Abstract:
An execution engine synchronizes test suite execution to run multiple independent operations concurrently or in parallel. The execution engine operates by stepping through the test suite. At each step it runs the required number of independent operations and waits for their completion. It then checks the results, and continues to the next step of the test suite. If at any step, the actual results are different from the expected results, then the exact sequence that triggered the fault is known. The sequence can be repeated when a correction is provided for the fault. Furthermore, the execution engine allows the user to interactively step through a test case when debugging a fault. A synchronization pattern generator may be incorporated in the system to generate minimal numbers of repetitions of test cases to guarantee varying degrees of interaction coverage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to software verification. More particularly, this invention relates to test program generation for validating the implementation of a specification of a software program that is capable of executing multiple independent operations.  
           [0003]    2. Description of the Related Art  
           [0004]    Concurrent and parallel software supports operations involving interdependent interactions between its classes and objects. Such operations may occur concurrently or in parallel. Concurrent operations occur in the same address space, but run in separate threads. Parallel operations occur in separate address spaces. Since the operations occur in separate address spaces, their associated objects may be distributed across multiple hosts.  
           [0005]    An important aspect of designing an advanced computer software system is the ability to thoroughly test its implementation in order to assure that the implementation complies with desired specifications. Usually, such verification requires the generation of test programs to verify that the system behaves properly under a wide variety of circumstances.  
           [0006]    Traditional software testing involves study of the software system by the tester, who then writes and executes individual test scenarios that exercise the software being tested. Testing may require validation of many functional levels, which substantially increases the complexity of the task. In the past, test scenarios were manually generated, but now this task is sometimes automated, using computer implemented test program generators.  
           [0007]    Test program generators are basically sophisticated software engines, which are used to create numerous test cases. By appropriate configuration, it is possible for test program generation either to be focused on very specific ranges of conditions, or to be broadened to cover a wide range 90 of logic. Today, large numbers of test cases can be created in the time that a single test case could be written manually.  
           [0008]    During the past decade, model-based random test program generators have become popular in processor architectural design verification and software testing. Model-based test generation involves the generation of a suite of tests from an abstract model of an application&#39;s behavior. The model is derived from a specification of the application. In many model-based testing situations, the behavior models are described as finite state machines (FSM). Such FSM models describe the possible states of the application and the transitions from state to state caused by operations or stimuli. Test suites generated from these behavior models cover different operation invocation patterns according to the testing goals.  
           [0009]    Typical of conventional approaches for generating test programs is U.S. Pat. No. 5,394,347 to Kita et al., which discloses a method of modeling a specification as an extended finite state machine, then performing a depth-first traversal of the resulting state diagram to generate a path file as a basis for a test program.  
           [0010]    In U.S. Pat. No. 5,918,037 to Tremblay et al., it is proposed to employ a test generator that automatically produces test programs based on a finite state machine model of the software. Limiting the number of test programs is achieved by controlling loop execution, and by appropriately setting the coverage level for the model, known as “transition cover testing”. This approach seeks to specify that each transition within the finite state machine model be exercised once. The generator is capable of specifying different coverage levels for selected portions of the program under test, so that critical portions might be exhaustively tested, while other portions receive less comprehensive testing.  
           [0011]    Test programs are repetitively executed by an execution engine, using suites of input data. The results of execution are compared in some way with design specification. Generic validation engines are known for this purpose. The validation engine need not have specific knowledge of the design, and can be used in the testing of many system components. Some validation engines maintain state tables for tracking outcomes, such as a sequence of events. Such an engine typically implements a predetermined algorithm for validating the outcome or sequence of events.  
           [0012]    The standard TTCN (ISO/IEC-9646) is a notation standardized by the International Organization for Standardization (ISO) and the Telecommunication Standardization Sector of the International Telecommunications Union (ITU-T) for the specification of tests for communicating systems. The standard TTCN has been employed in many test generation and execution environments. A TTCN-based model-based tool for the generation of test suites suitable for complex systems, known as the Telelogic Tau TTCN Suite, is produced by Telelogic AB, PO Box 4128, Kungsgatan 6, SE-203 12 Malmy, Sweden. This is an example of a commercial test execution engine, the input of which is an abstract TTCN test suite. The standard TTCN provides concurrency and synchronization primitives. However, a tester interested in validating concurrent aspects of a system-under-test is required to write specialized test cases.  
           [0013]    Another model-based generator is the GOTCHA-TCBeans Software Test Tool Kit, which has been developed by International Business Machines Corporation, New Orchard Road, Armonk, N.Y. 10504. This tool provides a framework designed to assist testers in developing, executing and organizing function tests directed against Application Program Interfaces (APIS) and software protocols written in Java™, C or C++.  
           [0014]    The product LoadRunner®, available from Mercury Interactive, Inc. 1325 Borregas Avenue 220, Sunnyvale, Calif. 94089, USA, emulates multiple users, called virtual users. The virtual users execute Vuser scripts, written in a proprietary testing language. The tester creates a Vuser script, parameterizes the script, and then configures the LoadRunner software to start the virtual user execution. If the tester wants to synchronize activity between the different virtual users he must add synchronization points to the scripts, known as rendezvous.  
           [0015]    Known test program generators are limited in their abilities to deal with systems having concurrent and parallel operations, for example multiprocessors and complex software suites such as client/server applications, database and transaction processing systems. The traditional approach to validating independent operations is either to model the independent behavior of the multiple operations or processes, or to run test cases in parallel.  
           [0016]    Modeling independent behavior is not a good solution, because it results in excessively large and intractable models. One of the biggest drawbacks of finite state machine modeling is state explosion. Modeling independent events exacerbates this problem. Running test cases concurrently or in parallel is even less effective, because it results in a lack of control and clarity. There may be many concurrent operations in a test case, and the location of a fault trigger that is uncovered by this technique is often obscure. Lack of control over the concurrent operations further complicates interactive fault debugging.  
