The railway transport sector is a key enabler of economic growth worldwide. The United Kingdom (UK) has a railway network of 17,732 km of track (the 17th largest in the world) which is spread over wide geographical areas throughout the country [1]. The number of railway passengers as well as freight volumes has increased significantly in recent years. According to recent statistics published by the Office of Rail and Road (ORR), a total of 1.654 billion journeys were made in 2014–2015, making the UK’s railway network the fifth most used in the world [2]. The growth of journeys is partly attributed to a shift away from private motoring due to increasing road congestion, but also to the improved quality of railway transport services. The British railway industry was privatised over the period 1994–1997, but nowadays most of the railway tracks are managed by Network Rail (NR) [3]. Nevertheless, the network is still confronted with serious problems caused by premature failure of assets that require costly and time-consuming maintenance work.
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The railway assets in general can be categorised into two types: The first one is the infrastructure which consists of fixed assets such as tracks, points and interlocking, bridges, signalling system, electrical units, etc. The other one is the rolling stock which includes assets that can move on railway, e.g. locomotives, passenger coaches, freight cars. A rolling stock is a multi-component system that consists of wheels, bogies, doors, power unit, brake control unit, coupler, compressor, pantograph, etc. Figure 1 illustrates the major components of a British Class 800 rolling stock asset and their relationships to one another. A failure of any of rolling stock components can cause a complete failure of the system and consequently lead to traffic delays and disruptions, passenger inconvenience and economic losses for train operating companies. Rolling stock failures may also result in the derailment of waggons and casualties of passengers and crew. For these reasons, it is crucial to develop practical methodologies for analysing and mitigating the risks associated with failure of various rolling stock components at a system level.
In recent years, a great deal of attention has been paid to the study of the failure/damage mechanisms for railway infrastructure assets. However, few attempts have been made by researchers to develop failure criticality assessment models for rolling stock components. There are several tools and techniques that are currently used to determine and evaluate the risk of failures occurring in engineering systems throughout their entire life cycle—from design to production, operation and maintenance. One of the widely used techniques in this regard is the failure mode, effects and criticality analysis (FMECA) which is an extended version of the failure mode and effects analysis (FMEA) method [4, 5]. In the FMECA technique, all potential failure modes that could occur in various components of a system are systematically analysed. The causes of each failure mode and their associated impact on system operation are identified. A “risk” or “criticality” measure is then calculated for each failure mode based on the rate of occurrence of failure and severity of the possible consequences. Finally, the failure modes are prioritised or classified according to their levels of criticality and some preventive actions are proposed to improve the reliability of the system.
In this paper, the potential risks of unexpected failures occurring in rolling stock are identified, analysed and evaluated using a FMECA-based approach. The criticality of a failure is measured as the product of the likelihood of occurrence of the failure mode (O) and the severity of damage caused by the failure (S), where O and S are allocated numbers from 1 to 10. According to criticality levels ranging from 1 (lowest) to 100 (highest), the most critical failure modes in the rolling stock with respect to both reliability and economic criteria are identified. Finally, several potential protective measures to eliminate the root causes of rolling stock failures are provided. The presented model is applied to a rolling stock passenger door system in a Scottish train operating company and the results are discussed.
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The remainder of this paper is organised as follows. Section 2 gives a brief overview of the risk evaluation in the railway industry. Section 3 presents a FMECA methodology for risk evaluation of rolling stock failures. In Sect. 4, a case study of the passenger train door system is described and the results are presented in detail. Finally, the paper is concluded in Sect. 5.
This chapter provides a basic methodology for testing the scalability and performance of Web applications throughout the life cycle. It outlines the process for selecting the appropriate tools and the recommended steps to perform effective scalability testing. This chapter is broadly divided into the following sections:
2.1Goals of Scalability Testing
The primary goals of a load test are as follows:
2.2Phases of Scalability Testing
The following are the different phases of load and scalability testing for a Web application:
Architecture Validation - tests the scalability of the architecture early in the development of a Web application, presumably after a prototype of the application has been created that can generate transactions to touch all tiers of the application. This allows the engineering organization to determine the viability of the architectural framework selected to build the Web application.
