Managing Sequences in RAC Environment

Agenda

• Introduction
• Application Sequences with Non-RAC Databases
• Oracle Sequences with Non-RAC Databases
• Application Sequence with RAC Databases
• Oracle Sequences with RAC Databases
• Sequences and Index Contention
• Migration Issues for Existing Appplications
• Sequences and Services
• Demonstration
• Questions & Answers

Oracle Sequences with RAC Databases

• Created and administered as in a Single Instance
• Real Application Clusters allows multiple instances to obtain unique numbers from the same sequence
• Sequences in Real Application Clusters:
  • Have the same options as single-instance databases plus the ORDER and NOORDER options
  • ORDER and NOORDER options control ordering of generated numbers
  • The Global Cache Service may need to coordinate sequences across instances
  • SQ Enqueue will be Global

Issues with Application Defined Sequences in Single Instance Databases

• Problems with this approach

  • Row level locking will cause contention
  • Update of sequence is not a recursive transaction
  • This may be handled by using Autonomous Transactions for the update of the sequence
  • At some level of concurrency contention will be a serious issue

• Advantages with this approach

  • Gaps may be eliminated by deferring the commit at the risk of additional contention but this can not occur with an Autonomous Transactions

Issues with Application Sequences in RAC Databases

• Problems with this approach

  • Same issues as with a Single Instance plus:
  • Extra Global Enqueue Services traffic due to Global TX and TM Enqueues for both Transaction and Table
  • Extra Global Cache Services traffic for shipping the dirty table block amongst involved instances

• Advantages with this approach

  • Same advantages as with a Single Instance

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