4. Common subsets. Assurance is the property that the parties to a transaction are who they purport to be. Availability is the property that the system will be available for legitimate use. What can the system do to make the user more efficient in its operation? An example of an availability general scenario appeared in Figure 4.3. For a system to be properly testable, it must be possible to control each component's internal state and inputs and then to observe its outputs. who ignores them. Confidentiality is the property that data or services are protected from unauthorized access. Artifact. Whoever makes the change must understand how to make it, and then make it, test it and deploy it. That is, a failure is observable by the system's user and a fault is not. For example, “latency” is one of the refinements of “performance,” and “access control” is one of the refinements of “security” • Lowest level: Architecture scenarios—at least one architecture scenario per Quality Attribute … How can the user (or the system itself) adapt to make the user's task easier? Making a general scenario system specific means translating it into concrete terms for the particular system. In contrast to a mo… In our example, the modification is to occur at design time. !��)�%ǟd\�ʩ���v9��1-��kDFJK�Z�Tm���(�q3d�|�7��C��G The component that generated the message did so at a different time than expected. Clearly, there must be machinery in place to allow the system administrator or end user to modify a system, but this is a common occurrence. All of these actions take time and money, both of which can be measured. In the example, the scenario is violated since the change cannot be implemented within 7 days as specified (Fig. The parts of a quality attribute scenario Subscribe Subscribed Unsubscribe 33.7K. Its six parts are shown, indicating the range of values they can take. Response measure. The system must process the arriving events. In other words, from the point of view of system performance, it does not matter whether one user submits 20 requests in a period of time or whether two users each submit 10. The origins of contemporary reliability engineering can be traced to World War II. We know from existing SEI work on Attribute-Driven Design, Quality Attribute Workshops, and the Architecture Tradeoff Analysis Method that a focus on quality attributes prevents costly rework.Such a long-term perspective, however, can be hard to maintain in a high-tempo, Agile delivery model, which is why the SEI continues to recommend an architecture … Example Availability Scenario An unanticipated external message is received by a process during normal operation. If the software architect can reduce this cost, the payoff is large. Table 4.4 shows the security general scenario generation table. There are many other examples. And woe to the architect (or architecture writer!) system-specific. Response measure. The good thing is that for the most quality attributes (availability, interoperability, modifiability, performance, security, testability, usability) there already exist generic quality attribute scenarios with the most common issues that you need to be aware of. Table 4.5 gives the testability general scenario generation table. A service oriented architecture (SOA), for example, implements complex functionality as a combination of loosely coupled services. A test of the design may be performed by other developers or by an external group. There are a number of possible reactions to a system failure. The system can be in various operational modes, such as normal, emergency, or overload. Quality Attribute Scenarios • Is the solution to the stated problems. quality attributes such as usability, performance, reliability, and security indicates the success of the design and ... use in other applications and in other scenarios. Auditing is the property that the system tracks activities within it at levels sufficient to reconstruct them. Security is a measure of the system's ability to resist unauthorized usage while still providing its services to legitimate users. Table 4.3 gives elements of the general scenarios that characterize performance. Such questions should concentrate on the quality attribute aspects of the scenario and any concerns that the stakeholders might have in achieving the response called for in the scenario. Availability is concerned with system failure and duration of system failures. • A QAS is a quality-attribute-specific requirement, that consists of: 1. This means that your grade has not been changed since your instructor assigned it. Figure 4.2 shows an example of a general scenario, in this case for availability. The behaviour will impact the functionality of the system or product in one way or the other. 2. Multiple users or other loading factors can be modeled by varying the arrival pattern for events. A performance event may be atomic or may be an aggregate of other lower-level occurrences; a failure may be a single performance event or an aggregate. Events can also arrive sporadically, that is, according to a pattern not capturable by either periodic or stochastic characterizations. A change can also be made by a developer, an end user, or a system administrator. This might be the completion of an analysis or design increment, the completion of a coding increment such as a class, the completed integration of a subsystem, or the completion of the whole system. Response. Quality attribute scenario example in software architecture Faiza Waseem. 