Simple .NET/ASP.NET PDF document editor web control SDK

LINQ to XML (LinqToXml) offers a new and easier way to work with XML than using the traditional XML DOM The SystemXmlLinq namespace contains everything you need to construct, load and save, manipulate, and query over XML documents You can reference the DLL containing this namespace using this: #I @"c:\Program Files\Reference Assemblies\Microsoft\Framework\v35" #r "SystemXmlLinqdll" Being a data format for tree-like structures, the XML trees are made up by a collection of XNode objects Structurally, there are two XNode descendants that can contain other nodes (and thus inherit from XContainer, a subclass of XNode): XDocument and XElement Therefore, all XML documents are represented either as an XDocument with nested XElement objects or simply as a collection of nested XElements.

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Buffered redo for the preceding two items has been generated in the SGA. Depending on the size of the preceding three items and the amount of time spent, some combination of the previous data may be flushed onto disk already. All locks have been acquired.

When we COMMIT, all that is left to happen is the following: A System Change Number (SCN) is generated for our transaction. In case you are not familiar with it, the SCN is a simple timing mechanism Oracle uses to guarantee the ordering of transactions and to enable recovery from failure. It is also used to guarantee read-consistency and checkpointing in the database. Think of the SCN as a ticker; every time someone COMMITs, the SCN is incremented by one. LGWR writes all of our remaining buffered redo log entries to disk and records the SCN in the online redo log files as well. This step is actually the COMMIT. If this step occurs, we have committed. Our transaction entry is removed from V$TRANSACTION this shows that we have committed. All locks recorded in V$LOCK held by our session are released, and everyone who was enqueued waiting on locks we held will be woken up and allowed to proceed with their work. Some of the blocks our transaction modified will be visited and cleaned out in a fast mode if they are still in the buffer cache. Block cleanout refers to the lockrelated information we store in the database block header. Basically, we are cleaning out our transaction information on the block, so the next person who visits the block won t have to. We are doing this in a way that need not generate redo log information, saving considerable work later (this is discussed more fully in the upcoming Block Cleanout section).

A fully qualified XML document (an instance of XDocument) contains meta-information such as declarations (added as a child XDeclaration object) and a DTD (added as a XDocumentType) and may contain various XML processing instructions (instances of XProcessingInstruction) Typically, you need an XDocument only if the XML you produce is exchanged with the external world (such as information passed through a web service, for instance); in any other case, you will be working with a collection of nested XElement objects These can have other XNode objects such as XText for storing text, which often represents binary data encoded using Base64 encoding mentioned in 10, or XComment for embedding comments On the other hand, XElements typically have attributes (XAttribute) key/value pairs, which are non-XNode objects Both the XElement names (the tag itself) and the XAttribute keys are XName objects.

As you can see, there is very little to do to process a COMMIT. The lengthiest operation is, and always will be, the activity performed by LGWR, as this is physical disk I/O. The amount of time spent by LGWR here will be greatly reduced by the fact that it has already been flushing the contents of the redo log buffer on a recurring basis. LGWR will not buffer all of the work you do for as long as you do it. Rather, it will incrementally flush the contents of the redo log buffer in the background as you are going along. This is to avoid having a COMMIT wait for a very long time in order to flush all of your redo at once. So, even if we have a long-running transaction, much of the buffered redo log it generates would have been flushed to disk, prior to committing. On the flip side is the fact that when we COMMIT, we must typically wait until all buffered redo that has not been written yet is safely on disk. That is, our call to LGWR is by default a synchronous one. While LGWR may use asynchronous I/O to write in parallel to our log files, our transaction will normally wait for LGWR to complete all writes and receive confirmation that the data exists on disk before returning.