.NET/ASP.NET/C#/VB.NET PDF Document SDK

A list of known countries won t change very often, but patient records might change while in use, particularly if the patient is currently in the hospital So when caching copies of information locally in a dictionary, you need to have some way of dealing with the fact that the information in the cache might be stale (For example, although caching patient records locally would be useful, your application might need to deal with the possibility that new test results become available while the patient is with the doctor) As with removal policy, detection of stale data requires application-specific logic For example, some data might never change accounting data usually works this way, because even when data is discovered to be incorrect, it s not usually modified Legal auditing requirements usually mean you have to fix problems by adding a new accounting record that corrects the old one.

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Figure 2-12. The form with the buttons Now locate the vertical spacer in the widget box (it is in the spacers group near the top). Drag the spacer to the dialog and place it in the gap between the upper three buttons and the lower one, as shown in Figure 2-13.

So for this sort of entry, you know that a cache of any given record will never be stale (Newer records might exist that supersede the cached record you have, but the cache of that record will still be consistent with whatever is in the database for that entry) Sometimes it might be possible to perform a relatively cheap test to discover whether the cached record is consistent with the information on a server HTTP supports this a client can send a request with an If-Modified-Since header, containing the date at which the cached information was known to be up-to-date If the server has no newer information, it sends a very short reply to confirm this, and will not send a new copy of the data.

Web browser caches use this to make web pages you ve previously visited load faster, while ensuring that you always see the most recent version of the page..

The ZedGraph fires an event upon rendering, which occurs when the page is loaded or refreshed. This event is called RenderGraph. In this case, the page is going to take two parameters, one for the ticker of the stock to be rendered and the other for the number of days to render. These will be used to make a call to the DataTier web service to get the DataTable back. The DataTable will then load the graph with the appropriate data. Listing 11-2 shows the full code for the ZedGraphWeb1_RenderGraph event handler. Listing 11-2. Rendering the Graph protected void ZedGraphWeb1_RenderGraph( System.Drawing.Graphics g, ZedGraph.MasterPane mPane) { int nDays = 0; int nRows = 0; GraphPane pane = mPane[0]; PointPairList pt = new PointPairList(); double nx; double ny; string days = (string)Page.Request.Params["days"]; string ticker = (string)Page.Request.Params["ticker"]; if (ticker != null) { ticker = ticker.Trim(); DataTier theDataTier = new DataTier(); if (days == null) nDays = 0; else nDays = Convert.ToInt32(days); DataTable dtTable = theDataTier.GetFullPriceHistory(ticker,nDays); nRows = dtTable.Rows.Count;

But you may simply have to guess. Sometimes the best staleness heuristic available to you might be something such as If the cached record we have is more than 20 minutes old, let s get a fresh copy from the server. But you need to be careful with this approach. Guesswork can sometimes lead to a cache that offers no useful performance improvements, or which produces data that is too stale to be useful, or both. Regardless of the precise details of your cache removal and staleness policies, the approach will look something like Example 9-5. (The Record type in this example is not a class library type, by the way. It s just for illustration it would be the class for whatever data you want to cache.)

Now select the four buttons and the spring, and then apply a vertical layout so that you get the form shown in Figure 2-15. You can select multiple items by clicking and holding the Shift key or by dragging a box containing the items that you want to select. Notice that you do not want to add the layout from the widget box. Instead, select the widgets that you want inside the layout and click the vertical layout button in the toolbar shown in Figure 2-14. The buttons are the following (from left to right): Apply horizontal layout places the widgets in horizontal row. Apply vertical layout places the widgets in a vertical row. Horizontal splitter places the widgets in a horizontal row, but also enables the user to adjust the size of the widgets. Vertical splitter places the widgets in a vertical row, but also enables the user to adjust the size of the widgets. Apply grid layout places the widgets in a stretchable grid. Break layout removes any current layout. Adjust size adjusts the size of the current layout so that the contained widgets fit. Try holding the pointer over the toolbar buttons to find the one with the tooltip Lay Out Vertically, which is the one you want.

class RecordCache { private Dictionary<int, Record> cachedRecords = new Dictionary<int, Record>(); public Record GetRecord(int recordId) { Record result; if (cachedRecords.TryGetValue(recordId, out result)) { // Found item in cache, but is it stale if (IsStale(result)) { result = null; } } if (result == null) { result = LoadRecord(recordId); // Add newly loaded record to cache cachedRecords[recordId] = result; } DiscardAnyOldCacheEntries(); } return result;