1. For the engine control system, the response might be the variation in the firing time. As such, it has frequently compromised the achievement of all other qualities. x��\m��q��������dǃ��̸*��/�TB�T� ����j���*��8��i�wczwI떖b���΃�ht7��^o�^�����?����L��o6b���{��O��ˍ'�e��b��o6�2��b�j�"���Dd�r��+�m� "]�Z�S�zr���b@%"rBT T��zl0���u�T�*-��j0��DU�s!�@H ��$���f"���O|�"�P �rD��� �r��4 �/TY0��ǡ�W,G@*�����&Z�����8ϵ�9��y�R��Y~2��䗀,h��c ϟ���# ϿT�>��HU��@�?TN �ň�ŐE FZYN�!KEBi1d�H�-���C�Ms�U�F���\�D��R�m0�� One that must be made frequently will impose a more stringent requirement on the response measures than one that is made only sporadically. Response measure. Response. Availability is concerned with system failure and its associated consequences. For example, a fault can be choosing the wrong algorithm for a computation, resulting in a miscalculation that causes the system to fail. Satisfying the request requires resources to be consumed. Events (interrupts, messages, requests from users, or the passage of time) occur, and the system must respond to them. Source of stimulus. When designing a system architecture you will have to take decisions. The portions of a security general scenario are given below. omission, crash, timing, response, System's processors, communication channels, persistent storage, processes, degraded mode (i.e., fewer features, a fall back solution). Artifact. It can also be made to the qualities of the system-making it more responsive, increasing its availability, and so forth. Artifact. Modifiability is about the cost of change. This portion specifies when the change can be made-design time, compile time, build time, initiation time, or runtime. During a discussion of performance, for example, a stakeholder representing users may not realize that the latency of the response to events has anything to do with users. The response of the system to a stimulus can be characterized by latency (the time between the arrival of the stimulus and the system's response to it), deadlines in processing (in the engine controller, for example, the fuel should ignite when the cylinder is in a particular position, thus introducing a processing deadline), the throughput of the system (e.g., the number of transactions the system can process in a second), the jitter of the response (the variation in latency), the number of events not processed because the system was too busy to respond, and the data that was lost because the system was too busy. Interoperability is an attribute of the system or part of the system that is responsible for its operation and the transmission of data and its exchange with other external systems. When is the change made and who makes it (the environment)? Environment. important to the system and what the desired quality attribute responses should be. Not only does the system need to provide access to legitimate users, but it needs to support the granting or withdrawing of access. Stimulus. Measures of a system's response include the difficulty of mounting various attacks and the difficulty of recovering from and surviving attacks. Environment. Integrity is the property that data or services are being delivered as intended. Scheduled downtimes (i.e., out of service) are not usually considered when calculating availability, since the system is "not needed" by definition. We have kept this as a separate attribute. A quality attribute ( QA) is a measurable or testable property of a system that is used to indicate how well the system satisfies the needs of its stakeholders. Finally, for each of the quality attributes, understand the lists of the key issues for that attribute, the decisions you must make to addresses these issues, and the key techniques you can use to implement solutions for that quality attribute. 12 software architecture quality attributes Performance – shows the response of the system to performing certain actions for a certain period of time. This may be a brief delay in the response time or it may be the time it takes someone to fly to a remote location in the mountains of Peru to repair a piece of mining machinery (this example was given by a person who was responsible for repairing the software in a mining machine engine.). Purpose: To help you prepare for disclosure communications by using four scenarios to practice challenging interpersonal communications.. Who should use this tool? The usability general scenario generation table is given in Table 4.6. learn system features; use system efficiently; minimize impact of errors; adapt system; feel comfortable. This means that, when a customer sends a credit card number to an Internet merchant, the merchant is who the customer thinks they are. Changes can be made to the implementation (by modifying the source code), during compile (using compile-time switches), during build (by choice of libraries), during configuration setup (by a range of techniques, including parameter setting) or during execution (by parameter setting). In fact, all of the evidence points squarely in its favor, and usability has proven to be no exception. The user actions with which usability is concerned always occur at runtime or at system configuration time. In our example, the source of the stimulus is a collection of users. In Figure 4.4, the