What is GES (Global Enqueue Service) in oracle RAC

 

GES (Global Enqueue Service) in Oracle is a critical component of the Oracle Real Application Clusters (RAC) architecture. It manages locks and resources across multiple instances in a RAC environment, ensuring that all nodes coordinate properly to avoid conflicts when accessing shared resources.

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Key Responsibilities of GES

1. Resource Management:

   • GES ensures that shared resources (such as rows, tables, and internal database structures) are accessed in an orderly and conflict-free manner across all RAC instances.

2. Lock Coordination:

   • It controls the allocation, tracking, and release of enqueue locks.
   • Enqueues are types of locks that protect access to shared resources.

3. Deadlock Detection:

   • GES identifies and resolves deadlocks that can occur when multiple instances wait indefinitely for resources held by each other.

4. Lock Mode Management:

• GES manages different lock modes (e.g., shared, exclusive) to ensure that resource access is appropriate to the operation being performed.

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How GES Works

1. Lock Request:

   • An instance requests a lock on a resource (e.g., a table or a row).
   • The GES checks if the lock can be granted based on the current state of the resource.

2. Conflict Resolution:

   • If another instance holds a conflicting lock, the GES either queues the request or triggers actions (e.g., converting lock modes, resolving deadlocks).

3. Global Enqueue Directory (GED):

   • GES maintains the Global Enqueue Directory, a centralized record of locks and resources across all instances.

4. Lock State Synchronization:

• GES ensures that all instances have a consistent view of lock states.

Examples of GES Enqueues

• DML Operations:

  • When one instance locks a row for update, GES ensures other instances cannot modify the same row until the lock is released.

• DDL Operations:

• When one instance alters a table, GES ensures other instances do not perform conflicting operations on the same table.

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Monitoring GES

You can monitor GES activity using Oracle dynamic views:

1. View Blocking Enqueues:

   
   SELECT * FROM gv$ges_blocking_enqueue;
   

2. View Resource Information:

   
   SELECT * FROM gv$ges_resource;
   

3. Deadlock Information:

• Deadlock-related information can be found in trace files or GV$ views like GV$EVENT.

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Common Issues GES Helps Avoid

1. Deadlocks:

   • Example: Instance A waits for a lock held by Instance B, while Instance B waits for a lock held by Instance A.

2. Lost Updates:

   • GES ensures that updates made by one instance are visible to others and prevents conflicting updates.

3. Resource Contention:

• By queuing or converting lock requests, GES ensures fair and efficient resource sharing.

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Summary

GES is a crucial part of Oracle RAC that handles global resource locking and ensures smooth coordination between multiple instances. While GCS focuses on data block sharing, GES is responsible for resource and lock management, helping Oracle RAC maintain consistency and prevent conflicts.