.NET/Java PDF, Tiff, Barcode SDK Library

from XML documents or extract data in XML. Data represented as XML carries various tags and meta-information that helps to identify what sort of data is contained within. This also amounts to a larger size, but typically this can be compensated for by applying compression on the XML text. As an example, consider the following classic XML example (contacts.xml): <contacts> <contact> <name>John Smith</name> <phone type="home">+1 626-123-4321</phone> </contact> </contacts> One way to represent and work with XML documents is via the XML Document Object Model (DOM) contained in the System.Xml namespace, and you saw how to work with this model in 9. Using the XML DOM constructors and methods, you can create the previous XML as follows: open System.Xml let doc = new XmlDocument() let rootNode = doc.CreateElement "contacts" doc.AppendChild rootNode |> ignore let contactNode = doc.CreateElement "contact" let nameNode = doc.CreateElement "name" let nameText = doc.CreateTextNode "John Smith" let phoneNode = doc.CreateElement "phone" phoneNode.SetAttribute("type", "home") let phoneText = doc.CreateTextNode "+1 626-123-4321" nameNode.AppendChild nameText |> ignore contactNode.AppendChild nameNode |> ignore contactNode.AppendChild phoneNode |> ignore phoneNode.AppendChild phoneText |> ignore rootNode.AppendChild contactNode |> ignore Here you are building up an XML document in a bottom-up fashion via a series of method calls that mutate the main XML document object. This means various XML elements cannot be constructed without this document container object and also construction by mutation makes the shape of the XML hard to read. Using XmlWriter is a bit more readable: let doc = new XmlDocument() let writer = doc.CreateNavigator().AppendChild() writer.WriteStartElement "contacts" writer.WriteStartElement "contact" writer.WriteElementString ("name", "John Smith") writer.WriteStartElement "phone" writer.WriteAttributeString ("type", "home") writer.WriteString "+1 626-123-4321" writer.WriteEndElement() writer.Close()

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Wait Time for Log File Sync (Seconds)

29.86 2.49 0.32 0.03 0.0

Here you don t have to worry about creating the structure of the document; instead, you simply output each element in a sequence. XmlWriter will also take care of the closing tags, even if you forget them before closing the writer.

As you can see, the more often you commit, the longer you wait (your mileage will vary on this). And the amount of time you wait is more or less directly proportional to the number of times you commit. Remember, this is just a single-user scenario; with multiple users doing the same work, all committing too frequently, the numbers will go up rapidly. We ve heard the same story, time and time again, with similar situations. For example, we ve seen how not using bind variables and performing hard parses often severely reduces concurrency due to library cache contention and excessive CPU utilization. Even when we switch to using bind variables, soft parsing too frequently caused by closing cursors even though we are going to reuse them shortly incurs massive overhead. We must perform operations only when we need to a COMMIT is just another such operation. It is best to size our transactions based on business need, not based on misguided attempts to lessen resource usage on the database. There are two factors contributing to the expense of the COMMIT in this example: We ve obviously increased the round-trips to and from the database. If we commit every record, we are generating that much more traffic back and forth. I didn t even measure that, which would add to the overall runtime. Every time we commit, we must wait for our redo to be written to disk. This will result in a wait. In this case, the wait is named log file sync.

So, we committed after every INSERT, we waited every time for a short period of time and if you wait a little bit of time but you wait often, it all adds up. Fully thirty seconds of our runtime was spent waiting for a COMMIT to complete when we committed 10,000 times in other words, waiting for LGWR to write the redo to disk. In stark contrast, when we committed once, we didn t wait very long (not a measurable amount of time actually). This proves that a COMMIT is a fast operation; we expect the response time to be more or less flat, not a function of the amount of work we ve done. So, why is a COMMIT s response time fairly flat, regardless of the transaction size It is because before we even go to COMMIT in the database, we ve already done the really hard work. We ve already modified the data in the database, so we ve already done 99.9 percent of the work. For example, operations such as the following have already taken place: Undo blocks have been generated in the SGA. Modified data blocks have been generated in the SGA.