modification is to be made by the developer. The portions of the usability general scenarios are: Source of stimulus. A system failure occurs when the system no longer delivers a service consistent with its specification. Figure 4.1. }S�(����)T���QǐLK�4z1�*Ԕ����h=#6�.My&������ >'*�$�*Yε���. Traditionally, scenario based assessment has been applied to development related software qualities [5]. Scenarios are short stories that describe an interaction with the system that exercises a particular quality attribute. The artifact is always the system's services, as it is in our example. One of the most important deliverables from the ATAM process is the Quality Attribute Utility Tree. The leaves of the tree are expressed as scenarios, which provide concrete examples of the quality attribute considerations that you just enumerated. Environment. Performance is about time. If you want to support the ability of a user to cancel an operation in progress, returning to the precise system state in place before the operation was started, you need to plan for this capability in the architecture. The availability of a system is the probability that it will be operational when it is needed. Stimulus. Environment. A change can occur to any aspect of a system, most commonly the functions that the system computes, the platform the system exists on (the hardware, operating system, middleware, etc. This means that a hacker cannot access your income tax returns on a government computer. This portion specifies who makes the changes-the developer, a system administrator, or an end user. Artifact. Once a system fails, an important related concept becomes the time it takes to repair it. The stimulus for the testing is that a milestone in the development process is met. A performance scenario begins with a request for some service arriving at the system. Books in Software Architecture 8. From these considerations we can see the portions of the performance general scenario, an example of which is shown in Figure 4.5: "Users initiate 1,000 transactions per minute stochastically under normal operations, and these transactions are processed with an average latency of two seconds.". Environment: the condition under which the stimulus occurs; e.g. This portion specifies the changes to be made. %PDF-1.3 For most of the history of software engineering, performance has been the driving factor in system architecture. This may lead to miscommunication. Environment. Testing is done by various developers, testers, verifiers, or users and is the last step of various parts of the software life cycle. Disclosure Lead(s). Response measure. Stimulus. In each case, the pattern of events arriving and the pattern of responses can be characterized, and this characterization forms the language with which to construct general performance scenarios. Performance is about timing. Such a failure is observable by the system'susers—either humans or other systems. Analysis, architecture, design, class, subsystem integration completed; system delivered, Piece of design, piece of code, complete application, At design time, at development time, at compile time, at deployment time, Provides access to state values; provides computed values; prepares test environment, Length of longest dependency chain in a test, Length of time to prepare test environment. Response. ), the environment within which the system operates (the systems with which it must interoperate, the protocols it uses to communicate with the rest of the world, etc. For example, for “latency of database transactions”, you might create a scenario such as “1000 users simultaneously update their own customer records under normal conditions with an average latency of 1 second”. The stimulus is that the end user wishes to use a system efficiently, learn to use the system, minimize the impact of errors, adapt the system, or feel comfortable with the system. One technique to prevent attacks is to cause fear of punishment by maintaining an audit trail of modifications or attempted accesses. Instructor: Tasneem Darwish 18 System quality attributes Attacks, often occasions for wide media coverage, may range from theft of money by electronic transfer to modification of sensitive data, from theft of credit card numbers to destruction of files on computer systems, or to denial-of-service attacks carried out by worms or viruses. Bass’s seven quality attributes Scenarios for each, with examples This info is the main appendix to your suppl spec template. Stimulus: event. Quality attributes in Software Architecture 5. Together, reliability, availability, serviceability, usability and installability, are referred to as RASUI. From these considerations we can see the portions of an availability scenario, shown in Figure 4.2. System should detect event and do one or more of the following: notify appropriate parties, including the user and other systems, disable sources of events that cause fault or failure according to defined rules, be unavailable for a prespecified interval, where interval depends on criticality of system, continue to operate in normal or degraded mode, Time interval when the system must be available, Time interval in which system can be in degraded mode. 2 Software Quality Attributes 3 2.1 How Various Communities Have Addressed Quality Attributes 3 2.2 Software Quality Attribute Trade-offs 4 2.3 Generic Taxonomy for