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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.

When Oracle reads a data block into the cache, it opens a GCS resource to coordinate concurrent access by multiple instances. Oracle coordinates and converts the resource into different modes and roles, depending on the following:

  • Whether the data block accessed will be modified or read

  • Whether a data block exists in the cache of only one instance or in multiple caches

Thus, a resource is a concurrency control mechanism on the data blocks. It is also called a GCS resource.


In a stand-alone Oracle instance, various locks are used to control data integrity and concurrency. Similarly, the multi-instance RAC architecture deals with cached data block mode roles and the controlling access levels. The cached data blocks acquire a global nature.


GCS resources comprise the concurrency control mechanism on data blocks. They include enqueues involving the transaction locks, table locks, library cache locks, and the dictionary cache locks. Global enqueue resources are normally held for a very short time and then quickly released. For example, the TX locks are acquired whenever a transaction starts. They are released immediately after the transaction commits or rolls back. Figure 2.9 shows the main groups of resources involved in the synchronization process.


Figure 2.9: Resources for Coordination


Now, the three main components - the global cache service, global enqueue service, and the global resource directory - will be examined more closely.

Global Cache Service

The main function of the Global Cache Service (GCS) is to track the status and location of the data blocks. Status is the resource role and the resource mode. The GCS is the main mechanism by which cache coherency among the multiple caches is maintained.


GCS maintains the modes for blocks in the global role and is responsible for block transfers between the instances. The LMS processes handle the GCS messages and carry out the bulk of the GCS processing. GCS resource coordination is explained in detail in later sections.

Global Enqueue Service

The Global Enqueue Service (GES) tracks the status of all Oracle enqueuing mechanisms. This involves all non-cache fusion intra-instance operations. The GES performs concurrency control on dictionary cache locks, library cache locks, and transactions. It performs this operation for resources that are accessed by more than one instance.


What exactly is an enqueue? Enqueues are shared memory structures that serialize access to the database resources. For example, when a user updates a row and gets a row-level lock, a TX enqueue is created for that user. If another user subsequently attempts to update that same row, that user's session will block or wait on the enqueue that the initial user created. They are sometimes referred to as positive waits.


In a single instance environment, enqueues are local to the instance.  With RAC, the enqueues can be global to the database. Enqueues are comprised of transaction locks, DML locks, SCN locks, and such.


Enqueues are associated with a session or a transaction. They are held longer than the latches, have finer granularity, more modes than the latches, and protect more database resources. For example, when a table lock is requested, the request is assigned to an enqueue. Oracle can use enqueues in any of three modes: null (N) mode, shared (S) mode, or exclusive (X) mode.


The GES controls access to data files and control files but not for the data blocks. GES processing includes the coordination for enqueues other than the data blocks. The resources managed by the GES include the following:

Transaction locks – It is acquired in the exclusive mode when a transaction initiates its first row level change. The lock is held until the transaction is committed or rolled back.

Library Cache locks - When a database object (such as a table, view, procedure, function, package, package body, trigger, index, cluster, or synonym) is referenced during parsing or compiling of a SQL, DML or DDL, PL/SQL, or Java statement, the process parsing or compiling the statement acquires the library cache lock in the correct mode.

Dictionary Cache Locks - Global enqueues are used in the cluster database mode. The data dictionary structure is the same for all Oracle instances in a cluster database as it is for instances in a single-instance database. However, in real application clusters, Oracle synchronizes all the dictionary caches throughout the cluster. Real application clusters use latches to do this, just as in the case of a single-instance Oracle database.

Table locks  – These are the GES locks that protect the entire table(s). A transaction acquires a table lock when a table is modified. A table lock can be held in any of several modes: null (N), row share (RS), row exclusive (RX), share lock (S), share row exclusive (SRX), or exclusive (X).


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