Performance Benchmarking - sets and creates the benchmark tests for the initial version of the application for all business transactions and gives the engineering and the quality assurance groups a set of metrics to quantify the scalability of the application. Based on the requirements specified, the development group will either maintain this scalability or improve upon it through the subsequent milestones.
Performance Regression - is the phase where the Web application is tested with the established benchmarks to ensure that the changes made to the application do not result in degradation of scalability. These tests are executed when key milestones have been reached or architectural modifications have been made during the development of the application. It is also common that the benchmark tests and the metrics originally set for the application be replaced or augmented with additional tests and newer metrics to reflect the improvements made to the application.
Acceptance and Scalability Fine Tuning - is the final load testing phase prior to the official launch of the Web application where all the different pieces of the Web application - including the hardware, load balancing components and all software components - are integrated and the scalability is validated. Different scenarios of real-life usage are emulated and the scalability of the final configuration is validated. These different scenarios are also used to configure the hardware and software components to yield optimal performance.
24x7 Performance Monitoring - after the application is deployed, it is essential to monitor the performance of the system under the real load generated by actual users so that crashes or slow-downs can be spotted before they become problematic. In this phase, data pertaining to real life usage can be collected to help refine future scalability tests for accurate emulation of load.
2.3Criteria for Accurate Scalability Testing
In order to emulate a realistic load that will correlate with real-life usage of the application, a load-testing tool must:
In addition, the following criteria are also important:
Overall Transactions-Per-Second Throughput Required - What is the overall transactions per second (TPS) throughput required for the load test? This can be computed based on the number of simultaneous business transactions and the duration of typical transactions.
Type of Error Handling - What type of error handling is required when executing the load test? Does the load test need to be stopped on encountering certain types of error or just log the error and continue? What types of error logging do we need to enable for each concurrent user and for the different components in the application architecture?
Type of Transaction and Performance Data Logging - What type of transaction and performance data needs to be logged for the various scripts?
2.8.3Planning the Scalability Tests
Developing detailed test plans before you actually create the tests is an important step in making sure the tests conform to the business analysis of the application and the defined criteria.
For each test that will perform a business transaction you need to plan and define the following information:
Steps for Scripts - Each script should have a detailed sequence of steps that define the exact actions a user would perform. Multiple scripts can be used. For example, you can define a specific script that performs user login, several scripts that perform specific business transactions, and another script that logs users off. For each script, you should define the expected results. Oracle OpenScript lets you quickly and easily record scripts that emulate a user's actions.
Run-Time Data - The test plan should specify any run-time data that is needed to interact with the application, for example, login user IDs, passwords, and other run-time data specific to the application.
Data Driven Tests - If the scripts require varying data at run-time, you'll need to have an understanding of all the fields that require this data. You also need to define the data sources and any tool(s) needed to either create fictitious data or extract real data from existing databases.
Oracle OpenScript Data Bank Wizard lets you specify and connect external data sources to the scripts.
2.8.4Planning the Load Test Scenarios
In addition to the business transaction details for each script, the test plan should also specify the different user groups and test scenarios that will be required for load testing. For each test scenario you need to plan and define the following information:
Type of User - Is this user a first-time user of the application or a repeat user? This is important if the application responds differently for a first-time user than it does for a repeat user and places more stress on the server. Oracle Load Testing scenarios can specify either a first-time user or a repeat user.
Transactions to Perform - Which business transaction(s) will this user perform? In what sequence? If the application requires a first-time user to perform some type of registration, then the user profile for first-time users should include a registration script.
Number of Users - How many virtual users with this user profile will run over the same time interval? Oracle Load Testing lets you specify the number of virtual users for each test scenario.
Which System - Which specific computer(s) will be used to generate the load for this user group? Oracle Load Testing can run virtual users on a single system or on multiple, distributed systems running Oracle Load Testing agents. Oracle Load Testing can specify which virtual user scenarios run on which workstations.
Which Browser - Which browser will this user group emulate? Oracle Load Testing can specify virtual user scenarios emulate either Internet Explorer or Netscape.