           [0017]    In view of the above-noted drawbacks of conventional approaches, it would be desirable to improve the methods and techniques for the validation of concurrent and parallel systems.  
         SUMMARY OF THE INVENTION  
         [0018]    It is a primary advantage of some aspects of the present invention that a software system, which is capable of multiple independent concurrent operations, can be modeled and its implementation tested using a finite state machine having a small number of states.  
           [0019]    It is another advantage of some aspects of the present invention that faults can be conveniently located when testing a system implementation, in which the system is capable of multiple independent concurrent operations.  
           [0020]    It is a further advantage of some aspects of the present invention that faults can be interactively debugged during validation of a software system, in which the system is capable of multiple independent concurrent operations.  
           [0021]    These and other advantages of the present invention are attained by an arrangement for validating concurrent and parallel systems by exploiting model based test generation. In a preferred embodiment of the invention, an execution engine executes test programs by stepping through the test suite. At each step it runs the required number of independent operations and waits for their completion. It then checks the results, and continues to the next step of the test suite. If the actual results at any step are different from the expected results, then the exact sequence that triggered the fault is known. The sequence can be repeated when a correction is provided for the fault. Furthermore, the execution engine allows the user to interactively step through a test case when debugging a fault.  
           [0022]    The invention provides an apparatus for verification of a system implementation, including a generator for generating a test program for execution thereof on a system-under-test, and a synchronizer that organizes a plurality of test cases of the test program into discrete synchronization steps. An execution engine is responsive to the synchronizer for stimulating the system-under-test according to the test program, to cause the system-under-test to concurrently execute the test cases synchronously in the synchronization steps. The apparatus further includes a validation engine for comparing a response of the system-under-test with a predicted result upon completion of one of the synchronization steps.  
           [0023]    According to an aspect of the apparatus, a behavioral simulator for the system-under-test is constructed by the generator using a behavioral model of the system-under-test.  
           [0024]    According to another aspect of the apparatus, the test cases are executed in a first cloned object and a second cloned object.  
           [0025]    According to a further aspect of the apparatus, the test cases comprise a plurality of different test cases that execute on a plurality of cloned objects.  
           [0026]    According to an additional aspect of the apparatus, the test cases comprise a plurality of different test cases that execute on a plurality of different objects.  
           [0027]    According to one aspect of the apparatus, the test cases comprise a plurality of cloned processes.  
           [0028]    According to another aspect of the apparatus, the test cases execute on a plurality of cloned hosts.  
           [0029]    According to yet another aspect of the apparatus, the test cases are executed in concurrent synchronization.  
           [0030]    According to still another aspect of the apparatus, the test cases are executed in sequential synchronization.  
           [0031]    In yet another aspect of the apparatus, a synchronization step of a first one of the test cases is executed in a different execution interval than a corresponding synchronization step of a second one of the test cases.  
           [0032]    According to still another aspect of the apparatus, the test cases include a gap occurring at one of the synchronization steps.  
           [0033]    According to still another aspect of the apparatus, the gap includes a plurality of gaps randomly distributed at different synchronization steps.  
           [0034]    According to an additional aspect of the apparatus, the gap includes a plurality of gaps that are systematically distributed among the synchronization steps.  
           [0035]    Yet another aspect of the apparatus includes a pattern generator, which outputs synchronization patterns to the execution engine, the synchronization patterns providing a predetermined independence coverage degree for the test cases.  
           [0036]    According to one aspect of the apparatus, the pattern generator pre-computes the synchronization patterns prior to operation of the execution engine.  
           [0037]    According to another aspect of the apparatus, the pattern generator generates the synchronization patterns dynamically.  
           [0038]    According to a further aspect of the apparatus, the pattern generator is adaptive to the response of the system-under-test.  
           [0039]    According to still another aspect of the apparatus, an input of the pattern generator also includes an alphabet of test steps, a string of the alphabet, and a specification of a number of concurrent processes to be executed in the system-under-test.  
           [0040]    According to an additional aspect of the apparatus, the alphabet includes a gap.  
           [0041]    According to a further aspect of the apparatus, the pattern generator includes an optimizer for minimizing a number of the synchronization patterns that are generated therein.  
           [0042]    The invention provides a method for verification of a system implementation, including the steps of generating a test program for execution on a system-under-test, producing a behavioral model of the system-under-test, organizing a plurality of test cases of the test program into discrete synchronization steps, stimulating the system-under-test according to the test program, to cause the system-under-test to concurrently execute the test cases synchronously, and comparing a response of the system-under-test upon completion of synchronization steps with a predicted result according to the behavioral model.  
           [0043]    An aspect of the method includes constructing a behavioral simulator of the behavioral model.  
           [0044]    In an aspect of the method, stimulating the system-under-test is performed by concurrently executing one of the test cases in a first cloned object and in a second cloned object.  
           [0045]    In yet another aspect of the method stimulating the system-under-test includes executing a plurality of different test cases on a plurality of cloned objects.  
           [0046]    In one aspect of the method, stimulating the system-under-test includes executing a plurality of different test cases on a plurality of different objects.  
           [0047]    In another aspect of the method, stimulating the system-under-test includes executing a plurality of cloned processes.  
           [0048]    In a further aspect of the method, stimulating the system-under-test includes executing the test cases on a plurality of cloned hosts.  
           [0049]    In yet another aspect of the method stimulating the system-under-test includes executing the test cases in concurrent synchronization.  
           [0050]    In still another aspect of the method stimulating the system-under-test includes executing the test cases in sequential synchronization.  
           [0051]    In an additional aspect of the method stimulating the system-under-test includes executing a synchronization step of a first one of the test cases in a different execution interval than a corresponding synchronization step of a second one of the test cases.  
           [0052]    In one aspect of the method stimulating the system-under-test includes separating two of the synchronization steps by a gap.  
