Caching in a High Availibility Cluster

As described in a previous article, the Oracle BI Server maintains a cache entry to improve the response time for further Analysis, which can be satisfied with the cached result. The cached result itself is a binary file, ending with “.tbl”.

Within a clustered Oracle BI environment, a Global Cache can be configured on a shared storage, but each Oracle BI instances will maintain it’s own “local” cached result.
For “regular” BI Answers and Dashboard usage, the cached results will not be propagated to other instances within the cluster. Only when using Oracle BI Agents with the cache seed as target, will be propagated from one instances to another.

Within the Official Documentation the following diagram is shown (e.g. for Version 12.2.1.2) to describe the Global Cache (path to be configured within Oracle BI Enterprise Manager) and the synchronization / propagation of cached results within the Cluster:

image

However, this only applies to Analysis run using Agents and with the target for seeding the Cache:

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The propagation of cached results will be working for Reports and Dashboard Pages.

Once, an Agent has run to seed the Cache, the cached result (.tbl) file will be placed (propagated/pushed) to the Global Cache (on the shared storage) and other BI instances within the cluster will pull the cached result into their own local cache directory to satisfy further Analysis based on their polling intervals. For regular BI Answer requests, these caching files (.tbl) will not be placed (pushed) on the Global Cache (shared storage), nor will the other BI Servers nodes pull the cache file!

Example Scenario:

Thus, a Business User may be logged into instances 1 and based on his Analysis a cached entry will be created for further Analysis. However, this cached result will not be copied (or propagated) from instances 1 to instances 2. Hence, the same User may log into the system later that day and will be logged into instances 2 and will not received the previously cached result from instances 1. Only if the Analysis has been seeded via a Agent, the cache will be copied and propagated between both instances to satisfy the e.g. second Analysis.

This is also documented with the following Technote to correct/precise the Official Documentation:

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Disabling the Query Cache for BICS

Similar to OBIEE the Query Cache can be disabled on Report Level within BICS.

However, the approach is slightly different to OBIEE, since BICS doesn’t provide the Pre or Post-fix box on the Advanced Tab of the Report Builder (Answers).

The below example shows the configuration and Behavior of disabling the Query Cache in a Step-by-step approach:

When creating a (simple) Query, a Cache entry is created for the returned result set:

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For any subsequent Report, the Cache is analyzed if it can be used to fulfill the subsequent Report. E.g. when adding a Filter for Channel “Catalog”, the above Result set could be used:

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Giving the following Result:

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When analyzing the Query log, the Cache hit is also shown in the log itself:

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In order to bypass the Query Cache, the Query Cache needs to be disabled for the respective Report by adding the following command (setting of internal server variable) into the Report:

SET VARIABLE DISABLE_CACHE_HIT=1;

To do this, the SQL displayed on the Advanced Tab of the Report Builder (Answers) needs to be copied to the clipboard and clicking the new Analysis button.

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The previously copied SQL needs to be pasted in the popup Windows with the Disable Cache Hit command as prefix and the 0 needs to be removed from the pasted SQL:

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The Presentation Server Cache can also by bypassed by checking the check-box at the top of the screen. When navigating to the Results tab of Answers, the Report is run again and the Query log can be checked again:

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The Query log also shows that now the Query is not retrieved from the Cache, but retrieved from the Database.

A Technote also exist for further information on Oracle Support.

Understanding Oracle BI Server Cache

To enable the Oracle BI Server Cache, the Cache needs to be activated within the Oracle Enterprise Manager:
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Additionally, the NQSConfig file contains additional parameter to steer the entry and usage of the Oracle BI Server Cache:
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If the Cache is enabled, the created Cache entries can be viewed on the hard disk, but also the BI Admin Tool. The Cache files are placed in:

C:\oracle\middleware\instances\instance1\bifoundation\OracleBIServerComponent\coreapplication_obis1\cache

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These Cache entries hold the information about the logical SQL used to created the Cache and the returned result set for the query. The Cache Entries can also be viewed from the BI Admin Tool to provide further information about the respective Cache Entry and e.g. the last usage:

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Some Reports/Analysis may not qualify for a creation of a Oracle BI Server Cache entry. This can also be seen in the Session log, with the following statement:

The logical query block fail to hits or seed the cache in subrequest level due to [[ only one subrequest ]]

In general, this is due to one of the following reasons given from the Official Documentation:

  • Noncacheable SQL element. If a SQL request contains CURRENT_TIMESTAMP, CURRENT_TIME, RAND, POPULATE, or a parameter marker, then it is not added to the cache.
  • Noncacheable table. Physical tables in the Oracle BI Server repository can be marked “noncacheable.” If a query references any noncacheable table, then the query results are not added to the cache.
  • Cache hit. In general, if the query gets a cache hit on a previously cached query, then the results of the current query are not added to the cache.
  • The exception is query hits that are aggregate roll-up hits. These are added to the cache if the NQSConfig.INI parameter POPULATE_AGGREGATE_ROLLUP_HITS has been set to YES.
  • Result set is too big.This situation occurs when you exceed the size set in DATA_STORAGE_PATHS, or if you have rows in excess of the number set in MAX_ROWS_PER_CACHE_ENTRY. See Section A.3.2, “DATA_STORAGE_PATHS” and Section A.3.3, “MAX_ROWS_PER_CACHE_ENTRY” for more information.
  • Query is canceled. This can happen by explicit cancellation from Oracle BI Presentation Services or the Administration Tool, or implicitly through timeout.
  • Oracle BI Server is clustered. Queries that fall into the ‘cache seeding’ family are propagated throughout the cluster. Other queries continue to be stored locally. Therefore, even though a query might be put into the cache on Oracle BI Server node 1, it might not be on Oracle BI Server node 2.

