IBM Website Monitoring Response Time

Transcription

1 IBM Website Monitoring Response Time Xiao Ming Hu CSI IBM China Development Lab Document version 1.0 Copyright International Business Machines Corporation All rights reserved.

2 IBM Website Monitoring Response Time Foreword: IBM Website Monitoring is a pure SaaS offering for monitoring the availability of your public Internet facing websites from multiple geographically distributed points-of-presence. This article describes how to interpret the response time data reported by IBM Website Monitoring and answer the question that customers might ask: Is the response time accurate? 1. How is response time data collected? IBM Website Monitoring is built upon industry leading Selenium technology where you can record your synthetic scripts using the Selenium IDE Firefox browser plugin. This section introduces the technologies used in the IBM Website Monitoring solution and outlines how the response time data is collected. 1.1 Technologies Selenese Runner is a Selenese script interpreter. The IBM Website Monitoring playback engine uses this technology to parse synthetic scripts uploaded by the end user and calls the Selenium web driver to execute the commands. Selenium WebDriver implements the W3C specification of the WebDriver API. It's usually built as a web browser plugin to control the behavior of the browser. BrowserMob Proxy works as a web browser proxy; it can manipulate HTTP requests and responses and capture HTTP traffic. IBM Website Monitoring uses it to retrieve sub HTTP metrics and HTTP breakdown views for end users. Navigation Timing is the W3C specification that defines an interface for web applications to access timing information related to navigation and elements. Most web browsers can implement the APIs defined in this specification. 1.2 IBM Website Monitoring tests for Response Time IBM Website Monitoring currently supports two different web page tests: Availability test o A single URL is required to start monitoring. User Behavior test o A synthetic script must be recorded and uploaded by the end user. In IBM Performance Management V , the availability test is converted into a synthetic script which contains two Selenese commands: open and verifytitle. Thus the two types of web page test share the same workflow. 1

3 1.3 IBM Website Monitoring Response Times This section describes how IBM Website Monitoring measures the response times for transactions, sub-transactions, and HTTP breakdown. Transaction Response Time Transaction Response Time is the overall time elapsed from the beginning of the playback instance to the end of the playback instance. It can be viewed by the end user from various widgets on the Application Performance Dashboard. Figure 1: Transaction response times displayed on the Application Performance Dashboard For each playback or test, setstarttime and setendtime are executed by Selenese Runner immediately before and after the playback. The delta endtime-starttime is used as the response time for the synthetic transaction. This is the execution time of the command set defined in the synthetic transaction. Figure 2: Calculating the transaction response time 2

4 Sub-Transaction Response Time A synthetic transaction is composed of Selenese commands, which are also called subtransactions. Each Selenese command has its own response time. Figure 3: Sub-transaction response times displayed on the Application Performance Dashboard Selenese Runner sets starttime before executing a command and sets endtime after finishing a command. In IBM Performance Management V , the response time of subtransaction is the command execution time. Figure 4: Calculating sub-transaction response times in IBM Performance Management V

5 In IBM Performance Management V , to improve the accuracy of the measured response time for commands that cause a page load like the open command, the Navigation Timing API is used to collect the page load time from the web browser via JavaScript instead of using the stopwatch method. The page load time performance.timing.loadeventend - performance.timing.navigationstart is taken as the response time of the sub-transaction. Figure 5: Calculating sub-transaction response times in IBM Performance Management V HTTP Breakdown Response Time Only sub-transactions that cause a page load or reload in the web browser have HTTP breakdowns. For each entry in the HTTP breakdown, there is a response time which is recorded by BrowserMob proxy. BrowserMob proxy works a HTTP network traffic monitor to capture the HTTP metrics when the browser processes the DOM content of a web page. 4

6 Figure 6: HTTP response times displayed on the Application Performance Dashboard 2. Does the response time test have overheads? Yes - the response time measured by IBM Website Monitoring is slightly greater than that experienced by the end user. IBM Website Monitoring aims to simulate a real end user to navigate a web page and get the web performance timing data at real time. During a script playback, a Firefox browser is opened and the web page is navigated to automatically, just like when a real end user opens Firefox browser and enters the web page URL. The website performance data is collected in the background. To achieve this, Selenium WebDriver is embedded as a plugin and BrowserMob Proxy is set as a HTTP proxy in Firefox, which increases the response time of synthetic transactions, in comparison to navigating to web pages as a real end user. Mozilla Firefox Caches: For each playback or test, a fresh Firefox instance is started to ensure that the current playback does not harvest web resources from previous playbacks. IBM Website Monitoring makes sure that previous playbacks never corrupt the current playback to get more accurate performance data. This means that no cache in memory and disk, and no session related data are reused as well. A real end user might never clean the caches, which may result in faster response. Preferences: Firefox is a program, the preferences in the profile has impacts on the behavior. Comparing the Firefox preferences launched by IBM Website Monitoring with the default Firefox preferences, there are differences between them. For instance, all the auto update services are disabled in the Firefox of Synthetic playback, also including all the features that could break automated navigation via Firefox web driver. The most import difference is that Firefox web driver plugin is installed before each test. Any plugin installed has overhead to the Web browser. 5

7 BrowserMob Proxy To get sub HTTP traffics, BrowserMob Proxy is used to capture HTTP metrics data for each sub HTTP request when loading a web page. BrowserMob proxy contributes to the response time overhead, as shown in the following graphic. Figure 7: BrowserMob Proxy overhead 3. Do any other factors contribute to response time? Background The following diagram illustrates the timing attributes defined by the PerformanceTiming interface and the PerformanceNavigation interface with and without redirect, respectively. Figure 8: W3C Navigation Timing 6

8 There are two candidates for web page response time: DOM content load time (domcontentloaded-navigationstart) and page load time (loadeventendnavigationstart). IBM Website Monitoring takes page load time as the response time as real end user time. Figure 9: Page load timeline The page load timeline shows that the response time of a web page is also affected by the network and web server. Network IBM Website Monitoring points-of-presence are deployed in SoftLayer data centers across the world. Faster networks can provide shorter response times, while slower networks result in longer response times. The network condition is outside the scope of IBM Website Monitoring. Web Server Note: When comparing the response times collected by different methods, we must make sure that they are measured under the same network conditions. The web server determines how quickly a request can be handled. 7