           [0053]    According to one aspect of the method, the gap includes a plurality of gaps randomly distributed at different ones of the synchronization steps.  
           [0054]    According to another aspect of the method, the gap includes a plurality of gaps that are systematically distributed among the synchronization steps.  
           [0055]    Another aspect of the method includes generating a plurality of synchronization patterns, the synchronization patterns providing a predetermined independence coverage degree for the test cases.  
           [0056]    In yet another aspect of the method generating the plurality of synchronization patterns is performed dynamically.  
           [0057]    In still another aspect of the method generating the plurality of synchronization patterns is performed responsive to the response of the system-under-test.  
           [0058]    In a further aspect of the method generating the synchronization patterns is performed by specifying an alphabet of test steps, defining a string of the alphabet, and specifying a number of concurrent processes to be executed in the system-under-test.  
           [0059]    According to yet another aspect of the method, the alphabet includes a gap.  
           [0060]    In still another aspect of the method step of generating the synchronization patterns includes minimizing a number of the synchronization patterns.  
           [0061]    The invention provides a computer software product for verification of a system implementation, including a computer-readable medium in which computer program instructions are stored, which instructions, when read by a computer, cause the computer to execute the steps of generating a test program for execution on a system-under-test, producing a behavioral model of the system-under-test, constructing a behavioral simulator for the behavioral model, organizing a plurality of test cases of the test program into discrete synchronization steps, stimulating the system-under-test according to the test program, to cause the system-under-test to concurrently execute the test cases synchronously in the synchronization steps, and comparing a response of the system-under-test upon completion of one of the synchronization steps with a predicted result according to the behavioral model.  
           [0062]    The invention provides a method for verification of a system implementation, including the steps of generating a test program for execution on a system-under-test, the test program including a plurality of processes, producing a behavioral model of the system-under-test, constructing a behavioral simulator of the behavioral model, generating a synchronization pattern for each of the processes, and stimulating the system-under-test according to the test program, wherein each of the processes execute concurrently in synchronization steps, according to the synchronization pattern. The method includes delaying during an execution interval until each of the processes has completed a respective one of the synchronization steps, and thereafter comparing a response of the system-under-test with a predicted result according to the behavioral model.  
           [0063]    The invention provides a computer software product, including a computer-readable medium in which computer program instructions are stored, which instructions, when read by a computer, cause the computer to perform a method for verification of a system implementation, including the steps of generating a test program for execution on a system-under-test, the test program including a plurality of processes, producing a behavioral model of the system-under-test, constructing a behavioral simulator based on the behavioral model, generating a synchronization pattern for each of the processes, stimulating the system-under-test according to the test program, wherein each of the processes execute concurrently in synchronization steps, according to the synchronization pattern. The method includes delaying during an execution interval until each of the processes has completed a respective one of the synchronization steps, and thereafter comparing a response of the system-under-test with a predicted result according to the behavioral model.  
           [0064]    The invention provides an apparatus for verification of a system implementation, including a test program generator for generating a test program for execution on a system-under-test, the test program including a plurality of processes, wherein the test program generator accesses a behavioral model of the system-under-test and responsive to the behavioral model, the test program generator constructs a behavioral simulator of the system-under-test. The apparatus includes a pattern generator for generating a synchronization pattern for each of the processes, an execution engine for stimulating the system-under-test according to the test program, wherein each of the processes execute concurrently in synchronization steps, according to the synchronization pattern. The execution engine delays the termination of an execution interval until each of the processes has completed a respective one of the synchronization steps. The apparatus includes a validation engine for comparing a response of the system-under-test with a predicted result according to the behavioral model. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0065]    For a better understanding of these and other objects of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein:  
         [0066]    [0066]FIG. 1 is a block diagram of a design verification system that is constructed and operable in accordance with a preferred embodiment of the invention;  
         [0067]    [0067]FIG. 2 is a block diagram illustrating in further detail the execution engine and the validation engine used in the system shown in FIG. 1;  
         [0068]    [0068]FIG. 3 is a diagram illustrating concurrent synchronization in accordance with a preferred embodiment of the invention;  
         [0069]    [0069]FIG. 4 is a diagram illustrating sequential synchronization in accordance with a preferred embodiment of the invention;  
         [0070]    [0070]FIG. 5 is a schematic illustrating a hierarchy of cloned entities in a system-under-test according to a preferred embodiment of the invention;  
         [0071]    [0071]FIG. 6 is a timing diagram illustrating an operation of a synchronizer of the execution engine shown in FIG. 2 according to a preferred embodiment of the invention;  
         [0072]    [0072]FIG. 7 is a timing diagram illustrating another operation of the synchronizer of the execution engine shown in FIG. 2 according to a preferred embodiment of the invention;  
         [0073]    [0073]FIG. 8 is a timing diagram illustrating another operation of the synchronizer of the execution engine shown in FIG. 2 according to a preferred embodiment of the invention;  
         [0074]    [0074]FIG. 9 is a diagram showing sequences representing the execution of two independent concurrent processes by an execution engine in a first mode of operation, in accordance with a preferred embodiment of the invention;  
         [0075]    [0075]FIG. 10 is a diagram showing sequences representing the execution of two independent concurrent processes by an execution engine in a second mode of operation, in accordance with a preferred embodiment of the invention;  
         [0076]    [0076]FIG. 11 is a block diagram illustrating in further detail the execution engine and the validation engine used in the system shown in FIG. 1 in accordance with an alternate embodiment of the invention;  
         [0077]    FIGS.  12 A- 12 B, collectively referred to herein as FIG. 12, are flow charts illustrating a method of model based test generation for validation of parallel and concurrent software in accordance with a preferred embodiment of the invention; and  
         [0078]    FIGS.  13 A- 13 B collectively referred to herein as FIG. 13, are flow charts illustrating a method of model based test generation for validation of parallel and concurrent software in accordance with an alternate embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0079]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art, however, that the present invention may be practiced without these specific details. In other instances well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail, in order not to unnecessarily obscure the present invention.  