Additionally, one of the following cases may also prevent from entries being created within the Cache:

  • Caching is not enabled (CACHE=N in the NQSConfig.INI file)
  • The WHERE clause in the logical SQL is not semantically the same, or not a logical subset of a cached statement
  • the columns in the SELECT list not yet exist in the cached query, or they are not able to be calculated from them
  • It has not equivalent join conditions, so that the resultant joined table of any incoming query is not the same as (or a subset of) the cached results
  • If DISTINCT is used, the cached copy has to use this attribute as well
  • Aggregation levels are not compatible, since they either have to be the same or more aggregated than the cached query
  • No further aggregation (for example, RANK,MAX) can be used in the incoming query
    (sub request / sub Report)
  • Any ORDER BY clause has to use columns that are also in the cached SELECT list

As an example, the following (main) Report:

cache main report

Is based on the results of another Analysis, which also uses an Aggregation:

cache sub report

The main Report is executed, but no Cache Entry is created:

cache monitor

This can also be analyzed from the Session log for the SQL for this statement:

session log

Since, the Sub Report also contains an Aggregation, no Cache Entry will be created for the Report. This can be seen in the Session log (may be required to set log level to 3 or even 4 to see below message):

cache message

To allow the Oracle BI Server to create a Cache Entry, the Analysis needs to be re-written. E.g. the Sub Report needs to be removed and needs to be part of the (main) Report. In the above case, by creating the Aggregate in the (main) Report and by the “BY” clause within the formula to specify the Group element.

Once, a Cache Entry is created for one User, it may also be shared to fulfill a request from another User. This can be seen from the Oracle BI Admin Tool within the Cache Manager Screen:

cache usage

Sometimes Users are also mislead in terms of Cache usage since Oracle BI provides two cache systems:

And even if the Oracle BI Presentation Server Cache is not enabled (by default not enabled) the Oracle BI Presentation Server still has a Cursor Cache which can be viewed from the Session Monitor (http://localhost:9704/analytics/saw.dll?Sessions) and which can not be disabled and only purged manually from the Session Monitor “Cancel All Cursor” Button.

Understanding Oracle BI EE Presentation Server Cache(s)

Within the Oracle BI System, multiple components provide a caching mechanism to re-use previously executed Reports to improve query performance of subsequent requests. The following diagram provides an Overview of the main Architecture and main caching components:

Oracle BI Server Cacheing cropped

By default, the Oracle BI Presentation Server Cache is not actived. However, the Oracle BI Presentation Server additionally provides a Oracle BI Presentation Server Cache which can not be deactived. This “Cursor” Cache can (only) be viewed from the Session Monitor Administration Page http://localhost:9704/analytics/saw.dll?Sessions:

session monitor

Thus, even when the Oracle BI Presentation Server Cache is not actived, subsequent execution of the same Analysis maybe retrieved from the Oracle BI Presentation Server Cursor Cache which may often mislead the perception of cacheing.

For the first execution of an Analysis a new Cursor and Cursor Cache maybe created eventually:

cache monitor

A subsequent execution of the same Report will return instantly (even with no Presentation Server Cache configured) since the Report can be retrieved from the Presentation Server Cursor Cache. No additional Cursor will be created and only the time of execution will be updated for the previous Cursor.

cache monitor 2

As said before, the Cursor Cache can not be disabled and the Cursor Cache can only be purged manually from the Session Monitor. However, a Report/Analysis can be foreced to bypass the Presentation Server Cursor Cache (and Presentation Server Cache) by checking the bypass Presentation Server Cache option on the Advanced Tab of a Analysis.

bypass

The disk directory of the Cursor Cache is not documented by Oracle, but they are stored under the query cache folders of:

C:\oracle\middleware\instances\instance1\tmp\OracleBIPresentationServicesComponent\coreapplication_obips1\querycache_0

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Which places a result set file for the respective Presentation Server Cursor Cache:

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To active the regular Presentation Server Cache, the instanceconfig.xml needs to be edited. The instance config can typically found here (for 11g):

C:\oracle\middleware\instances\instance1\config\OracleBIPresentationServerComponent\coreapplication_obips1

To active the Caching the following xml elements need to be added to the file:

<ServerInstance>
  <Cache>
    <Query>
      <MaxEntries>100</MaxEntries>
      <MaxExpireMinutes>60</MaxExpireMinutes>
      <MinExpireMinutes>10</MinExpireMinutes>
      <MinUserExpireMinutes>10</MinUserExpireMinutes>
    </Query>
  </Cache>
<ServerInstance>

After restarting the Presentation Server, the Cache Files will be created in the following directory:

C:\oracle\middleware\instances\instance1\tmp\OracleBIPresentationServicesComponent\coreapplication_obips1\obis_temp

Summary: Even with no Presentation Server Caching enabled, the Presentation Server Cursor Cache will cache result sets to fulfill subsequent Analysis requests from the Client.