This article discusses Oracle 9i Scalability Enhancements - what's changed in Oracle 9i to enable your databases to support more users without adding more resources.
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Introduction
Real Application Clusters
Scalable Session State Management
Automatic Resource Management
Summary
Further Reading/Other resources
The previous five articles looked at the enhancements that have been made to the Oracle database in release 9i and the impact these have on business and technical issues. In this issue the focus is on the changes made to enable Oracle databases to support more users with clustering and other changes enabling more users to be supported without having to add more hardware. For more information about the new features see the resources section which reviews books for further reading and contains links to information on OTN and other places.
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http://www.smart-soft.co.uk/Oracle/oracle-9i-security-enhancements.htm
http://www.smart-soft.co.uk/Oracle/oracle-9i-manageability-enhancements.htm
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Oracle have always boasted of the ability of their databases to support large numbers of users, but scalability is more than just being able to support many users, it is also a case of how easy it is to increase the maximum number of users that can be supported. Oracle 9i has performance enhancements that reduce resource requirements somewhat but there is also the need to be able to add more resources to support more users and to provide redundancy in case of failure. Real Application Clusters allow exactly that, thereby making scalability simply a case of adding more servers to the cluster.
Oracle 7 and 8 supported a form of clustering called Oracle parallel server, but this was complex to implement because the application had to be aware that it was running in a parallel server configuration. Real Application Clusters in Oracle 9i are a replacement for and an enhancement of the previous parallel server technology. Applications no longer have to be aware that they are running in a cluster and new servers can be added to or removed from the cluster at any time without having to shut down the database at all.
The major business benefits of Real Application Clusters (RACs) are :
In technical terms the new features include a new "cache fusion architecture" which as the name suggests treats the memory caches of all the servers in the cluster as a single cache. This means that reads from disk only have to take place when none of the servers has the requested data in memory. If one of the servers does have the required data in memory, it is transferred across the high-speed bus connecting the servers, to the memory of the server that made the request.
Other new features include :
The amount of resources required by Oracle 9i databases for each user has been reduced by improvements to the network layer, shared server (what used to be called multithreaded server) enhancements, improvements for the support of Java sessions and shared memory improvements. Together these improvements allow more users to be supported on the same hardware.
Oracle 8i introduced the facility to allocate resource quotas to various groups of users (known as consumers) by use of the database resource manager. This enhanced the ability of the dba to share resources appropriately amongst the users by giving them the ability to manage mixed workloads (e.g. reporting and transaction processing) and to control system performance. The resources that could be controlled were the percentage of CPU time, the degree of parallelism and priorities
Oracle 9i improves on this by providing the dba with more granular control over resources, adding the following new resource plan directives:
Let's look at these in more detail.
The undo pool quota controls the maximum amount of undo allowed for a consumer group. When the limit is reached, the active DML statement is terminated and no other members of the consumer group can perform any DML until undo space is released from the pool by other sessions within the group or the dba manually increases the undo quota for the group.
This directive controls the maximum number of active sessions allowed for a consumer group. When the limit is reached, new sessions can be queued until a session becomes available. A time-out period can also be specified to determine the maximum length of time a new session will wait until it is aborted with an error.
This enables the dba to set an execution-time limit for an operation. If the database estimates that an operation will exceed this limit, the operation is aborted and an error returned to the user. This error can be trapped and the operation rescheduled if needed.
Analogous to the execution time quota is the ability to specify that a session should be switched automatically to a different consumer group if it is active (i.e. running and consuming resources, not idle) for more than the specified time. This switch will occur even if the active session quota for the new group has already been reached. One the previously switched session becomes idle again, it is switched back to its original consumer group. The session can also be switched prior to starting execution if the database estimates that the operation will take longer than the specified time limit and the use of estimates is specified.
The biggest change in terms of scalability is undoubtedly the introduction of Real Application Clusters for Windows, Linux and UNIX. This feature provides load-balancing, scalability and hot-standby all rolled into one without requiring any application redesign or database shutdown to add more resources to the cluster.
The enhancement of resource management to include undo pool, execution time and active session quotas is also useful for heavily used systems as is the reduction in resources required to support a user.
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