         [0080]    Software programming code, which embodies aspects of the present invention, is typically maintained in permanent storage, such as a computer readable medium. In a client/server environment, such software programming code may be stored on a client or a server. The software programming code may be embodied on any of a variety of known media for use with a data processing system, such as a diskette, or hard drive, or CD-ROM. The code may be distributed on such media, or may be distributed to users from the memory or storage of one computer system over a network of some type to other computer systems for use by users of such other systems. The techniques and methods for embodying software program code on physical media and distributing software code via networks are well-known and will not be further discussed herein.  
         [0081]    Definitions.  
         [0082]    As used herein, the term “system” or “system under test” means a software system or a hardware system, the implementation of which is to be verified.  
         [0083]    Architectural Overview.  
         [0084]    Turning now to the drawings, reference is made to FIG. 1, which is a block diagram of a verification system that is operable in accordance with a preferred embodiment of the invention. A verification system  10 , used for verifying a software or hardware implementation, has several basic interacting components. Those components of the verification system  10  that are located above a broken line  11  are dependent on the specification of the implementation being verified, while those located below the line  11  are independent of the specification.  
         [0085]    The verification system  10  enables the creation of tests that have various degrees of randomness. The ability of the verification system  10  to introduce random unspecified values is fundamental, since design flaws in practice are usually unpredictable.  
         [0086]    An abstract behavioral model  14  holds a formal description of the specification of the system. This specification may be stored in a database, which may also incorporate testing knowledge of the system design (in our system these may be testing constraints and coverage criteria). The integration of all the information stored in the behavioral model  14  is referred to herein as the knowledge base of the verification system  10 .  
         [0087]    A generic test program generator engine  22  has a user input  20 , which influences the test program generator engine  22 . The influence of the input  20  includes, for example, the identity of the test instructions, their relative order, and various events relating to the instructions.  
         [0088]    A behavioral simulator  26  is preferably constructed by the test generator from the behavioral model  14  by the test program generator engine  22 . This construction can be performed “on-the-fly”, or as a separate off-line procedure. In either case, the behavioral simulator  26  is transparent to the user. The behavioral simulator  26  is used to develop the behavioral model  14 , and to predict the results of instruction execution in accordance with the specification of the system being verified.  
         [0089]    The test program generator engine  22  may also receive some generic knowledge of the design specification, and can exploit this knowledge so as to generate sequences of instructions to form the test programs  30 . The test programs  30  are executed by an execution engine  12  on an implementation of the system under test. The system can be a complex software implemented system, for example middleware, or a hardware simulator. Indeed, the system itself may be a simulator.  
         [0090]    Execution of the test programs  30  produces a response  34  from the system. The response  34  is submitted to a validation engine  36 , which has knowledge of the expected response, validates the response  34 , and produces validation results  38 .  
         [0091]    Reference is now made to FIG. 2, which is a block diagram illustrating the execution engine  12  (FIG. 1) and the validation engine  36  in further detail. The description of FIG. 2 should be read in conjunction with FIG. 1. The execution engine  12  is initially programmed by the test program generator engine  22  with one of the test programs  30 , which comprises a suite of test cases  40 . The test cases  40  act as sequences of stimuli to a system-under-test  42  (SUT). The execution engine  12  thus maps operations described by the behavioral model  14  to a stimulus for the system-under-test  42 . The system-under-test  42  is represented in FIG. 2 by a server  44  having a plurality of clients  46 . However, as noted above, the system-under-test  42  could be any form of software. For example, the system-under-test  42  could be a non-traditional type of software, such as a hardware simulator. In addition to providing input for the execution engine  12 , the test cases  40  include expected results of the execution of the test case by the execution engine  12 . These expected results are provided to the validation engine  36 . The output of the validation engine  36  is logged as validation results  38 .  
         [0092]    The execution engine  12  is provided with a synchronizer  48 , which produces synchronizing control signals that are adapted to synchronize operations specified by each of the test cases  40 . The execution engine  12  is responsive to the control signals of the synchronizer  48 , and regulates its operations such that concurrently executing test cases, or multiple concurrently executing instances of a particular test case, stimulate the system-under-test  42  in discrete synchronization steps or phases. This is referred to herein as “step synchronization”. The terms “concurrent” and “parallel”, as used herein encompass both real and simulated concurrency and parallelism, as employed in practical computing environments. The phases or synchronization steps may be defined by application-specific indicators of progress of execution, for example by predefined events, or by time intervals. In any case, the intervals in which synchronization steps occur are referred to herein as “execution intervals”.  
         [0093]    Corresponding steps of independent test suites, or of different instances of the same test suite can execute in phase, or the corresponding synchronization steps of two processes can be offset from one another, meaning that they execute in different execution intervals. For instance, the test suites generated by a database read/write behavior model could be run alongside a backup and recovery behavior model of the same database.  
         [0094]    In this embodiment the synchronization pattern developed by the synchronizer  48  is developed in accordance with a predetermined algorithm, so as to provide a desired degree of test coverage, which optionally is exhaustive. However, in order to limit the number of tests, the synchronization steps may include a given number of gaps. The use of gaps is a flexible technique that insures that a desired degree of test coverage using a minimum number of test runs. In some embodiments, the gaps are inserted randomly. In other embodiments, gaps can be inserted systematically, for example at regular intervals, and in series in which the intervals vary in a predetermined manner. For example, in a test suite, gaps could be inserted in alternate phases of execution in a first series of test runs, and at every third phase of execution in a second series of test runs.  
         [0095]    Distribution, Multiplication and Synchronization of SUT Objects.  
         [0096]    Continuing to refer to FIG. 2, in complex systems the system-under-test  42  could be a subsystem. In such a case, it is possible to replicate the system-under-test  42 , so that it could execute on many remote hosts. The functional test suites executed by the execution engine  12  can be readily reused in tests of a larger system.  
