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 Multi-Version Consistency Model Architecture
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Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Oracle’s multi-version consistency model architecture distinguishes between a current data block and one or more consistent read (CR) versions of the same block. It is important to understand the difference between the current block and the CR block. The current block contains changes for all the committed and yet-to-be-committed transactions. A consistent read (CR) block represents a consistent snapshot of the data from a previous point in time. Applying undo/rollback segment information produces consistent read versions. Thus, a single data block can reside in many buffer caches under shared resources with different versions.


Multi-version data blocks help to achieve read consistency. The read consistency model guarantees that the data block seen by a statement is consistent with respect to a single point in time and does not change during the statement execution. Readers of data do not wait for other writer's data or for other readers of the same data. At the same time, writers do not wait for other readers for the same data. Only writers wait for other writers if they attempt to write. As mentioned earlier, the undo, i.e. rollback segment provides the required information to construct the read-consistent data blocks. In case of a multi-instance system, like the RAC database, the requirement for the same data block may arise from another instance. To support this type of requirement, past images of the data blocks are created within the buffer cache. Past images will be covered later in the chapter.

Process Architecture

Oracle uses several processes to execute the different parts of Oracle code and to spawn additional processes for the users. Whenever a user connects to the database, a new server process is created on behalf of the user session. Functions of the server process include the following:

  • Parse and execute SQL statements issued through the application

  • Read the necessary data blocks from the disk data files into the shared database buffers of the SGA if the blocks are not already present in the SGA

Interact and return results in such a way that the application can process the information

There are many additional processes that are automatically spawned whenever the instance starts. These processes are called background processes, and they perform the Oracle kernel functions.

Locking Mechanism

Locking is another important requirement of a multi-version consistency model. Oracle also uses a locking mechanism to control concurrent access to the data blocks. Locks help prevent destructive interaction between users accessing data blocks.


As seen earlier, when a user intends to update a data block that has already been updated by another user but is still in an uncommitted state, the update event has to wait. Without a lock mechanism, the data integrity would have been lost in this case. Locks also ensure that the data being viewed or updated by a user is not changed by other users until the user is finished using the data.


In the case of a RAC system, new and improved components or services namely, the Global Cache Service (GCS) and Global Enqueue Service (GES) handle the lock and access management functions better than the earlier Oracle Parallel Server distributed lock manager (DLM). GES is covered in more detail later in this chapter.


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