proxy was already well tested

• 6 days of work, mostly writing generators

• 5 faults found:– One in Megaco message encoding/decoding– Four command sequences crashed

Error-provoking Sequences

•

•

•

Add Mod

Add Sub

Add ModAdd

Streams must have two ends

Here today, gone tomorrow…

…with differing numbers of streams

The Best Error!

Add Add Sub Add Sub Add Sub

Due to data corruption here

Shrinking reduced 160 commands to seven!

No one in their right minds would test this!

”Most Likely Bug” Effect

• Once a bug is found, almost every QuickCheck run finds the same one!– (or at least, shrinks to it)

• Add bug preconditions to the model to avoid it and find the next one– Formulate a ”bug hypothesis”– Test the hypothesis!– Document it as a precondition

Real bugs or not?

• Most of these sequences are not sent by Ericsson’s Media Gateway Controller– Controller and Proxy were tested together!

• But the Interwork Description doesn’t say so!

• One of the IWD/the code was buggy!

Using QuickCheck Earlier?

• Testing an older version– Using the same generators and model

• 9 errors found in 6 hours! – (2 TRs for this version)

• Most of the work: writing bug preconditions

QuickCheck is Moreish…

• Follow-up projects under way

• Testing non-Erlang systems (the big one!)

Conclusions

• QuickCheck + Erlang– Simple declarative models of the SUT– Concise, maintainable test code

• Testing: a great application for FP– Performance irrelevant– No need to commit to Erlang in the product

• A winning combination?