         [0097]    In general the system-under-test  42  consists of different entities, which need to be modeled in order to achieve adequate validation of the system as a whole. In the case of a database system, models are developed for the user, file, client, server, and database organization. In a practical database system, there are many files, users, and even servers referring to the same database. It may be expected that, for example, the behavior of a system having two users, five files, two clients, and one server would be the same as a system having five users, ten files, eight clients and two servers.  
         [0098]    When the number of objects does not affect the behavior of the whole system, the number of objects is a parameter of execution. Such objects are referred to as “mutually independent objects”. Their replication is called “object multiplication” or “cloning”.  
         [0099]    For example, if the system-under-test is a time server and a client, the client can be cloned. The clones connect to the time server and request the time. The behavior of such a system does not depend on the number of clients or clones. Each clone is the exact copy of the other. However, the individual clones can be initialized differently.  
         [0100]    There are two types of synchronization of cloned objects: concurrent and sequential.  
         [0101]    Concurrent synchronization means that each transition of a test case affects all clones at the same time. The next transition does not start until the previous one is ended on all clones.  
         [0102]    Reference is now made to FIG. 3, which is a diagram illustrating concurrent synchronization in accordance with a preferred embodiment of the invention. A time server  50  is accessible by a client  52  which has been replicated into three clones  54 ,  56 ,  58 . Only one replicated client, the client  52 , is shown for purposes of clarity. However, it will be understood that the time server  50  could be accessed by many different clients, each having different numbers of its own clones. The arrows in FIG. 3 represent transitions. Dots represent points of synchronization. Thus operation of the time server  50  is initiated during a common execution interval defined by synchronization points  60 ,  62 . Next, during an execution interval defined by synchronization points  62 ,  64 , the clones  54 ,  56 ,  58  all connect simultaneously to the time server  50 . Next, during a common execution interval defined by synchronization points  64 ,  66 , the clones  54 ,  56 ,  58  request the time from the time server  50 . Then, during a common execution interval defined by synchronization points  66 ,  68 , the clones  54 ,  56 ,  58  disconnect from the time server  50 . The time server  50  then terminates operation during an execution interval defined by synchronization point  68 ,  70 .  
         [0103]    Sequential synchronization means that a transition of one clone does not begin before it ends on a previous clone.  
         [0104]    Reference is now made to FIG. 4, which is a diagram illustrating sequential synchronization in accordance with a preferred embodiment of the invention. FIG. 4 is similar to FIG. 3, in which like components are given like reference numerals, and the significance of the arrows and dots is the same. Thus, operation of the time server  50  is initiated during an execution interval defined by synchronization points  72 ,  74 . Next, during an execution interval defined by synchronization points  74 ,  76  the clone  54  connects to the time server  50 . Next, during an execution interval defined by synchronization points  76 ,  78 , the clone  56  connects to the time server  50 . Next, during an execution interval defined by synchronization points  78 ,  80  the clone  58  connects to the time server  50 . It will be noted that the three intervals are exclusive.  
         [0105]    After all of the clones  54 ,  56 ,  58  have connected to the time server  50  requests for time occur. First, during an execution interval defined by synchronization points  80 ,  82  the clone  54  requests the time from the time server  50 . Next, during an execution interval defined by synchronization points  82 ,  84  the clone  56  requests the time from the time server  50 . Next, during an execution interval defined by synchronization points  84 ,  86  the clone  58  requests the time from the time server  50 .  
         [0106]    The clones  54 ,  56 ,  58  now disconnect sequentially. First, during an execution interval defined by synchronization points  86 ,  88  the clone  54  disconnects from the time server  50 . Next, during an execution interval defined by synchronization points  88 ,  90  the clone  56  disconnects from the time server  50 . Next, execution during an interval defined by synchronization points  90 ,  92  the clone  58  disconnects from the time server  50 . Finally, during an execution interval defined by synchronization points  92 ,  94  the time server  50  terminates its operation.  
         [0107]    Generally a system-under-test consists of units-under-test or processes, which are composed in turn of objects-under-test, and which can run on different host machines. Three levels of multiplication can be defined. Host multiplication occurs when all processes running on the one host machine are cloned to another machine. Process multiplication occurs when a process, including all of its objects, are cloned on the same host machine. Object multiplication occurs when an object is cloned within a process.  
         [0108]    Reference is now made to FIG. 5, which illustrates a hierarchy  96  of entities in a system-under-test. The hierarchy  96  includes hosts  98 ,  100 , processes  102 ,  104 , and objects  106 ,  108 . In test execution directives provided to the execution engine  12  (FIG. 1), an initialization set can be assigned to each entity of the hierarchy  96 . When individually specifying values in each initialization set, values of the initialization set of the higher levels of the hierarchy  96  may be referenced. For example, as shown in of Listing  1 , which represents fragments of initialization sets, an object initialization set symbolically references elements of both a process initialization set and a host initialization set.  
                                         Listing 1                                    Host Initialization Set:            IP = 9.148.32.112           Process Initialization Set:            USER = Joe           Object Initialization Set             IP = HOST.IP             USER = PROCESS.USER                      
 
         [0109]    It should be noted that the goal of object multiplication and cloning is to validate the functional behavior of the system under test, rather than to test its performance. This is an efficient approach since it reuses an existing test suite. When object multiplication is performed, it will generally be the case that the test suite has previously run successful without object multiplication. Thus, any problems discovered subsequently can be attributed to the introduction of more objects into the system-under-test.  
         [0110]    Initialization.  
         [0111]    Continuing to refer to FIG. 2, when the execution engine  12  in cooperation with the synchronizer  48  creates the various above-described entities (FIG. 5) in the system-under-test  42 , it initializes them with an initialization set. This may be empty, but generally, it contains name-value pairs.  
