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When a web site stops working correctly, the time it is unavailable can often be measured in dollars. The example we ll use in this chapter is a web site monitor, a program used to ensure that a web site is in a state such that it can service user requests in a timely manner. One approach to web site monitoring is to have a predefined page that returns an indication of health. The page is retrieved at regular intervals. This page often exercises various objects or assemblies, perhaps accesses a database, and returns a reasonable estimation of the health of the web server the page resides on. Listing 5.1 uses the System.Net.WebClient object to retrieve a page referenced by a URL.
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Listing 5.1 Retrieving a web page using WebClient (VB.NET)
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Imports System.Net Imports System.Threading Public Class WebSiteMonitor Private Private Private Private URL As String MonitorThread As Thread SleepTime As Integer LastRequestHowLong As TimeSpan
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Public Sub New(ByVal URL As String, ByVal SleepTime As Integer) Me.URL = URL Me.SleepTime = SleepTime MonitorThread = New Thread(AddressOf ThreadMethod) MonitorThread.Name = "WebSiteMonitor" End Sub . . . Private Sub ThreadMethod() Dim Notify As Boolean
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While True Notify = False Dim client As New WebClient()
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Dim data As Byte() Dim StartTime As DateTime = System.DateTime.Now Retrieves ' Retrieve the Page the page data = client.DownloadData(URL) Dim StopTime As DateTime = System.DateTime.Now LastRequestHowLong = StopTime.Subtract(StartTime) Dim Results As String Results = System.Text.Encoding.ASCII.GetString(data) If Results.IndexOf("OK") < 0 Then Notify = True End If If Notify Then ' Let someone know End If Thread.Sleep(SleepTime) End While End Sub End Class
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Processing begins by creating an instance of the WebClient class. The current time is recorded so that the time required to retrieve the page can be calculated. The DownloadData method is used to return the contents of the page as an array of Bytes. Once the page is downloaded, the time is recorded. In order to easily interact with the contents page, we must convert it from a Byte array to a string using the GetString method of the System.Text.Encoding.ASCII class. If the resulting string does not contain "OK" the Notify flag is set to true, indicating that someone should be notified that the web site is in an unhealthy state. The idea is that a dynamic page will return the status of the web site. If the system is in a healthy state, the page will return. Notification could be through the addition of an entry to the NT Event Log, or some other means. Figure 5.1 gives a high-level view of how the application logically functions. When we monitor a web site, we generally pause between each check. If the pause is too short, we have written a web site stress-testing tool instead of a monitor. If we pause too long, we may miss something important. We ve seen the Sleep method in previous chapters. It causes a thread to pause for a period of time. We will examine
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EXAMPLE: WEB SITE MONITORING
the Sleep method in detail in section 5.3.1. At times we wish to check the state of the site immediately; perhaps someone has reported that the web site is down. In threading terms, this is accomplished using the Interrupt method. When the connection from the monitoring machine to the Internet goes down, often the best approach is to stop checking the site until the connection has been reestablished. This relates to the Suspend and Resume methods that we cover in section 5.5. At the point it becomes apparent that the connection is down, the thread polling the site should be suspended. Once it is determined that polling should continue, Resume should be called on the suspended thread. When multiple sites are being checked, some are likely more important than others. We may want to ensure that the thread that is checking the more important site is given the opportunity to do its work first. In section 5.6.1 we cover how to adjust the thread s priority. We most likely will want to have a thread whose job is to notify someone when there is a problem. That notification is important and needs to happen at a higher priority than the monitoring. Often the computer doing the monitoring is not dedicated to that task. In that case it is desirable to control how many of the resources of the computer are used. We may wish to control what processors in a multiple-processor machine can be used to check the status of the web sites. In this case processor affinity lets us control what processors a process utilizes. Processor affinity is an advanced topic, and not something generally done. We ll cover it in section 5.7 since it relates to thread control. If you limit what processor a process uses, you ve also limited what processor a thread can use.
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