Pacing mode - What pacing mode will be used for the user group? Will the testing be performed using recorded think times, a range of times, or as fast as possible? Oracle Load Testing virtual user scenarios let you specify recorded, random, or no pacing.
Delay Between Business Transaction Runs - What delay time will be included between business transactions, if any? Oracle Load Testing lets you specify the amount of delay time between transaction runs.
With or Without Images - Will the user group run with images or without images? You may want to create different user groups that perform load testing both with and without images for comparison. Oracle Load Testing provides this capability.
2.8.5Create and Verify the Test Scripts
After planning the scripts, you will use Oracle OpenScript to create and verify each script.
Create the Scripts - This process is defined by Oracle OpenScript (recording user actions) and the individual test plans for each script. When creating the script, you specify the following information as defined in the test plan:
Verify the Scripts - Once each script is created, you should verify that the script performs as expected and produces the desired result. Each script should be verified independent of any other scripts and in a controlled manner to simplify script debugging.
2.8.6Create and Verify the Load Test Scenarios
Once the individual scripts have been created and verified, you can create and verify the load test scenarios. It will save you a lot of time and aggravation if you perform a number of simple verification steps before your full-blown load test.
Verify scripts with Multiple Virtual Users - Before combining multiple scripts into a single load test scenario, you should verify that you can successfully run a single script as multiple virtual users. Each script should perform as expected as defined by the criteria for the application.
Oracle Load Testing Autopilot lets you run multiple scenarios with different virtual user characteristics.
Verify distributed test execution on multiple machines - You should verify the load test tool's ability to execute the individual scripts properly in a distributed environment if you plan to use multiple CPU's for load generation. This usually involves a master system controlling the virtual user execution on multiple workstations on the network. This can help you isolate any installation or networking-related issues.
Verify real-life scenarios that include one of each user group - Before executing the full load test you should create and verify a scenario that includes one virtual user of each user group you wish to run at the same time. That is, before you run a test with 20 VU's of group A, and 20 VU's of group B, and 60 VU's of group C, you should first run one VU of group A, one VU of group B, and one VU of group C, and make sure that the results are as expected.
Create real-life scenarios - This process should be defined in the test plans for each scenario. When creating the individual scenarios, you specify the following information as defined in the test plan:
2.8.7Execute the Tests
Once you have created and verified the basic load test scenarios above, you can begin to run the load test scenarios with many virtual users, and expect that the test results will be valid.
Run basic tests to ensure scaling - run tests with a minimal amount of virtual users to ensure that the system scales up correctly.
If the above two scenarios execute without any problems, the next step is to execute the full load test with the full number of virtual users of each user-group type.
Run the real-life scenarios - Run each of the real scenarios as outlined in the previous steps:
Re-Run these scenarios with a real user - While the load test is running, a real person should access the system through a standard browser and report performance observations:
2.8.8Evaluate the Results
For each of the load test scenarios, examine the following performance data and validate the results against the expected criteria:
View Run graph options let you evaluate performance in real-time.
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Save the erroneous HTML when problems occur to help the development group debug the errors.
Installation Steps For Virtual Wire Mode Evaluation Criteria Example2.8.9Generate Analysis Reports
Document the performance by generating the various reports that may be required for acceptance and deployment of the application. The following are some examples of the types of reports that can be generated from a load test:
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Oracle Load Testing graphs in the Create Reports tab let you view performance and error data from the load test in multiple formats.
2.9SummaryPalo Alto Virtual Wire Mode Limitations
Load testing throughout the development cycle has become an essential part of the process of designing scalable, reliable Web applications. Developers and QA professionals now rely on load testing tools as a means to validate system architectures, tune applications for maximum performance, and assess the impact of hardware upgrades. Consequently, it is critical that the load test results can be used with confidence as the basis for key decisions about application readiness and potential changes to the system's hardware and software. Using the methodology embodied in this guide along with accurate load testing tools such as Oracle Load Testing, you now have a systematic approach to ensure the performance of your Web applications. With load testing established as a routine part of the application lifecycle you can be sure to avoid costly 'scalability surprises' when your application goes live for the first time or after any subsequent release.
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