         [0112]    Cloning.  
         [0113]    Continuing to refer to FIG. 5, each entry of an initialization set for different elements of the same level of the hierarchy  96  can be given different values, in order to initialize clones differently, for example, the clones represented by the objects  106 ,  108 . Values are assigned to the clones in sequence. Thus, a first value could be assigned to the initialization set of the object  106 , and a second value to the object  108 . If a value list representing the clones is exhausted, then the assignment cycles back to the beginning of that value list in order to continue the process.  
         [0114]    In another example, an object “lamb” is to be cloned, and four clones are desired. The initialization set for the objects is shown in Listing  2 .  
                                             Listing 2                                    FUR_COLOR:    white           EYE_COLOR:   blue, green           NAME:    Mercury, Venus, Earth                      
 
         [0115]    Applying the assignments of the values cyclically as described above yields four white lambs: Mercury with blue eyes, Venus with green eyes, Earth with blue eyes, and Mercury with green eyes.  
       EXAMPLES  
       [0116]    Reference is now made to FIG. 6, which is a timing diagram illustrating an exemplary operation of the synchronizer  48  in cooperation with the execution engine  12 . FIG. 6 models an example of database read/write behavior, wherein the symbol “R” refers to a read operation, and the symbol “W” refers to a write operation, and can be understood in conjunction with FIG. 2. According to stimuli issued by the execution engine  12  to the system-under-test  42 , different clients  46  are executing the same sequence of read/write operation. The read and write operations of a first process, shown as a sequence  110 , and those of a second process, shown as a sequence  112 , are shown executed in phase.  
         [0117]    Reference is now made to FIG. 7, which is a timing diagram illustrating another exemplary operation of the synchronizer  48  in cooperation with the execution engine  12 , similar to the behavioral model  14 . Now different clients  46  are executing the same sequence of read/write operations. Now, however, the read and write operations of a first process, shown as a sequence  116 , and those of a second process, shown as a sequence  118 , are executed in different execution intervals. For example, the first operation of the first and second processes occurs respectively in successive execution intervals  120 . They are offset from one another by one execution interval.  
         [0118]    Reference is now made to FIG. 8, which is a timing diagram illustrating another exemplary operation of the synchronizer  48  in cooperation with the execution engine  12 . Two different test cases, one a read/write operation, and the other a backup and recovery operation, are used to stimulate the system-under-test  42 . Here the symbols “R” and “W” have the meanings given above. The symbol “C” means “configure backup”; “S” means start backup, and “E” means “inject error. A first process, the read/write operation, is shown as a sequence  122 , and a second process, the backup and recovery operation, is shown as a sequence  124 .  
         [0119]    Referring again to FIG. 2, in another mode of operation, the execution engine  12  can “split” any of the test programs  30 . That is to say, the execution of the test programs  30  can be interrupted for any length of time, and then resumed. To illustrate this mode of operation, reference is now made to FIG. 9, which illustrates sequences representing the execution of two independent concurrent processes. Steps of a first concurrent process  126  are represented by upper case letters, and steps of a second concurrent process  128  are shown in lower case letters. FIG. 9 shows three tests, in which the execution engine  12  executes the two test cases with different beginning offsets. In a first test  130 , the two processes begin simultaneously. In a second test  132 , the second process is initiated one execution interval prior to the initiation of the first process. In a third test  134 , the first process is initiated two execution intervals prior to the initiation of the second process.  
         [0120]    Reference is now made to FIG. 10, which illustrates sequences representing the execution of two independent concurrent processes according to another mode of operation of the synchronizer  48 . FIG. 10 is similar to FIG. 9, except now splits or gaps, represented by the symbol “−”, have been introduced in the tests.  
         [0121]    In a first test  136  two concurrent processes begin simultaneously. A gap  138  has been introduced in the first process between synchronization steps “C” and “D”. A gap  140  has been introduced between synchronization step “i” and synchronization step “j” in the second process. Corresponding synchronization steps of the two processes are executed out of step between the gaps  138 ,  140 . Following the gap  140 , corresponding synchronization steps of the two processes again execute in the same execution interval. Gaps can be inserted at the beginning of a test case, at the end, or between two synchronization steps.  
         [0122]    In a second test  142 , the two concurrent processes begin simultaneously, and execute in step until the introduction of a gap  144 . Subsequent corresponding synchronization steps of the two processes execute out of phase, separated by one execution interval. Thus, synchronization step “K of the first process executes concurrently with synchronization step “1” of the second process.  
         [0123]    In a third test  146 , a gap  148  precedes the first synchronization step “a” of the second process, so that the first process actually begins executing one execution interval prior to the second process. The two processes are out of phase by one execution interval until synchronization steps “I” and “i” of the first and second process respectively. Synchronization has been stored by inserting a gap  150  between synchronization step “H” and synchronization step “I” of the first process.  
         [0124]    From a consideration of FIG. 10, it can be appreciated that many more possibilities of interaction can be tested, as compared with the arrangement of FIG. 9. For example, although only one gap has been introduced into any one of the sequences shown in FIG. 10, several consecutive gaps could be introduced, thus creating any desired delay in the execution of a given process. Advantageously, in the mode of operation illustrated in FIG. 10, problems resulting from race conditions are likely to be detected. If the number of gaps is limited, such problems can be detected with less computation than in the mode of operation illustrated in FIG. 9.  
         [0125]    Alternate Embodiment.  
         [0126]    In the discussion of the first embodiment the use of model based test generation techniques is disclosed, wherein a simple sequential test case is converted into a set of concurrent test cases, which can be used to validate a system-under-test without producing an impractical number of additional test cases. In this alternate embodiment, synchronization of concurrent test cases is extended to cover a much wider variety of process state combinations.  