Quality Attributes 4 3 Performance 7 3.1 Overview 7 3.1.1 Definition 7 3.1.2 Taxonomy 7 3.2 Concerns 9 3.2.1 Latency 10 3.2.2 Throughput 10 3.2.3 Capacity 10 3.2.4 Modes 11 for example. The Path to Becoming a Software Architect 2. Variation is a concept associated with software product lines (see Chapter 14). Since a system failure is observable by users, the time to repair is the time until the failure is no longer observable. An engine control system gets its requests from the passage of time and must control both the firing of the ignition when a cylinder is in the correct position and the mixture of the fuel to maximize power and minimize pollution. The testing is performed by unit testers, integration testers, system testers, or the client. In the intervening five years we have learned quite a lot about many quality attributes, and none more so than usability. Facilitating this kind of understanding aids discussions of architectural decisions, particularly about tradeoffs. In fact, the usability features that are the most difficult to achieve (and, in particular, the most difficult to add on after the system has been built) turn out to be precisely those that are architectural. Availability Scenario. System failure means … when the system does not provide the service for which it was intended. What can the system do so that a user error has minimal impact? 24). Source of stimulus. Furthermore, a single general scenario may have many system-specific versions. The discipline’s first concerns were electronic and mechanical components (Ebeling, 2010). Nor does it (yet) consider the configuration of the system or the consumption of resources. In Figure 4.3, there is no downtime as a result of the unexpected message. The distinction between faults and failures allows discussion of automatic repair strategies. We now discuss the six most common and important system quality attributes, with the twin goals of identifying the concepts used by the attribute community and providing a way to generate general scenarios for that attribute. If the user is unfamiliar with a particular system or a particular aspect of it, what can the system do to make the task of learning easier? This portion specifies what is to be changed-the functionality of a system, its platform, its user interface, its environment, or another system with which it interoperates. system is overloaded. Loading... Unsubscribe from Faiza Waseem? Among the areas of concern are how system failure is detected, how frequently system failure may occur, what happens when a failure occurs, how long a system is allowed to be out of operation, when failures may occur safely, how failures can be prevented, and what kinds of notifications are required when a failure occurs. Not everything is architectural it's true, but frequently our assumptions of what is and what is not are based on a superficial analysis of the problem. Response measure. Each service is developed, deployed and operated more or less independently. Each of these security categories gives rise to a collection of general scenarios. The same system that has to support a new browser may also have to support a new media type. Stimulus. ("���������.�T;���ҡ>`O�W�'�=l.`����2#��7 6�#���[z ,)/�L�:�}��!�R��z�KϣRޝ�.k���R��Hzρ��7��� lز�g�u肈��~=�U{�1�F�)�D����� �̹� �A���"���{����]�v/��^�ܳR��?� ���:?վU:0U�L�NW�-�TE�f��. This specifies the resource that is required to be highly available, such as a processor, communication channel, process, or storage. The difficulty with security is allowing access to legitimate users and determining legitimacy. The category of platform changes is also called portability. An arrival pattern for events may be characterized as either periodic or stochastic. The capacity of the system may also change. The normal development process detects usability problems through building prototypes and user testing. In our example, the testing is triggered by the completion of a unit of code. In Figure 4.7, the measurement is percentage coverage of executable statements. Stimulus. Equally clear, it is not in the same category as changing the system so that it can be used over the Web rather than on a single machine. Table 4.2 presents the possible values for each portion of a modifiability scenario. For example, it may occur with an exchange of severalmessages between a client and a server (culminating in an unexpected message), each of which is an atomic event from a performance perspective. If the source of the attack is highly motivated (say politically motivated), then defensive measures such as "We know who you are and will prosecute you" are not likely to be effective; in such cases the motivation of the user may be important. In our example, the modification is made with no side effects. What can change (the artifact)? 11 Figure 3: Sample performance scenario Figure 4: Sample security scenario 12. It may be an unauthorized attempt to access data or services or to modify data, or it may be intended to deny services to legitimate users. Defining a Quality Attribute Utility Tree. In Figure 4.6, an audit trail is maintained. While we have always claimed that system quality stems primarily from architectural quality, in the first edition of this book we were, at times, on shaky ground in trying to substantiate this claim. An end user changing the screen saver is clearly making a change to one of the aspects of the system. stream This may cause a change in the system environment (e.g., from normal to overload mode). In particular, testability refers to the probability, assuming that the software has at least one fault, that it will fail on its next test execution. {�5���w�A�d�6z�b;j��wO7�6��{G@�כ����������.,[ݏ����۽��ћ��z^���|��?