         [0127]    Referring again to FIG. 9, it is shown that the processes  126 ,  128  can be tested with different initial synchronization step offsets. In order to test the independence of all pairs of steps in the two processes  126 ,  128 , it is necessary to run the test cases together m+n times, where m is the number of synchronization steps of the process  126 , and n is the length of the process  128 . However, as more processes are tested together, the number of repetitions required to test for the independence of all 3-tuples in the case of three concurrent processes, or 4-tuples in the case of four concurrent processes, increases as the square or cube of the number of synchronization steps respectively.  
         [0128]    Similarly if there are p processes, and it is desired to test for all patterns of p steps interacting together, the number of possible patterns is m p , assuming that each test case has m distinct synchronization steps. It would require at least m p−1  repetitions to evaluate all such combinations, since at most m combinations can be observed at any one repetition.  
         [0129]    Reference is now made to FIG. 11, which is a block diagram illustrating in further detail the execution engine and the validation engine used in the system shown in FIG. 1 according to an alternate embodiment of the invention. The embodiment of FIG. 11 is similar to that of FIG. 2, and like elements are given like reference numerals. However, in this embodiment a synchronization pattern generator  152  generates synchronization patterns  154 , which are input to the execution engine  12 .  
         [0130]    In some embodiments, the synchronization patterns  154  are developed in advance of the operation of the execution engine  12 . In such embodiments the pattern generator  152  operates off-line, and stores pre-computed synchronization patterns. Appropriate logic is provided in the pattern generator  152  for the timely selection of the synchronization patterns  154 .  
         [0131]    In other embodiments, the synchronization patterns  154  are dynamically generated. The pattern generator  152  receives feedback input from the execution engine  12  or the validation engine  36 , as indicated by the dashed line  156 . In such embodiments, the synchronization patterns  154  are influenced by the results of execution by the execution engine  12 . Adaptive generation of synchronization patterns is important in two cases. In the first case, the model of the system-under-test is non-deterministic. That is, the behavioral model is a more abstract representation of the specification than the implementation. In this case, the model may predict one or more outcomes of a particular stimulus. The execution engine and the validation engine could observe which of the valid or invalid responses were received, and decide whether to insert a gap, based on both the observed response, and the current coverage criteria. In the second case, the test system itself is non-deterministic. Here too, the observed responses and current coverage situation could motivate the test execution engine to insert a gap in order to increase the likelihood of reaching an uncovered synchronization task later in the test case.  
         [0132]    In this embodiment, repetitions and gap insertions of each test case are generated by the pattern generator  152 , and the synchronizer  48  is responsive, in order for the test program generator engine  22  to cover a prescribed degree of independence, referred to as an “independence coverage degree”. An independence coverage degree could be pairs of steps in two or more concurrent processes, triples of steps, and all possible combinations of steps.  
         [0133]    The issue of practical synchronization pattern generation can be explained as follows: The input is:  
         [0134]    1. A finite alphabet of symbols called test steps, denoted by an alphabet of size n;  
         [0135]    2. A finite string of symbols over the alphabet (called a test case), having a length m;  
         [0136]    3. A number of concurrent processes k; and  
         [0137]    4. A coverage requirement (independence coverage degree).  
         [0138]    The output is a set of test runs or synchronization patterns. A test run is a set of k strings over the alphabet extended with the symbol gap. Each of the strings in a run consists of the original test case with gap symbols inserted.  
         [0139]    A test run is said to cover a pair of alphabet members {x, y}, if the member x occurs at a position i in one of the sequences in the run and the member y also occurs at the same position i in a different sequence of the run. This notion of coverage is extended to triples and generally to n-tuples.  
         [0140]    A good test synchronization solution is one with a small number of runs.  
         [0141]    A special case of the test synchronization problem may be attacked using “difference sets” and “difference families”, which are known from standard references on combinatorial design theory. If gaps are only allowed to be inserted at the beginning of a test case, and moreover, the test case can be repeated cyclically, and the test case consists of n different symbols. Then, a cyclic difference family with parameters 2−(n,k,1) can be used to construct a set of runs which cover all pairs of distinct alphabet symbols. A 2−(n,k,1) difference family is a set of k-subsets of the set of integers modulo n, with the property that the set of all differences between members of the same k-subset is precisely the set of all non-zero integers modulo n, each one occurring once, corresponding to the value 1 in the notation 2−(n,k,1)).  
         [0142]    The name “cyclic balanced incomplete block design” is mathematically synonymous with a 2−(n,k,m) difference family.  
         [0143]    The independence coverage degree can sometimes be satisfied using cyclic balanced incomplete block design, but in general this is not possible, as there are numerous restrictions. Gaps can occur only at the beginning of the test case. Repeats can only occur at the end or the beginning. The parameters m and n must be equal, and there can be no repeated steps in the test case. Furthermore, difference families can only exist when certain divisibility conditions are satisfied, e.g., n−1 must be divisible by k−1. For these and other reasons, difference families are impractical.  
         [0144]    To illustrate an exemplary operation of the pattern generator  152 , which produces a test having an independence coverage degree consisting of pairs, reference is made to Table 1. Table 1 illustrates three concurrent processes. A test case pattern for each process consists of the steps “A B A C D”. It is required that all combinations of pairs of steps occur together during the tests. That is, each of the pairs of steps AA, AB, AC, AD, BB, BC, BD, CC, CD, and DD must occur together at some interval of the tests. Table 1 shows a succession of intervals, each occupied either by a gap, or by concurrently occurring steps of the processes.  
                                               TABLE 1                           Process 1   —   A   B   A   C   D   —   —       Process 2   —   —   —   A   B   A   C   D       Process 3   A   B   A   C   D   —   —   —                  
 
         [0145]    In this relatively simple example, using gaps, the pattern generator  152  can generate runs of the test case to achieve full coverage of these ten pairs in only two repetitions. The first repetition is shown in Table 1. A second repetition is shown in Table 2.  
                                               TABLE 2                           Process 1   —   A   B   —   —   A   C   D       Process 2   A   B   A   C   D       Process 3   A   B   A   C   D   —   —   —                  
 
         [0146]    Inspection of Table 1 shows that the pairs AB, AA, AC, BC, BD, CD, and AD are tested. For example, the pair AB is tested in the second and third intervals from the left. The pair AD is tested in the third interval from the right. The pairs BB, CC, and DD are not tested.  
         [0147]    In the second repetition, shown in Table 2, the pairs AA, AB, BB, CC, and DD are tested. These pairs include the pairs BB, CC, and DD, which were not covered in the first repetition. The pairs AA and AB are incidentally retested. Thus, all ten pair sets are covered in only two repetitions.  
         [0148]    Referring again to FIG. 11, the pattern generator  152  can be realized as a tool such as the above-noted GOTCHA-TCBeans Software Test Tool Kit or other finite state machine based test generators. A model can be readily created, which receives as input the number of concurrent processes, a single test case provided as a pattern of steps, (e.g., A B C D A E F), the length of the test case (in the foregoing pattern the length is  7 ), and an independence coverage degree requirement. The generator tool is modified to optimize the sequences by strategic insertion of gaps combined with variation of the process offsets, so as to satisfy the independence coverage degree, and stimulate the system-under-test with a minimum number of test repetitions. This operation can be done automatically, and may employ well-known optimization techniques such as simulated annealing and genetic algorithms.  
         [0149]    Methods of Operation.  
         [0150]    The steps of the following methods are presented in a particular order. However, it will be understood by those skilled in the art that many of these steps can be performed in different orders, or even simultaneously.  
         [0151]    Reference is now made to FIG. 12, which is a flow chart illustrating a method of model based test generation for validation of parallel and concurrent software in accordance with a preferred embodiment of the invention, in which synchronization patterns are determined during operation of the test system. The process begins at initial step  158 , where a behavioral model is prepared from the specifications of the system-under-test. A behavioral simulator is created from the behavioral model during initial step  158 .  
         [0152]    Next, at step  160  test coverage goals and test constraints are established. Control then proceeds to step  162 , where a test suite is generated for a single process. In a test case of the test suite, a finite alphabet of symbols called test steps, denoted by an alphabet of size n is used. The test case is a finite string of symbols over the alphabet, having a length m; and a number of concurrent processes k.  
         [0153]    Next, at step  164 , the number of clones of the process for which the test suite was generated in step  162  is determined.  
         [0154]    Next, at step  166  all clones, the number of which was determined in step  164 , are spawned. The execution engine begins executing a test case of the test suite that was generated in step  162  on all the clones.  
         [0155]    At step  168  the execution engine sends a stimulus, or a gap to the processes. This is done according to a decision algorithm, in which at least the information obtained in the previous steps, including the test coverage requirement, is employed in order to optimize the synchronization patterns, such that the system-under-test can be validated with a minimum number of test case repetitions. The processes execute step-wise in accordance with the synchronization patterns generated by step  168 .  
         [0156]    At step  170  responses to the stimulus of step  168  are observed and validated. Any implementation flaws that are detected during validation are recorded.  
         [0157]    Next, at decision step  172  a determination is made whether there are steps remaining to be executed in any of the processes. If the determination at decision step  172  is affirmative, then control proceeds to step  174 .  
         [0158]    If the determination at decision step  172  is negative, then control proceeds to decision step  176 , which is disclosed below.  
         [0159]    At step  174  the next stimulus of the synchronization pattern is determined, based at least in part on the response of step  170 . Whether this is to be a gap or a stimulus corresponding to another member of the alphabet is established according to the coverage criteria currently in force. Control then returns to step  168 .  
         [0160]    At decision step  176  it is determined if the current test case needs to be repeated. This determination is made according to the coverage goals that were input in step  160 . If the determination at decision step  176  is affirmative, then control returns to step  166 .  
         [0161]    If the determination at decision step  176  is negative, then control proceeds to decision step  178 . Here a determination is made whether more test cases of the test suite remain to be processed.  
         [0162]    If the determination at decision step  178  is affirmative, then control proceeds to step  180 , where another test case is selected. Control then returns to step  166 .  
         [0163]    If the determination at decision step  178  is negative, then control proceeds to final step  182 , and the procedure ends.  
         [0164]    Reference is now made to FIG. 13, which is a flow chart illustrating a method of model based test generation for validation of parallel and concurrent software in accordance with another preferred embodiment of the invention, in which optimized synchronization patterns are computed prior to operation of the test system. Steps in FIG. 12 and FIG. 13 that are identical are given like reference numerals, and their descriptions are not repeated in the interest of brevity. The process begins at initial step  158 . Initial step  158 , step  160 , step  162  and step  164  are performed in the same manner as disclosed above with reference to FIG. 12.  
         [0165]    Next, at step  184  the test suite that was created in step  162  is expanded by computing optimized synchronization patterns for each test case of the test suite, using the methods disclosed hereinabove. Control then passes to step  166 , the disclosure of which is given above.  
         [0166]    Next, at step  186  the execution engine sends a stimulus, or a gap to the processes. The stimulus is selected from elements of the synchronization patterns that were pre-computed in step  184 . As in step  168  (FIG. 12), the processes execute step-wise in response to the stimulus.  
         [0167]    Control next passes to step  170 , and then to decision step  172 , which are disclosed above. If the determination at decision step  172  is affirmative, then control proceeds to step  188 , where the next stimulus is selected. As the stimuli and their order are pre-computed, determined. The next stimulus can be read from a table or list. Control then returns to step  186 .  
         [0168]    If the determination at decision step  172  is negative, then control proceeds to decision step  176 , and thence to decision step  178 , step  180 , and final step  182 , the disclosures of which are given above in connection with FIG. 12.  
         [0169]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art which would occur to persons skilled in the art upon reading the foregoing description.