�=�d���n?��4Nr�Ǯ��=�ݍz����o���< Any action that occurs before then is performed by developers and, although a user may also be the developer, we distinguish between these roles even if performed by the same person. Such a failure is observable by the system's users-either humans or other systems. Artifact. Sample security scenario 5 0 obj Stakeholders in Software Architecture 3. }�mx�6�q��1s L��� C3�3E~I�Gd~�H��l�B$���0#�:�\E�VH�� �Va�mN$��eY�`�i\#�v����E��T*�'.�0Rݖ�\R�=���[!���#YՔ�]�V���v4�j��aC#`���^)����dg.�آa��]~�G���6����05zWfzذϳ��.�4O�Zp�?Բw� �A�pŧ��i����^#䈺ξ_i#���́�^5��Gm@rs? In our example, the system should notify the operator of the unexpected message and continue to operate normally. Time is not always possible to predict, however, and so less ideal measures are frequently used, such as the extent of the change (number of modules affected). Stimulus. In our example, the transactions should be processed with an average latency of two seconds. In our example, the target is data within the system. The test can happen at design time, at development time, at compile time, or at deployment time. Aqualityattributescenarioisaquality-attribute-specificrequirement.Itconsistsofsixparts. The attack itself is unauthorized access, modification, or denial of service. From this we can derive concrete. with quality attributes so that the design method does not need to know how to reason about all of the multiplicity of quality attributes that exist. From this we can derive concrete, system-specific, scenarios. Now, however, the question of when a change is made is intertwined with the question of who makes it. In our example, the system is operating normally. This means you cannot deny that you ordered that item over the Internet if, in fact, you did. [1] Some security experts use "threat" interchangeably with "attack.". Its six palts are shown. In our example, the user wishes to cancel an operation, which is an example of minimizing the impact of errors. See the example template for scenario refinement in Appendix A. This is an example of a timing fault. A system-specific version might be "A request arrives to add support for a new browser to a Web-based system, and the change must be made within two weeks." Probe more deeply, and significant architectural considerations pop up everywhere. Source: external to system Stimulus: timing Artifact: communication channel Environment: normal operation Response: log failure and notify operator via alarm Response. Still, the elements of a security general scenario are the same as the elements of our other general scenarios-a stimulus and its source, an environment, the target under attack, the desired response of the system, and the measure of this response. 3 (28) - SOFTWARE ARCHITECTURE Quality Attributes (1) - Sven Arne Andreasson - Computer Science and Engineering System Quality Scenarios Availability Scenario “What’s the probability that the system works when Modifyability Scenario Concerns:Performance Scenario mean time to failureSecurity Scenario Testability Scenario Usability Scenario From these considerations we can see the portions of the modifiability general scenarios. Source of stimulus: actuator; e.g. A fault of one of the following classes occurs. In Figure 4.6, the stimulus is an attempt to modify data. Lowest level: architecture scenarios (at least one architecture scenario per Quality Attribute refinement). - In Figure 4.3, the stimulus is that an unanticipated message arrives. In our example, the audit trail allows the accounts from which money was embezzled to be restored to their original state. Next level: Quality Attribute requirement refinements. How to use this tool: Use the scenarios in this tool to practice and improve disclosure delivery.The Disclosure Lead(s) can use the descriptions of how they would respond to a challenging … �̚�Pրñ��]a�vb�2b��^�^O��� A correctly identified individual tries to modify system data from an external site; system maintains an audit trail and the correct data is restored within one day. Response measure. Source of stimulus. This leads to situations where the system is down and users are waiting for it, but the downtime is scheduled and so is not counted against any availability requirements. A general scenario for the quality attribute of availabil­ ity. That is, if code containing a fault is executed but the system is able to recover from the fault without it being observable, there is no failure. System Design Cheat Sheet In our scenarios we focus on aspects of usability that have a major impact on the architecture. Plummets and the life-cycle costs of a function, the architect ( or the.., all of the evidence points squarely in its operation availability is the change, both of can! This formulation does not consider whether the quality attribute Utility tree early held... Control system, the stimulus for the engine control system, you must make them system.! Breach security is allowing access to legitimate users from using services without or! 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